Estuaries
Vol. 23, No. 6, p. 793–819
December 2000
Estuaries of the South Atlantic Coast of North America: Their
Geographical Signatures
RICHARD DAME1,*, MERRYL ALBER2, DENNIS ALLEN3, MICHAEL MALLIN4, CLAY MONTAGUE5,
ALAN LEWITUS3, ALICE CHALMERS2, ROBERT GARDNER3, CRAIG GILMAN1, BJÖRN KJERFVE3,
JAY PINCKNEY6, AND NED SMITH7
1
Coastal Carolina University, Conway, South Carolina 29528
University of Georgia, Athens, Georgia 30602
3Baruch Marine Laboratory, University of South Carolina, Georgetown, South Carolina 29442
4University of North Carolina at Wilmington, Wilmington, North Carolina 28403
5University of Florida, Gainesville, Florida 32611
6Texas A&M University, College Station, Texas 77843
7Harbor Branch Oceanographic Institute, Fort Pierce, Florida 34946
2
ABSTRACT: Estuaries of the southeastern Atlantic coastal plain are dominated by shallow meso-tidal bar-built systems
interspersed with shallow sounds and both low flow coastal plain and high flow piedmont riverine systems. Three general
geographical areas can be discriminated: the sounds of North Carolina; the alternating series of riverine and ocean
dominated bar-built systems of South Carolina, Georgia, and northeast Florida, and the subtropical bar-built estuaries
of the Florida southeast coast. The regional climate ranges from temperate to subtropical with sea level rise and hurricanes having a major impact on the region’s estuaries because of its low and relatively flat geomorphology. Primary
production is highest in the central region. Seagrasses are common in the northern and southern most systems, while
intertidal salt marshes composed of Spartina alterniflora reach their greatest extent and productivity in South Carolina
and Georgia. Nuisance blooms (cyanobacteria, dinoflagellates, and cryptomonads) occur more frequently in the northern
and extreme southern parts of the region. Fishery catches are highest in the North Carolina and Florida areas. Human
population growth with its associated urbanization reaches a maximum in Florida and it is thought that the long-term
sustainability of the Florida coast for human habitation will be lost within the next 25 years. Tidal flushing appears to
play an important role in mitigating anthropogenic inputs in systems of moderate to high tidal range, i.e., the South
Carolina and Georgia coasts. The most pressing environmental problems for the estuaries of the southeastern Atlantic
coast seem to be nutrient loading and poor land use in North Carolina and high human population density and growth
in Florida. The future utilization of these estuarine systems and their services will depend on the development of
improved management strategies based on improved data quality.
framework related to estuarine type and location.
Anthropogenic influences resulting from increasing human population density, urbanization, industrial development, and changing agricultural
practices are then addressed. Finally, we synthesize
these characteristics into geographical signatures
for southeastern estuaries and point out important
informational gaps. The paper as a whole seeks to
provide a current overview of the status of southeastern Atlantic estuaries and to serve as an informational starting point for those interested in the
structure and function of these ecosystems.
Introduction
The southeastern coast of the United States is a
broad coastal plain bordered with barrier islands
and beaches interspersed with tidal inlets and rivers. After centuries of relatively low human population density, this highly productive area is now
undergoing intense development that is rapidly
impacting the structure and function of regional
estuarine ecosystems. In this review, we first describe the location and types of estuaries in the
region. Then the available published data on the
physical parameters, i.e., salinity, water fluxes, and
nutrient fluxes, influencing these systems are presented comparatively. The biotic components of
these estuaries are examined in a trophic dynamic
Geographical and Geological Setting
The southeastern coastal plain of the United
States is divided into two relatively large geographical reaches. The South Atlantic Bight (SAB) is located between Cape Hatteras, North Carolina
(35.38N and 75.58W) and Cape Canaveral, Florida
(28.48N and 80.68W) and the southeastern Florida
* Corresponding author: address: Marine Science, P.O. Box
261954, Coastal Carolina University, Conway, South Carolina
29528; tele: 843/349-2216; fax: 843/349-2926; e-mail:
dame@coastal.edu.
Q 2000 Estuarine Research Federation
793
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R. Dame et al.
coast stretches from Cape Canaveral to the Florida
Keys (25.58N). This trailing edge coast is distinguished by mainland Pleistocene terraces and
Pleistocene or Holocene barrier islands further
seaward, and a broad, shallow continental shelf.
The ocean boundary is often demarcated by the
presence of the Gulf Stream (Florida current south
of Cape Canaveral), a major oceanographic current that flows northward along this area transporting heat, materials, and organisms (Fig. 1).
The northern portion (the SAB) is a gently curving bight of about 1,000 km (along coast). A band
of salt marshes, intersected by networks of tidal
creeks, occupies low elevation areas between the
mainland and the barrier islands. The salt marshes
attain their greatest width (approximately 12 km)
at the apex of the SAB between Beaufort, South
Carolina and Brunswick, Georgia, where tides for
the region reach their maximum range of 3 m.
Along its entire extent, the SAB receives input
from many rivers of piedmont and coastal plain
(blackwater) origin.
Two distinct estuarine areas dominate the southeastern Florida portion of the region: the Indian
River Lagoon and Biscayne Bay. These systems are
also confined to the area between the barrier islands and the mainland, but instead of salt marshes, mangrove wetlands predominate. Indian River
lagoon system, including Mosquito Lagoon and Banana River lagoon, stretches north-south from
Ponce de Leon Inlet (29.28N), just north of Cape
Canaveral, to Jupiter Inlet (26.98N) or about 248
km. Biscayne Bay is bounded to the north by Dumfoundling Bay (25.98N) and to the south by Florida
Bay (25.28N). These southeastern Florida estuaries
drain the interior of the state east of the northward-flowing St. John’s River and the southwardflowing ‘‘river of grass’’ through the Everglades, via
an extensive network of drainage canals and small
streams.
Without the strong and dedicated service of a
cadre of scientists working within the region, it
would be difficult to report on the estuaries of the
southeast Atlantic coast. For the most part these
investigators are associated with university research
laboratories, state and federal laboratories, and
specific research or protected areas (Fig. 2). While
Fig. 2 is not an all-inclusive list, it does show that
there is extensive coverage of the estuarine and
coastal environments across the region.
Types of Estuaries
BAR-BUILT
AND
LAGOONAL
Bar-built or lagoonal systems form behind offshore barrier sand islands and usually have small
adjacent upland watersheds. Tidal inlets that occur
between the barrier islands allow for the exchange
of water between the estuaries and the sea. In some
cases, the water body behind the barrier islands is
extensive and referred to as a sound, i.e., Pamlico
Sound, North Carolina. In others, where smaller
expanses of water occur behind the islands, the
term lagoon is used, (e.g., Mosquito Lagoon, Florida). Throughout the region bar-built systems with
extensive marshes and minimal areas of open water are found, (e.g., North Inlet, South Carolina).
Astronomical tides are the major forces controlling
circulation and water height in inlets, while wind
or freshwater runoff is more controlling in sounds.
Water flow in the shallow interior portions of lagoons is often controlled by wind action. Because
the areas behind the barrier islands are subject to
less wave action and are generally depositional environments, their embayments are dominated by
extensive wetlands. From North Carolina to Cape
Canaveral in Florida, Spartina salt marshes dominate, while further south mangrove swamps prevail. Vernberg et al. (1992) reported that there are
more than 320 small bar-built systems (, 60 3 106
m2) between Cape Fear and Cape Canaveral. They
also calculated that the total area of these small
systems in South Carolina exceeds the total combined area of the major riverine systems, (e.g.,
Winyah Bay, Santee, Charleston Harbor, and Port
Royal Sound). Because these systems are smaller
individually than riverine systems, their surface
area to volume ratio is higher than that of riverine
estuaries. A higher ratio suggests that these small
systems may play a greater role in coastal ecological
processes than previously thought because they potentially have a greater proportion of surface area
over which exchanges of materials can take place.
PIEDMONT
One type of estuary originates in the hilly areas
of the piedmont. These estuaries have extensive
watersheds, receive substantial freshwater discharge, usually develop two-layered gravitational
circulation in a longitudinally compressed saline
zone, may carry suspended sediment loads of clay
particles, and have a relatively small proportion of
the watershed covered by wetlands. Because high
freshwater flow rates (average . 120 m3 s21) carry
high loads of suspended clay particles, these systems are sometimes high flow estuaries. The piedmont estuaries common to the region are the
Neuse and Cape Fear River in North Carolina,
Winyah Bay, Santee, and Edisto systems in South
Carolina, and the Savannah and Altamaha of Georgia. Several of these systems are composites of
piedmont and coastal plain tributaries. The Cape
Fear River originates in the piedmont and is supplemented by two coastal plain streams. Winyah
Southeast Atlantic U.S. Estuaries
795
Fig. 1. A sea surface temperature image of the Southeast Coast of the United States (April 14, 1996) showing the Gulf Stream.
Cooler waters are in blue and warmer waters are in red. Numbers indicate tidal range in meters and mean monthly minimum
temperature in 8C.
796
R. Dame et al.
Southeast Atlantic U.S. Estuaries
Bay is a complex system where both coastal plain
rivers (Sampit, Black, and Waccamaw) and piedmont rivers (Little Pee Dee and Great Pee Dee)
mix and enter the Atlantic Ocean through the bay.
Also, the Ogeechee estuary in Georgia originates
in the piedmont, but most of its flow comes from
the coastal plain.
COASTAL PLAIN
The other type of riverine estuary that occurs in
the region is known as a coastal plain system. In
this case, the rivers drain watersheds entirely contained within the coastal plain. These systems have
sluggish flow rates with low and highly variable discharge, occupy smaller watersheds, possess a larger
proportion of wetlands, and generally contain a
more extensive saline zone. These systems are often called blackwater rivers because the high concentrations of humic and tannic acids resulting
from tree roots and decaying vegetation give their
waters a clear black or tea colored appearance. Examples of these estuaries are the New River in
North Carolina, the Cooper in South Carolina,
Ogeechee, Satilla, and St. Marys in Georgia, and
the St. Johns in Florida.
FRESHWATER INFLOW
Most of the estuaries in the continental U.S. are
impacted by human manipulation of their freshwater inflow (Dynesius and Nilsson 1994). In a survey of river fragmentation, 12 of the 19 large rivers
(flow . 350 m3 s21) that flow through the continental U.S. were considered strongly impacted, 6
were considered moderately impacted, and 1 (the
Pascagoula) was considered unimpacted by channel fragmentation and flow regulation (Dynesius
and Nilsson 1994). The survey considered four rivers in the southeast. Two of these, the Santee and
the Savannah, were classified as strongly impacted,
with 5 dams in each system. The Pee Dee, with 4
dams, and the Altamaha, with 3, were each classified as moderately impacted. The region also includes some of the few remaining unimpounded
rivers in the country, and many of the smaller
797
coastal plain rivers (e.g., Waccamaw, Edisto, Ogeechee, Satilla) are free-flowing (Table 1).
In addition to the presence of dams and diversions, freshwater, in the form of both groundwater
and surface water, is withdrawn from the watersheds of all of the rivers in the region. For example, coastal Georgia has long been dependent on
groundwater from the Upper Floridian Aquifer,
which underlies all of Florida, coastal Georgia, and
parts of eastern Alabama. However, parts of the
aquifer are subject to saltwater intrusion (e.g.,
Brunswick, Savannah, Hilton Head Island) (Clarke
et al. 1990). In 1997 the Georgia Environmental
Protection Division imposed upper limits on
groundwater use in several coastal counties, and in
some cases set goals for reductions in groundwater
withdrawal (Environmental Protection Division
1997).
Traditionally, the water cycle has been presented
as water flowing from the land to the sea via rivers
and estuaries. Recent evidence from groundwater
studies that examined the concentrations of the radioactive isotope 226Ra in the SAB region (Moore
1996) generate estimates of groundwater fluxes to
the coastal ocean that are comparable to those of
the regions rivers. The magnitude of these fluxes
has a number of implications (Church 1996). First,
our current estimates of the chemical mass balance
in coastal waters may be radically altered. Second,
as noted above, increasing human population densities in the coastal zone are making ever greater
demands on the coastal aquifers and this results in
larger interaction between saline and freshwater
aquifers. Further, increasing sea level and the consequential need to maintain navigational channels
at even greater depths are also likely to enhance
this interaction. As the coastal human population
continues to grow, this means that any new or expanded water use in the region will require an alternative water source. In other words, future development along the coast will likely be coupled to
surface water withdrawal. Several studies are now
underway to understand how increased surface water withdrawal will impact the region’s estuaries.
←
Fig. 2. Distribution of estuarine data sources along the southeastern Atlantic coast. KEY: ECU 5 East Carolina University, NEP 5
an Environmental Protection Agency’s National Estuary Program site, DUKE 5 Duke University Marine Laboratory, NMFS 5 National
Marine Fisheries Service, UNC 5 University of North Carolina, NCSU 5 North Carolina State University, UNC-W 5 University of
North Carolina at Wilmington, NERR 5 National Oceanic and Atmospheric Administration’s National Estuarine Research Reserve,
CCU 5 Coastal Carolina University, USC 5 University of South Carolina, BARUCH 5 Baruch Marine Field Laboratory, UCHARLESTON 5 University of Charleston/College of Charleston, SC-DNR 5 South Carolina Division of Natural Resources Laboratory, NOAA
5 National Oceanic and Atmospheric Administration, SKIDAWAY 5 Skidaway Institute of Oceanography, UGA-SAPELO 5 University
of Georgia Marine Laboratory at Sapelo Island, LMER 5 a National Science Foundation’s Land-Margin Ecosystem Research, LTER
5 Long Term Ecological Research site, JU 5 Jacksonville University, UFL-WHITNEY 5 University of Florida—Whitney Marine Laboratory, FIT 5 Florida Institute of Technology, HARBOR BRANCH 5 Harbor Branch Institute of Oceanography, UMIAMI-ROSENSTIEL 5 University of Miami—Rosenstiel School of Marine and Atmospheric Sciences.
798
R. Dame et al.
TABLE 1. Physical and hydrologic characteristics of southeastern Atlantic coast riverine estuaries. NC 5 North Carolina, VA 5
Virginia, SC 5 South Carolina, GA 5 Georgia, and FL 5 Florida.
Drainage Area (km2)
Estuarine Zones (km2)
State
Estuarine
Fluvial
Tidal
Fresh
NC, VA
NC, VA
NC
NC
SC
SC
SC
SC
SC, GA
GA
GA
GA
GA
FL
FL
FL
12,585
5,051
5,208
11,176
24,671
1,818
3,089
3,809
3,263
3,752
3,907
8,219
4,386
15,840
3,093
6,746
32,451
5,679
8,859
12,413
22,288
0
38,028
8,454
24,760
8,381
33,055
2,023
0
7,375
0
0
Estuary
Albemarle Sound/Chowan/Roanoake Rivers
Pamlico/Pungo Rivers
Neuse River
Cape Fear River
Winyah Bay/Pee Dee/Black Rivers
North/South Santee Rivers
Charleston Harbor/Wando/Cooper/Ashley Rivers
St. Helena Sound/S. Edisto/Coosaw Rivers
Savannah River
Ossabaw Sound/Ogeechee River
Altamaha River
St. Andrew/St. Simons Sounds/Satilla River
St. Marys River/Cumberland Sound
St. Johns River
Indian River
Biscayne Bay
Mixing
598 1,899
0
452
5
451
1
76
12
59
0
18
1
58
0
111
7
37
11
39
5
29
7
103
0
7
511
156
0
0
0
94
Seawater
AverFW
Estuary age Volume Inflow Tidal
Area Depth (m3
(m3 Range
(km2)
(m) 3 109) s21)
(m)
0 2,497
0
452
0
456
23
100
17
89
0
18
25
85
92
203
77
121
38
88
5
39
67
176
57
64
17
684
866
866
608
702
4.3
2.7
3.7
3.4
3.0
2.1
4.9
4.0
4.6
4.0
3.4
4.0
6.1
3.4
2.1
2.4
10.7
1.3
1.6
0.3
0.4
0.0
0.6
1.0
0.8
0.4
0.1
0.8
0.5
2.3
1.7
1.6
317
69
118
221
465
366
159
85
344
86
396
65
20
151
34
121
0.6
0.5
0.2
1.1
0.8
1.1
1.4
1.7
2.1
1.9
1.9
2.1
1.8
0.7
0.3
0.5
* Coastal Assessment and Data Synthesis System 1999.
SALINITY
Understanding the impacts of freshwater input
to estuaries and the resulting changes in salinity
regimes may be one of the most important challenges facing coastal scientists and managers. Salinity is a primary indicator of estuarine circulation
because of its conservative character; it is also a
significant determiner of biological productivity,
faunal distributions, and habitat structure. Orlando et al. (1994) synthesized salinity data from 15
estuaries in North Carolina, South Carolina, and
Georgia by characterizing salinity structure and
spatial and temporal variability. A very abbreviated
summary of Orlando et al. (1994) findings are provided in Table 2. Three Georgia estuaries, the Altamaha, Satilla, and Ogeechee Rivers are among
the most variable in salinity. River input of freshwater is the primary causative agent of salinity variability in these estuaries. Contrastingly, Albemarle
Sound, Charleston Harbor, and the Broad River
are among the most stable. The latter two systems
are shallow coastal plain estuaries with minimal
freshwater input, while one of the two rivers that
drains into Albemarle Sound is heavily regulated
by dams.
MATERIAL FLUXES
The major materials of carbon, nitrogen, and
phosphorus are rapidly translocated, transformed,
and remineralized within estuarine ecosystems.
Abiotic factors including river flow, tides, and windgenerated waves and currents are major components in translocating materials. Primary production and consumer feeding and metabolism generate both particulate and dissolved materials that
influence estuarine nutrient cycles (Dame and Allen 1996). In general, estuaries are large filters or
traps (Biggs and Howell 1984) for materials that
can be transformed by resident processes.
Because of the high productivity of southeastern
estuaries, there have been extensive investigations
into the fate of materials produced in these systems. In riverine estuaries, transport of dissolved
and particulate materials is generally seaward due
to the net flux of water down slope due to gravity.
Estimates of physical and hydrological parameters
for a number of southeastern coastal plain and
piedmont riverine estuaries are given in Table 1.
The average annual water discharge data clearly
distinguishes the two types.
To estimate the amount of nutrients being loaded into southeastern riverine estuaries, data on dissolved inorganic nutrient concentrations were
compiled from water quality surface sampling stations in 8 of the major riverine systems in the SAB.
Water quality data were obtained from EarthInfo
(1997), and daily discharge data were obtained
from the United States Geological Service (USGS)
web site (water.USGS.gov). Only those rivers with
systematic sampling programs and consistent analysis protocols were included in this analysis. Stations chosen were located the furthest downstream
in each system. In most cases, dissolved inorganic
nitrogen (NH4, NO31NO2) records ranged from
1979 through 1994 and dissolved orthophosphate
(PO4) records covered from 1981 through 1994.
When examined on a quarterly basis, concentrations of ammonium and nitrate plus nitrite did not
vary greatly over the course of the year (Fig. 3a,b).
However, maximum concentrations were observed
Southeast Atlantic U.S. Estuaries
799
TABLE 2. The character of salinity structure and variability in southeastern Atlantic coast estuaries. Data from Orlando et al. (1994).
Salinity variability: Very Low 5 ,2‰; Low 5 3–5‰; Medium 6–10‰; High 5 11–20‰.
Estuary
Albemarle/Pamlico Sounds, North Carolina
Bogue Sound (Beaufort, North Carolina)
New River, North Carolina
Cape Fear River, North Carolina
Winyah Bay/North Inlet, South Carolina
North and South Santee Rivers, South Carolina
Charleston Harbor, South Carolina
St. Helena Sound, South Carolina
Broad River, South Carolina
Savannah River, Georgia
Ossabaw Sound/Ogeechee River, Georgia
St. Catherines/Sapelo Sounds/Satilla River, Georgia
Altamaha River, Georgia
St. Andrews/St. Simons Sounds, Georgia
St. Marys River/Cumberland Sound, Georgia
Dominant Temporal Scale
Months-Years
Hours-Years
Hours-Years
Hours-Years
Hours-Years
Hours-Years
Hours-Years, Episodic
Hours-Years
Hours-Years
Hours-Years
Hours-Years
Hours-Years
Hours-Years
Hours-Years
Hours-Years
during the spring quarter (April–June). Orthophosphate concentrations were more variable (Fig.
3c). These were highest during the summer
months ( July–September) and lowest during late
winter ( January–March).
Average annual nutrient concentrations (Fig. 4)
were multiplied by average annual discharge or
freshwater inflow (Fig. 5) to obtain an estimate of
the loads of dissolved inorganic nutrients in each
river (Fig. 6). It should be noted that these loads
reflect what enters the upper end of the estuaries
and not what is loaded to the coastal ocean. Many
of these systems have extensive estuarine areas,
with additional nutrient sources located downstream of the riverine USGS sampling stations (i.e.,
point-source discharges, and tidal and non-tidal
creeks draining areas of significant human usage).
Moreover, the amount of time river water spends
in the estuary before being discharged to the coastal ocean will influence the amount of material processing that occurs within the estuary. For example, in the Richmond River estuary in Australia,
Eyre and Twigg (1997) found that flushing times
controlled whether nutrients were internally processed or discharged to the shelf. In a study of
flushing times in the Georgia riverine estuaries, Alber and Sheldon (1999) found that median annual
flushing times ranged from 6 d in the Savannah to
72 d in the St. Marys, but that there was considerable interannual and intra-annual variability in
these estimates. The highest inorganic nutrient inputs enter into the estuaries of the Cape Fear, Pee
Dee, and Savannah Rivers. The Pee Dee and Savannah are the two largest systems considered, with
large fluvial drainage basins that likely contribute
nutrients in the form of fertilizer runoff. The Cape
Fear, although a relatively large river, has about
half of the fluvial drainage area and a lower river
Mechanism(s)
Tides, winds
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides, Winds
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Freshwater, Tides
Variability
Low-Medium
Medium-High
Medium-High
Medium
Medium
Medium
Very Low-Low
Low-Medium
Low
Low-Medium
Medium-High
Medium
Medium-High
High
Low-Medium
discharge rate than the Pee Dee and Savannah.
However, the highest concentrations of all nutrients were reported in the Cape Fear River, which
averaged 0.80 mg l21 N and 0.11 mg l21 P as compared to an average of 0.27 mg l21 N and 0.05 mg
l21 P in the remaining 7 rivers. The Cape Fear River basin contains more than one-quarter of North
Carolina’s population, and is the most industrialized river basin in the state (641 National Pollution
Discharge Evaluation System monitored discharges). Agricultural nutrient sources are also abundant, with approximately 24% of the land usage
devoted to agricultural operations (North Carolina
Division of Water Quality 1996). The basin also
contains over five million head of swine (North
Carolina Division of Water Quality 1996), mainly
in concentrated animal operations (CAOs) that
have only primitive animal waste treatment facilities (Mallin 2000).
Recently, Alberts and Takacs (1999) did a similar
compilation of USGS water quality data to estimate
dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) loads into southeastern
U.S. estuaries. They had data from the same rivers
used above for inorganic nutrients, with the exception of the Cape Fear and the addition of the Altamaha. DOC concentrations were higher in rivers
that originate in the Coastal Plain (average concentrations in the Satilla and St. Marys Rivers were
19.1 and 27.9 mg C l21, respectively) than in those
that originate in the Piedmont (average concentrations in the Altamaha and Pee Dee were 8.5 and
5.9 mg C l21, respectively). DON concentrations
followed similar patterns, with average concentrations of 0.75 and 0.66 mg N l21 in the Satilla and
St. Marys Rivers, respectively, and 0.35 and 0.46 mg
N l21 in the Altamaha and Pee Dee, respectively.
Although bulk DOC measures generally indicate
800
R. Dame et al.
Fig. 4. Average quarterly nutrient concentrations (with SE)
for selected coastal plain and piedmont estuaries on the southeast Atlantic coast.
simple conservative mixing of freshwater dissolved
organic material (DOM) through estuaries, the use
of fluorescence signals to trace terrestrial DOM
within the Georgia estuaries suggests a highly dy-
Fig. 3. Average annual concentration (with SE) of inorganic
nutrients in selected coastal plain and piedmont estuaries on
the southeast Atlantic coast.
Fig. 5. Average discharge of selected coastal plain and piedmont estuaries on the southeast Atlantic coast.
Southeast Atlantic U.S. Estuaries
Fig. 6. Average nutrient loads in metric tons (with SE) for
selected coastal plain and piedmont estuaries on the southeast
Atlantic coast. A. ammonium; B. nitrate 1 nitrite; C. orthophosphate.
namic picture of the DOM pool, with inputs from
freshwater sources, intertidal marsh sources, and
even marine sources for some systems (Moran personal communication).
The concentrations of particulate material in an
estuary can range widely. For example, observed
concentrations of suspended sediment in Georgia
estuaries ranges from 8–200 mg l21, depending
upon sampling depth, tidal stage, and current
speed (Georgia Rivers Long Term Ecological Research [GARLMER] data). In the Ogeechee River
Estuary, Alber (2000) demonstrated that the biological constituents of the suspended sediment
were preferentially associated with fine-grained,
801
slowly settling material, which are likely transported out of the estuary with the net residual flow.
Meade (1972) argued that most of the fine-grained
sediments that reach the Atlantic Continental
Shelf are actually transported back into the estuaries. This contention is supported by an observed
rapid decrease in the concentration of inorganic
material between the inner and outer shelf, which
suggests that river sediment is not being moved
seaward (Manheim et al. 1970) and evidence for
net landward transport of fine material (Meade
1969; Windom et al. 1971; Mulholland and Olsen
1992). Meade (1972, 1982) further suggested that
coastal marshes are important traps for finegrained sediment along the Atlantic coast of North
America. This is supported by the observation that
marsh elevations are generally keeping up with the
relative rate of sea level rise (Vogel et al. 1996),
which suggests they are sites of net deposition.
However, material that is deposited in marshes may
undergo a much longer period of net deposition
and resuspension in exchange with tidal creeks
(Chalmers et al. 1985) before it is eventually buried (Stevenson et al. 1986, 1988).
In a recent study of carbon exchange between
the Georgia riverine estuaries and the extensive intertidal marshes that border these systems, Cai et
al. (1999) noted that CO2 concentrations measured in estuarine waters could not be explained
by respiration within the estuarine water column
itself. They demonstrated that CO2 and HCO3 are
produced in the marsh, and then funneled into
the estuarine water by tidal action. Interestingly,
the outwelling of organic matter to the estuaries
was minor in comparison to the release of inorganic carbon. Their data show that in a brackish
water marsh in the Satilla River Estuary, nitrate
that is brought to the marsh via estuarine water is
removed, most likely via denitrification (Cai et al.
in press).
Bar-built and lagoonal systems usually lack significant river flow and have extensive vascular
plant subsystems (Kjerfve 1989). In these systems,
the transport of particulate material is typically governed by the flux of tidal waters. In the Duplin
River, Georgia (Chalmers et al. 1985) and North
Inlet, South Carolina (Dame et al. 1986), the tidalvelocity asymmetry, also known as ebb dominance,
leads to the seaward transport of suspended materials (Ward 1981; Kjerfve 1989). In some systems,
particulate materials may be imported probably as
a result of passive and active filtration by subsystems, i.e., salt marshes, oyster reefs, etc. (Dame et
al. 1989). In an effort to explain this diversity of
material transport, Dame et al. (1992) proposed
the geohydrologic continuum theory of marsh-estuarine ecosystem development. In this theory, ma-
802
R. Dame et al.
ture components are at the ocean-estuary interface
and export particulate and dissolved materials.
Young or immature subsystems are at the land-estuary interface and import particulate and dissolved materials. Intermediate systems are midaged and import particulate materials and export
dissolved materials. Some bar-built estuaries may
have all three developmental stages or subsystems,
e.g., North Inlet (Dame et al. 1992), while others
may only have one or two subsystems. In lagoonal
systems, many materials are processed and retained by the estuary so that only some substances
are transported to the adjacent sea. Which materials are processed, retained, or transported depends on the specific subsystem. Tidal marshes, for
example, function as giant filters or traps removing
and processing large quantities of suspended particulate and dissolved materials from tidal waters
and releasing other materials into ebbing tides or
during rain events at low tide (Chalmers et al.
1985; Dame 1989). This high retention efficiency
is well documented in North Inlet, where more nitrogen and phosphorus are recycled within the
marsh than are transported to or from the adjacent water column (Dame et al. 1991). In contrast,
Northern Indian River Lagoon has little opportunity for export because there is little tidal flux or
river input (Montague and Odum 1997).
As sea level rises, the active and passive retention
of materials by salt marshes is important in maintaining the size and elevation of the major estuarine subsystems. Vogel et al. (1996) have calculated
that the amount of inorganic sediments exported
by South Carolina riverine systems is about equal
to the amount of this material necessary for salt
marsh elevation to keep pace with sea level rise.
In an effort to develop regional nitrogen budgets and riverine nitrogen and phosphorus fluxes
for drainages to the North Atlantic Ocean, Howarth et al. (1996) estimated discharge and material exports for the southeastern Atlantic coast. Total phosphorus fluxes were estimated to be 0.011
Tg yr21 and total nitrogen was 0.24 Tg yr21or about
32 and 676 kg km22 yr21 respectively. Our loading
estimates (Fig. 3), were not for the whole region
(we only used rivers with consistent water quality
records), and only took into account dissolved inorganic nutrients. They are therefore much less
than the totals reported in Howarth et al. (1996).
It should be noted that both of these estimates are
for material reaching the head of the estuaries
only, and so do not take into account either additions or subtractions that may take place as the water moves to the coastal ocean. They do not account for potential inputs from lagoonal systems
with little or no river flow, which dominate the
southeastern Atlantic coast. At present, it is unclear
whether these systems represent net sources or
sinks of nutrients to coastal water. Finally, groundwater flow needs to be taken into account, because
it can be an additional, direct source of nutrients
to the nearshore (Moore 1996).
Vogel et al. (1996) computed a first approximation of the relative contribution of tidal water
exchange by bar-built and lagoonal systems and
discharge by riverine systems in South Carolina.
They estimated that tidal water exchange was over
an order of magnitude higher than river discharge
and that nutrient exports by bar-built systems were
at least an order of magnitude greater than riverine delivery to the coastal ocean. If the Howarth
et al. (1996) estimates for the southeast Atlantic
coast are increased an order of magnitude then
this region would be more comparable to the Mississippi Basin of the Gulf Coast in material fluxes
and water discharges.
Climate
The climate of the southeastern coast is Carolinian temperate from Cape Hatteras to north Florida
and transitional to subtropical from there to Biscayne Bay. Water temperatures in shallow estuaries,
lagoons, and sounds often exceed 308C during July
and August and can drop to less than 108C at the
northern extremes of the region during February.
Seasonality is well defined in the Carolinas and
Georgia. Along the south Florida coast there is a
pronounced wet period between May and October
and a dry period from November to April. The
wet-dry season periodicity in south Florida results
in an alternation between brackish and hypersaline
conditions in many parts of the Indian River Lagoon.
HURRICANES
The entire region is vulnerable to tropical
storms, hurricanes, and tornadoes. Occasionally,
tropical storms cross the Gulf Stream and make
landfall in the region. During the past century,
Florida has been the most hurricane-prone state in
the U.S. with 57 strikes (including 24 category 3
or greater) making landfall (National Oceanic and
Atmospheric Administration 1997). In the last decade, hurricanes Hugo (1989) near Charleston,
South Carolina, Andrew near Miami, Florida
(1992), and Bertha and Fran (1996), Bonnie
(1998) and Dennis and Floyd (1999) near Wilmington, North Carolina, struck the southeast
coast. These storms brought significant winds,
waves, flooding, erosion, and associated damage to
maritime forests and property. High river discharge associated with rainfall, rerouting of human
sewage because of power outages, and extensive
animal waste pollution from industrial hog farms
Southeast Atlantic U.S. Estuaries
in river floodplains combined to produce severe
anoxia/hypoxia for weeks following these events
(Van Dolah and Anderson 1991; Mallin et al. 1999;
Mallin 2000).
Within estuaries, hurricanes have been shown to
build washover fans on barrier islands, form new
tidal inlets by cutting barrier islands, move large
quantities of sediments both into and out of estuarine areas, destroy forest, alter adjacent watersheds, and dramatically modify estuarine circulation due to extensive water runoff from rain and
storm surges. The impact of a given storm depends
on its size, location, speed, the state of the tide,
shoreline configuration, and slope of the adjacent
continental shelf (Hayes 1978). Besides these factors, the water quality damage associated with hurricanes depends on the amount and type of floodplain development, and whether the hurricane impacts well-flushed systems or poorly-flushed rivers
and estuaries (Mallin et al. 1999).
For example, in 1989, the eye of Hurricane
Hugo struck the South Carolina coast just north of
Charleston Harbor estuary and about 75 km south
of Winyah Bay and North Inlet estuaries. This Category 4 storm, with sustained winds of 39 m s21
(140 km h21) and a central pressure of 935 mbars,
moved onto the coast at a velocity of 12 m s21 at
high tide resulting in massive property damage
along the coast (Gardner et al. 1992). In Charleston County where there were wind gusts over 200
km h21 and a storm surge of up to 9 m above sea
level, Tissue et al. (1994) reported that the storm
not only resuspended and winnowed sediments,
and eroded channel banks, but also scoured and
exported older deposits in large quantities. At
North Inlet, the geomorphology of the landscape
and the estuary showed little change, although several small new washover fans were formed on the
adjacent barrier islands. The nearby maritime forest, however, experienced extensive wind damage,
tree mortality, and loss of terrestrial animals in the
surge-affected forest immediately after the storm.
Major changes in the age structure of the forest
and related ecological functions due to this hurricane will be recognized for decades (Gardner et
al. 1992).
ENSO
Inter-annual global climate changes also affect
the southeastern Atlantic coast of North America.
The best know of these cycles is the El Niño-Southern Oscillation (ENSO) climate event. El Niño and
La Niña are terms used to denote opposite phases
of this cycle. This climatic cycle occurs every 2 to
7 yr and generally begins in October and ends in
March (Ropelewski and Halpert 1986). Individual
ENSO events are quite variable in North America
803
and this variability has been attributed to differences in Eastern Pacific sea surface temperatures
and internal atmospheric processes in North
America. The latter is thought to account for a majority of the variability in the southeastern U.S.
(Hoerling and Kumar 1997). ENSO events are
thought to influence the southeastern Atlantic
coast in a number of ways. First, during El Niño
years, winter temperatures are cooler and precipitation may exceed normal values by 3 cm or more
(Ropelewski and Halpert 1986; Philander 1990;
Hanson and Maul 1991). Although higher than
normal freshwater discharge to southeastern estuaries with a higher groundwater table should be
expected during these events, these features are
only statistically observed from the northeastern
coast of South Carolina to Florida (Kuhnel et al.
1990). Second, hurricane frequency in the Atlantic
is usually reduced during El Niño years and that
implies fewer storm impacts along the southeast
coast. During the La Niña phase, the southeastern
Atlantic coast experiences warmer winter temperatures with drier conditions when compared to average years. A recent synthesis by Sun and Furbish
(1997) found that El Niño and La Niña are responsible for up to 40% of the annual precipitation variation and up to 30% of the river variations
in Florida. During the recent 1997–1998 El Niño,
few Atlantic hurricanes were observed and rainfall
was heavy throughout the region from December
through March. Florida and the coastal zone of the
Carolinas received over 200% of normal precipitation for this period, while inland areas received
150% of normal precipitation. In North Inlet, a
normally euhaline estuary, the increased rainfall
depressed salinities below 20 psu for over 3 mo and
below 10 psu for one extended period, setting a
20-yr record.
SEA LEVEL
Sea level is another indicator of global climate
change both on seasonal and annual time scales.
Mean sea level varies seasonally due to changes in
surface pressure (inverse barometer effect) and
steric variations, largely the result of heating of estuarine and shallow shelf waters during the summer months and cooling during winter months
(Patullo et al. 1955). Measurements of water level
at North Inlet, South Carolina indicate that sea level in mid-September on average is 0.25 m higher
than mean sea level in February (Kjerfve et al.
1978). As a result, complete flooding of salt marshes occurs most commonly during high tides in September. Similarly, water level in Indian River is 0.27
m higher in October than in most of the rest of
the year. This results in a high tide level in winter
that is lower than the low tide level in October.
804
R. Dame et al.
Long-term water level measurements at Charleston, South Carolina and calculations of mean sea
level there indicate that relative sea level has increased 0.25 m since 1922 (3.3 mm yr21), both as
a result of eustatic sea level rise (50%) and coastal
land subsidence (50%). Recent analyses of cores
using the lead-210 technique at North Inlet, South
Carolina show a rate of sea level rise of about 3.2
mm yr21 (Vogel et al. 1996) or about the same rate
as the Charleston data. Water level records (1913
to present) at Key West show an average rise of 1.7
mm yr21, almost entirely the result of eustatic sea
level rise. There is little doubt that sea level is rising along the southeastern Atlantic coast and it will
gradually force the transgression of the barrier islands and estuaries of the region upslope along the
coastal plain (Dame et al. 1992). These are the
same areas where human population density is rapidly increasing with associated increases in buildings and infrastructure. Eventually, sea level rise
and human development will compete for the
same coastal landscape, but before that significant
damage due to storms will likely increase.
Primary Producers
Primary production is relatively high in southeastern estuaries, and is partitioned amongst a variety of plant and algal communities. A predominant contribution by vascular plants is suggested
by their vast aerial coverage over much of the
southeastern coast, especially in South Carolina
and Georgia. Compared to more northern temperate estuaries, the relative expansiveness of
marsh area and high vascular plant productivity in
South Carolina and Georgia has been attributed to
characteristically high tidal amplitude, low tidal energy, the prevalence of barrier islands, high solar
input, a lower seasonal temperature differential,
and the absence of snow cover or ice rafting influence (Turner 1976; Haines and Dunn 1985). Measurements of macrophytic primary productivity
have generally exceeded algal-based estimates in
those southeastern estuaries studied (see Total Primary Production section below), but too few systems have been adequately examined to draw
strong general conclusions. Although, in North
Carolina and Florida estuaries, research efforts targeting phytoplankton production are somewhat
equitable with those examining vascular plant production, our understanding of the relative importance of these groups in South Carolina and Georgia estuaries suffers from a dearth of information
on phytoplankton community production (Bricker
et al. 1999). Studies on benthic microalgal production are even scarcer, but recently have drawn attention as potentially significant sources of fixed
carbon (e.g., Pinckney and Zingmark 1993a; Sig-
mon and Cahoon 1997; Cahoon et al. 1999). Even
less is known about the relative activity of macroalgae, traditionally considered insignificant to overall estuarine production, but more recently recognized as seasonally important primary producers
in some systems (see below). Generally, there are
no submerged seagrasses in South Carolina and
Georgia because of restricted light penetration.
MICROALGAE
Microalgal communities from the southeastern
area can be broadly classified as phytoplankton or
benthic microalgae (which includes periphyton
and edaphic algae). Tidal flushing and wind events
in shallow waters may result in benthic microalga
resuspension, effectively mixing the two communities. While several estuaries have been well-studied, there remain many systems in which little or
nothing is known about the microalgae. Species
composition and community dynamics vary between estuaries, and are dependent on salinity distribution, turbidity, light penetration, nutrient
loading, and mixing characteristics. Phytoplankton
biomass rarely exceeds 50 mg chl a m23, but extensive phytoplankton blooms do occur in some
systems, particularly in North Carolina and Florida
(Mallin 1994; National Oceanic and Atmospheric
Administration 1996). Although nitrogen is the
macro-nutrient most commonly limiting to phytoplankton population growth in southeastern estuaries, limitation by silicate or phosphorus has also
been reported, particularly in the more southern
regions (south of Cape Romain; National Oceanic
and Atmospheric Administration 1996). Light can
also be an important limiting factor, and may be a
more influential regulatory factor than nutrients in
shallow and very turbid South Carolina and Georgia salt marsh estuaries (Pomeroy et al. 1981; Vernberg 1993; Lewitus et al. 2000). Microzooplankton
grazing and iron have been also been suggested as
limiting factors in certain South Carolina salt
marsh estuaries (Kawaguchi et al. 1997; Lewitus et
al. 1998).
In North Carolina, phytoplankton production
and abundance typically exhibit peaks in the
March–May and July–September periods (Carpenter 1971; Thayer 1971; Mallin 1994). However, periodic winter blooms of estuarine dinoflagellates
and cryptomonads, linked to nitrate loading pulses, occur in some estuaries in North Carolina (Mallin 1994; Mallin and Paerl 1994; Pinckney et al.
1998). Diatoms generally dominate the spring
bloom phytoplankton communities in North Carolina (Dardeau et al. 1992; Mallin 1994; Tester et
al. 1995), but dinoflagellates and cryptophytes can
also be prevalent during all seasons and can dominate during the summer (Mallin 1994). Although
Southeast Atlantic U.S. Estuaries
805
TABLE 3. Phytoplankton primary production in southeastern Atlantic estuaries.
Estuary
Pamlico, North Carolina
Neuse, North Carolina
Beaufort, North Carolina
Murrells Inlet, South Carolina
North Inlet, South Carolina
Sapelo Island, Georgia
Indian River, Florida
Biscayne Bay, Florida
Primary Production
(g C m22 yr21)
Source
288–500
280–370
52–68
76
100–355
190–375
328–441
13–46
Mallin (1994)
Mallin (1994)
Thayer et al. (1975)
Lewitus unpublished data
Pinckney and Zingmark (1993); Lewitus unpublished data
Pomeroy et al. (1981)
Jensen and Gibson (1986)
Roman et al. (1983)
phytoplankton dynamics in South Carolina and
Georgia estuaries have been evaluated for only a
few systems (Table 3), some distinctions from
North Carolina estuaries are suggested below.
However, because several North Carolina estuaries
are moderately-to-highly eutrophic as a result of
anthropogenic practices (Bricker et al. 1999), it is
difficult to sort differences due to anthropogenic
influences from those due to geography. Nutrient
loading may be a much more influential driving
force for regulating phytoplankton dynamics in
North Carolina, especially in poorly flushed areas,
than in the South Carolina and Georgia systems
that are weakly-to-moderately eutrophic and typically well-flushed. In this respect, the prevalence of
high dinoflagellate and cryptophyte populations
intermittently throughout the year in North Carolina estuaries may result largely from the influence of weather-driven loading of anthropogenically-derived nutrients.
In those South Carolina and Georgia estuaries
studied, diatoms were the predominant contributors to phytoplankton biomass during much of the
year (Marshall 1985; Verity et al. 1993; Lewitus et
al. 1998). However, during the summer bloom, the
relative contribution of phytoflagellates approached that of diatoms, and, in terms of abundance, the phototrophic nanoflagellates (several
groups such as chrysophytes, prasinophytes, prymnesiophytes, cryptophytes, and dinoflagellates)
and picoplankton (primarily coccoid cyanobacteria
such as Synechococcus spp.) can exceed that of diatoms. Lewitus et al. (1998) found that microplanktonic diatom abundance in North Inlet estuary decreased from spring to summer, while phototrophic nanoplankton increased, suggesting that the
bloom formed as a result of an increasing standing
stock of the latter group. The bloom was characterized by a ‘‘microbial loop’’ structure; similar to
summer bloom communities in more northern
temperate estuaries such as Chesapeake Bay. Phytoplankton population growth was not limited by
nitrogen, but rather controlled by microzooplankton grazing. In comparison to more northern tem-
perate estuaries, the relative predominance of summer phytoflagellate-dominated bloom communities, with microbial loop dynamics and microzooplanktonic grazing control, may be a result of the
shorter spring/prolonged summer characterizing
the lower latitudes, which favors the regeneration
of N-nutrients such as NH4 and DON, bacterial
production, the growth of flagellates over diatoms,
and grazing activity of microzooplankton over macrozooplankton (Glibert et al. 1982; Sherr et al.
1986, 1992; Keller et al. 1999; Lomas and Glibert
1999).
Benthic microalgae are found on tidal flats, the
surface of salt marshes, and subtidal areas. Their
biomass may be as high as 250 mg chl a m22 in the
upper 5 cm of sediment, but typically ranges from
10 to 100 mg chl a m22 depending on sediment
type and depth of overlying water (Pinckney and
Zingmark 1993b; Sigmon and Cahoon 1997; Cahoon et al. 1999). A study of five salt marsh habitats (Pinckney and Zingmark 1993b) found the
highest benthic algal biomass in sediments within
Spartina habitats and on intertidal mudflats, and
lowest biomass in shallow subtidal areas. In a broad
scale study that included the subtidal and intertidal
areas of eight North Carolina estuaries, Cahoon et
al. (1999) found a negative relationship between
the proportion of fine sediments and benthic microalgal biomass.
Annual benthic microalgae production ranges
from about 50 to over 200 g C m22 (Pomeroy 1959;
Pinckney and Zingmark 1993a). In North Inlet,
South Carolina, combined microalgal productivity
can be as high as 50% of the total macrophyte primary production (Pinckney and Zingmark 1993a).
Additional primary production can be attributed
to epiphytic microalgae. The production of epiphytic microalgae in southeastern estuaries is highly variable and species-specific (Coleman and Burkholder 1994), but can range from 20% to 200% of
that of host seagrasses or marsh macrophytes (Penhale 1977; Mallin et al. 1992).
806
R. Dame et al.
TABLE 4. Distribution of salt marshes along the southeastern Atlantic coast.
Area (ha)
State
North Carolina
South Carolina
Georgia
Florida
Spartina
Juncus
Total
42,923
112,766
121,335
10,536
40,712
36,882
30,337
28,304
83,635
149,648
151,672
38,840
HARMFUL ALGAL BLOOMS
Increasing anthropogenic nutrient inputs from
a rapidly growing human population in the coastal
zone, industrial and municipal effluents, changing
land-use practices, and atmospheric nitrogen deposition are rapidly exceeding the processing ability of impacted systems (Paerl 1997). Symptoms of
eutrophication (nuisance and toxic algal blooms,
hypoxia/anoxia, fish and shellfish mortality) have
been documented in nearly half of major southeastern estuaries (Bricker et al. 1999). Harmful algal blooms such as red tides or brown tides occur
infrequently in most southeastern Atlantic coastal
systems. Outbreaks of red tides caused by Gymnodinium breve are more common along the southern
portion of the area; however, in 1987 one was carried by the Gulf Stream as far north as Onslow Bay
in North Carolina (Tester et al. 1991). Cyanobacterial (blue-green algal) and dinoflagellate blooms
have resulted in significant ecological and economic damage in some rivers and estuaries of North
Carolina and Florida. In the late 1980s, a group of
toxic dinoflagellate species were found in several
North Carolina estuaries (Burkholder et al. 1992).
One recognized species in this group, Pfiesteria piscicida, has been identified as the causative agent of
approximately 50% of the fish kills in the Pamlico,
Neuse, and New River estuaries of North Carolina
in the past decade (Burkholder and Glasgow 1997;
Burkholder et al. 1995; Burkholder 1998). A second, less virulent toxic Pfiesteria species has also
been cultured, identified, and tested for toxicity
from waters of this region (Burkholder and Glasgow 1997). Both species have also been found in
sub-estuaries of the Chesapeake Bay (Lewitus et al.
1995).
Reports of harmful algal blooms in South Carolina and Georgia estuaries are rare. To our knowledge, Pomeroy et al.’s (1956, 1972) report of a dinoflagellate bloom (Kryptoperidinium sp.) in the Sapelo Island salt marsh estuary is the only potential
harmful algal bloom documented in Georgia estuarine waters. The first reported red tide localized
to a South Carolina estuary occurred in Spring
1998, when a new species of dinoflagellate (Scrippsiella carolinium, Morton personal communication)
formed a bloom in Bulls Bay, McClellanville (Lew-
Source
Critcher 1967
Alexander et al. 1986
Alexander et al. 1986; Eleuterius 1976
Eleuterius 1976
itus et al. in press). This same species formed red
tides in several sites in spring 1999 ranging from
North Inlet to Hilton Head Island estuaries over
100 miles apart. Although the environmental factors promoting harmful algal blooms are not wellunderstood, and events are unpredictable and vary
with species, the relative lack of harmful algal
blooms in the estuaries of South Carolina and
Georgia may relate to the high tidal flushing, short
water residence time, and low anthropogenic nutrient loading that characterize these systems. The
recent, recurrent (1998–1999), and widespread appearance of the Scrippsiella red tide in South Carolina is worrisome, and it is unknown whether its
origin was related to natural events or anthropogenic influence. In contrast, high nutrient loading
and long water residence times characterize the
sites where Pfiesteria-induced fish kills tend to occur
(Burkholder et al. 1995; Burkholder and Glasgow
1997). Tidal flushing and short water residence
times in many southeastern systems may minimize
the occurrence of both toxic and nuisance phytoplankton blooms. In order to prevent or control
harmful algal blooms in the region, the development of ecologically sound and economically feasible watershed-based nutrient management strategies must be addressed in the coming years.
MACROALGAE
The southeastern Atlantic coast of the U.S. has
been characterized as inhospitable to macroalgae
(seaweeds) and without its own flora (Humm 1969;
Coutinho 1987). For example, in Georgia and
South Carolina estuaries, macroalgae is primarily
found in shallow creeks during the winter months
when turbidity levels are low. The typical distribution of macroalgae in estuaries is that of marine
species colonizing most of the length of the estuary
with freshwater species predominating only near
the head (Wilkinson 1980). In North Inlet, South
Carolina, the highest numbers of macroalgal species were found in winter, with peak reproductive
activity occurring during the spring (Coutinho
1987). The area averaged annual net primary production of macroalgae in North Inlet is 197 g C
m22 (Coutinho 1987). The magnitude of macroal-
Southeast Atlantic U.S. Estuaries
807
TABLE 5. Vascular plant and macroalgal primary production for several estuaries along the southeastern Atlantic coast.
Estuary
Beaufort, North Carolina
North Inlet, South Carolina
Form
Sapelo Island, Georgia
seagrass
marsh grass
macroalgal
marsh grass
Indian River, Florida
Biscayne Bay, Florida
seagrass
seagrass
gae production is usually low relative to vascular
plant production in the area (Table 4).
VASCULAR PLANTS
Vascular plants common to estuaries along the
southeastern Atlantic coast include salt marshes,
sea grasses, and mangroves. The intertidal salt
marsh grass Spartina alterniflora is the dominant
species in most estuaries from North Carolina
through Georgia (Table 4). In north Florida, Juncus marshes tend to dominate at higher intertidal
elevations. Salt marshes reach their greatest extent
in Georgia and South Carolina. South of Daytona
Beach, the black mangrove, Avicennia germinans, in
the intertidal zone, gradually replaces S. alterniflora. Spartina alterniflora is the most common salt
marsh plant with distinctive height forms depending on location in the marsh. The dominant vegetation shifts as salinity changes from salt to freshwater (Odum et al. 1984). Spartina alterniflora, S.
cynosuroides, and Juncus roemaerianus are important
in brackish water, and Zizaniopsis milacea, Zizanaia
aquatica, Pontederia cordat, Scirpus validus, Eleocharis
albida, Peltandra virginicus, and Typha dominogensis
occur in tidal freshwater marshes (Gallagher and
Reimold 1973; Schubauer and Hopkinson 1984).
Hefner et al. (1994) recently reported that there
was almost no salt marsh wetland loss along the
southeastern Atlantic coast in the 1970s to 1980s.
Annual net primary production of S. alterniflora
(above 1 belowground) ranges from 0.1 to over
2.5 kg C m22 in short form stands and 0.8 to 2.1
kg C m22 for stands of the tall form (Dame 1989;
Montague and Wiegert 1990; Dai and Wiegert
1996). Highest values are in South Carolina and
Georgia with low values in North Carolina and
Florida (Table 5).
In south Florida, Rhizophora mangle, red mangrove, is the most typical mangrove plant, particularly in areas heavily influenced by seawater. The
structure of mangrove swamps is usually attributed
to topography, substrate, tidal action, and freshwater hydrology (see Lugo and Snedaker 1974).
Moving up riverine systems into lower salinities
and higher intertidal elevations, other vascular
Primary Production
(g C m22 yr21)
134–700
800–2,100
197
815–2,000
749–1,421
159
520–2,000
Source
Thayer et al. 1975; Penhale 1977
Dame 1989; Coutinho 1987
Pomery et al. 1981;
Dai and Wiegert 1996
Jensen and Gibson 1986
Roman et al. 1983
plant species besides R. mangle can be found. Avicennia, black mangrove, and Laguncularia, white
mangrove, are common to this group. Mangrove
annual net primary production ranges from 0.4 to
4.4 kg C m22 with an average of 1 kg C m22 (Mitsch
and Gosselink 1993; Odum and McIvor 1990).
Seagrasses are found in the northernmost estuaries of North Carolina and in Florida. In many
areas, there has been a drastic decline in seagrass
abundance that has been attributed to declining
water quality. These subtidal vascular plants inhabit
soft sediments and appear to be light limited (Day
et al. 1989) which may be a primary cause for their
absence in South Carolina and Georgia estuaries.
In North Carolina, the dominant form is eelgrass,
Zostera marina, and in Florida turtlegrass, Thalassia
testudinum, dominates. Seagrass annual net primary production (Table 5) in North Carolina ranges
from less than 0.1 to 0.7 kg C m22 and in Florida
from 0.1 to 2.5 kg C m22 (Thayer et al. 1975; Penhale 1977; Zieman and Wetzel 1980; Zieman
1982).
TOTAL PRIMARY PRODUCTION
There have been estimates of the various components of total primary production in only a few
locations within the southeastern Atlantic coastal
region (Table 6). In the estuarine area near Beaufort, North Carolina, Thayer et al. (1975) estimated a total annual primary production of 0.2 kg C
m22 distributed among eelgrass (64%), phytoplankton (28%), and salt marsh grass (8%). At
North Inlet, South Carolina, estimated total annual
primary production including belowground rootrhizome production is 1.0 to 1.6 kg C m22 (Dame
1989; Pinckney and Zingmark 1993a). Using minimal values and adjusting for the total system, annual production is partitioned as S. alterniflora
(46%), benthic microalgae (29%), macroalgae
(13%), and phytoplankton (12%). In the Duplin
River system near Sapelo Island, Georgia, total annual primary production was estimated to be 1.4
kg C m22 apportioned as S. alterniflora (84%), benthic microalgae (10%), and phytoplankton (6%).
Pomeroy et al. (1981) suggests that on the bottom
808
R. Dame et al.
TABLE 6. Relative primary production at five estuaries along the southeastern Atlantic coast. Data sources given in text.
Relative Primary Production (%)
Estuary
Beaufort,North Carolina
North Inlet, South Carolina
Sapelo, Georgia
Indian River, Florida
Biscayne Bay, Florida
Phytoplankton
Benthic
Microalgae
33.7
12.0
7.6
3.5
6.7
and in the water column, turbidity limits light penetration and, thus, photosynthesis. In Biscayne Bay,
Florida, Roman et al. (1983) showed that phytoplankton (27 g C m22 yr21) accounted for less than
10% of total system production with the seagrass
Thalassia and seaweeds accounting for the majority
(0.6 kg C m22 y21).
From these widely distributed sites, it appears
that macrophytes are by far the dominant producer of fixed carbon in southeastern estuaries. Phytoplankton production is relatively low except at
Beaufort, North Carolina where it accounts for
over one-fourth of the total production of the system. Some eutrophic systems such as the Neuse
River differ from the above systems; the main
source of fixed carbon is phytoplankton because
seagrasses are rare or absent and salt marshes are
confined to river fringes in lower reaches of the
estuary.
The importance of vascular plant primary production in many southeastern estuarine ecosystems
supports Peters and Schaaf’s (1991) empirical
modeling exercise that suggests that there is insufficient microalgae production to support the current catch of finfish and shellfish along the Atlantic
coast of the U.S. Opposing views exist that argue
the significance of algae (particularly edaphic microalgae) to carbon trophic transfer in southeastern estuarine food webs. The Peters and Schaaf
(1991) model probably underestimated the relative contribution of algae to fish production by using a 20% trophic transfer efficiency, which does
not account for the need for vascular plant detritus, and is high relative to the prevalent 10% efficiency found by Pauly and Christensen (1995) in
their worldwide survey of primary productivity
needs for fishery sustenance, and failing to account for the production of edaphic microalgae in
salt marshes and seagrass beds (Mallin et al. 1992).
Also, stable isotope analyses from the Sapelo Island
salt marsh (Haines 1976; Peterson and Howarth
1987; Sullivan and Moncreiff 1990) indicated that
algae accounted for 50% of the transfer of fixed
carbon to fauna, despite their much lower absolute
primary production compared to vascular plants;
i.e. suggesting that the efficiency of trophic trans-
29.0
14.4
14.6
Macroalgae
13.0
Vascular
Prorated Total
(g C m22 yr21)
66.3
46.0
78.0
81.9
93.3
202
1,600
1,445
441
686
fer is higher in algae than vascular plants in this
system.
Consumers
With their high primary production, estuaries of
the southeastern Atlantic coast also have abundant
and often commercially important consumers. Because macrophytic production dominates the estuaries of the region, both detrital and grazing
food paths are well represented. Frequently, many
macro-consumers are feeding on organisms from
both of these pathways. Furthermore, many consumers are predominantly found in either bottom
sediment or water column habitats, but mechanisms coupling these subsystems abound.
MICROCONSUMERS
The decomposer food web in coastal waters is
dominated by microorganisms that are either using DOM in the water column or particulate organic matter (POM). The community of attached
microbial organisms and POM is usually referred
to as detritus. Organic carbon enters estuaries
from other environments and from macrophytes,
epiphytes, benthic microalgae, macroalgae, and
phytoplankton.
In North Inlet, DOC concentrations range from
1.0 to 13.0 g C m23 and particulate organic carbon
(POC) concentrations vary greatly about seasonal
mean concentrations of 1.2 to 1.5 g C m23 (Dame
et al. 1986). In the Duplin River near Sapelo Island, DOC varies over the year from 2 to about 11
g C m23 and POC range from 3 to 11 g C m23 over
the same period (Chalmers et al. 1985). Both systems trap sufficient particulate carbon (organic
and inorganic) to maintain elevation as sea level
rises and export carbon to the adjacent ocean.
The source of the majority of dissolved and particulate carbon in southeastern Atlantic coast estuaries is usually attributed to the decomposition
of organic materials. Odum and de la Cruz (1967)
gave the original description of the aerobic decomposition of Spartina alterniflora for conditions at Sapelo Island. After the initial leaching stage of decomposition, during which a large pulse of DOM
is released from post senescent tissues (Valiela et
Southeast Atlantic U.S. Estuaries
809
TABLE 7. Comparison of means zooplankton abundance and biomass for estuaries along the southeastern Atlantic coast.
Estuary
Beaufort area, North Carolina
Neuse River, North Carolina
North Inlet, South Carolina
Biscayne Bay, Florida
Plankton Net
Mesh Size
(mm)
156
76
76
60
156
153
64 and 200
239
Density
(No. m23)
Sample Period
1970
1971
30 mo
20 mo
1989
22 mo
20 mo
6 mo (summer)
1978
14 mo
al. 1985), fibrous tissues are initially colonized by
fungi and bacteria that structurally weaken the tissues so that macroconsumers can process them
into smaller particles (Newell 1996). As the particles get smaller the surface area increases and the
microbes increase their colonization. The original
vascular plant material is gradually converted into
microbial biomass and CO2 as a byproduct. Various
detrital feeders then ingest these small particles,
the microbes are removed, and the POM repackaged as fecal pellets that are egested, and then recolonized by microbes with the particles again ingested by consumers. A comparison of vascular
plant decomposition shows that the seagrasses decompose the fastest, followed by red mangroves,
and salt marsh grasses the slowest (Odum and de
la Cruz 1967; Heald 1969; Zieman 1975).
In addition to the decomposition of organic material aerobically, much vascular plant material is
also buried in estuarine sediments where it decomposes anaerobically and plays a role in a number
of bacterially mediated biogeochemical cycles.
In the water column, the microbial food web has
been identified as a major pathway for processing
of DOM (Azam et al. 1983). In this web, DOM produced by various sources, both living and non-living, is taken up by bacteria that are then consumed
by protozoans and finally the protozoans are eaten
by macroconsumers (LeGall et al. 1997). DOM
from producers may also be converted, via microbial mediation, to organic aggregates that are in
turn consumed by metazoa (Alber and Valiela
1995).
The role of microzooplankton (e.g., protists
such as flagellates and ciliates) in processing bacterial or primary production is unknown for many
estuaries in the southeast, and in fact most information comes from two salt marshes; Sapelo Island, Georgia, and North Inlet, South Carolina. In
both systems, estimates of abundances and grazing
rates of microzooplankton were at the high end of
those reported in the literature (Sherr et al. 1986,
4,000
8,400
21,000
34,530
31,224
137,150
9,257
21,555
47,000
Biomass
(mg dry
wt m23)
14.0
21.0
47.8
15.3
17.2
38.7
16.1
—
25
53.4
Source
Thayer et al. (1974)
Thayer et al. (1974)
Fulton (1984)
Mallin (1991)
Mallin (1991)
Mallin and Paerl (1994)
Lonsdale and Coull (1977)
Houser and Allen (1996)
Roman et al. (1983)
Woodmansee (1958)
1991; Capriulo 1990; Zeitzschel 1990; Sanders et
al. 1992; Wetz et al. 1999). Sherr et al. (1986, 1989,
1991) estimated that, depending on season and
proximity to the land-margin, small (, 20 mm)
aloricate ciliates or heterotrophic flagellates were
the major bacterivore. Lewitus et al. (1998) demonstrated the importance of microzooplankton
grazing in limiting phytoplankton population
growth during the summer bloom in North Inlet.
Heterotrophic flagellate abundance peaked at
6,000 to 9,000 cell ml21 during the summer in several North Inlet tidal creeks (Wetz et al. 1999).
These numbers are exceptionally high relative to
literature values, consistent with the high bacterial
abundance typically measured during the summer
in North Inlet (up to 108 cell ml21; Lewitus et al.
2000). Research is need to determine whether the
relatively high heterotrophic flagellate, ciliate, and
bacterial abundances, and the predominance of
microzooplankton grazing in controlling bacterial
and phytoplankton population growth, are characteristic of other southeast estuaries (and, thus,
may reflect the generally higher temperatures and
prolonged summer compared to more northern
temperate estuaries).
MACROCONSUMERS
Macroconsumers of southeastern Atlantic coast
estuaries are abundant, diverse, and often commercially important. In the water column, the macroconsumers are mainly zooplankton and nekton.
Many of these animals are meroplanktonic with larval stages that live in the water column and adult
stages that are nektonic or benthic (Dame and Allen 1996). There are only a few estimates of annual
zooplankton density and biomass in southeastern
estuaries; most of these being from North and
South Carolina (Table 7). These data represent
contrasting estuarine geophysical types, with Beaufort, North Carolina estuaries consisting of several
interconnected high-salinity sounds, the Neuse a
mesohaline riverine system, North Inlet a high-sa-
810
R. Dame et al.
linity marsh-estuarine system, and Biscayne Bay a
subtropical open body of water. Recorded zooplankton abundance was highly dependent upon
net mesh size used in collections, with use of smaller (60–76 mm) mesh sizes yielding much greater
abundance and biomass (Table 7). Season was also
important in these systems with summers yielding
the highest abundances. Fulton (1984), Mallin
(1991), and Mallin and Paerl (1994) found a positive correlation between water temperature and
zooplankton abundance and biomass. At the
southern end of the range, Roman et al. (1983)
attributed the higher zooplankton concentrations
in Biscayne Bay to the availability of large amounts
of macroalgal detritus. It should also be noted that
these subtropical estuarine waters are considerably
less turbid than those of the more temperate systems.
Mallin and Paerl (1994) estimated that zooplankton grazed between 38% and 45% of daily
phytoplankton production on an annual basis in
the Neuse River Estuary in North Carolina. Holoplanktonic consumers (e.g., copepods) are primary sources of food for fish (Weinstein 1979; Bozeman and Dean 1980) and zooplanktivorous fishes
(Allen et al. 1995) that arrive in the estuarine nursery ground in the spring. Predation peaks in the
spring and summer when the transfer of phytoplankton carbon to zooplankton is highest, and
most fishes depart in the autumn when phytoplankton productivity and zooplankton abundance
have decreased (Mallin and Paerl 1994).
Benthos composition and abundance generally
reflects the latitude, type, and geomorphology of
estuaries within the region. Sediments, currents,
salinity, temperature, and many other abiotic and
biotic factors influence the benthos over a wide
range of spatial and temporal scales.
Infauna and epifauna associated with salt marshes are similar from North Carolina to Florida. Species composition and abundance varies within each
estuary as a function of elevation within the tidal
range, bottom type, and distance from the ocean.
Low energy environments (mud-silt) typically support higher diversities of meiofauna than higher
energy sandy habitats (Coull 1985). Although macrobenthos biomass often surpasses that of meiofauna, the smaller animals are more diverse and
have much higher turnover rates (Coull and Bell
1979). Microcrustaceans and oligochaetes are the
primary source of food for shrimps, fishes, and
other epibenthic species that use the estuarine
nursery. Polychaetes dominate the macrobenthos
in mud bottoms, whereas amphipods and small bivalves are most abundant on sand bottoms, and
echinoderms may dominate carbonate sediments
(Dardeau et al. 1992). Seasonal variations in nu-
merical abundance and species richness may be
large with maxima usually occurring between fall
and spring (Service and Feller 1992).
Hard bottoms are not abundant within the region, but where they do occur, they are often very
important. Benthic filter feeding bivalves often
comprise the most obvious consumer community
forming dense beds and, in the case of oysters,
reefs with complex structural attributes. Intertidal
oyster reefs are common to bar-built and lagoonal
systems throughout the region. In North Inlet they
have been shown to be capable of removing more
phytoplankton from the water column than is produced by the estuary, resulting in a regular total
import of phytoplankton from the adjacent ocean
(Dame et al. 1986, 1989). At North Inlet, Dame et
al. (1989) calculated that zooplankton grazing was
several orders of magnitude below oyster grazing
on phytoplankton. Because of their high rates of
water pumping and filtration, oyster reefs have also
been shown to play a major role in the cycling of
nutrients in these systems (Dame 1996).
Benthic diversity and productivity are especially
high in vegetated subtidal areas. Seagrass beds in
North Carolina and south Florida are habitats for
hundreds of species of benthic, epiphytic, and mobile invertebrates (Dardeau et al. 1992). Dominated by polychaetes and amphipods, the density and
diversity of benthos in these habitats increases towards the south (Virnstein et al. 1984). Mangrove
habitat supports a distinct, speciose, and productive benthos and epifauna in the southern portion
of the region (Odum and Heald 1972).
Mobile macroconsumers or nekton are also
prominent components of southeastern Atlantic
estuaries. Whereas, many of these animals are permanent (year round) residents, warm season transients (mostly young of the year fish and shrimps)
often account for most of the mobile annual consumers in these estuaries. Estuarine nekton use the
shallow tidal creeks, marshes, mangrove swamps,
and vascular plant grass beds as nursery grounds
and habitat (Hettler 1989; Gilmore 1995; McIvor
and Rozas 1996). Resident and transient nekton
can potentially move production from these shallow habitats into the subtidal estuary and coastal
ocean (Kneib 1997). Nelson et al. (1991) surveyed
20 mostly riverine estuaries on the southeastern Atlantic coast for the distribution and abundance of
fishes and shellfish. Sufficient information about
commercial value, recreational value, indicators of
environmental stress, and ecological value for
about 40 species were available to develop a general analysis. Penaeid shrimps and blue crabs dominated the invertebrate component of the nekton,
and both groups have life cycles that strongly couple the estuary to the coastal ocean. Blue crabs
Southeast Atlantic U.S. Estuaries
811
Fig. 7. A comparison of fisheries landings in North Carolina,
South Carolina, Georgia, and Eastern Florida for 1988–1998.
Fig. 9. Total commercial fish and shellfish landings in Georgia from 1988–1998.
were abundant or highly abundant in all systems
across the region, while the abundance of shrimp
varied with different species dominating across the
region. The most abundant finfish (the bay anchovy, spot and striped mullet) occur across a wide
range of salinities and dominate most southeastern
estuaries. In a 16-year continuing study, Allen (personal communication) found that young of the
year of dominant nekton species in North Inlet varied considerably in abundance and biomass from
year-to-year, but that the total production of nekton seemed to be much less variable.
A review of commercial landings of finfish and
shellfish (shrimp, crabs, and bivalves) data from
1988 to 1998 for each state in the region showed
that total catch per year was stable with North Carolina’s loadings being greater than those of the
other three states combined (Fig. 7). Finfish dominate the catches in North Carolina and Florida
(Figs. 8 and 9), while the greater proportion of
catch in South Carolina and Georgia is shellfish
(Figs. 10 and 11). In North Carolina, South Carolina, and Georgia, estuarine related finfish and
shellfish species dominated the catch, but in Florida, catches were slightly dominated by marine species (Fig. 12). The commercial landings data support the idea that the South Carolina and Georgia
fisheries are very similar both in terms of total
catch and proportion of shellfish and estuarine
species, that North Carolina’s fisheries are dominated by finfish, have greater annual yields compared to South Carolina and Georgia, but are also
primarily based on estuarine species, and that Florida’s fisheries are more equally divided between
shellfish and finfish, but estuarine species account
for less than half the catch. This characterization
of the southeastern estuaries by states corresponds
to the geomorphological distribution of sounds
dominating in North Carolina; barrier islands with
bar-built estuaries most common in South Carolina
and Georgia and marine factors dictating in Florida. These data indicate a relationship between dif-
Fig. 8. Total commercial fish and shellfish landings in North
Carolina from 1988–1998.
Fig. 10. Total commercial fish and shellfish landings in eastern Florida from 1988–1998.
812
R. Dame et al.
Fig. 11. Total commercial fish and shellfish landings in
South Carolina from 1988–1998.
fering levels of primary production and fisheries
harvest across the region, which is consistent with
ecological theory regarding food chain structure
and productivity (Pauley and Christensen 1995).
At the top of southeastern estuarine food webs
are a number of highly mobile predators including
raptorial birds, sharks and rays, turtles, alligators
and crocodiles, as well as marine mammals, the
most dominant of which is the bottle nosed dolphin (Hoese 1971). Without any significant predators, these animals with highly developed sensory
systems and behaviors are well adapted to feeding
on larger fish, shrimp, and turtles in the highly
turbid shallow estuarine waters of the region.
Anthropogenic Effects
Human population density has steadily increased along the southeastern Atlantic coast since
Europeans first colonized it centuries ago. Over
the 30-yr period between 1980 and 2010, human
population density in coastal counties of the region
is expected to continue to rise (Fig. 13) with the
average growth rate by state expected to range between 15% and 45% with Florida leading the way
(Culliton et al. 1990). In Florida, coastal counties
occupy 57% of the land, but contain 78% of the
population (Montague 1997; Montague and Odum
1997). With a human population growth rate of
over 600 individuals per day, Florida is already exhibiting environmental stress in many areas. At this
rate of human population growth, it is projected
that Florida will lose the ability to sustain its estuarine environments within the next 20 years (Montague and Odum 1997). In contrast to Florida,
Georgia and South Carolina have some of the
smallest coastal human populations in the country,
but their growth rates are still high (10% to 20%
per decade). In the Carolinas and Georgia, there
Fig. 12. Proportion of total fisheries landings comprised of
finfish and shellfish species associated with estuaries along the
southeastern Atlantic coast for 1988–1998.
is also a well-defined seasonal change in population due to recreational tourism. The net effect of
millions of tourist visiting this area annually is the
doubling of the average daily population. Development of infrastructure has had significant environmental impact. Along the northeast coast of
South Carolina there are over 110 golf courses that
are essentially intensely maintained grasslands developed in response to human recreational demands. In addition to problems associated with
routine management, there is mounting concern
that golf course ponds will not be able to contain
their contents during a significant hurricane induced storm surge.
Fig. 13. Past and projected human population numbers in
coastal counties of the southeastern Atlantic coast. Data sources:
North Carolina Office of State Planning, State Demographics;
South Carolina Budget and Control Board, Office of Research
Statistics; Georgia GIS Data Clearinghouse and State Data and
Research Center; and Florida Office of Economic and Demographic Research, The Florida Legislature.
813
Southeast Atlantic U.S. Estuaries
TABLE 8. Southeast Atlantic estuarine eutrophication assessment. L 5 LOW, ML 5 Moderate Low, M 5 Moderate, MH 5 Moderate
High, H 5 High, IH 5 Improved High, IL 5 Improved Low, NC 5 No Change, WL 5 Worse Low, WH 5 Worse High (Coastal
Assessment and Data Synthesis System 1999; National Oceanic and Atmospheric Administration 1999).
Eutrophication Symptoms
Estuary
L
ML
Albemarle Sound
Pamlico Sound
Pamlico/Pungo Rivers
Neuse River
Bogue Sound
New River
Cape Fear River
Winyah Bay/North Inlet
North/South Santee Rivers
Charleston Harbor/Wando/Cooper/Ashley
Rivers
Stono/North Edisto Rivers
St. Helena Sound/S. Edisto/Coosaw Rivers
Broad River
Savannah River
Ossabaw Sound/Ogeechee River
St. Catherines/Sapelo Sounds
Altamaha River
St. Andrew/St. Simons Sounds/Satilla River
St. Marys River/Cumberland Sound
St. Johns River
Indian River
Biscayne Bay
X
X
South Atlantic Coast
11
1
M
MH
Anthropogenic Influences
H
L
ML
M
MH
H
Prognosis (2020)
IH
IL
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Estuarine water quality is often used as an indicator of environmental stress in coastal areas with
nutrient concentrations being the most common
measure (National Oceanic and Atmospheric Administration 1996). The sources of additional nutrients to estuaries are point (e.g., wastewater treatment, industry, etc.) and non-point (e.g., agriculture, golf courses, lawns, etc.) discharges. These
inputs have both direct effects (i.e., algal blooms,
increased turbidity, decreased oxygen concentrations, etc.) and indirect effects (i.e., decreases in
commercial fisheries, impacts on recreation and
public health risks) (National Oceanic and Atmospheric Administration 1996).
A recently published National Oceanic and Atmospheric Administration (1996) report focused
on existing information on nutrient loading or eutrophication in 21 south Atlantic estuaries (Table
8). Medium concentrations (. 5 mg l21) of chl a
are observed periodically in 20 estuaries and 11 of
these systems had high or hypereutrophic (. 20
mg l21) concentrations episodically. Medium or
greater (. 0.1 mg l21) concentrations of nitrogen
are found in 19 estuaries. Phosphorus concentrations of medium or greater (. 0.01 mg l21) are
observed in 18 estuaries. During the summer
months small percentages of 13 estuaries are hypoxic and 11 systems exhibit anoxic occurrences.
The trends data were sparse, but the Neuse River
3
X
X
X
X
X
X
X
X
5
2
5
WH
X
X
X
X
X
WL
X
X
X
NC
5
8
2
1
0
3
8
4
7
appeared to have a number of increasing and decreasing trends depending on location within the
estuary. In contrast, data from a number of South
Carolina estuaries indicated a decrease in nitrogen
and phosphorus loading.
As noted earlier, water flow and watershed physical characteristics play a major role in estuarine
dynamics. Biggs et al. (1989) developed a classification scheme designed to evaluate the potential
ability of an estuary to flush pollutants. From this
scheme, the Santee River in South Carolina is most
susceptible to population, heavy industry, and agricultural activities. Fortunately, the Santee has no
industry or large cities, but is mainly an agricultural watershed. The St. Johns River is projected to
be most susceptible to population and agricultural
activities. Charleston Harbor would be most susceptible to heavy industry, while Winyah Bay, Albemarle Sound, and the Neuse River would be
most susceptible to agricultural activities. Unfortunately for the latter 5 estuaries, the scheme predicts these system’s actual situation.
Conclusions
This review of the characteristics of southeastern
Atlantic estuaries has taken an ecosystem approach
to include as many components and cross discipline interactions as possible. Three types of estuaries play major roles in the region. The marshes
814
R. Dame et al.
in all three system types (bar-built, coastal plain,
and piedmont) have high primary production and
provide extensive nursery grounds for secondary
consumers. Both coastal plain and piedmont systems flush extensive, highly productive freshwater
wetlands. Piedmont estuaries, with large water
flows, connect the coast to large interior watersheds. These highly productive estuarine systems
can be both directly and indirectly influenced by
management decisions both locally and as far away
as the foothills of the Appalachian mountains.
The discharges of the southeastern riverine systems carry large quantities of dissolved and particulate materials to the coastal ocean. Lagoonal and
bar-built systems that are tidally dominated may export large amounts of inorganic and organic materials. Water fluxes in all three estuarine types appear to play a major role in the accumulation or
lack of accumulation of anthropogenic materials,
nuisance algae blooms, and marine diseases.
Vascular plant production dominates estuaries of
the region and is decomposed through extensive
foodwebs in both the water column and the sediment. This high primary production combined
with extensive vascular plant habitat supports large
populations of commercially and recreationally important secondary consumers.
The estuaries of the southeastern Atlantic coast
can be divided into three general areas. The North
Carolina coast is dominated by extensive and poorly flushed sounds. These estuaries are characterized by modest primary production and they support the largest estuarine fishery in the southeastern Atlantic region. Although human population
density is low and growth is moderate, high nutrient loading results from poor land use for industrial animal production. This area harbors several
of the most polluted estuaries in the country.
The central region that makes up most of the
South Atlantic Bight is dominated by the estuaries
of South Carolina and Georgia. Well-flushed barbuilt and riverine estuaries are common. Primary
production is very high and dominated by emergent salt marshes. Commercial fishery landings are
relatively low, but shellfish dominate catches and
estuarine species comprise more than 75% of the
catch. Human population density and growth are
low to moderate and nutrient loading reflects this
land use pattern.
The Florida coast, particularly south of Jacksonville, is dominated by moderate to poorly flushed
bar-built estuaries with little freshwater input. A variety of plant types support a modest level of primary production. Fisheries catches are moderate,
and equally divided between shellfish and finfish,
but estuarine species are much less important than
marine species in the landings. Human population
density and growth rate are high and probably
cause high nutrient loading. With continued high
population growth, urbanization, and habitat destruction, Florida presents the scenario of potential
environmental collapse.
One of the points to emerge from this synthesis
is that, although a few representative bar-built systems are very well studied (e.g., North Inlet, South
Carolina and Duplin River, Georgia), very little is
known about most of the estuaries in the southeast
Atlantic region. In contrast to other areas of the
country, this region has received relatively little attention and the data do not exist for evaluating
historic trends for many parameters.
As reflected in several of the federal government
reports that are the results of scientific panels, the
future utilization of these estuarine systems and
their essential services will depend on the development of improved management strategies based
on enhanced data quality. These approaches must
be increased comprehensive long-term environmental monitoring linked to integrated regional
planning in order to maintain the viability and usability of southeastern Atlantic estuarine and coastal systems. In particular the magnitude and sources
of freshwater inputs, the nature of material processing by the different types of estuaries and how
coastal fisheries are coupled to these estuaries
clearly need comprehensive study. Most importantly there is an evident need for large or regional
scale approaches focused on the estuarine coupling of the landscape to the sea.
ACKNOWLEDGMENTS
The authors wish to thank the many state, federal, and private
resources that have supported their research on southeastern
estuaries in the past and present. This work was heavily supported by National Science Foundation awards DEB-95095 to
Coastal Carolina University and the Georgia Rivers Land Margin
Ecosystem Research DEB-94120898 to the University of Georgia.
This is publication number 1252 of the Belle W. Baruch Institute for Marine Biology and Coastal Research. This work was
presented at the Estuarine Federation Meeting in Providence,
Rhode Island, October 1997.
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