CHAPTER 12—COASTAL WATERS AND MARGINAL SEAS
I.
Coastal Waters
A. Primary difference between coastal waters and open ocean is depth
i. Changes in the nature of water concerning distance and time are
much greater in the shallow coastal waters
ii. River runoff and tidal currents have a significant effect on the
nature of coastal water
B. Salinity
i. River runoff reduces salinity
1. reduces surface layer where mixing is not significant
2. reduces throughout the water column where mixing occurs
3. runoff of the rivers will be at maximum during seasons of
maximum precipitation (rain dominant)
4. other river runoff will be maximum in the summer (melting
of ice/snow dominant)
ii. Prevailing offshore winds counteract the effect of runoff in some
coastal regions
1. these winds lose most of their moisture over the continents
2. these winds evaporate water as they move across the
surface of the coastal waters—increase in evaporation rate
increases the surface salinity
C. Temperature
i. ranges in temperature vary on a yearly basis
ii. Sea ice forms in many of the high-latitude coastal areas (-2*C,
28.4*F)
iii. Maximum surface temperature in low-latitude coastal water
reaches 45* C (113*F)
iv. Midlatitude regions show the greatest variance in temperatures
D. Coastal Goestrophic Currents
i. Caused by:
1. wind blowing parallel to the coast, causing water to pile up
along the shore—forms a “hill” of water
2. under the influence of the Coriolis effect the water must
eventually run back down the slope of the hill toward the
open ocean (due to gravity)
3. in the Northern Hemisphere, the Coriolis effect causes that
water to veer north on the western coast and to the south on
the eastern coast
ii. Also caused by:
1. Runoff of large quantities of fresh water that gradually mix
with the oceanic water
2. produces a surface wedge of surface water moving away
from the shore
3. seaward slope(movement) is associated with salinity and
density gradients as both increase seaward
4. the currents depend upon the wind and amount of runoff for
their strength
5. kept inline by the steadier boundary currents of ocean open
gyres
6. local geostrophic currents frequently flow in the opposite
direction of the boundary current (like in the case of the
Davidson Current)
a. large amounts of precipitation and strong winds in
the Pacific Northwest occur during the winter
months—combined effect produces a relatively
strong northward-flowing geostrophic current
between the southward flowing California Current
and the North American continent
II.
Estuaries
A. Intro:
i. Estuaries—semi-enclosed coastal bodies of water in which the
ocean water is significantly diluted by fresh water from land runoff
ii. Mouths of large rivers form the most economically significant
estuaries—many are ports and centers of ocean commerce
iii. Many support important commercial fisheries as well
iv. Many bays, inlets, gulfs, and sounds may be considered estuaries
on the basis of their structure
B. Origin of Estuaries
i. All estuaries today owe their origin to the fact that in the last
18,000 years, sea level has risen approximately 120 meters (394
feet) owing to the melting of the glaciers
ii. Four major classes of estuaries, based on their origin, are:
1. Coastal plain estuaries—formed as the rising sea level
caused the oceans to invade existing river valleys
(sometimes referred to as drowned river valleys)
2. Fjords—glaciated valleys that are U-shaped with steep
walls. Usually have a moraines located near the ocean
entrance.
3. Bar-built estuaries—shallow estuaries separated from the
open ocean by bars of sand deposited parallel to the coast.
Lagoons separating barrier islands from the mainland are
bar-built estuaries
4. Tectonic estuaries—produced by faulting or folding, which
causes a restricted down-dropped area into which rivers
flow. Ex: San Francisco Bay (like Coastal plain estuaries,
but formed tectonically)
C. Water Mixing in Estuaries
i. Freshwater runoff that flows into an estuary moves as an upper
layer of low-density water across the estuary toward the open
ocean
ii. Inflow from the ocean takes place below the upper layer
iii. Mixing takes place at the contact between these two water masses
iv. classification of estuaries is based on the degree of mixing as
determined by the distribution of water properties
1. Vertically Mixed
a. Shallow, low-volume estuaries
b. Net flow always proceeds from the river at the head
of the estuary toward the mouth (toward the sea)
c. Salinity at any point will be uniform from the
surface to the bottom because of even mixing of the
river water with ocean water—due to eddy diffusion
at all depth
d. Salinity increases from the head to the mouth of the
estuary
2. Slightly Stratified
a. Somewhat deeper estuary
b. Salinity increases from the head to the mouth at any
depth
c. Two basic water layers: less saline upper water
from river, and deeper marine water separated by a
zone of mixing
d. Begin to see the typical estuarine circulation pattern
develop—net surface flow of low-salinity water into
the ocean and a net subsurface flow of marine water
toward the head of the estuary
3. Highly Stratified
a. Typical of deep estuaries in which the salinity in the
upper layer increases from the head of the estuary to
the mouth, where it reaches a value close to that of
open-ocean water
b. Deep-water layer has a rather uniform marine
salinity at any depth throughout the length of the
estuary
c. The mixing that occurs at the interface of the upper
water and the lower water is such that the net
movement is from the deep-water mass into the
upper water
d. Less saline surface water does not seem to dilute the
deep-water mass and simply moves from the head
toward the mouth of the estuary, having its salinity
increased as water from the deep mass joins it
e. Relatively strong haloclines develop at the contact
between the upper and lower water masses
f. At times of maximum river flow, these haloclines
can reach magnitudes approaching 20ppt (parts per
thousand)
4. Salt Wedge
III.
a. Saline wedge of water intrudes from the ocean
below that of the river water
b. Typical of the mouths of deep, large-volumetransport rivers
c. No horizontal salinity gradient at the surface in
these deep estuaries
d. Water is essentially fresh throughout the length of,
and even beyond, the estuary
e. There is a horizontal salinity gradient at depth and a
very pronounced vertical salinity gradient
manifested as a strong halocline ate any station
throughout the length of the estuary
f. Halocline will be shallower and more highly
developed near the mouth of the estuary
5. the mixing patterns described often cannot be applied to an
estuary as a whole. Mixing within an estuary may change
with:
a. longitudinal distance
b. season
c. tidal conditions
v. Chesapeake Bay
1. more populated estuary
2. coastal plain estuary produced by the drowning of the
Susquehanna River
3. most fresh water enters the bay along the western margin
via rivers draining the slopes of the Appalachian Mountains
4. Salinity distribution is also effected by the Coriolis effect
acting on marine water entering the bay from the south, and
the southward flow of river water
5. maximum river flow in the spring develops a strong
halocline preventing mixing of the fresh surface water and
saltier deep water
6. beneath the pycnocline (can be as shallow as 5m or 16ft)
water may become anoxic from May through August
7. major kills of blue crabs and oysters as well as other
bottom-dwelling organisms occur during these events
8. the number of kills have increased over the last 40 years
Wetlands
A. Wetlands are biologically productive strips of land delicately in tune with
natural shore processes; they border estuaries
B. two types (both are intermittently submerged by ocean water and are
characterized by oxygen-poor mud and peat deposits):
i. Salt Marshes
1. characteristically inhabited by a variety of grasses
2. known to occur from the equator to as high as 65 degrees
latitude
IV.
3. believed to serve as nursery grounds for over half the
species of commercially important fish in the southeastern
United States
4. Other fish (like flounder and bluefish) use them for feeding
and overwintering
5. fisheries of oysters, scallops, clams, and fishes such as eels
and smelt are located directly in the marshes
ii. Mangrove Swamps
1. Mangrove trees are restricted to latitudes below 30 degrees
latitude
2. once mangroves colonize an area, they normally outgrow
and replace marsh grasses
C. More than half of the nation’s wetlands have been lost
i. Of the Original 215 million acres of wetlands that once existed in
the United States, only about 90 million acres remain
ii. Environmental Protection Agency (EPA) established an Offices of
Wetlands Protection (OWP) in 1986
iii. At that time wetlands were being lost to development at a rate of
300,000 acres a year
D. Important characteristics of wetlands is their ability to remove inorganic
nitrogen compounds and metals from groundwater polluted by land
sources
i. Most removal is probably achieved through adsorption on claysized particles
ii. Some nitrogen compounds trapped in sediment are decomposed by
denitrifying bacteria—release nitrogen to the atmosphere as
nitrogen gas
iii. Remaining nitrogen is used for plant production. When these
plants die, the organic nitrogen compounds are either:
1. incorporated into the sediment and converted to peat
2. Broken up and become food for bacteria, fungi, or detritusfeeding fin and shell-fish.
Lagoons
A. Lagoons are protected, shallow bodies of water that lie landward of the
barrier island salt marshes
B. Restricted circulation between lagoons and the ocean causes three distinct
zones within the lagoon
i. Freshwater zone near the mouths of rivers that flow into the lagoon
ii. Transitional zone of brackish water (salinity between fresh and
oceanic water)
iii. Saltwater zone close to the entrance, where the maximum tidal
effects within the lagoon can be observed (usually detectable in the
freshwater zone)
C. Salinity is determined by factors other than position relative to the mouth
of the lagoon:
V.
i. In latitudes with seasonal variations in temperature and
precipitation, ocean water will flow through the entrance during a
warm, dry summer to compensate for the volume of water that is
lost through evaporation—results in increased salinity within the
lagoon
ii. May become hypersaline in arid regions where the inflow of
seawater is not sufficient to keep pace with evaporation that is
taking place at the surface of the lagoon
iii. During the rainy season, the lagoon will become much less saline
as the amount of freshwater runoff increases
D. Laguna Madre
i. Laguna Madre is a hypersaline lagoon (may become hypersaline
due to little tidal flushing and seasonal variability in freshwater
input. High evaporation rates and low freshwater input can result
in very high salinities) along the coast of Texas between Corpus
Christi and the mouth of the Rio Grande
ii. Protected from the open ocean by Padre Island
iii. Formed about 6000 years ago as sea level was approaching its
present height
iv. Less than 1 meter in depth
1. shallowness of water makes possible a very great seasonal
range of temperature and salinity in this semi-arid region
2. high temperatures in the summer may fall below 5 degrees
C in the winter
3. Salinities range from 2-100 parts per thousand
a. Infrequent storms provide large volumes of fresh
water in a short period of time
b. High evaporation keeps the salinity generally well
above 50 parts per thousand
c. Marsh is replaced by an open sand beach on Padre
Island because marsh grasses cannot withstand such
high salinities
v. Circulation is opposite of circulation for estuaries
1. inflow from the ocean is as a surface wedge over the denser
lagoon water
2. outflow from the lagoon occurs as a subsurface flow
Mediterranean Sea
A. Not strictly coastal—however, climate and pattern of circulation are
similar in the Atlantic Ocean
i. East-west trending body of water that has a very irregular coastline
dividing it into subseas with separate circulation patterns
ii. Surrounded by land with two very small connections to other
bodies of water
1. Strait of Gibraltar to the Atlantic Ocean
2. Bosporus to the Black Sea
VI.
3. man-made Suez Canal connects the Mediterranean Sea to
the Red Sea
iii. Divided into two major basins separated by a sill with a 400 meter
depth, extending from Sicily to the coast of Tunisia
1. strong currents run through the Strait of Messina
2. much broader connection through the Strait of Sicily
3. Atlantic water enters through the Strait of Gibraltar as a
surface flow—replaces water evaporated at a high rate in
the eastern end of the Mediterranean Sea
iv. Circulation between the Mediterranean Sea and the Atlantic Ocean
is typical of closed restricted basins in areas where evaporation
exceeds precipitation
1. restricted basins will always lose water at a very high rate
from the surface through evaporation
2. water will have to be replaced by surface inflow from the
open ocean
3. evaporation of water flowing in from the ocean increases
its salinity to very high values—causes sinking and the
return to the open ocean as a subsurface flow
4. during periods of hypersalinity, Laguna Madre exhibits this
pattern of circulation between itself and the Gulf of
Mexico—this type of circulation is called Mediterranean
circulation (opposite of circulation in estuaries)
Pollution stresses the Coastal Waters
A. Petroleum
i. Spills have occurred after tanker accidents and blowouts of oil
wells being drilled
ii. Some consider these hydrocarbons to be the least damaging
pollutants of the ocean because they are organic substances bio
degradable by microorganisms
iii. The problem because these hydrocarbons are very complex and
usually have additional components
iv. West Falmouth Harbor
1. oldest well-studied oil spill in the United States occurred on
September 16, 1969 near Buzzards Bay, MA
2. the barge Florida came ashore and ruptured
3. currents carried fuel north into Wild Harbor
a. initial kill was almost total for intertidal and
subtidal animals
b. severe reduction in species diversity
c. rapid increase in population of polychaete worms
resistant to the oil
d. species diversity did not increase until well into the
third year after the spill
e. conditions have returned to near normal, but oil can
still be found in the fine sediment of some of the
area
v. Chedabucto Bay
1. February 4, 1970—the vessel Arrow spilled in Nova Scotia
2. 1/3 of the initial oil spilled 200 km off shore was clear after
three months
a. cleaning was primarily the result of wave action on
the rocky shore
b. by 1973, mechanical cleaning by waves,
evaporation, photochemical decomposition, and
bacterial decomposition had cleaned all but the
restricted low-energy estuarine marshes; 75% of the
shore was clean
3. half-life for removal of the visible oiling in the bay as a
whole is about two years, but it appears that the half-life for
removal from fine sediment will be up to 25 years
vi. Argo Merchant
1. when oil spills do not come ashore, the negative effects are
not so obvious
2. Argo Merchant ran aground on Fishing Rip Shoals and
sank in 1976
3. winds kept the oil from coming ashore
4. surface slick moved east out to the sea and was gone by
mid-January
5. primary obstacle effect was in pelagic fish eggs
6. numerous oiled birds washed ashore at Nantucket and
Martha’s Vineyard
vii. Prince William Sound
1. 1989—Exxon Valdez went aground on rocks out of Valdez,
Alaska
2. largest spill to occur in United States waters
3. Spread to the Gulf of Alaska and damaged 1775 km of
shoreline
4. killed at least 900 otters and 34,000 birds
5. Exxon spent $10 million to spread phosphorus and
nitrogen-rich fertilizers on Alaskan shorelines to boost the
development of indigenous oil-eating bacteria
viii. Largest spill of all time occurred in 1979—Petroleus Mexicanos
(PIMEX) in the bay of Campeche off the Yucatan peninsula,
Mexico, blew out and caught fire
B. Sewage
i. Clean Water Act of 1972 prohibited dumping of sewage into the
ocean after 1981—the high cost of treating and disposing of it on
land resulted in extended waivers being granted to these areas
ii. Sewage that washed ashore on the Atlantic coast beaches during
the summer of 1988 hurt the tourist business and was responsible
for legislation again being passed to terminate disposal of sewage
in the ocean—ironically, this sewage has not been disposed of in
the ocean
C. Halogenated Hydrocarbons (DDT and PCBs)
i. DDT(dichorodiphenyltrichorethane) and PCBs (polychlorinated
biphenyls) are found throughout the marine environment
ii. Persistent, biologically active chemicals that have been put into the
oceans entirely as a result of human activities
iii. Main route DDT and PCBs travel to enter the ocean is through the
atmosphere
iv. Concentrated in the thin surface slick of organic chemicals at the
ocean surface—gradually sink to the bottom attached to sinking
particles
v. Antarctic marine organisms contain measurable quantities of
halogenated hydrocarbons—substances have been transported
from distant sources by winds and ocean currents
D. Mercury
i. First tragic occurrence of mercury poisoning happened at a
chemical factory in Minamata Bay, Japan
ii. First ecological changes were reported in 1950, human effects
were noted in 1953, and the mercury poisoning known as
Minamata disease became epidemic in 1956
iii. Mercury was not declared responsible for the disease until 1968
iv. Also occurred at a factory in Niigata, Japan
v. Outbreaks led to the establishment of safety levels of mercury
content in fish to be marketed by considering three variables:
1. fish consumption rate of the human population under
consideration
2. mercury concentration of the fish being consumed by that
population
3. minimum ingestion rate of mercury that induces symptoms
of disease