Spatial Distribution of Sedimentary Fatty Acids in the Coastal mid-Atlantic Potomac River Basin
(USA)
Cassi L.Walls and Gregory D. Foster1
Department of Chemistry, George Mason University, MSN 3E2, Fairfax, VA 22030
1
Corresponding author: email to gfoster@gmu.edu; fax, 703/993-1055
1
Abstract
The molecular composition of fatty acids in sediments collected along a 320 km
longitudinal transect of the Potomac River (mid-Atlantic United States) was investigated to
identify the primary sources of lipid organic matter in sediments along the hydrologic continuum
of the physiographically diverse Potomac River basin. Total fatty acid concentrations in
sediments ranged from ~ 10 to 400 g g-1 dry weight. In all samples, the short-chain C16 fatty
acid dominated although longer-chain, terrestrially derived moieties were also present. Ratios
based on the fatty acid chain structure such as short/long, branched + odd/ long even,
polyunsaturated/saturated were used to determine the relative contribution of terrestrial,
bacterial, or algal inputs, respectively. Factor analysis identified two suites of fatty acid
compounds that account for the majority of variability. These fatty acid suites represent
allochthonous and autochthonous derived organic matter. Samples collected from the upland
freshwater sites had prominent terrestrial fatty acid signatures while samples collected from the
tidal region showed greater algal and bacterial contributions. The fatty acid profiles revealed
changes in the molecular composition of fatty acids along a downstream gradient ranging from
the upland Blue Ridge Province to the Coastal Plain of the Potomac River basin.
Keywords: Fatty acids; Potomac River Watershed; Lipid biomarker compounds; Sediments;
Factor Analysis
1. Introduction
Natural organic matter incorporated into alluvial sediments of coastal rivers is derived
from a variety of sources, principally terrestrial vegetation and fluviatile and marine primary and
2
bacterial production. Organic matter in natural waters can be apportioned into several major
categories of organic substances (Drever, 1988), including in approximate distributions fulvic
and humic acids (50%), neutrals (15%), bases (5%), acids (25%), and contaminants (<5%). The
organic matter composition in surficial sediments reflects depositional profiles and early
diagenetic alterations of carbon compounds at the watershed scale, which can be used to
apportion sources of organic matter that characterize carbon cycling and biogeochemistry of a
specific category of organic matter (i.e., lipids, which consist of neutrals and acids). Important
anthropogenic impacts such as changes in the ecological landscape of rapidly urbanizing regions,
soil erosion, and the mode of transport and distribution of organic contaminants in river runoff
may be understood in unique ways through the analysis of molecular markers, broadly defined as
chemical assemblages that serve as useful geochemical and ecological indicators of carbon
sources and alterations. Coastal watersheds near large population centers undergo substantial
modifications in surface lithography and ecology during conversion from natural forests to urban
structures such as roads and buildings. Such large-scale landscape changes may be recorded in
space and time through sedimentary composition of biogenic organic substances, such as lipids,
in coastal rivers.
Fatty acids represent a group of molecular markers that differentiate among terrestrial,
fluviatile and marine autochthonous lipid carbon sources in aquatic environments across spatial
and temporal scales (Canuel, 2001), and can be used comparatively throughout a watershed to
determine first-order changes in organic matter cycling and transport in coastal watersheds.
Fatty acids were identified in sediments of the Potomac River, the second largest tributary of the
Chesapeake Bay, from distinct hydrologic zones to characterize and compare sources and sinks
of lipid organic matter in the watershed. Sampling was conducted with a primary consideration
3
of spatial resolution to identify upstream to downstream profiles in a transitional coastal river
system. The central study hypothesis was that sedimentary fatty acid composition is resolvable
into distinct molecular profiles among the hydrologic zones of the Potomac River basin as
related to ecology, landscape, land use and the anthropogenic impacts of urbanization.
Mechanisms of organic matter transport and source to sink processing at the watershed scale
were evaluated by using lipid molecular markers in aquatic sediments.
1.1. Fatty Acids as molecular markers
Molecular markers are unambiguous compounds whose structures can be related to
specific biological sources due to their own biosynthesis (Brassel & Eglinton, 1986; Grimalt &
Olive, 1993; Hedges & Prahl, 1993). In addition to source specificity, ideal biomarkers are
reasonably long-lived and relatively stable in the aquatic sedimentary environment.
Microbiologists have used biomarkers to identify and quantify microbial populations in situ
within sediments (Baird & White, 1985; Dachs et al., 1999; Gillan & Sandstrom, 1985; Parkes,
1987; Parkes & Taylor, 1985; Perry et al., 1979; Rajendran et al., 1992; White et al., 1979).
Microbial marker studies also have been essential to help explain the various inputs and
diagentic changes within sediment (Brassel & Eglinton, 1984; Van Vleet & Quinn, 1979).
Molecular biomarkers have been used to characterize the source and fate of organic matter in
marine and coastal sedimentary environments (Boon & Duineveld, 1996; Boon et al., 1999;
Carrie et al., 1998; Dachs et al., 1999; Duan, 2000; Harvey, 1994; Prahl et al., 1994; Wakeham
& Canuel, 1988; Yunker et al., 1995), as well as, in several estuaries, for example the Mackenzie
River estuary in Canada (Yunker et al., 1995), the microtidal Krka estuary in Croatia
(Laureillard & Saliot, 1993; Scribe et al., 1991), the Changjiang mesotidal estuary in China
4
(Sicre et al., 1993; Sicre et al., 1994; Tian et al., 1992), the macrotidal estuaries of the Loire
River, Brittany, France (Relexans et al., 1988; Saliot et al., 1988b; Tronczynski et al., 1986), the
macrotidal Seine estuary of France (Bodineau et al., 1998; Thoumelin et al., 1997), the Conwy
Estuary of North Wales (Mudge & Norris, 1997), the main stem Chesapeake Bay (Canuel, 2001;
Zimmerman & Canuel, 2001), the St. Lawrence estuary (Gearing et al., 1994; Rodier & Khalil,
1982), and the Morlaix River, Brittany France (Quemeneur & Marty, 1992; Quemeneur &
Marty, 1994).
Membrane lipids and their associated fatty acids are particularly useful biomarkers
because they are essential components of every living cell and have great structural diversity
coupled with high biological specificity (Parkes, 1987). Fatty acids are essential constituents of
cell membrane lipids that assist in the formation of the lipid bilayer, which is the most
fundamental structure of cell membranes. Membrane lipids typically have an amphipathic
structure, where one end of the lipid biomolecule contains a polar group (i.e., phosphate, amine,
or alcohol) and the other a hydrophobic, long chain hydrocarbon.
In phospholipids, hydrocarbon chains are in the form of esterified fatty acids. The length
of the chain varies from as few as 8 to as many as 30 carbons. The number of double bonds per
fatty acid commonly ranges from none to as many as six. Of the two possible isomers for the
double bonds found in fatty acids, the cis isomer is the most commonly found in biological
membranes. Furthermore, it is interesting to note that in the case of fatty acids with multiple
bonds, the double bonds are almost never conjugated (Yeagle, 1993).
1.2. Fatty acid sources
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Chemotaxonomic features of organisms can be used to define input sources of organic
matter in various environments through molecular signatures. Although few biogeolipids can be
linked unequivocally to one biological source, some useful correlations have been developed for
differentiating terrestrial, algal, and bacterial sources by the analysis and identification of fatty
acid distribution patterns, which are summarized in Table 1.
1.2.1. Terrestrial
The external surface of many higher plants comprises of a cuticular layer covered by a
waxy deposit that functions in preserving the water balance of the plant. These waxes are known
to be complex mixtures of long chain (>C22) alkanes, alcohols, ketones, aldehydes, acetals, esters
and acids (Eglinton & Hamilton, 1967; Eglinton et al., 1974), and are characterized by a
predominance of odd carbon number chains for n-alkanes and even carbon number chains for
fatty acids (Caldicott & Eglinton, 1973; Cranwell, 1974; Kolattukudy, 1976; Wannigama et al.,
1981). Therefore, even carbon numbered, saturated fatty acids from 24 to 36 carbon atoms are
commonly used as indicators of allochthonous terrestrial plant inputs in sediments (Cranwell,
1981; Kolattukudy & Walton, 1973; Leenheer & Meyers, 1983).
1.2.2. Algal
Algae produce mostly even numbered fatty acids with C12 to C22 carbon chains
containing 1 to 6 double bonds. Normally, the double bonds are in the cis configuration. Unlike
bacteria and fungi, algae do not appear to synthesize fatty acids with unusual functional groups
such as hydroxy, epoxy, cyclopropanoic and cyclocpropenoic acids (Pohl, 1982).
Polyunsaturated fatty acids (PUFAs) with 16, 18, 20 and 22 carbons can often be confidently
6
assigned as algal molecular markers (Ackman et al., 1968; Chuecas & Riley, 1969; Morris &
Culkin, 1976; Pohl, 1982; Saliot et al., 1982; Volkman et al., 1980; Wakeham et al., 1984). The
presence of PUFAs indicates a fresh algal input because they are liable to degradation processes.
There are >1,300 known species of diatoms (Dunstan et al., 1994), and diatom fatty acid
profiles are remarkably similar despite significant differences in species diversity. Unlike many
other algal classes, diatoms exhibit characteristic fatty acid compositions with a predominance of
16:17, 16:0 and 20:53 together with significant quantities of 14:0 and 20:46, and with lesser
contributions of 22:63, 16:41, 16:34 and 16:24 (Chuecas & Riley, 1969; Dunstan et al.,
1994; Orcutt & Patterson, 1975; Skerratt et al., 1995; Volkman et al., 1980). Unlike some other
algal species, diatoms are low and rarely abundant in C18 fatty acids (Orcutt & Patterson, 1975;
Volkman et al., 1980). C16 and C20 PUFAs are present in diatoms, especially 16:34 and
20:53 (Chuecas & Riley, 1969; Volkman et al., 1989; Volkman et al., 1980).
The major fatty acid components in green algae cultures have been identified as 16:0,
18:33, 16:43, 18:19, 18:26, and 16:17 in order of decreasing relative abundance.
Although variations have been reported in the fatty acid composition of unicellular green algae,
the abundance of C18 unsaturated acids and of 16:43 is generally typical of this algal division
(Chuecas & Riley, 1969; Volkman et al., 1989; Volkman et al., 1980). Low abundance of C20
PUFAs and 16:17 contrasts with the diatom distribution where these acids predominate
(Volkman et al., 1980). C16 and C18 PUFAs can represent 5-43% of total fatty acids in green
algae (Volkman et al., 1989).
The fatty acids abundant in dinoflagellates are 18:5 and 22:63 (Ahlgren et al., 1990;
Hama, 1991; Joseph, 1993; Kattner et al., 1983; Mayzaud et al., 1989; Parrish et al., 1993; Pohl,
7
1982; Volkman et al., 1989; Wakeham & Lee, 1989). C18 PUFAs including 18:33 and 18:43
are also typical of dinoflagellate fatty acid signatures.
Because the fatty acids profiles for specific classes of algae overlap, and multiple classes
of algae are found in the Potomac River, the algal source of lipids were not separated into
individual algae classes. Therefore, the fatty acids of all algal classes are combined for use as a
source marker for overall algal input.
1.2.3. Bacterial
Bacteria contain distinct fatty acid compositions, with high proportions of C13 to C21 odd
numbered fatty acids that are often branched with at most one site of unsaturation. A significant
difference between bacteria and most other organisms is that bacteria do not usually have
polyenoic fatty acids (Parkes, 1987). Branched compounds in the iso and anteiso configuration
with 15 and 17 carbon atoms have been identified in bacterial cultures where they can account
for up to 70% of the total fatty acids (Gillan et al., 1983); these fatty acids are present only in
trace amounts in most algae and are not involved in either vegetal or animal metabolism in
marine or freshwater organisms. Cyclopropyl (17:0 and 19:0) fatty acids are also found
predominately in bacteria (Harwood & Russell, 1984; Parkes & Taylor, 1983; Perry et al., 1979).
For these reasons, such compounds have been widely used for identifying the importance of
bacterial input into marine and estuarine sediments and particles (Cooper & Blumer, 1968;
Federle et al., 1983; Fevrier et al., 1983; Gillan & Hogg, 1984; Gillan et al., 1983; Goutx &
Saliot, 1980; Johns et al., 1977; Parkes & Taylor, 1983; Perry et al., 1979; Saliot et al., 1988a;
Saliot et al., 1982; Schultz & Quinn, 1977; Van Vleet & Quinn, 1979; Volkman et al., 1980;
Wakeham et al., 1984). Although the individual fatty acids are characteristic of bacteria they are
8
not unique to these organisms (Perry et al., 1979), and, hence, it is the combination of these fatty
acids that can be considered as a general marker for bacteria within sediments.
Another fatty acid that appears to be a promising bacterial biomarker is vaccenic acid
(18:17) (Gillan et al., 1983; Perry et al., 1979). However, this compound is also found in other
organisms such as diatoms (Volkman et al., 1980), and Thoumelin (1997) observed that
C18:17 was not an unambiguous bacterial biomarker. The fatty acid 16:110 also has been
suggested as a bacterial marker (Sicre et al., 1988). However, Nichols (1990) suggests that this
fatty acid may also originate from zooplankton or invertebrates. Therefore, these latter fatty
acids were not used as bacterial markers in the present study.
2. Materials and methods
2.1. Watershed description
The Potomac River originates at Fairfax Stone, WV at an elevation of ~1,000 m above
sea level. It stretches ~650 km southeast from its headwaters and gradually widens to ~18 km at
its convergence with Chesapeake Bay at Point Lookout, MD. The Potomac River is the second
largest tributary of Chesapeake Bay, with a watershed of 37,600 km2 and a historical annual flow
of 330 m3/s at Chain Bridge, Washington, D.C. (Mason & Flynn, 1975). Acting as the principal
boundary between the states of Maryland and Virginia, the river flows through six physiographic
provinces from its headwaters to Chesapeake Bay (Fig. 1).
The six physiographic regions of the Potomac River basin include the (1) Allegheny
Plateau, (2) Ridge and Valley, (3) Great Valley, (4) Blue Ridge, (5) Piedmont , and (6) Coastal
Plain (Stanton, 1993). The Allegheny Plateau, also referred to as the Appalachian Plateau, is the
westernmost province of the Potomac River watershed, and it consists of low permeable soils,
narrow valleys, and steep ridges with some reaching elevations of over 1,000 m where large
9
amounts of coal can still be found. This province is predominately forested and has a low
population density (Fig. 2).
The Ridge and Valley, the Great Valley, and the Blue Ridge Valley provinces are similar
geologically to the Allegheny Plateau, but they lack the presence of coal. The Ridge and Valley
Province contains a series of narrow fertile valleys separated by long steep ridges. The Great
Valley Province is an important agricultural area in the eastern United States. The Blue Ridge
Province reaches elevations up to 1,200 m and forms a narrow boundary of rock 16 to 20 km
wide. All of these upstream provinces are predominately forested and have low population
densities (Fig. 2). The types of vegetation found in these first four provinces are quite similar,
with forests comprised primarily of oak and hickory, but also including red maple, tulip,
pawpaw, dogwood, white pine, pitch pine, hemlock, and holly trees (Shosteck, 1968). Cropland
and developed areas begin to become more abundant in the more eastern Great Valley and Blue
Ridge Provinces.
The Piedmont Province is approximately 60 km wide and is characterized by lowland
areas consisting of rolling and hilly terrain. The physiographic boundary between the Piedmont
and Coastal Plain provinces is known as the fall line. At this point, the Potomac River descends
sharply in elevation at Great Falls before reaching Chain Bridge, Washington, D.C. Population
density increases in the Piedmont Province largely due to proximity to major urban centers (Fig.
2).
The Coastal Plain province is located below the river fall line and is characterized by flat
lowland plains with nutrient poor, hydric soils. Here the Potomac River becomes a tidal river
and expands into a broad estuary for ~150 km before discharging into the mainstem Chesapeake
Bay. The tidal region of the Potomac River has three distinct hydrodynamic zones including the
10
upper tidal freshwater zone (from Chain Bridge to Quantico, VA), the transition (mixing) zone
(to Morgantown, MD), and the saline zone (to Point Lookout, MD) (Lippson, 1979). Similar to
the Piedmont province, much of the Coastal Plain is highly developed, although cropland area is
not as abundant (Fig. 2).
There is a great diversity of plant life within the Piedmont and Coastal Plain Provinces,
which is due to the difference in topographic features and chemical and physical properties of the
soils. The region of the Potomac River located in the Piedmont province often flows swiftly
through narrow steep-sided gorges from 30 to 60 m in height, whereas the portion of the river
flowing through the Coastal Plain flows more slowly and is bordered by stretches of mudflats
and marshes. Although many species of plants are common to both provinces, plants adapted to
sweet soils are confined to the Piedmont Province due to the presence of lime, whereas acidloving plant species are confined to the Coastal Plain Province (Shosteck, 1968). Furthermore,
the close proximity to the Chesapeake Bay and oceanic climatic conditions greatly affect the
types of vegetation that grow along the river in the Coastal Plain Province. Common trees
include loblolly pine, red, black and pin oaks, sweet and black gum, yellow popular, hickory,
soft maple, and holly (U.S. Department of the Interior, 1970). Plant life along the estuary
gradually changes from freshwater marsh vegetation to those that are adapted to soils and water
containing high salt concentrations. Freshwater marsh vegetation consists of bulrush, cordgrass,
wild rice, reeds and cattails while, plant life of the salt marshes consists of cordgrass, saltwort,
and orache. The riverine ecosystem and hydrogeochemistry varies throughout the physiographic
provinces and the three tidal regions of the Potomac River estuary. The Potomac River
watershed has expanding urban and suburban development, especially in the Piedmont and
11
Coastal Plain regions near Washington, D.C., with a metropolitan population of nearly 5 million,
yielding significant ecological alterations within the lower watershed.
2.2. Sediment collection
In June 2000, Potomac River sediments were collected from upper Potomac River in the
Piedmont Province (P), freshwater tidal (FT), transition tidal (TT), and saline tidal (ST) river on
board a Boston Whaler using a petite Ponar grab (Wildco, Saginaw, MI). Since the Allegheny
Plateau, Ridge and Valley, Great Valley and Blue Ridge Provinces all contain similar types of
vegetation, they were grouped together and labeled hereafter as Blue Ridge (BR) samples. Ponar
grabs raised onboard the whaler were released in clean stainless steel trays, where surficial
sediments (top 2 cm) were transferred to amber glass jars, sealed with Teflon lined caps,
wrapped with aluminum foil, labeled, placed in a zip lock plastic bag, and stored in an ice chest
until return to the laboratory. Sediments from the Blue Ridge were collected using the petite
Ponar while wading in the deeper pools of the river, and the sediments were stored as described
above. Upon arrival at the laboratory, sediment samples were stored at –40 oC in a freezer prior
to chemical analyses.
Eight to ten sediment samples were collected throughout each region, except for the Blue
Ridge where fewer samples were collected. In order to assess variability, one station in each
distinct hydrologic region was sampled at least in triplicate. Additionally, four to five samples
each were collected throughout two small sub-basins. These samples were obtained from an area
that is heavily impacted by urban development (the urban Anacostia River) and the other that is
forested and relatively undeveloped (Chopowamsic Creek). A total of 36 sediment samples were
collected from the Potomac River (Table 3).
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2.3. Moisture content
A sub-sample of thawed sediment was centrifuged at 1,500 rpm for 10 min to remove
bulk interstitial water. Wet sediment was tared, dried to a constant weight in an oven at 60oC,
and weighed to determine the percentage of water present.
2.4. Grain size analysis
Five-grams of thawed sediment was dried to a constant weight, tared and sieved through a
63 m stainless steel sieve using distilled water. The residual sand was collected from the sieve,
dried, and weighed to determine the sand content. The balance of the sieved mass was
considered to represent the silt/clay fraction.
2.5. Total organic carbon
One gram of sediment was treated with 50 mL of 2 M HCl(aq) to degas inorganic carbon
as carbon dioxide. The acidified mixture was allowed to stand for 24 hr, and the treated
sediment was allowed to dry in an oven at 60 oC to a constant weight. The treated dry sediment
was analyzed for total organic carbon using a Perkin-Elmer Series II CHNS/O (Model 2400)
Analyzer. The elemental analyzer was calibrated using acetanilide before each new batch of
sediment analysis.
2.5. Lipid extraction
Fatty acids were extracted from sediments using procedures described by Harvey (1994).
All solvents used were high purity (Burdick and Jackson brand, Fischer Scientific, Pittsburgh,
PA). Prior to analysis, all glassware was fired to 450 oC for 4 hours and solvent rinsed (acetone
13
and hexane) to remove impurities. A thawed 10 g (wet weight) sediment sample was mixed with
10 ml of dichloromethane (DCM): methanol (MeOH) (1:1) and bath-sonicated three times
sequentially to extract the lipids. The extracts were combined and evaporated to dryness using
rotary flash evaporation. The extracted residue was subjected to mild alkaline hydrolysis using
0.5 M KOH/MeOH and gentle heating (70 oC for 30 min). After the sample was cooled, the
neutral lipids were partitioned from the alkaline solution into 2 ml of hexane, which was
separated and stored for further analysis. The remaining aqueous layer containing the fatty acid
salts was acidified to pH 2, where the fatty acids in this polar-lipid fraction were partitioned
separately into an additional 2 ml of hexane.
The polar lipid fraction containing the fatty acids was evaporated to dryness using rotary
flash evaporation and treated with 10 ml of 12% BF3/MeOH (Sigma Aldrich, St. Louis, MO)
while heating at 70 oC for 30 minutes to form the fatty acid methyl esters (FAMEs). The FAMEs
were subsequently partitioned from the reaction solution into 2 ml of hexane. The hexane layer
was evaporated to dryness, and the extract was then re-dissolved into DCM for chromatographic
analysis.
2.7. Chromatographic analysis
Analysis of the final extracts was performed by gas chromatography/mass spectrometry
(GC/MS) using a Hewlett-Packard (Wilmington, DE) 5890 GC coupled to an HP 5970 mass
selective detector. The inlet was operated in the splitless mode with total and purge flows
adjusted to 30 and 3 mL/min, respectively. The GC/MS was fitted with an HP-5 60 m x 0.25
mm (id) fused-silica capillary column (0.25 m film), with the He carrier gas flow rate through
the column adjusted to 1 mL/min at 100 oC. A two-stage temperature program consisting of 50-
14
120oC at the rate of 10oC min-1, followed by a 3oC min-1 rate to 300oC was used in all
separations. All gas chromatographic data were collected and quantified using MSD Enhanced
ChemStation (Version B.01.00). Fatty acid quantitation was performed using nonadecanoic acid
(Sigma Aldrich) as the internal injection standard. Structural identification of the fatty acids was
determined by comparison of retention times with both internal and external standards
(AccuStandard, New Haven, CT and SUPELCO, Bellefonte, PA) and mass spectral
interpretation.
2.8. Statistical Analysis
Fatty acid data were statistically evaluated using MINITAB (MINITAB Inc.; release 12,
University Park, PA). Parametric analyses, including ANOVA, Tukey’s pairwise comparison
and the Student’s t, were used to identify differences in sediment bulk properties and fatty acid
compositions among the hydrographic regions. The results of these tests were used to evaluate
relative differences among sample sites and were further correlated to ecological or
hydrogeochemical processes.
The data were also evaluated using principle components analysis (PCA) to identify the
underlying factors associated with fatty acid distributions within the Potomac River basin. Only
those variables with mean values and standard deviations of similar magnitude were selected for
factor analysis. Cluster analysis (K-Means) was also used to group sample sites together based
on the relative abundance of each fatty acid identified in each region.
15
3. Results
3.1. Bulk sediment composition
In general, the fine-grained sediments had the highest moisture contents among all the
sediments collected (Table 2). There was a significant correlation between the moisture content
and sediment texture (r = 0.75, p<0.001). Regional differences in the moisture content and
texture existed (ANOVA, p<0.05) for the samples collected in the saline tidal region, which had
higher moisture and higher silt/clay contents relative to the samples collected in the Blue Ridge
or Piedmont regions.
Sediment total organic carbon (TOC) concentrations ranged from 1.5 to 6.8% (wt/wt)
with the lowest regional averages found in the tidal Potomac River. Although there was no
significant difference (ANOVA, p>0.05) in TOC contents among all sediments collected,
excluding sites B1 and B4 produces a clearly identifiable trend of decreasing TOC
concentrations along the downstream transect (Fig. 3). The texture of sediments collected at
sites B1 and B4 was 99% (wt/wt) sand (Table 2), yielding very little organic matter for
analysis. The downstream trend in TOC was in agreement with sediments collected in the
mainstem Chesapeake Bay by Zimmerman and Canuel (2001), who found higher TOC
concentrations in the upper (less saline) mid-Chesapeake Bay relative to the southern (more
saline) Bay.
Significant differences among C/N ratios (Fig. 3) existed for sediments in the upland and
tidal regions of the mainstem Potomac River (ANOVA, p<0.001), with the order (Tukey's
Pairwise Comparisons) Piedmont (mean of 16.1 with B2 and B3 sites included) > freshwater
tidal (including transition tidal, mean of 10.5)  saline tidal (mean of 8.7). The C/N ratios for
sediments in the Anacostia River (mean of 16.3) and Chopawamsic Creek (mean of 15.0) were
16
similar (ANOVA, p>0.05), but both tributaries had C/N ratios in sediments greater than the
saline tidal region of the mainstem Potomac River (ANOVA, p<0.05, Tukey's). Furthermore,
the Anacostia River C/N ratios were greater than that found in the freshwater tidal mainstem
Potomac River sediments (Tukey's).
There was no significant difference among any of the other bulk sediment properties
among Potomac River sediments, or between any of the bulk properties in the Chopowamsic and
Anacostia River sediments.
3.2. Fatty acid composition
The total-FA concentrations ranged from 7 to 410 g g-1 dry wt in the mainstem Potomac
River, with the highest mean concentrations observed in the saline tidal region (233  112 g g-1)
and the lowest mean concentrations in the Blue Ridge regions (42  48 g g-1). The high
standard deviations among these concentrations are not surprising given the substantial
variability in sediment properties (e.g., grain size and TOC) in the Potomac River. Furthermore,
there was a significant correlation (R = 0.44, p<0.01) between the moisture content and total FA
concentration, suggesting a link with grain size distribution of the sediment that has been
previously reported (Mudge & Norris, 1997). When expressed as a percent of sediment TOC,
total-FA concentrations were greater in the marine and upstream freshwater end-members of the
Potomac River than in the freshwater and tidal transition regions (Fig. 3b), where FA
concentrations were lower but more consistent among sediments.
The total-FA concentrations found in the two tributaries ranged from 46 to 194 g g-1 dry
wt, with the highest mean concentration detected in Chopowamsic River sediments (108  58 g
g-1) and the lowest mean concentration found in Anacostia River sediment (89  46 g g-1).
17
There was no measurable difference in the total-FA sediment concentrations between these two
tributaries (Student's t, p>0.05).
A total of 105 individual fatty acids FAs were identified in the mainstem Potomac River,
Chopowamsic Creek and Anacostia River sediments, representing a range of lipid organic matter
sources. When the 101 individual FA concentrations were consolidated into 53 isomer groups,
the most abundant FAs found in the mainstem Potomac River were 16:0, 16:1, 18:0 in all the
freshwater and saline tidal regions (Fig. 4). Furthermore, 14:0 showed enrichment in the saline
tidal sediments, while 22:0, 24:0 and 26:0 were distinctly more abundant in the freshwater
(upstream and tidal) sediments. The FAs detected in Chopowamsic Creek and Anacostia River
sediments were similar to the mainstem Potomac River freshwater regions in composition. The
FA isomer groups shown in Fig. 4 represented >95% by weight of the total-FAs present.
The FAs were further arranged into categories that related to directly to sources (Table 1)
for spatial comparisons. It should be noted that site T7 (Gunston Cove) was not included in
spatial comparisons because concentrations of polycyclic aromatic hydrocarbons (PAHs) in the
sediments were so high (P. McEachern, personnal communication) that they interfered with the
FA identifications. In addition, because sediments at sites B1 and B4 were predominantly sand,
the low concentrations of FAs detected made comparisons difficult, and as a result were also not
included. The FAs grouped according to source specificity represented an average of 62  3%
(mean  SD) of the total FA composition in each sample for the mainstem Potomac River
sediments, and 66  3% (mean  SD) of the total-FAs for the Anacostia and Chopowamsic River
sediments.
Although considerable spatial variation within the distribution of FAs in the mainstem
Potomac River sediments existed throughout the freshwater regions of the Potomac River, a few
18
trends emerged (Table 3). The FAs grouped according to source and normalized to TOC showed
relatively similar, with some variation, proportions from LCSAs, branched and normal oddnumbered-carbon fatty acids (BNFAs), and PUFAs in the upstream Blue Ridge and Piedmont
locations (Fig. 3a). Further downstream, a progression to a slight predominance of LCSA and
BNFAs occurred in the freshwater tidal region, and to a more pronounced enhancement of LCSA
and BNFAs in the transition tidal region. In the transition tidal region, PUFAs were depleted in
sediments relative LCSAs and BNFAs. In the saline tidal region, BNFAs and PUFAs were
predominate, progressively increasing in relative abundance further downstream of site T17 (Fig.
3a). The percentage of FAs in TOC increased sharply in the tidal saline region. The relative
abundance of LCSA, derived from terrestrial plants, remained relatively stable throughout the
entire river, with the notable exception of site T17 that showed a pronounced increase up to
0.10% (Fig 3a).
Although there were no significant differences (Student's t, p>0.05) among LCSA,
BNFA, or PUFA concentrations normalized to TOC between Chopowamsic Creek and
Anacostia River sediments, a shift in the relative abundances of the sedimentary FAs was
evident. The Chopowamsic Creek sediments showed the same relative abundance of
LCSA>BNFA>PUFA in all sediments at the four sites (Fig. 5a). Conversely, the Anacostia
River sedimentary FA profiles showed changes in distribution trough the river transect. The
upstream-most site in the Anacostia River showed a FA pattern similar to the Chopowamsic
Creek sediments, while in downstream sites the FA profiles were altered to a greater relative
abundance of PUFAs. In some cases, such as at site A4, PUFAs predominated, while at site A5
BNFAs predominated. The higher PUFA and BNFA relative abundances in the downstream
19
Anacostia River correlated with a greater degree of channelization, shoreline development, and
urban runoff through combined sewer outfall discharges in the river.
The biomarker ratio C16/C24 was used to identify the importance of allochthonous versus
authchthonous lipid sources in Potomac River basin sediments. All of the ratios were >1,
excluding site T17, indicating that Potomac River sediments receive or retain more lipid organic
matter through autochthonous than allochthonous sources for all river sediment except for site
T17, located in the mixing zone. In the freshwater regions of the Potomac River, relatively high
ratios (i.e., >10) were observed at P5 and T6, and in the marine region from T20 to T25. When
the ratios were compared by region, there was no significant difference between any of the ratios
for the Blue Ridge and saline tidal regions (ANOVA, p>0.05), although the Blue Ridge and
saline tidal ratios were significantly greater than those in the Piedmont, freshwater tidal and
transition tidal segments of the Potomac River (ANOVA, Tukey Analysis, p<0.001).
The higher autochthonous ratios at P5 and T6 correlated with the FAs 20:53, a diatom
biomarker, and (16 + 18)monounsaturated fatty acids (MUFA) , nonspecific algal biomarkers,
although the relative contributions of two the algal sources varied between the two sites (Fig.
3c). A proportionately greater contribution of non-diatom derived FAs was observed at the nontidal P5 site (i.e., 16MUFA minus 20:53 in Fig. 3c) relative to the tidal T6 site, indicating
differences in algal population dynamics throughout the major regions of the Potomac River.
The tidal marine region of the Potomac River showed substantial increases in 20:53 and (16 +
18)MUFA that correlated with the enhanced autochthonous sources identified through the
biomarker ratios.
The C16/C24 biomarker ratio followed the same trend (Fig. 5b) in the Chopowamsic Creek
and Anacostia River, with ratios >1 indicating that sediments from both tributaries receive lipid
20
organic matter inputs primarily from autochthonous sources. However, the Anacosita River
sediments had significantly higher biomarker ratios than that found in Chopowamsic Creek
(Student's t, p<0.05), representing another indicator of enhanced deposition of algal and bacterial
FAs relative to terrestrial sources in the more urbanized sub-basin.
3.3. Spatial variability
Spatial variability in FAs was assessed through the analysis of replicate sediment samples
at selected sites to compare within and among site mean total-FA concentrations in each of the
major hydrologic regions of the mainstem Potomac River (Fig. 6). Variances in total-FA
concentrations were equal for all regions (F-ratio Test, p<0.05) except the saline tidal (where
mean total-FA concentrations were compared using Mann-Whitney U). The highest variability
in FAs was observed in the upstream Blue Ridge and Piedmont sediments, where the within site
means differed from the regional means (Student's t, p<0.05). Although total-FA concentrations
within all of the tidal regions were not statistically different between the within and among site
comparisons, the freshwater and transition tidal sediments showed the greatest overall
consistency in FA concentrations.
3.4. Factor analysis
Factor analysis using principal components analysis (PCA) was used to identify
meaningful geochemical trends in FA sediments in the Potomac River basin. PCA was
conducted using individual FAs or FA groups normalized to sediment TOC. Because the
method requires fewer variables than observations, variables were grouped to reflect a common
source. Therefore, the complete data set was not used in order to meet the assumption of
21
independence. Following the approach of Zimmerman and Canuel (2001), only those variables
with mean values and standard deviations of similar magnitude were selected for PCA.
Two factors identified by factor analysis (Fig. 7) accounted for 69% and 5% of the total
variance in the FA data, respectively. A large portion of the total variance remained unaccounted
for, suggesting there are many additional minor factors that contribute to the high variability in
this complex system. However, the first two factors combined accounted for 74% of the
variability and appeared to represent interpretable, geochemical factors. All of the variables
except for LCSA were heavily correlated to factor 1, including all of the mixed algal and
bacterial biomarkers being autochthonous in nature. Factor 2 was highly correlated with LCSA,
which is the group of terrestrial fatty acids suggesting that Factor 2 represented allochthonous
inputs.
The factor score plot (Fig. 5) shows the relative influence of each factor on the sample
composition. Most of the samples are located around the origin, suggesting that they are equally
dominated by allochthonous and autochthonous input. Site T17, however, has a very high
positive score for Factor 2 indicating that it was dominated by allochthonous input. Samples
T22, T23, T24, and T25 were all located in the southern most saline tidal region and had
relatively high scores for Factor 1, which is indicative of autochthonous input. The sample
scores validated the interpretation of the factors as indicators of lipid organic matter sources and
their ability to represent spatial variations in surficial sediments.
The results of the cluster analysis (K-means), based on FA concentrations normalized to
TOC, showed that only two predominant clusters of FA profiles existed along the downstream
continuum of the Potomac River. The first cluster consisted of all BR, P, and T samples except
for T17 and the downstream most sites T22 – T25; the second cluster consisted of T22 - T25.
22
The FA profiles were very similar though the entire freshwater region of the Potomac River up to
the transition zone. Below the transition zone, the FA profiles changed to a predominantly
marine algae source. Although the terrestrial LSCA biomarkers were fairly consistent between
the two clusters, there was a clear difference in algal and bacterial biomarker concentrations
between the two regions. The saline tidal cluster had much higher algal and bacterial biomarker
concentrations.
4. Discussion
4.1. Bulk organic matter in the mainstem Potomac River
The gradual decrease in sediment TOC from the Blue Ridge regions to the freshwater-salt
water transition (mixing) zone in the Potomac River results from the hydrologic dispersion and
early diagenesis of terrestrially derived organic matter entering the river principally in the upland
regions of the basin. Organic matter undergoing land-to-sea transport is dispersed over the wide
river bottom in the tidewater region of the Potomac River, mixing with fresh autochthanous
sources also settling to the tidal bottom sediments. In rivers, sedimentary material has a very
active depostion-resuspension cycle in relation to river flow and bottom shear forces
(XXXXXX), especially in steep gradient geological regimes such as the upland Potomac River,
that serve to rapidly transport sediments in the downstream direction. Sedimentary TOC in the
Potomac River follows a dilution trend along the downstream gradient in a fashion similar to
riverine dissolved organic carbon (XXXXX).
Sedimentary TOC in the Blue Ridge and Piedmont regions was variable, but was the
highest observed in the Potomac River basin. Below the river fall line TOC remained relatively
uniform throughout sediments in the freshwater and transition tidal regions of the Potomac
23
River. The decrease in TOC in the tidal river appeared to minimize near the beginning of the
mixing zone, and increased slightly in a progressive fashion through several sites on seaward
end. The low TOC at site T23 (Kingscopsico Point) was due to the sandy texture of sediment at
this location (~90% sand), reflecting the sediment texture heterogeneity encountered in the
Potomac River sediments. Although TOC was not significantly correlated to sediment texture
throughout the basin (R2 =?, p>0.05), sediments having a very sandy texture did show low
TOCs, generally. Inputs of organic matter, as reflected by river sediments, appeared to be the
highest at the upstream and marine end-members of the Potomac River basin. A similar endmember trend was observed for the organic carbon composition of suspended particles in the
tidal reaches of the York River, a more southern tributary of Chesapeake Bay (XXXXXX).
The sedimentary C/N ratios showed sharp deceases at the transitions of the upland to
freshwater tidal and freshwater tidal to saline tidal regions of the Potomac River, reflecting a
progressive downstream enrichment of auotchatanous carbon sources along the river transect.
The C/N ratios declined sharply between sites T17 to T20, where primary production increases
markedly in the saline tidal region of the river. Allochthanous sources of organic matter in soils
have been reported to have C/N ratios >20 on average, while autochthanous sources typically
provided C/N ratios <10 (Herczeg, 2001 #181). Thus, sediments in the Blue Ridge regions of
the Potomac River incorporate predominantly terrestrial organic matter, having an average
upstream end-member C/N ratio of 22.6 (from the two Blue Ridge sites). The Piedmont
sediments (average C/N ratio of 13.9) show a mixed but predominantly autochthanous source
composite, comprising ~70% (wt/wt) of the total organic matter pool, as estimated by applying a
simple two component mixing model with the Blue Ridge and saline tidal (8.7) average endmember C/N ratios. The C/N ratios integrate the total organic matter pool in Potomac River
24
sediments, and source profiles characterized by C/N ratios may differ from that identified using
lipid biomarkers.
It is noteworthy that there appeared to be a gradual increase in C/N ratios from site T1 to
a maximum at site T17, near the boundary of the mixing zone, likely arising from fresh inputs of
organic matter in the tidal regions derived from vascular plants. The freshwater tidal regions of
the Potomac River support a great abundance of submerged aquatic vascular plants (SAV),
estimated via imagery data in 1990 to support roughly ten thousand tons during the peak growing
season (Orth, 1991 #182). The dominant species of SAV in the Potomac River is Hydrilla
verticillata, often regarded as a nuisance to navigatibility in the river. That vascular plants have
larger C/N ratios than algae coupled with the reduced level of primary production observed in
the transition tidal reach of the Potomac river may account for the upward trend in C/N ratios
seen in the freshwater and transition tidal Potomac River. It is likely that SAV contribute a
substantial amount of organic matter to sediments in areas where they are abundant.
4.2. Fatty acids in the mainstem Potomac River
Sediment TOC and total-FA concentrations were not correlated (R2 = 0.02, p>0.05) in
the mainstem Potomac River, although the concentrations of total-FAs were greatest in
sediments found in freshwater and marine end-members of the Potomac River basin. A positive
relation between TOC and FA’s seemed to exist on a regional basis, but not among individual
sediment sites. Gearing (1994) found a similar trend among sediments in the St. Lawrence
Estuary where FAs were highest at the landward and seaward ends of the estuary because of a
postulated increased algal production in these areas. Therefore, primary production is greatest in
the Piedmont and saline tidal regions of the Potomac River, as evidenced by total-FA
25
concentrations in sediments. Because FAs made up a very small fraction of the TOC, less than
0.5% by mass in all Potomac River basin sediments, changes in lipid sources and abundances
have little impact on the quantity of sedimentary TOC. The composition of the TOC in
freshwater sediments is predominantly in the form of humic and fulvic acids, tannins and other
recalcitrant biogenic substances, derived from terrestrial plant material through diagenetic
alteration or the slow breakdown of lignin (XXXXX).
The use of compositional ratios may reveal additional patterns that are not clearly seen in
relative abundance profiles. Compositional ratios assisted in identifying the relative importance
of various organic matter source inputs. As previously discussed, the autochthonous (algae and
bacteria) to allochthonous (terrestrial) ratios tended to decrease moving from upstream to
downstream and then they dramatically increased in the tidal saline region. The results of the
ANOVA and Tukey analyses showed that BR and T III regions were not significantly different
compared to each other; P, T I, and T II regions were not significantly different compared to
each other; however, T III compared to P, T I, and T II is significantly different (P < 0.001). As
also observed by Gearing (1994), this suggests that the algal and bacterial input are important
contributors to the organic matter in the landward and seaward regions. This observation was
not evident when simply evaluating the relative abundance of the individual fatty acids.
The lipid derived organic matter in the Potomac River sediments in early summer appears
to originate primarily from autochthonous sources along the entire river transect, based on all
biomarker ratios C16/C24 being >1, with only one exception at site T17 (Mathias Point).
Biomarker ratios assist in identifying source inputs into sedimentary organic matter and have
been widely used (Boon & Duineveld, 1996; Colombo et al., 1996; Fukushima & Ishiwatari,
1984; Meyers et al., 1984; Mudge & Norris, 1997). The pattern of C16/C24 biomarker ratios
26
showed that autochthanous inputs to sediments varied spatially in a seemingly rhythmic fashion
of peaks and valleys along the river transect in response to particle settling and
resuspension/dispersion processes acting on sediments in fluvial and bed load transport. Particle
settling zones in the river were clearly evident at sites P5, T6, and to a lesser extent T9 and T16
(Fig 5b), being areas where PUFA concentrations, derived from the deposition of suspended
algal cells, showed marked to small increases in sediments. Selected FA biomarkers such as
20:53 and (16+18)MUFA indicated a shift in algal community dynamics along the river
transect in the particle settling zones early summer, in which diatoms were more prominent in
tidal river relative to the Piedmont sediments. A ratio of diatom to mixed algae biomarkers
[20:53/(16+18)MUFA] ranged from <1 in Piedmont sediments to ~1 in tidal freshwater and
saline river sediments in the major particle focusing zones. A substantial amount of primary
production occurs throughout the entire length of the Potomac River, with carbon dynamics
being dependent on the deposition cycles in selected areas of the river. Several investigators
have confidently used 18, 20 and 22 PUFAs as algal biomarkers (Ackman et al., 1968; Morris &
Culkin, 1976; Saliot et al., 1982; Volkman et al., 1980; Wakeham et al., 1984). 18-PUFAs, the
presence of which could be attributed to green algae, are present in lower amounts in the
maximum turbidity zone (T17) an observation which correlates with the decrease in primary
productivity (Saliot et al., 1988a) (Fig.11). The concentrations of 20:53, 14:0 and 22:63which
are abundant in numerous diatoms and other algae (Smith et al., 1983; Volkman et al., 1980), as
well as 18 PUFAs are at their maximum in the saline waters (T III), corresponding to the
maximum marine phytoplankton productivity (Fig.12).
Typically, BNFAs tracked PUFAs in sediments, which is due to the affinity of
heterotrophic bacteria to a favorable source of organic matter for anabolic metabolism. As
27
reported by Thoumelin (1997), maximum proportions of bacterial fatty acids were found
simultaneously to those of algal PUFAs, and this possibly indicates a bacterial population
associated with algal cells. Similarly, Saliot (1988a) observed that maximum microbial
(bacterial) imprints are present in association with high planktonic/algal biomasses. However,
allochthonous sources contributed more to sediment lipid pools in certain regions of the river,
especially at site T17 in the mixing zone. The high LCSA content of FAs at site T17 was most
likely due to a localized source of organic matter from either terrestrial plants or submerged
aquatic vegetation (SAV), which are abundant in the tidal portion of the Potomac River. The
lipid composition of SAV contains substantial amounts of >C22 FAs that are normally associated
with the waxes of land plants. The significant abundance of SAV in the tidal Potomac River
could influence carbon dynamics and profiles of FAs in sediments in the transition tidal region.
The collection sites in this study were made within the main channels of the Potomac River and
its tributaries as much possible where organic matter was expected to be well mixed to avoid any
shoreline effects dominating source comparisons. Saliot (1988a) indicated, continental material
is closely associated with large particles in riverine transport, where terrigenous imprints are high
in the maximum turbidity zone, and FA deposition is regulated by enhanced particle coagulation
in the mixing zone, whereupon the less hydrophilic LSCAs are preferentially sorbed to river
particles relative to shorter chain FAs leading to enrichment in sediments. In addition,
enhancement of dissolved organic matter from released pore waters during resuspension events
probably induces aggregation of particles into larger sizes. Organic matter “glues” mineral
particles together, and this results either from a decrease of the repulsive forces between
electrically charged particles and/or from microbial action on organic matter (Baretta-Beckker et
al., 1992). The combination of these processes acts to preferentially trap the LSCAs in
28
sedimentary material at the mixing zone of coastal rivers, although the importance of enhanced
deposition affecting the FA composition of organic matter at site T17 seems unlikely because of
localized nature and apparent aberration of FA composition at this site. TOC in site T17
sediments was also low because of the high current velocity of the river at this location,
indicating it was not a depositional zone in the river. It appeared that the LCSA represented a
consistent sedimentary FA composition in the Potomac River mainstem, continuing through to
the saline Chesapeake Bay.
The lipid dynamics in the mainstem Potomac River seemed to deviate from the normal
associations between PUFAs and BNFAs in the transition tidal region, which were the most
unusual in the river. Here, BNFAs showed substantial increases through sites T11 to T14 even
as PUFAs remained at low concentrations. Heterotrophic bacteria were utilizing a alternative
carbon source in this region of the river, one likely derived from fresh vascular plant sources
based on elevated LCSA concentratioins through the same sites. Such a trend would seem to
pinpoint some role of SAV and/or localized allochtanous inputs on carbon cycling in the
transition tidal region of the Potomac River. The results of several studies examining
phytoplankton production in estuarine environments correlate with the present observations
made on FAs in Potomac River sediments (Fisher et al., 1988; Harding et al., 1986; McPherson
et al., 1990). McPherson (1990) observed that spatial factors in transitional rivers resulting from
the interaction of salinity, nutrients, and watercolor that resulted from the mixing of freshwater
inflow and seawater, effects phytoplankton productivity. Although freshwater inflow increases
the availability of nutrients in low saline waters, the highly colored freshwater restricts light
penetration and, thus, limits primary production. Maximum productivity and biomass occurs
where color associated with the freshwater inflow was diluted by seawater so that light and
29
nutrients are both more bioavailable. Both Harding (1986) and Fisher (1988) observed that the
chlorophyll and productivity maximum in the Chesapeake Bay occurred seaward of the turbidity
maximum where light penetration increased and sufficient nutrients were present to support
active phytoplankton growth. Marine phytoplankton abundance was clearly evident in by the
increased relative abundance of phytoplankton, especially diatomaceous, FAs in the sediment
organic carbon pool downstream of the transition zone.
The source of lipid organic matter in Potomac River sediments undergoes distinct
changes in the tidal region at the marine and freshwater boundary, with a switch to almost
exclusively autochthonous sources in the marine region. A mass balance of lipid biomarkers is
illustrated in Fig. 9 to highlight the changing dynamics of organic matter transport through a
coastal river. Such changes in the origin of organic substances in the river can affect the sources
and fate to anthropogenic contaminants during land to sea transport.
4.3. Fatty acids in the Potomac River tributaries
The compositional ratios also revealed indistinguishable patterns in the Anacostia versus
Chopowamsic analysis. The results of the t-tests showed that the two regions were not
significantly different in any of the individual fatty acid concentrations. However, the tests did
show significant differences in the three autochthonous vs. allochthonous ratios. This suggests
that terrestrial source is a more important input into the Chopowamsic lipid organic matter than
in the Anacostia organic matter. Conversely, there was a change in the predominance of lipid
biomaker sources in the Anacostia River through the downstream transect (Fig. 9). The
upstream tidal Anacostia River site showed a lipid profile very similar to Chopowamsic Creek,
where the terrestrial and bacterial lipids predominated, but further downstream algal sources
30
became the most abundant. The downstream Anacostia River is the most urban-developmentimpacted region of the Potomac River basin, and here the river is completely channelized and
surrounded by development. Several combined sewer outfalls, which discharge raw sewage and
impervious surface runoff in response to atmospheric precipitation, provide heavy loads of
pollutants to the river (XXXX).
4.4. Implications of lipid biomarkers
Sediments of recent origin act as ‘sinks’ or reservoirs of pollutants entering the aquatic
ecosystem, where they tend to persist and accumulate. The bacteria within these sediments play
a vital role in the mineralization reactions that are essential parts of the cycles of nitrogen,
phosphate, carbon, sulfur and other nutrients, thus microbial populations can be altered by
pollutants; bacteria will respond rapidly to even small environmental changes (Parkes, 1987).
Characterization of sedimentary microbial communities could provide important information
regarding the impact or degree of pollution in an aquatic environment. Analysis of biological
membrane constituents may provide information concerning community structure, including
total biomass, bacterial biomass, or the biomass of specific bacterial groups. Therefore, it was
unexpected to observe was no significant difference in the bacterial to terrestrial ratio between
the forested Chopowamsic Creek and the urban/polluted Anacostia River. However, this
analysis could only be related to the bacterial biomass. Biomass estimates, which may be
equitable to activity for higher organisms, do not necessarily reflect metabolic activity when
applied to bacteria (Meyer-Reil et al., 1978). Therefore, the bacteria population in the Anacostia
River may essentially be the same magnitude in size as the population in the Chopowamsic
Creek, but it could be more metabolically active. Since bacterial biomass measurements do not
31
necessarily reflect bacterial activity, assessment of different types of activity must be an
important part of any attempt to characterize bacterial populations within polluted sediments
(Parkes, 1987).
5. Conclusions
Graphical analyses, compositional ratios, factor analysis and cluster analysis of fatty acid
biomarkers were used to reveal spatial variability in the origin of lipid organic matter sources
along the hydrographic continuum of the Potomac River. The use of all of these tools allowed a
more complete geochemical interpretation of lipid organic matter source distributions.
The use of graphical analyses and compositional ratios identified the maximum turbidity
zone along the Potomac River. The factor analysis identified two factors that account for 74% of
the variability. Factor 1 represents allochthonous (terrestrial) input, while Factor 2 represents
autochthonous (algal and bacterial) input. Furthermore, the factor analysis and cluster analysis
revealed that there are only two distinct regions, in addition to the maximum turbidity zone, that
have unique fatty acid biomarker signatures. One grouping consists of all BR, P and T (except
T22-T25) samples while the other grouping consists of the southernmost samples T22, T23, T24,
and T25 located in the saline tidal region of the Potomac River. The predominate difference
between these two groupings is that the phytoplankton productivity maximum occurs seaward of
the turbidity maximum (T22, T23, T24, and T25) where there is a strong algal fatty acid
signature as compared to the up-river locations.
The use of compositional ratios also revealed information about the relative importance
of the of source specific fatty acids into the organic matter. The algal and bacterial input are
more important contributors to the organic matter in the landward and seaward regions of the
32
Potomac River. Additionally, terrestrial sources are more important contributors in the forested
Chopowamsic Creek compared to the urban Anacostia River.
Acknowledgements
Sincere appreciation is given to Dr. Rodger Harvey and his research group of the
University of Maryland for technical assistance in the lipid extraction method. Mr. Tom Huff
and Mr. Phil McEachern of George Mason University (GMU) provided critical laboratory
coordination and support. The coordination of sediment collection by Dr. Paige Doelling Brown
(GMU) and the statistical assistance of Dr. Leila Barraj (
) were invaluable. Financial support
for this study was provided by the Jeffress Memorial Trust (Grant No. J-559).
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