Final Report

A Survey of the Shallow Water and Intertidal Benthic Invertebrates

 at Three Sites in the Vicinity of The Chalk Point Steam Electric Station

A Study Conducted for The Patuxent River Damage Assessment

 of the Chalk Point Oil Spill

by 

The Academy of Natural Sciences

Estuarine Research Center

10545 Mackall Road

St. Leonard, Maryland 20685

Telephone: 410-586-9700

Fax: 410-586-9705

Principal Investigator

Richard W. Osman, Curator

Benthic Ecology
1

A Survey of the Shallow Water and Intertidal Benthic Invertebrates

 at Three Sites in the Vicinity of The Chalk Point Steam Electric Station

In April 2000, oil was released into Swanson Creek at the site of the Chalk Point

Steam Electric Station (Figure 1). The oil spread into the Patuxent River and was

carried down the river more than 25 km, potentially impacting coastal habitats within

this area. This study was designed as a simple survey to measure the abundance of

infaunal invertebrates in sediments from three low intertidal - shallow subtidal sites in

the vicinity of the release. The three sites that were sampled included: 1) a reference

or control site in Hunting Creek that was up-river from the release point and not

impacted by oil, 2) a moderately-oiled site in Trent Hall Creek down-river from the

release point, and 3) a heavily-oiled site in Swanson Creek, the location of the release.

Samples were collected in late September 2000, approximately 5 months after the oil

was released.

In order to compare the fauna among the three sites, 10 replicate core samples

were taken at each of the three sites. To minimize the natural variation among sites

every effort was made to select sampling locations within each creek that were at

similar tidal heights, were located adjacent to marsh habitat, and had similar sediment

types. All 3 sites were sampled on the same day within 3 h of each other. 

Methods

Field Collections

All samples were collected at the three sites on 20 September 2000. To prevent

any possible contamination of samples, the sites were sampled in order of their

exposure to oil. Samples were first taken at the control site in Hunting Creek (76( 39'

26" Long, 38( 33' 26" Lat), then in Trent Hall Creek (76( 40' 52" Long, 38( 28' 45" Lat),

and finally in Swanson Creek (76( 41' 34" Long, 38( 32' 39" Lat). At each site we

measured water temperature and salinity using a YSI 85 meter. Next a 20-m transect

parallel to the shore was marked at water depths of 15 - 37 cm. Because the samples

were collected on a receding tide the mean water depth also was decreased from a

mean of 35.5 ± 1.0 (SE) cm in Hunting Creek to 25.5 ± 0.6 cm in Trent Hall Creek to

18.6 ± 0.8 cm in Swanson Creek in an effort to sample at the same depth relative to

Mean Low Water. Ten sites along each transect were chosen randomly and marked. 

At each marked sampling site a 15-cm diameter core sample was taken at a distance of

50 cm perpendicular to the transect. The shoreward or seaward direction of the 50-cm

displacement was chosen randomly. 

For each sample we pushed the corer approximately 20 cm into the sediment. 

Before the corer and sediment core were extracted, we measured the water depth and

then took a 200 g sediment sample adjacent to the core tube. This sample was given
2

to a representative of the Trustee Council for potential chemical analysis. After this

sample was taken, the corer was extracted. Each core sampled the sediment to a

depth of approximately 20 cm resulting in our sampling 3.5 l of sediment. The sediment

collected from each replicate core was placed in a separate plastic lidded container,

returned to the laboratory and refrigerated. 

Laboratory Analyses

Initial processing of the cores, consisting of sub-sample collection and sieving of

the bulk of the sediment for macroinvertebrates, was done within 24 h of collection. 

Sub-samples were taken from the mixed core sediments in each container. Subsampling included the collection of 100 g of the sediment for grain size analysis and 20

ml that was frozen for subsequent analysis of organic matter content. These three

types of samples were collected as follows:

1. Macroinvertebrate Samples. After the sub-samples were taken, the

remaining bulk of the sediment was sieved using a 500 μm screen. All

material retained on the screen was fixed in formalin and stained with

Rose Bengal. After approximately 1 wk samples were transferred into

alcohol. Subsequently, samples were examined using a dissecting

microscope and all macroinvertebrate organisms were identified and

counted.

2. Sediment Grain Size Analysis. The 100 g samples were sieved using a

series of stacked screens with mesh sizes of 2000, 1000, 500, 250, 125

and 62 μm (Butler and McManus 1979, Buchanan 1984). The sediment

was sieved wet to avoid aggregation of the finer particles as a

consequence of drying. All water and sediment passing through the 62

μm sieve were retained and filtered to retrieve the fine silt - clay

sediments (Butler and McManus 1979). The proportion of the sample in

each of the 7 size classes was determined by volume and dry weight and

used to characterize the sediment and any differences within and among

the three field sites. 

3. Organic Matter Content. The 20-ml frozen samples were analyzed using the

ignition method (McCave 1979, Rosa et al. 1991) for organic matter

content. Each sample was dried at 60(C, weighed, combusted at 450( C,

and then re-weighed. The difference before and after combustion was

used to estimate the proportion of organic matter in each sample. 

Combustion was at 450( C to avoid the combustion of any shell

(carbonate) material in the samples (Rosa et al. 1991).
3

Data Analyses

The objective of all statistical analyses was to compare the three sites and

determine whether they differed from one another. All analyses were conducted using

JMP 4.0 statistical software (SAS Institute 2000).

The main type of analysis conducted was a standard one-way analysis of

variance (ANOVA). Differences among the three stations for individual taxa, trophic

groups, and species richness were all analyzed using one-way ANOVA. For all

analyses in which a significant difference was found (p<0.05), a Tukey multiple

comparison test (Tukey 1991, Kramer 1956, SAS Institute 2000) was used to contrast

the individual stations.

The similarity in community composition among the samples collected at the

three sites was also analyzed using hierarchical clustering using Ward's minimum

variance method for linking clusters (SAS Institute 2000). Three types of analyses

were conducted: 1) similarity among individual samples based on the abundances of all

taxa in the samples, 2) similarity among individual samples based on the abundances

of the 8 most abundant or dominant species, and 3) similarity among the dominant

species based on their abundances in the samples.

Results

Environmental Differences

A limited number of environmental parameters were measured for each site. 

These included the water temperature and salinity at each site and the sediment grain

size distribution and organic matter content of each sample. Water temperature varied

slightly from 21.3(C at Hunting Creek to 23.8(C at Trent Hall to 25.5(C at Swanson

Creek. These differences probably resulted from the order in which the sites were

sampled with Hunting Creek being sampled first in the morning and Swanson Creek

being sampled in the mid-afternoon when air temperatures had increased. Also the

sampling day was sunny and solar radiation could be expected to warm the shallow

water at the sites. Unlike temperature there was a substantial difference in salinity

among the sites with the salinity at Trent Hall (9.6 ppt) being almost double that at

Hunting Creek (5.3 ppt) and Swanson Creek (5.6 ppt).

The analysis of sediment grain size indicated that there was little difference

among the sites in the types of sediments that were present. When analyzed by dry

weight (Figure 2), Swanson Creek had a significantly lower mean percent of it's

sediment in the 125 )m class than the other two sites and Trent Hall had a significantly

lower percentage in the >2000 )m class. Likewise when analyzed by volume (Figure

3), Swanson Creek had a higher percentage of silt-clay (<63 )m) than the other sites,

but this was balanced by lower percentages in the 63 )m and 125 )m classes. Overall,
4

these differences indicate little, if any, meaningful differences in the sediments at the

three sampling sites.

The analyses of organic matter content in the sediments also indicated little

difference among the sites (Figure 4). The mean percent organic matter was highest in

the sediments from Swanson Creek (5.3% ± 1.4 SE) compared to those from Hunting

Creek (3.2% ± 0.7) and Trent Hall Creek (2.6% ± 0.8). However, these differences

were not statistically significant.

Distribution of Dominant Taxa

The fauna sampled at the three sites was fairly typical of shallow subtidal areas

within the Patuxent River and adjacent Chesapeake Bay. Given the high spatial and

temporal variability in the abundances of these species, the densities observed at the

three sites were within the ranges seen in the region (Holland 1985, Holland et al.

1977a, 1977b, 1980), The principal macrofaunal invertebrate groups were polychaete

annelids, nemerteans, molluscs, and crustaceans. Meiofaunal groups (e.g. copepods,

nematodes) were also sampled. However, because we used a 500 )m screen in

processing the samples, the much smaller organisms in these groups were underrepresented. Below we compare the abundances of the dominant taxa among the

three sites. All statistical inferences are based on one-way ANOVA's with p<0.05.

Polychaetes. Polychaetes were the most abundant taxa found at all three

sites. The polychaete fauna consisted of two species, the sub-surface deposit feeding

Heteromastus filiformis and Nereis succinea, an omnivore which also preys on other

infauna. Surprisingly, no other polychaete species were found, including any surface

deposit-feeding species.

Both species of polychaetes were significantly more abundant in Trent Hall

Creek (Figure 5). Their densities were almost 3 times greater at this site than the other

two sites. They were the two most abundant species at all three sites.

Nemerteans. The only nemertean found at the sites was the predator Micrura

leidyi. This species was in fairly low abundance (Figure 5) with no significant difference

found among the three sites.

Molluscs. Three species of molluscs were found in samples from the three sites. 

These included the bivalves Macoma balthica and Rangia cuneata and the gastropod

Hydrobia sp. Rangia is a suspension-feeder typically most abundant in low salinity

areas. Although Macoma can also suspension-feed it is principally a surface depositfeeder, especially in muddy sediments (Olafsson 1986, 1989). Hydrobia is an

omnivorous grazer on the surface sediments.
5

The distribution of Macoma was similar to the two species of polychaetes (Figure

6). It was significantly more abundant in Trent Hall Creek, with densities > 3 times

higher there than in the other two sites. Rangia exhibited a different pattern with its

density being significantly higher in Hunting Creek than in Trent Hall Creek (Figure 6). 

Densities in Swanson Creek were intermediate between the other two stations and not

significantly different from either. Finally, Hydrobia was found only in Swanson Creek

and its densities were significantly higher there than at the other two sites (Figure 6).

Arthropods. Arthropods formed the most diverse taxonomic group of

macrofauna at the three sites, including the cumacean Leucon americanus, the amphipod

Leptocheirus plumulosus, the isopods Edotea triloba and Cyathura polita, the grass shrimp

Palaemonetes pugio, the mud crab Rhithropanopeus harrisii, chironomid insect larvae, and

the barnacle Balanus improvisus. Except for the suspension-feeding barnacle, all of

these species graze on the sediment surface with Leucon and Leptocheirus considered

to be surface deposit-feeders and the remaining species considered omnivorous

(Chesapeake Bay Program 2000).

Of the arthropods, the two isopods, Edotea and Cyathura were most abundant. 

Cyathura exhibited no significant difference among the three sites (Figure 7). Edotea, on

the other hand, was significantly less abundant in Trent Hall Creek than at the other

two sites (Figure 7).

The only other arthropod found to vary significantly among the sites was the

cumacean Leucon. Leucon was significantly more abundant in Trent Hall Creek than in

Hunting Creek, where it was absent (Figure 7). Its abundance in Swanson Creek was

intermediate and not significantly different from either of the other two sites.

The remaining arthropod species were in fairly low densities and exhibited no

significant differences among the three sites (Figure 8). The amphipod Leptocheirus

was found only in Hunting Creek and the mud crab Rhithropanopeus was found only in

Trent Hall Creek and Swanson Creek.

Meiofauna. Both meiofaunal groups, copepods and nematodes, were more

abundant in Trent Hall Creek than the other two sites (Figures 9). However, these

differences were not statistically significant.

Distribution of Larger Taxonomic Groups

The distribution among the three sites of each of the major taxonomic groups

(polychaetes, molluscs, arthropods, and meiofauna) was also compared. Polychaetes

were greater than 50% of the total fauna at all three sites with their proportional

abundance being significantly greater in Trent Hall Creek than at the other two sites

(Figure 10). Molluscs comprised 10-20% of the fauna at all three stations (Figure 10)

and meiofauna were 5-10% of the sampled fauna (Figure 10). Neither of these groups
6

differed significantly among the sites. Finally, arthropods were >20% of the fauna in

Swanson and Hunting Creeks (Figure 10). These percentages were significantly

greater than the <10% found for Trent Hall Creek.

Trophic Group Differences

The taxa in each sample were also pooled by trophic group or guild in order to

examine whether any potential differences existed among the three sites in trophic

dynamics. The feeding guild of each taxon was assigned using the Chesapeake Bay

Program classification system (Chesapeake Bay Program 2000). Based on this system

the sites had four major trophic groups; deep, sub-surface deposit feeders

(Heteromastus), surface deposit feeders (Leptocheirus, Leucon, Macoma),

omnivores/carnivores (chironomids, Cyathura, Edotea, Hydrobia, Micrura, Nereis,

Palaemonetes, Rhithropanopeus), and suspension feeders (Rangia, Balanus).

Both the sub-surface deposit feeder (Figure 11) and surface deposit feeder

(Figure 11) groups were significantly more abundant in Trent Hall Creek. Much of this

results from the high densities of Heteromastus and Macoma in Trent Hall Creek. 

Omnivores (Figure 11) were significantly more abundant in Trent Hall Creek (largely

from high Nereis densities) than in Hunting Creek with Swanson Creek being

intermediate and not significantly different from either of the other two stations. Finally,

suspension feeders (Figure 11) were significantly more abundant in Hunting Creek (as

a consequence of high Rangia densities) than Trent Hall Creek with Swanson Creek

being intermediate.

Community Patterns

Overall, total faunal densities in Trent Hall Creek were approximately double

those found at the other two sites and significantly higher (Figure 12). This was largely

a consequence of the high abundances of Heteromastus, Nereis, and Macoma in Trent

Hall Creek. However, this difference in abundances was not reflected in the species

richness of the sites. The mean number of species found at the three sites varied

between 7 and 9 with no significant differences found among the sites (Figure 12).

Hierarchical cluster analyses generally showed the same types of patterns for

communities as was seen for individual taxa. Cluster analyses were done using the

abundance data for all taxa (Figure 13) and also with only the most abundant

macrofaunal taxa (Nereis, Heteromastus, Macoma, Rangia, Micrura, Leucon, Hydrobia,

Edotea, Cyathura) to weight the analysis towards dominant species (Figure 14). In

general, the results for both analyses were similar and demonstrated the community

differences among the three stations. When all taxa were included (Figure 13) 5 of the

10 Trent Hall samples cluster together as a group distinct from all other samples. 

Likewise, 7 of the 10 Swanson Creek samples along with 1 Hunting Creek sample

formed a second distinct grouping. The remaining samples formed a larger, mixed
7

group dominated by samples from Hunting Creek. However, within this group there are

4 distinct sub-groups of a) 5 Hunting Creek samples, b) 3 Trent Hall samples, c) a

single Hunting Creek sample, and d) a mixed group of the remaining samples. When

the analysis was repeated with only the dominant species (Figure 14) 7 of the Trent

Hall samples formed a distinct cluster. The cluster of 7 Swanson Creek samples and 1

Hunting Creek sample did not change and there was a grouping of 2 Hunting Creek

samples. The remaining grouping had similar but less distinct sub-groupings as in the

previous analyses. Overall these analyses suggest distinct differences in the

communities at the three sites but with a great deal of overlap.

Finally, the clustering of species based on their distributions among the 30

samples was also examined, one comparing all species (Figure 15) and the other

(Figure 16) contrasting the most dominant taxa (Nereis, Heteromastus, Macoma, Rangia,

Hydrobia, Edotea, Cyathura, and nematodes). Both analyses show much the same

pattern. When all species were included (Figure 15), 5 major clusters were found

including a) a distinct cluster with Heteromastus and Nereis, the two most abundant

species that were also in greatest abundance in Trent Hall Creek, b) a cluster of three

species that were dominant in Hunting Creek (Rangia, Edotea, and Cyathura), c) a

cluster of lower density taxa that were also most abundant in Trent Hall Creek (Macoma,

nematodes, and copepods), d) Hydrobia that was only found in Swanson Creek, and e)

a large group of the remaining species that generally had low densities and a mixed

distribution among the sites. When the 8 most dominant taxa were analyzed separately

(Figure 16) the pattern was largely the same with a) the distinct Trent Hall dominant

Heteromastus, Nereis cluster, b) the Hunting Creek dominant Rangia, Edotea cluster, and

c) a mixed cluster species more prevalent in Trent Hall Creek (Cyathura, Macoma, and

nematodes) or Swanson Creek (Hydrobia).

Discussion

This study compares the abundances of sediment-dwelling invertebrates at three

sites on one sampling date. Five months prior to the sampling date, the three sites

selected varied in their exposure to oil with Swanson Creek having heavy exposure,

Trent Hall Creek moderate exposure, and Hunting Creek no exposure. Except for

salinity, most other environmental variables measured appeared to be similar between

the sites. Sediment grain size distributions differed slightly but sediments at all three

sampling sites can be classified as muddy sands, similar to western shore sands and

muds of Chesapeake Bay (Ryan 1953). Swanson Creek did have a slightly higher

percentage of silt-clay. Organic matter content did not vary significantly among the

three sites. Tidal heights of the sampling sites and measured water temperatures also

varied little. The only measured environmental parameter that did differ among the

sites was salinity with Trent Hall Creek having a salinity much higher than the other two

sites. Given the location of Trent Hall Creek farther down the Patuxent River, it is likely

that it has a higher mean salinity than the other two sites.
8

All three sites had taxa that are characteristic of shallow sub-tidal and intertidal

habitats within the region (e.g. Holland 1985, Holland et al. 1977a, b, 1980, Marsh and

Tenore 1990, McErlean 1964, Kemp et al. 2000) as well as in other estuarine systems

(e.g. Sanders et al. 1965). Species composition at the sites was also similar to that

found in earlier Academy of Natural Sciences surveys of shallow subtidal areas in the

immediate vicinity of the present study (e.g. ANS 1971, 1978a, 1978b) as well as in

intertidal habitats in some Patuxent River marshes (Osman and Whitlatch 1998). In a

study of infaunal populations in the lower Patuxent River, Marsh and Tenore (1990)

found that four species, Nereis, Macoma, Leptocheirus, and the spionid polychaete

Streblospio benedicti, accounted for >90% of the community biomass and two of these,

Nereis and Macoma were among the 3 most dominant species at the study sites. The

other two species in the lower Patuxent study are opportunistic species that recruit in

early summer. They often suffer high mortalities that result in low abundances by late

August (Marsh and Tenore 1990), prior to the time of sampling in our study. Nereis and

Macoma recruit in the late summer and fall (Holland et al. 1977b), making it likely for

them to be in higher densities at the time we sampled. McErlean (1964) also found 

Macoma densities in the Patuxent River to be highly variable but he reported similar

densities to those we found in the vicinity of the three sites.

Although many of the same species occurred at all three sites, the densities of

many taxa at the three sites were clearly different with Trent Hall Creek having much

higher overall densities (Figure 12) than the other two sites. All three sites were

dominated by polychaetes, but this dominance was significantly greater in Trent Hall

Creek (Figure 10). Both Heteromastus and Nereis had densities in Trent Hall Creek that

were 3 times greater than at the other two sites (Figure 5). Mollusc densities did not

differ among the sites (Figure 10), but the dominant species varied among the sites. 

Macoma was significantly more abundant in Trent Hall Creek, Rangia in Hunting Creek,

and Hydrobia in Swanson Creek (Figure 6). Although arthropods had the greatest

number of species, most species were in fairly low abundance at the sites. Overall,

arthropods were a significantly greater part of the fauna in Hunting and Swanson

Creeks than in Trent Hall Creek (Figure 10). The isopod Edotea was three times more

abundant in Hunting and Swanson Creeks than in Trent Hall Creek and the cumacean

Leucon was significantly more abundant in Swanson Creek (Figure 7).

These differences in faunal distributions also resulted in differences among the

sites in trophic group composition. Because of the dominance of Heteromastus in Trent

Hall Creek, deep sub-surface deposit feeders formed a significantly greater part of the

fauna at this site (Figure 11). Omnivorous species formed the most abundant group at

all three sites (Figure 11). However, in Trent Hall Creek this resulted mostly from the

high densities of the sub-surface feeding Nereis while at the other sites omnivorous

surface-feeding arthropods were more important. Surface deposit feeders were

significantly more abundant in Trent Hall Creek, largely as a consequence of higher

densities of Macoma, while the high abundances of Rangia in Hunting Creek made

suspension feeders a significantly greater faunal component at this site. Site
9

differences were also reflected in the community analyses. Although there were clear

overlaps among the sites, the cluster analyses suggest that samples from the same site

were more similar to each other than to samples from other sites. Using all of the taxa

(Figure 13) or just dominants (Figure 14) made little difference.

Even though there were clear faunal differences among the three sites, it is not

possible to determine the cause or causes of these differences. First, the sedimentdwelling shallow water and intertidal invertebrate fauna exhibits significant spatial (e.g.

McErlean 1964) and temporal (Holland 1985, Marsh and Tenore 1990) variability. In

addition, the sampling occurred at the end of the summer recruitment season and at a

time when species such as Nereis and Macoma may be recruiting in high numbers

(Holland et al. 1977b). Natural population variability coupled with the normally high

spatial variability in recruitment could have contributed to the observed faunal

differences among the sites. 

Secondly, the large salinity difference observed between Trent Hall Creek and

the other two sites may be indicative of natural environmental differences among the

sites. Gradients in salinity have been shown to cause large differences in fauna along

a river (e.g. Sanders et al. 1965). In addition, the significantly higher abundances of

higher salinity species such as Nereis, Heteromastus, and Macoma (Figures 5, 6) in Trent

Hall Creek and the significantly higher densities of lower salinity species such as

Rangia (Figure 6) in Hunting Creek are consistent with a salinity effect.

Thirdly, it is possible that the differences in faunal distributions resulted from

effects of the released oil. However, the evidence for this is quite mixed. The absence

of Leptocheirus at the sites in both Swanson and Trent Hall Creeks is suggestive of oil

effects. Leptocheirus is a species widely used in toxicity studies (e.g. McGee et al.

1998, Fuchsman et al. 1998, Norton et al. 1999, Horne et al. 1999) and might be

expected to be a good indicator of oil effects. However, even though field populations

seem to be more sensitive to toxic compounds than those reared in the laboratory

(McGee et al. 1998), Leptocheirus has been found to be insensitive to some pollutants in 

field sampling studies (e.g. Horne et al. 1999). Other taxa used in toxicity testing such

as Hydrobia (e.g. Forbes et al. 1995) and chironomids (e.g. Fuchsman et al. 1998) had

distributions that did not correlate well with the known oil distribution. Hydrobia had

significantly higher densities in Swanson Creek (Figure 6) and chironomids had lower

densities (but not significant) in Swanson and Trent Hall Creeks (Figure 8). Finally, the

distributions of broader taxonomic and trophic groups are also known to be affected by

pollutants. Horne et al. (1999) in their study of salt marsh sediments found that highly

contaminated areas were dominated by polychaetes and surface deposit feeders. 

Polychaetes and surface deposit feeders were significantly more abundant in Trent Hall

Creek, but not in Swanson Creek (Figures 10, 11). Omnivores, which included many

surface feeding taxa were more abundant at the two oiled sites than in Hunting Creek

(Figure 11). Thus these results are also mixed in their support of an effect of oil on the

distribution of the infauna at the three sites.
10

Fourthly, it is also possible that the process of cleaning oil from the intertidal

sites could have had an effect on the faunal distributions. However, there is no way to

distinguish this from the effects of the oil itself (or other environmental parameters)

using the data collected.

Fifthly, the distributions of these sediment-dwelling invertebrates could also

have been influenced by other toxic substances not associated with the oil spill. For

example, Riedel et al. (1998, 2000) have shown that cadmium and other toxic trace

elements vary spatially and temporally in the Patuxent River. In particular, cadmium

was found in the highest concentration in oysters taken near Chalk Point and steadily

declined towards the mouth of the river (Riedel et al. 1998). Given that species such

as Leptocheirus have been shown to be affected by cadmium (e.g. McGee et al. 1998), it

is possible that the decreases in many species from Hunting and Swanson Creeks to

Trent Hall Creek could result from effects of contaminants, such as cadmium, not

associated with the oil spill.

Finally, the differences in distributions could result from a combination of any or

all of the factors described above. For example, if a significant acute mortality of

infauna resulted from the oil spill or the process of cleaning intertidal areas, then the

higher densities at Trent Hall could have resulted from recruitment in the 5 months after

the spill. The absence of competitors and predators at this site could have enabled

high recruitment of opportunistic species such as Heteromastus and Nereis. The lower

numbers of these species at Swanson Creek could be attributed to the interacting effect

of salinity on the distribution of these species.

Thus, it is impossible to know the causes of broad spatial differences in fauna

from a set of one time samples taken at only three sites. This study has shown that

significant, and sometimes large, differences do exist among the three sites, but it

cannot make any inferences regarding the cause or causes of these differences. The

design of the study was inadequate to determine such differences. Given the large

spatial and temporal variability in environmental parameters (e.g. salinity) and biota

inherent in estuarine habitats, a single, one-time set of samples cannot differentiate

between causes.

The best and most direct approach to identifying and quantifying the effects of

the oil spill on the biota would have been to have taken samples prior to the spill and

compared these to those taken over time intervals after the spill. Given that the oil

spread down-river over a period of many days, this could have been done at many sites

and it is unfortunate that it was not.

Given the lack of sampling before sites were impacted, the best way to have

identified oil damage would have been to sample many sites, both oiled and unoiled

immediately and then at time intervals after. Samples taken immediately after the spill

in April would have identified those species present. Individuals killed would not have
11

been lost immediately. Comparing these samples to those taken later could have been

used to distinguish differences resulting from the mortality of residents from those

resulting from later recruitment. Differences or similarities seen in one-time sampling

can be ephemeral (e.g. resulting from random recruitment differences that may be

changed by subsequent recruitment or mortality) or they can be long-lasting. It is

impossible to know the dynamic response of such a system based on a single set of

samples.

Finally, the strong environmental gradients in an estuarine system such as the

Patuxent River produce similar gradients in fauna. Natural spatial and temporal

variability in populations can also result from random variation in recruitment, growth,

and survival. These processes make it imperative that multiple sites in each class (e.g.

control, moderately-oiled, heavily-oiled) be sampled so that oil effects can be isolated

from natural differences. These sites should also be sampled repeatedly over a time

range that can account for differences in recruitment as well as short- and long-term

variations in mortality (including population structure changes, growth and reproductive

output). By sampling only one site in each category it is not possible to determine

whether a site is different because of the categorical difference (i.e. oil exposure) or

just natural variability.

In summary, although this study can identify differences among the sites, it is

inadequate to determine what caused those differences. It is also inadequate for 

evaluating whether any long-term damage to the intertidal and shallow subtidal fauna

resulting from the oil spill has occurred. 
12

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116. 
15

Figure 1. Map of Patuxent River showing the three sampling locations near the Chalk

Point oil spill site.
0

10

20

30

0

10

20

30

% of Total

0

10

20

30

<63 63 125 250 500 1000 2000

<63 63 125 250 500 1000 2000

<63 63 125 250 500 1000 2000

Grain Size (μm)

Trent Hall Creek

Swanson Creek

Hunting Creek

*

*

*

16

Figure 2. Comparison of grain size distributions at the three sampling sites. Means

(±SE) are shown for each size class. * Indicates a significant difference

from the other sites for the size class.
0

10

20

30

0

10

20

30

% of Total Volume

0

10

20

30

<63 63 125 250 500 1000 2000

<63 63 125 250 500 1000 2000

<63 63 125 250 500 1000 2000

Grain Size (μm)

Trent Hall Creek

Swanson Creek

Hunting Creek

*

*

*

17

Figure 3. Comparison of grain size distributions at the three sampling sites based

on volume. Means (±SE) are shown for each size class. * Indicates a

significant difference from the other sites for the size class.
0

246

Hunting Trent Hall Swanso

%Organic Matter

18

Figure 4. Comparison of the % Organic Matter in sediment samples from the three sampling sites. 

shown for each site.
500

1000

1500

2000

500

1000

1500

Density (No .m-2

)

25

50

75

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

Heteromastus

*

Nereis *

Micrura

19

Figure 5. Comparison of the densities of Heteromatus filiformis, Nereis succinea, and

Micrura leidyi among the three sampling sites. Means (±SE) are shown

for each site. * Indicates a significant difference from the other sites.
100

200

300

400

100

200

300

Density (No .m-2

)

50

100

150

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

Macoma

*

Rangia *

Hydrobia

*

A

B

AB

20

Figure 6. Comparison of the densities of Macoma balthica, Rangia cuneata, and

Hydrobia sp. among the three sampling sites. Means (±SE) are shown for

each site. * Indicates a significant difference from the other sites. Letters

indicate sites with no significant difference.
100

200

300

100

200

300

400

Density (No .m-2

)

25

50

75

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

Cyathura

Edotea

Leucon

A

B

AB

*

21

Figure 7. Comparison of the densities of Cyathura polita, Edotea triloba, and Leucon

americanus among the three sampling sites. Means (±SE) are shown for

each site. * Indicates a significant difference from the other sites. Letters

indicate sites with no significant difference.
15

30

153045

1020

153045

Hunting Trent Hall Swanson Hunting Trent Hall Hunting Trent Hall Swanson Hunting Trent Hall 

Leptocheirus Chironom

Palaemonetes Rhithropanopeus

Density (No

.

m

-2

)

22

Figure 8. Comparison of the densities of Leptocheirus plumulosus, Chironomids, Palaemonetes pugio, 

Rhithropanopeus harrisii among the three sampling sites. Means (±SE) are shown for eac
100

200

300

100

200

300

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

Nematodes

Copepods

Density (No .m-2

)

23

Figure 9. Comparison of the densities of Nematodes and Copepods among the

three sampling sites. Means (±SE) are shown for each site.
0

306090

0

1020

0

102030

048

12

Hunting Trent Hall Swanson Hunting Trent Hall Hunting Trent Hall Swanson Hunting Trent Hall 

% Polychaetes

*

% Molluscs

% Arthropods

*

% Meiofauna

24

Figure 10. Comparison of the relative abundances of Polychaetes, Molluscs, Arthropods, and Meio

three sampling sites. Means (±SE) are shown for each site. * Indicates a significant diffesite and others.
500

100015002000

150300450

500

100015002000

100200300

Hunting Trent Hall Swanson Hunting Trent H

Hunting Trent Hall Swanson Hunting Trent Ha

Deep Deposit Feeders

*

Surface Deposit Fee

*

Omnivores

A

B

AB

Suspensi

A

B

Density (No

.

m

-2

)

25

Figure 11. Comparison of the densities of four trophic groups among the three sampling sites. Mea

shown for each site. * Indicates a significant difference between the site and others. Letwith no significant differences.
1500

3000

4500

Density (No .m-2

)

0

3

6

9

No. Species

Hunting Trent Hall Swanson

Hunting Trent Hall Swanson

All Taxa

*

26

Figure 12. Comparison of the densities of All Taxa and the Number of Species

among the three sampling sites. Means (±SE) are shown for each site.
0 2 4 6 8 10

H1

H5

H4

H6

H8

H2

T5

H0

S7

H3

T8

S5

S6

T3

T7

T0

H9

H7

S0

S1

S2

S3

S4

S8

S9

T1

T2

T6

T9

T4

Figure 13. Dendrogram showing similarity among sites based on the densities

of all taxa. Higher links indicate greater dissimilarity. H = Hunting

Creek, S = Swanson Creek, T = Trent Hall Creek.
0 2 4 6 8 10

H1

H7

H2

H3

H0

S7

S5

H5

H4

H6

T5

H8

T8

S6

T0

S2

S8

S4

S3

S0

S9

H9

S1

T1

T4

T2

T3

T6

T9

T7

Figure 14. Dendrogram showing similarity among sites based on the densities

of the dominant taxa. Higher links indicate greater dissimilarity

H = Hunting Creek, S = Swanson Creek, T = Trent Hall Creek.
0 2 4 8 18

Nereis

Hetermastus

Micrura

Leptocheirus

Palaemonetes

Insect

Balanus

Rhithropanopeus

Choronomid

Leucon

Hydrobia

Copepods

Nematodes

Macoma

Edotea

Cyathura

Rangia

~~

Figure 15. Dendrogram showing similarity among all species based on their

distribution among sites. Higher links indicate greater dissimilarity.
0 2 4 6 8 10 12

Nereis

Heteromastus

Hydrobia

Nematodes

Macoma

Cyathura

Edotea

Rangia

Figure 16. Dendrogram showing similarity among dominant species based on their

distribution among sites. Higher links indicate greater dissimilarity.
31

APPENDIX I

FAUNAL AND SEDIMENT SAMPLING DATA FOR EACH STATION
32

Hunting Creek

Sample 1 2 3 4 5 6 7 8 9 10 Mean

Water Depth (cm) 34 31 34 42 37 39 36 34 33 35 35.5 

Time of collection 11:00 11:00 11:00 11:00 11:00 11:00 11:00 11:00 11:00 11:00 11:00 

Salinity ppt 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 

Temperature C 21.3 21.3 21.3 21.3 21.3 21.3 21.3 21.3 21.3 21.3 21.3 

 Nemerteans

 Micrura leidyi 1 0 0 1 0 1 3 1 0 0 0.7 

 Annelids

 Nereis succinea 11 6 4 10 3 12 10 15 26 10 10.7 

 Heteromastus filiformis 21 13 12 13 3 3 13 6 13 1 9.8 

 Bivalve Molluscs

 Rangia cuneata 4 2 0 6 6 6 11 9 6 1 5.1 

 Macoma balthica 0 1 0 0 2 0 1 0 1 1 0.6 

 Gastropods

 Hydrobia sp. 0 0 0 0 0 0 0 0 0 0 0.0 

 Cumaceans

 Leucon americanus 0 0 0 0 0 0 0 0 0 0 0.0 

 Barnacles

 Balanus improvisus 0 0 0 0 0 0 0 0 0 0 0.0 

 Amphipods

 Leptocheirus plumulosus 1 0 0 0 1 0 0 0 2 0 0.4 

 Isopods

 Edotea triloba 10 6 3 3 1 6 17 5 4 1 5.6 

 Cyathura polita 3 1 1 2 2 3 0 1 1 1 1.5 

 Decapods

 Palaemonetes pugio 2 0 0 0 0 0 0 0 0 0 0.2 

 Rhithropanopeus harrisii 0 0 0 0 0 0 0 0 0 0 0.0 

 Insects

 Chironomids 0 0 1 0 0 0 0 1 3 0 0.5 

 Adult Insects 0 0 0 0 0 0 0 0 0 0 0.0 

 Meiofauna

 Copepoda 3 0 0 0 1 1 5 0 3 0 1.3 

 Nematoda 0 1 0 3 0 1 0 0 8 0 1.3 

 % Organic Matter 3.2 1.9 1.6 2.3 4.8 3.2 1.7 2.5 1.6 8.8 3.2 

Table A1. Abundances of macrofauna in ten core samples from the control site in Hunting

Creek. Numbers in each core and the station means are shown. Incidental

meiofauna numbers and the % organic matter are also shown.
33

Trent Hall Creek

1 2 3 4 5 6 7 8 9 10 Mean

Water Depth (cm) 26 26 22 24 25 25 28 29 24 26 25.5 

Time of collection 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 

Salinity ppt 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 

Temperature C 23.8 23.8 23.8 23.8 23.8 23.8 23.8 23.8 23.8 23.8 23.8 

 Nemerteans

 Micrura leidyi 1 2 0 2 2 0 0 1 1 1 1.0 

 Annelids

 Nereis succinea 36 23 23 25 10 31 34 17 37 29 26.5 

 Heteromastus filiformis 52 42 21 50 9 34 14 12 24 15 27.3 

 Bivalve Molluscs

 Rangia cuneata 1 0 0 4 3 1 0 1 1 1 1.2 

 Macoma balthica 3 6 8 13 0 10 3 5 10 5 6.3 

 Gastropods

 Hydrobia sp. 0 0 0 0 0 0 0 0 0 0 0.0 

 Cumaceans

 Leucon americanus 1 2 1 0 0 1 0 0 0 0 0.5 

 Barnacles

 Balanus improvisus 0 0 0 0 0 0 1 0 0 0 0.1 

 Amphipods

 Leptocheirus plumulosus 0 0 0 0 0 0 0 0 0 0 0.0 

 Isopods

 Edotea triloba 2 4 0 1 3 1 0 1 2 0 1.4 

 Cyathura polita 5 8 0 10 1 5 1 4 4 1 3.9 

 Decapods

 Palaemonetes pugio 0 0 1 0 0 0 0 0 0 0 0.1 

 Rhithropanopeus harrisii 0 1 0 0 0 0 3 0 0 0 0.4 

 Insects

 Chironomids 1 0 0 0 0 0 0 0 0 0 0.1 

 Adult Insects 1 0 0 0 0 0 0 0 0 0 0.1 

 Meiofauna

 Copepoda 12 3 0 19 1 5 1 1 4 0 4.6 

 Nematoda 16 1 1 10 1 1 0 0 0 1 3.1 

% Organic Matter 1.6 1.9 2.3 2.5 3.5 1.2 9.4 1.4 0.9 1.4 2.6 

Table A2. Abundances of macrofauna in ten core samples from the moderately-oiled site

in Trent Hall Creek. Numbers in each core and the station means are shown. 

Incidental meiofauna numbers and the % organic matter are also shown.
34

Swanson Creek

1 2 3 4 5 6 7 8 9 10 Mean

 Water Depth (cm) 20 18 18 19 22 23 18 16 15 17 18.6 

 Time of collection 13:15 13:15 13:15 13:15 13:15 13:15 13:15 13:15 13:15 13:15 13:15 

 Salinity ppt 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 

 Temperature C 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 

 Nemerteans

 Micrura leidyi 1 0 1 1 0 1 0 0 0 1 0.5 

 Annelids

 Nereis succinea 21 16 8 15 9 14 4 15 11 18 13.1 

 Heteromastus filiformis 15 17 1 6 1 5 0 3 4 9 6.1 

 Bivalve Molluscs

 Rangia cuneata 5 1 1 3 3 3 0 2 1 6 2.5 

 Macoma balthica 0 0 3 2 2 2 0 0 0 3 1.2 

 Gastropods

 Hydrobia sp. 3 2 1 2 1 0 0 3 5 3 2.0 

 Cumaceans

 Leucon americanus 2 1 0 2 0 0 0 0 2 2 0.9 

 Barnacles

 Balanus improvisus 0 0 0 0 0 0 0 0 0 0 0.0 

 Amphipods

 Leptocheirus plumulosus 0 0 0 0 0 0 0 0 0 0 0.0 

 Isopods

 Edotea triloba 9 11 13 3 2 0 3 3 0 16 6.0 

 Cyathura polita 8 2 4 4 4 3 3 4 1 0 3.3 

 Decapods

 Palaemonetes pugio 0 0 0 0 1 0 0 0 1 0 0.2 

 Rhithropanopeus harrisii 0 0 1 0 0 0 0 0 0 1 0.2 

 Insects

 Chironomids 3 0 0 0 0 0 0 0 0 0 0.3 

 Adult Insects 0 0 0 0 0 0 0 0 0 0 0.0 

 Meiofauna

 Copepoda 1 2 0 2 0 0 1 1 1 7 1.5 

 Nematoda 11 0 0 0 0 0 0 1 0 1 1.3 

 % Organic Matter 2.9 3.0 6.8 2.5 4.6 2.5 16.4 2.1 6.4 5.5 5.3 

Table A3. Abundances of macrofauna in ten core samples from the heavily-oiled site in

Swanson Creek. Numbers in each core and the station means are shown. 

Incidental meiofauna numbers and the % organic matter are also shown.
35

STATION Hunting Creek Trent Hall Creek Swanson Creek

Dry Weight (mg)

 <62.5 )m 9.0 10.1 11.6 

 62.5 )m 6.4 8.2 5.0 

 125 )m 27.3 22.5 12.4 

 250 )m 21.0 23.0 18.0 

 500 )m 27.6 28.2 34.3 

 1000 )m 2.7 3.6 8.4 

 2000 )m 6.0 4.3 10.3 

 Sand (>62.5 mm) 91.0 89.9 88.4 

Silt-Clay (<62.5 mm) 9.0 10.1 11.6 

Volume (ml)

<62.5 )m 8.8 10.0 14.7 

62.5 )m 6.3 7.9 4.2 

125 )m 22.7 21.3 10.9 

250 )m 17.2 21.7 16.0 

500 )m 24.4 27.4 34.4 

1000 )m 7.4 4.5 8.1 

2000 )m 13.3 7.3 11.6 

Sand (>62.5 )m) 91.2 90.0 85.4 

Silt-Clay (<62.5 )m) 8.8 10.0 14.6 

Table A4. Grain size analysis for sediments collected at the three sites. The mean

percentage by size class is shown for each site. The means are based on 10

samples collected at each field site. Percentages by dry weight and by volume

are shown.