Benthos
4.1 Background
Benthos are identified as those organisms living in or on the
bottom. In estuarine systems, the benthic community is dominated primarily by species that
burrow into the sediments (infauna), either living within tubes or burrow systems. Taxa
dominating the infauna in most estuaries include small worms (polychaetes and
oligochaetes), amphipod crustaceans, clams, and insect larvae, depending on salinities. In
addition to infauna there are also a number of epibenthic organisms that reside on the
sediment surface at least for part of their lives. Epibenthic species include mysid
shrimp, some amphipods and isopods. Benthic animals generally consume detrital or
planktonic food sources, with some predatory species present, and are in turn prey for
larger fish, shrimp and crabs. In many estuarine systems there is a strong link between
timing of predator recruitment (e.g. larval fish) and their benthic prey.
Benthic fauna are considered important indicators of water quality and
are used in a variety of monitoring programs to assess overall estuarine health and to
follow long-term trends in estuarine communities related to anthropogenic impacts (Boesch
et al. 1976, Aschan and Skullerod 1990, Simboura et al. 1995, Hyland et al. 1996). From a
monitoring perspective, benthos offer 3 positive attributes: 1) they are relatively
sedentary and long-lived, 2) they occupy an important intermediate trophic position, and
3) they respond differentially to varying environmental conditions. After settlement, most
benthos remain within a relatively constrained area, often less than 5 m2, for
their entire adult lives. Therefore, unlike many other biotic or chemical measures,
benthos reflect conditions at a specific location. Many benthos are also relatively
long-lived, with lifespans generally ranging from weeks for some opportunistic worms to
months or years for many larger taxa, leading to a community structure that reflects
average conditions integrated over a time period of months. However, benthos vary greatly
in their responses to changes in water quality. Some taxa are relatively tolerant of
organic enrichment and low dissolved oxygen while others are quickly eliminated under low
DO conditions (Boesch et al. 1976, Simboura et al. 1995). Increased nutrient inputs can
strongly affect abundances of some species through indirect and direct pathways, while not
affecting others. Similarly, there is a wide variation in tolerance to pesticides and
metal contaminants such as mercury or cadmium. By examining shifts in the benthic
community over time, one can gain an understanding of the major environmental processes
affecting the local biota (Hyland et al. 1996). A variety of indices have been developed
to quantify the health of estuarine systems based on the relative proportions of species
tolerant or susceptible to specific water quality parameters (e.g.; EPA Benthic Index; Ampelisca
toxicity tests; suspension feeder : deposit feeder ratios; deep burrower : shallow
burrower ratios; the Chandler Score, and the BMWP Score Index) (Whitehurst and Lindsey
1990). However, application of most indices requires long-term monitoring sufficient in
duration to separate seasonal or annual variations from variations due to changes in water
quality. Benthic community studies are a major component of the national EPA Environmental
Monitoring and Assessment Program for estuaries as well as regional monitoring efforts,
such as in Chesapeake Bay, Florida Bay, Long Island Sound, Pamlico Sound, and Tampa Bay.
In Spring 1996, the Benthic Ecology Laboratory of the UNCW Center for
Marine Science Research began long-term studies of benthic infauna in the Cape Fear River
as part of the basic monitoring plan of the Cape Fear River Program. The benthic
monitoring component has three major long-term objectives: 1) characterize the benthic
communities in the lower Cape Fear River and compare them with that of other southeastern
and mid-Atlantic river-dominated estuaries to gain a first-order assessment of estuarine
health, 2) determine seasonal, annual and spatial patterns of variability in these benthic
communities, and 3) establish a baseline for detecting changes in the estuarine community
through examination of changes in abundances of specific indicator taxa and eventual
application of standard benthic indices.
Although infaunal sampling for the Lower Cape Fear River started in
Spring 1996, the Benthic Ecology Laboratory took preliminary samples in winter of 1996
(late January). Gratis epibenthic samples were collected by the Benthic Ecology Lab in the
initial stages of the Cape Fear River Program and, starting in 1998, epibenthic trawls
were included as part of enhanced monitoring funded by the North Carolina General
Assembly.
4.2 Methodology
Four stations are sampled as part of basic monitoring for benthic
infauna: NCF6 in the Northeast Cape Fear River, NAV in the mainstem Cape Fear River, and
M54 and M31 in the lower estuary (Figure 4.1). These stations span the oligohaline to
mesohaline/polyhaline zones of the estuary and correspond to stations sampled as part of
water quality monitoring. The basic monitoring of benthos involves quarterly sampling at
each station. Samples are collected in winter (late January-February), spring (April-May),
summer (July-August), and fall (October-November). Following hurricanes Bertha and Fran,
additional samples were also taken in September 1996, December 1996, and January 1996 to
observe recovery patterns. This additional sampling was supported by the Benthic Ecology
Laboratory. When data from these additional samples are presented they will be indicated
by the month and year of collection, while data from regular sampling will be denoted by
the season and year they were collected. Post-hurricane samples collected on 1 October
1996 (denoted October 1996) were only taken from stations M54 and M31 because of debris at
the other 2 stations.
At each sampling station, benthic infaunal samples were taken with a
Petite Ponar grab, 15 cm x 15 cm opening (0.023 m2) and 15 cm depth. Five grab
samples were taken at each sampling location on each sampling date. Grabs were retained
only if the grab was full in order to standardize volume sampled. Grab samples were taken
from a boat at stations specified by GPS coordinates and all sampling locations were in
approximately 6-8 feet of water (studies in other estuaries have indicated greater
abundances in shallow areas as compared to deep channels within an estuary). Immediately
after collection, the samples were sieved through a 0.5 mm mesh screen, preserved in 10%
buffered formalin with rose bengal dye added, and transferred to 70% ethanol after 3 days
for later sorting and identification. Separation of animals from remaining sediment was
done under a dissecting microscope. All animals were identified to the lowest reliable
taxonomic level, with random specimens verified by outside taxonomists. These procedures
follow standard formats for benthic sampling outlined by the EPA Environmental Monitoring
and Assessment Program and used in other monitoring programs (Hyland et al. 1996, Posey et
al. 1997).
Patterns of infaunal community composition were compared among sites
for numerically common taxa (those comprising at least 1% or 3% of the total fauna
collected at that site), for higher taxonomic groupings, and for guild groupings.
Comparison of higher taxonomic groupings (polychaete, amphipod, bivalve, oligochaete)
allows observation of patterns for morphologically similar organisms. Guild analyses allow
observation of general patterns for species that share similar feeding types or living
positions. Major feeding types distinguished here are deposit feeders (directly consuming
bacteria and/or dead plant and animal material), suspension feeders (filtering food from
the water column), and grazers (microalgal grazers, other herbivores, predators). Changes
in feeding type may reflect availability of organic detritus, quality of planktonic food
sources, and proportion of predatory species (reflecting food web strength). Major living
position types were epibenthic (living on or near the substrate surface, including
tube-building forms), shallow burrowing (living within 2 cm of the substrate surface), and
deep burrowing. Living position reflects availability to fish, crab and shrimp predators.
All guild classifications were made based upon Fauchauld and Jumars (1979) and Posey et
al. (1996) and species were not classified according to feeding type or living position if
there was conflicting or inadequate information. Shannon-Weiner Diversity Index was also
calculated for each site and date. This index measures both the species richness (number
of species) and eveness (relative abundance) components of diversity. Low diversity
reflects few species and/or dominance by only one or a few taxa while high diversity
reflects many species and/or co-dominance by many species. As another measure of whether
the community is dominated by only a few taxa or a variety of co-dominant species, the
number of taxa comprising at least 1% of the total individuals at a site was recorded.
Sampling of epibenthos was conducted at 4 stations during winter and
spring 1998 corresponding to areas sampled during a 1995 U.S. Army Corps of Engineers
study (Figure 4.1, site labels following Army Corps designations). These sites are all
located below the confluence with the Brunswick River, with the northernmost site located
near the Wilmington Port and the southernmost site located near M31. Because epibenthos
are often dominated by larval fish, shrimp and crabs, concentration in the lower estuary
provides a better opportunity to examine influx of these organisms. Sampling was conducted
with an epibenthic sled towed from a UNCW vessel. The sled has a rectangular frame opening
(0.5m wide x 0.3m high) with a 1 mm mesh, 2m long net attached to the frame. Attached to
the end of the net is a 1.4 l removable collection bucket. The sled frame rests on 3 metal
skis that allow the net to be pulled just above the bottom. At each location on each day,
a single tow is made for 1 minute at 1200 rpm speed against the current in a shallow area,
2m depth, and in an adjacent deep area, 8-10 m depth. A General Oceanics Model 2030
mechanical flow meter is attached to the mouth of the sled to measure the water volume
sampled for later standardization of analyses (note: there were no significant differences
in volumes sampled between depths, locations, or dates during the study). Epibenthic
samples are taken twice within a one week period coinciding with infaunal grab samples.
4.3.a. Spatial Patterns and Site Characterization
A total of 132 taxa have been collected since Spring 1996 (Table 4.1, though note that several groupings are
combined for descriptive purposes). However, less than half of these species occur at any
specific site (Table 4.2) and most species are
relatively rare (Tables 4.3-4.6). The NAV site had the highest mean infaunal density
(114/grab, 4960 m-2; Table 4.2), but was characterized by lower diversity than
the other sites (Table 4.2; Figure 4.6). This lower diversity reflected
somewhat lower overall numbers of species (51 total) as well as dominance by relatively
few taxa. Only 6 of the 51 taxa occurring at this site comprised more than 1% of total
individuals and the site was strongly dominated by 3 taxa, the polychaete Maranzellaria,
oligochaetes, and insect larvae of the genus Procladius. Thus, most of the taxa at
this site did not comprise a major numerical component of the benthic community. The other
oligohaline site, NCF6, also had fewer species than lower estuarine sites (Table 4.2), but
was characterized by greater diversity and co-dominance by a greater number of species (Fig. 4.6). Eighteen species at NCF6 comprised at
least 1% of the total faunal and the site was dominated by 6 taxa, including chironomid
larvae, Maranzellaria, oligochaetes, the polychaete Polydora, and the
insects Polypedilum and Procladius (Table
4.2-4.6). In contrast to this pattern of greater
diversity, NCF6 had the lowest overall infaunal abundance of any of the sites (13.5/grab,
587 m-2). Site M54 is located between the oligohaline stations of NCF6 and NAV
and the more saline M31 station. This station is also located near the turbidity maximum
for the Cape Fear River. M54 was characterized by low infaunal abundances but was
intermediate with respect to diversity and numbers of common taxa. Overall number of
species was 59, with 10 comprising greater than 1% of total individuals collected at the
site. Dominant taxa included species common at the NAV and NCF6 sites (Maranzellaria
and oligochaetes) and others common at the M31 site (the polychaetes Mediomastus
and Streblospio). Site M31 is the lowest and most saline site and was dominated
primarily by polychaete worms (Table 4.2-4.6). This site had the highest overall
diversity, though diversity varied seasonally (Fig. 4.6). High diversity at M31 was due to
both higher numbers of taxa present and the presence of several common taxa.
Examination of guild distributions indicated more similarity among
sites. All sites were dominated by deposit feeders, reflecting the abundance of a variety
of deposit feeding oligochaetes and polychaetes at the four sites (though the exact
species varied between sites; Figs. 4.2-4.5). Seasonal
influx of insect larvae enhanced grazer abundance at the oligohaline sites on certain
dates. Living positions varied, with deep burrowing and epibenthic (including
tube-dwelling) forms being most common.
The general patterns for diversity and species composition at the four
Cape Fear River sites are similar to other river-dominated estuaries in the mid Atlantic
and southeast region (e.g. Apalachicola Estuary, Winyah Bay, James River, Potomac River,
upper Chesapeake Bay). Although not high, diversity is within the range observed for other
estuaries in the region and the pattern of increasing diversity with increasing salinity
is well documented elsewhere. The dominant species observed here are characteristic of
many moderately disturbed estuarine systems and are generally considered opportunistic in
nature. Although present, many species sometimes considered characteristic of more
pristine, relatively undisturbed environments (e.g. maldanids, terebellids, certain
bivalves) were not common. Mesohaline to oligohaline portions of the James and Potomac
Rivers overlap extensively in species composition with the Cape Fear River, but differ
qualitatively by having greater abundances of clams (Shaffner et al. 1987, Holland et al.
1987). In a five year study including four seasons per year of sampling, both overall
abundances and number of species were lower in the Potomac compared to the Cape Fear River
Estuary (Holland et al, 1987). A one-year study of the oligohaline Susquehannah Flats area
of the upper Chesapeake Bay (Posey et al. 1993) also indicated strong overlap in species
composition, similar total abundances in unvegetated areas, but a lower overall number of
species compared to the Cape Fear system. Oligohaline portions of Winyah Bay, South
Carolina, were dominated by three species of oligochaetes, chironomids, Gammarus palustris,
Cyathura, and a spionid polychaete - all taxa common or at least present in Cape
Fear River sites (Posey et al. 1997). In a five year study of mesohaline reaches of the
Apalachicola Estuary in Florida, overall abundances were similar to that observed in the
Cape Fear River and among the community dominants were capitellid polychaetes, including Mediomastus,
spionid polychaetes, including Streblospio, as well as chironomids (Mahoney and
Livingston 1982).
4.3.b. Temporal Patterns
The strongest temporal pattern observed over the two years of
sampling was the change in faunal abundance associated with Hurricane Fran. Significant
declines were observed for most taxonomic groups at both the NCF6 and M54 sites for
several months after the hurricane (Figs. 4.2, 4.4). Recovery did not occur at these sites until winter
or spring 1997. Shorter-term effects were observed at the NAV and M31 sites, with a
depression in overall abundance relative to other fall samples and changes in the relative
abundance of species (Figs. 4.3, 4.5).
In order to assess seasonal and yearly variations in faunal composition
and abundance, data is needed for each season over two or more successive years. Because
of disturbance from Hurricane Fran, replicate seasonal information is available only from
summer and possibly spring, limiting any conclusions about seasonality. However, several
temporal patterns are apparent from the limited data. First, a winter/spring peak in
abundance for the polychaete Maranzellaria dominates both oligohaline stations and
the M54 station. This polychaete lives in near-surface tubes and its strong spring
recruitment, followed by a summer decline, may indicate a potential importance as a food
resource to juvenile fish and crabs entering the Cape Fear River estuary at this time. The
decline in abundance for this and other taxa in summer probably reflects predation by fish
and crabs. A second temporal pattern is that while all four sites are dominated by similar
functional groups, the exact species filling that group changes between seasons and years.
Relatively few taxa are persistent for all sampling periods, rather we tend to see
replacement of species as seasons and years progress. Species replacement on a temporal
scale tends to be with functionally similar species (feeding type and/or living position).
Explanations for temporal variations in taxa most likely involve variations in recruitment
between years and differential mortality. Many species observed here rely on either
planktonic recruitment or dispersal by flying adults, both of which can depend on weather
conditions. Interestingly, one of the taxa with the most consistent distributions over
time are oligochaetes, which are generally characterized by direct development.
Preliminary analysis of variations in abundance over time emphasize
variations among seasons and years. Attempts to understand community dynamics in the Cape
Fear estuary must clearly take into account such variability. Limited sampling during only
certain seasons, over one or a few isolated years, or on an episodic basis will not
provide accurate representations of community dynamics and changes in this estuarine
system.
4.3c. Epibenthos
Epibenthos were dominated by larval fish (especially croaker), mysid
shrimp, amphipods, and bivalves. The presence of bivalves in these samples, including many
species reported from other estuarine systems (e.g. Mulinea and Macoma)
indicate that these clams are not absent from the Cape Fear River, but merely uncommon
enough to not be a major component of grab samples. With only two seasons of epibenthic
samples, it is difficult to make firm conclusions about the nature of the community.
However, several trends are apparent. First, there is strong variation between sites and
between sampling periods. Secondly, there is segregation in the benthic community by
depth, with certain taxa more common in deeper areas and other taxa more common in shallow
areas. Finally, the high abundances of epibenthos in winter and spring, especially
amphipods and mysid shrimp, suggest their importance as food for recruiting fish.
4.4 Summary
Initial results indicate that the benthic fauna in the Cape Fear River is typical in species composition, spatial patterns and temporal variability to benthic communities in other river-dominated estuaries in the southeastern and mid Atlantic areas of the United States. As is typical of such estuaries, the fauna is dominated by a variety of opportunistic and widespread taxa that are capable of recovery from a variety of disturbances. From a benthic community perspective, these patterns suggest that the Cape Fear River is subject to periodic disturbances (salinity, storm inputs, and possibly sediment, DO, and organic stresses), but that the general condition of the Cape Fear River is similar to that of other estuaries with similar levels of development. Strong responses in the benthic community to Hurricane Fran indicate that that the estuarine system can be negatively affected by upstream inputs, possibly with strong ecosystem consequences, and that long-term trends in estuarine health need to be closely monitored. Strong temporal variability in faunal composition and abundances emphasizes the need to conduct monitoring over several seasons and years in order to understand and identify important ecosystem dynamics.
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