Benthos
3.1 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.
Benthic animals generally consume detrital or planktonic food sources, with some predatory
species present, and are in turn prey for finfish, shrimp and crabs.
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 most 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 variations 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 basic monitoring for the Cape Fear River Program. Benthic monitoring 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 the 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. We began addressing the first objective with completed analysis of spring 1997
samples. Analysis of seasonal and annual patterns as well as detection of long-term trends
will require at least 2-3 years of background data because of natural variations in
recruitment between years and effects of extreme weather events. Although not part of the
Cape Fear River Program basic monitoring plan, the Center for Marine Science Research and
the UNCW Benthic Ecology Laboratory have also initiated studies of epibenthos in the Cape
Fear River to assess stocks of juvenile fish, crabs, shrimp and mysid shrimp. The
monitoring of epibenthos is complementary to the basic benthic program and will allow
observation of trophic changes in the Cape Fear Estaury in conjunction with water quality
shifts.
3.2 Methodology
Four stations were 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 3.1). These stations span the oligohaline to
mesohaline/polyhaline zones of the estuary and correspond to stations sampled as part of
water quality monitoring. Basic monitoring for benthos includes quarterly sampling at each
station; however, monthly sampling (supported by the CMSR) was instituted from October
1996-May 1997 to observe the changes and recovery associated with Hurricane Fran. Monthly
sampling was considered necessary after Hurricane Fran because of the strong effects the
storm had on water quality in the Cape Fear River and resulting fluctuations in the
benthic community due to time lag effects and fall recruitment. Benthic samples were taken
during 10 time periods at the M31 and M54 stations: 10 January 1996, 28 March 1996, 19
August 1996, 1 October 1996, 31 October 1996, 17 December 1996, 22 January 1997, 24
February 1997, 17 March 1997, 18 April 1997, and 20 June 1997. At NAV and NCF6, no samples
were taken during 1 October 1996 because of debris on the bottom and only one successful
grab was taken at NCF6 during 19 August (this one grab was excluded from subsequent
analyses of temporal and spatial patterns). Sorting, identification, and analyses have
been completed for samples taken through January 1997. Identifications and analyses are
ongoing for subsequent sampling periods.
At each sampling station, benthic infaunal samples were taken with a
Petite Ponar grab, 15cm x 15cm opening 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 are 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 utilized in other monitoring programs (Hyland et al. 1996, Posey et
al. 1997). Differences in infaunal abundances were compared among sampling dates (through
January 1997) for common taxa (comprising at least 1% of the individuals taken) and higher
taxonomic groups using Analysis of Variance on log-transformed abundances. Because of
differences in faunal composition at the 4 sites (reflecting salinity and other riverine
gradients), comparisons between dates were conducted separately for each site.
Sampling of epibenthos was conducted at 4 stations corresponding to
areas sampled during a 1995 U.S. Army Corps of Engineers study (Figure 3.1, 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 much of the epibenthos is dominated by larval
forms, concentration in the lower estuary provides a better opportunity to examine influx
of shrimp, fish and crab larvae. Sampling was conducted during summer and fall of 1995 as
part of an Army Corps funded study and was begun again on a monthly basis beginning 1
October 1996 in an effort to monitor the effects and recovery from Hurricane Fran.
Sampling was conducted with an epibenthic sled towed from a UNCW vessel. The sled had 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 was a 1.4 l removable collection bucket. The
sled frame rested on 3 metal skis that allowed the net to be pulled just above the bottom.
At each location on each day, a single tow was 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 was attached to the mouth of the sled to
measure the water volume sampled for later standardization of abundances (note: there were
no significant differences in volumes sampled between depths, locations, or dates during
the study).
During the first year of benthic sampling, unusual storm events
(Hurricane Bertha and Hurricane Fran) significantly disturbed the benthic communities and
made assessment of community composition, spatial patterns, and temporal patterns
tentative at best. It is difficult at this time to determine how representative the late
summer and fall data are of natural community patterns.
Although the diversity was not high relative to other estuarine
systems, the benthic infaunal species collected in the Cape Fear River estuary were
typical of southeastern and mid Atlantic river-dominated estuaries. Among the major
taxonomic groups, polychaete worms were dominated by a variety of opportunistic species,
including capitellids (Mediomastus and Heteromastus), Nereis, Streblospio,
and Maranzellaria (Table 3.1). These worms are
relatively euryhaline, quick to recolonize disturbed areas, and tolerant of sedimentation,
organic inputs and nutrient inputs (Boesch et al. 1976, Van Dolah et al. 1984, Simboura et
al. 1995). Although less common, larger tube-dwelling taxa such as maldanids and
terrebelids (sometimes considered indicators of lower average organic inputs) were also
present. The amphipods and bivalves were also dominated by a variety of widespread,
opportunistic taxa that are both euryhaline and tolerant of a variety of environmental
conditions and/or able to quickly recolonize after disturbances. Among the dominant
amphipods were widespread oligohaline/mesohaline taxa such as Monoculoides, Cyathura,
Gammarus mucronatus, and Gammarus palustris (Table 3.1). Most
bivalves collected were juveniles, with Corbicula and Mulinea the most
common adult forms.
Although Hurricane Fran complicated spatial and temporal analyses,
there appeared to be a significant change in benthic community composition as one moves
down the estuary. The upper sites were dominated by taxa typical of oligohaline regions of
the estuary. At the NCF6 site, polychaetes (mainly Maranzellaria) were common
during one sampling period, but a diversity of insect larvae, amphipods and oligochaetes
dominated during most of the study (Figure 3.2). Though shifts occurred in the relative
abundance of specific amphipods and insects (especially Gammarus, Cyathura
and Procladius), this may be related to hurricane effects. The NAV site had similar
overall abundances of polychaetes and amphipods as the NCF6 site, but had densities of
insect larvae and oligochaetes over 2x that of the NCF6 site, leading to strong numerical
domination by these taxa (Figure 3.3). Polypedilum and Procladius were the
dominant insect larvae at this site. Lower in the estuary, at M54, insects become less
important (probably because of higher salinity) and the community was dominated primarily
by a mix of polychaetes, with oligochaetes and amphipods as the primary numerical
subdominants (Figure 3.4). The dominant polychaetes were capitellids and Maranzellaria,
the dominant amphipod was Monoculoides. Insects were common at M54 in October and
November 1996, probably reflecting lowered salinity effects from Hurricane Fran. Overall
benthic abundances were higher at the lowest estuarine station relative to the mid station
and the community was dominated primarily by a diverse mix of polychaete worms (over 75%
of the total individuals collected) (Figure 3.5). Remaining taxa included primarily
bivalves and amphipods. This zonation pattern as one moves down the estuary, changing from
insect and oligochaete dominated to polychaete dominated, is typical of most riverine
estuaries, with broad taxonomic patterns consistent with other southeastern systems.
In addition to spatial variations in abundance, there were also strong
temporal variations in abundance attributable to both seasonal and Hurricane Fran effects.
Strongest temporal variations in infaunal abundance were observed at the M54 site, which
is located near the turbidity maximum zone representing a critical boundary for many
infauna organisms. Total amphipods, total polychaetes, and total fauna were all
significantly less abundant in the three sampling period immediately following Hurricane
Fran compared to before the hurricane or equivalent times during the previous year (Table
5.2). However, all three groups exhibited recovery towards pre-hurricane levels by January
samples. Aside from hurricane effects, increased abundances of certain polychaetes during
winter and summer may reflect late fall or late spring recruitment events. The lower
estuarine M31 site showed less temporal variability in abundances. Amphipods and one
polychaete species (Maranzellaria) declined in numbers after Hurricane Fran (Table
5.2), both showed recovery by January 1997 samples, and bivalves exhibited variation in
numbers over the year following no definable seasonal pattern. At the NAV site, there was
strong interannual variability, with January 1996 abundances differing significantly from
January 1997 abundances for three species of insect larvae and one species of polychaete
(though total insect and total polychaete numbers did not differ). Insect larval
abundances exhibited an expected peak in winter/spring samples (many midges overwinter as
aquatic larvae). Interannual variability may represent natural variation between years or
a residual effect of Hurricane Fran. At NCF6, spring was dominated by strong recruitment
of the polychaete Maranzellaria and by winter/spring increases in insect larvae.
Although there were temporal variations in abundance of other taxa at the NAV site, the
sampling is still not sufficiently extended to determine patterns of variation. As
sampling continues, we will be able to identify background oscillations in abundances and
infaunal composition.
Analysis of epibenthic data is being conducted on a gratis basis and is
still ongoing. The epibenthic community in the lower Cape Fear River is dominated by 4
higher taxa: crab larvae, amphipods, mysids and fish larvae (Figure 3.6). Mysids are small
shrimp-like animals that are important food sources for juvenile fish. Neomysis was
the dominant mysid in the river with Mysidopsis comprising the major subdominant
species. Research conducted in other estuaries indicates mysids often comprise a large
portion of the epibenthic community, with peaks in spring and early summer. Amphipods were
dominated by Gammarus species and showed a strong seasonal peak in January. Crab
larvae included blue crabs as well as fiddler crabs (Uca) and mud crabs (Sesarma).
Blue crab larvae and juveniles were most common in the lower stations and exhibited a peak
in mid fall. Fish larvae were initially dominated by larval spot (October and early
November) and later by larval croaker (December through January), with several of other
species (i.e. larval flounder) also being represented. Greatest catches of juvenile fish
were made in the mid stations.
3.4 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 such as salinity, storm inputs, 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.
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