4.0.
Benthic Community Patterns in the Lower
Cape Fear River System
Martin Posey and Troy Alphin
Benthic Ecology Laboratory
University of North Carolina at Wilmington
4.1
Summary
4.2
Background
Benthic
organisms (benthos) are 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), often 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, with actual
dominance changing based on region of the estuary and salinities. Benthos may
also include larger animals such as rock crabs and blue crabs. Many benthic
animals consume detrital or planktonic food, although some species are
predatory, and are in turn prey for larger fish, shrimp and crabs.
In many estuarine systems there is a strong link between timing of fish
recruitment and their benthic prey, indicating that this group is a crucial link
in these food webs.
Benthic
fauna are considered important indicators of general water quality conditions.
They are used in a variety of monitoring programs to assess overall estuarine
health and to follow long-term trends in estuarine communities, especially
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 features: 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.
Although a few opportunistic species may live for only a few weeks, most benthic
animals have life spans ranging from months to over a year, leading to a
community structure that reflects average physical conditions over a time period
of months. However, benthos vary greatly in their responses to changes in
physical conditions, such as 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 influences on food availability and sediment conditions, while not
affecting others. Similarly, there is a wide variation in tolerance to
pesticides and metal contaminants such as mercury and cadmium. In general,
sediment type (clays, silts, sands, etc.), organic content, deposition rates of
sediments (from upstream erosion), dissolved oxygen, salinity and temperature
are considered most important in determining abundances and types of animals in
bottom communities. By examining shifts in the benthic community over time
(years), 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.
Preliminary samples were also taken during winter 1996 as part of other research
efforts. The benthic monitoring component has four major long-term objectives:
1) characterize the benthic communities in the lower Cape Fear River and compare
them with that of other river-dominated estuaries to gain a first-order
assessment of estuarine health, 2) determine seasonal, annual and spatial
patterns of variability, 3) correlate benthic abundances and physical measures
over short and long time periods, and 4) 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.
In
the 2001-2002 annual report, we continue to present data from our long-term
monitoring of patterns in benthic community composition, abundance of benthic
organisms, benthic species diversity, and numbers of species (richness). We also
present information on juvenile blue crab abundances within the lower Cape Fear
estuary. Sampling of juvenile blue crabs has been conducted at 4-6 sites in the
lower estuary since spring 1998, initially funded by North Carolina Sea Grant
(grant #R/MRD-40) and funded since 1999 by the National Science Foundation
(grant #9978613). While not funded by the Lower Cape Fear River Program, we
present our juvenile blue crab data here because of its relevance to an
important fishery and key species in southeastern North Carolina.
4.3
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. These stations span the oligohaline to mesohaline/polyhaline
zones of the estuary and correspond to stations sampled as part of water quality
monitoring. Complete descriptions of these stations are given in previous Cape
Fear River reports. The basic monitoring of benthos involves quarterly sampling
at each station. Samples are collected in winter (January-February), spring (mid
March-May), summer (July-early September), and fall (October-November).
Following hurricanes Bertha and Fran in late summer 1996 and Hurricane Bonnie in
September 1998, additional samples were also taken to observe recovery patterns
(September 1996, October 1996, December 1996, February 1997, September 1998 and
November 1998). This additional sampling was supported by the Benthic Ecology
Laboratory. Post-hurricane samples collected on 1 October 1996 were only taken
from stations M54 and M31 because of debris at the other 2 stations. During
1998, samples were collected in January, April/May, August, September (to
examine effects of Hurricane Bonnie), and October/November. Confirmation of
identifications for the summer and fall 2001 samples is ongoing and the results
from the 2001 sampling will not be reported until all identifications have been
completed, confirmed, and QA/QC completed (data will be presented in the next
annual monitoring report).
At
each sampling station, benthic infaunal samples are taken with a Petite Ponar
grab, 15cm x 15cm opening (0.023m2) and 15cm 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 are 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 is 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).
For
the purposes of this report, we have defined dominant taxa as any taxa that
comprises greater than 3% of the individuals collected at a given site on a
specific date. Most species collected in this monitoring effort were represented
by relatively few individuals and are thus unlikely to be important to food web
dynamics in the Cape Fear system. Diversity is calculated using the standard
Shannon Weiner formula (diversity [H’] = S pi(logpi))
and includes all taxa collected. Species richness is calculated as the total
number of species collected at a location over a specified time period. We
calculated both diversity and species richness on a per sample basis to compare
long-term trends among years because of some variability in sampling effort
between hurricane and non-hurricane years. In both cases they represent the
average number of species or average diversity per grab sample at a site during
a season’s sampling. We used principal components analysis to identify
distinct groupings of samples based upon site, year or season. This method
allows us to reduce a complicated data set (262 taxa X 6 years X 4 seasons X 4
sampling locations) into a more easily interpretable graphic form.
As
part of a larger study on blue crab biology, juvenile blue crab abundances have
been variously monitored at 8 locations within the Cape Fear estuary since
spring 1998. Here we report on abundances at 6 sites with the most continual
collection history. These include 3 sites at Southport, one along the waterway
near the Southport Marina (SPWW), one near an old canning plant along the main
river shoreline between downtown Southport and the Bald Head Island Freight
Ferry terminal (SPFC), and the third along the shore within the Southport Marina
(SPMA). Two sites are located in the mid river area, one near Carolina Beach
State Park (CBSP), along the river shoreline to the south of the marina
entrance, and the other along the shore of a dredge spoil island immediately
west of Carolina Beach State Park (CFCM). Our 2 upper stations were located near
M51, one along the eastern shore where the powerlines cross the river (POWL) and
the other along the shore of a dredge spoil island west of that location (CFPM).
Abundances of juvenile blue crabs were monitored using standard net sampling
approaches (seines and sweep nets) and more recently suction sampling. Ten
replicate samples were collected at each site on a bimonthly basis from spring
1998 to summer 1999 and monthly from summer 1999 to the present. This effort is
part of a larger study that has examined larval blue crab recruitment, blue crab
growth, blue crab prey availability, predators on blue crabs and aspects of blue
crab physiology in both the Cape Fear and New Rivers.
4.4
Results and Discussion
A
total of 262 taxa have been collected since winter 1996. In general, less than
half of the species were collected at any single site and most species are
relatively rare. Below we summarize major physical characteristics as they may
affect benthos, general patterns of species dominance, diversity and species
richness, and patterns of juvenile blue crab distribution and abundance.
4.4a.
Site descriptions
Physical
conditions at all 4 sites were described in Mallin et al. (1999, 2000) and
additional water quality data is provided in the current report. General site
characteristics that are important for interpreting benthic community dynamics
are summarized here.
NCF6
is located in the Northeast Cape Fear River. This site is characterized by low
salinity and fine sediments. Salinity for the months in which benthos were
sampled averaged 1.8 o/oo, with lower salinity during winter months (0.7 o/oo)
and higher salinity during summer (5.5 o/oo) and towards the end of 2000
(related to the start of drought conditions). Water temperature ranged from 5.9 oC
to 28.3 oC. Dissolved oxygen varied seasonally, with a winter average
of 8.8 and a summer average of 3.7. DO exhibited severe declines immediately
after the passage of Hurricanes Fran (September 1996) and Bonnie (September
1998) though less so after Hurricane Floyd (September 1999) (Mallin et al. 1998,
1999, 2000). Sediments are fine sands to sandy silts, with macrodetritus present
on some sampling dates.
NAV
is located in the mainstem Cape Fear River. Like NCF6, this site is
characterized by low salinity and fine sediments. Salinity for the months in
which benthos was sampled averaged 1.2 o/oo, with lower salinity during winter
months (0.3 o/oo) and higher during summer (4.0 o/oo), though salinities
increased towards the end of 2000. Water temperature ranged from 4.2 oC
to 28.3 oC. Dissolved oxygen varied seasonally, with a winter average
of 9.2 and a summer average of 3.9. DO exhibited declines immediately after the
passage of Hurricanes Fran and Bonnie (Malin et al. 1999).
M54
is located in the mainstem Cape Fear River below the confluence of the Cape Fear
and Northeast Cape Fear rivers. It is characterized by higher but more variable
salinities than NCF6 and NAV sites. Salinity for the months in which benthos was
sampled averaged 5.2 o/oo but ranged from 0 to >20 o/oo. Salinity was lower
during winter months (3.4 o/oo) and higher during summer (11.9 o/oo). Water
temperature ranged from 8.4 oC to 28.2 oC. Dissolved
oxygen varied seasonally, with a winter average of 8.9 and a summer average of
4.8 for the months sampled. DO exhibited severe declines immediately after the
passage of Hurricanes Fran and Bonnie. Sediments are fine sands with a layer of
flocculent detritus sometimes present over the surface.
M31
is located in the Cape Fear estuary and is the furthest downstream site sampled
as part of the basic monitoring program. It is characterized by higher
salinities than any of the other sites. Salinity for the months in which benthos
was sampled averaged 10.8 o/oo, ranging from 0.1 (immediately after Hurricane
Bonnie) to >24 o/oo. As with other stations, salinity was lower during winter
months (8.9 o/oo) and higher during summer (18.5 o/oo). Water temperature ranged
from 6.8 oC to 28.6 oC. Dissolved oxygen varied
seasonally, with a winter average of 9.3 and a summer average of 5.0. DO
exhibited a decline immediately after the passage of Hurricane Bonnie, but less
so after Hurricane Fran (Mallin et al. 1988, 1999). Sediments are predominantly
fine sands, with a flocculent layer overlying the substrate surface containing
larger detritus.
4.4b
Community Patterns
A
total of 262 taxa have been collected from the 4 permanent benthic infaunal
sampling stations (Tables 4.1-4.5), however only 12 new taxa (all rare) have
been collected since winter 2000. As noted in previous reports, while both NAV
and NCF6 have similar salinities (at least through 2000), they differ in
community patterns. NAV is strongly dominated by oligochaetes (including the
oligochaete species Tubificoides heterochaetus) and/or the polychaete Marenzellaria
viridis during all sampling periods (Table 4.5). In contrast, oligochaetes
often constitute a much smaller proportion of the total fauna at NCF6.
This site is usually dominated by a mix of midge larvae (Procladius,
Polypedilum, Chironomids), amphipods (Gammarus, Monoculodes)
and polychaetes (Marenzellaria, Spio, Polydora).
Estuarine polychaetes begin to dominate more at the more saline M54 site,
including the worms Mediomastus, Streblospsio, Marenzellaria,
and Malacoceros. Amphipods (Gammarus and Monoculodes)
remain present at this site but oligochaetes are much less abundant than at NAV
and insect larvae are generally uncommon (Table 4.5). Polychaete worms dominate
the lowest site, M31, with Mediomastus, Parandalia, Streblospio
and Marenzellaria being most common. Principal components analysis
indicated relatively similar patterns among sites, with the only notable
exceptions being the distinct nature of the NAV site, especially during
1996-1998, and the strong dominance of polychaetes at M31 making that site
different from the other sites (Figure 4.1). NAV site is distinguished through
its dominance by only a few taxa.
The
dominant taxa at all 4 sites are relatively opportunistic species capable of
quick recovery after disturbances, often short-lived with good recruitment
ability. Among the polychaetes, Streblospio, Mediomastus and Marenzellaria
are widespread in occurrence, found in moderately disturbed estuarine
environments throughout the mid Atlantic and southeast United States. Midge
larvae (Chironomids, Procladius, Polypedilum) can colonize quickly
because of aerial dispersal and the amphipods Gammarus and Monoculodes
are also known to colonize quickly after disturbances when predation is low.
Notably uncommon were bivalves, though juveniles were found and the small clam Mulinea
was abundant on some dates. Also uncommon were larger burrowing or tube-dwelling
fauna that are often considered characteristic of more stable and less impacted
estuarine environments.
Total
faunal density at the sites showed variable patterns. NCF6 was generally
characterized by low abundances except for summer 1999, when it had the highest
total faunal abundance among all sites (Figure
4.2). Fall 1998 samples were
taken after the passage of Hurricane Bonnie and had low total faunal density at
all sites. However, fall 1996 (after Hurricane Fran) and fall 1999 (after
Hurricane Floyd) did not have consistently low abundances at all 4 sites and
fall 2000 exhibited declines in total faunal abundance at all 4 sites (with no
hurricane in 2000). There were no consistent declines at any site over the 5
years of sampling, though abundances at NAV were greatest in the first 2 years
of sampling compared to subsequent years.
Diversity
remained consistent over time, with no significant change in diversity aside
from periodic spikes and lows from 1996 to 2000 (Figure
4.3). Species richness
showed a general decline at most sites from 1996 through 1998 (significant only
for M31), but 1999 was a period of higher species richness at NAV, NCF6 and M54
compared to previous years. As with total abundance, species richness declined
in summer-fall 2000 (Figure 4.4).
4.4c.
Juvenile Blue Crab Abundance Patterns
Juvenile
blue crabs in the Cape Fear River showed a seasonal pattern consistent with most
mid-Atlantic estuaries: peak abundance from late summer into early fall
(October) with low numbers from January-March (Figures
4.5-4.6). This pattern
has generally been attributed to peak larval recruitment during August-September
followed by movement of juveniles into deeper areas or burial into bottom
sediments during winter. Spatially, highest abundances were observed at the
Southport Marina site (Figure 4.7), but we suggest this may be affected by bait
and fish material left in the marina area. Among other sites, highest numbers of
crabs in the sweep net catches occurred in the mid estuarine sites, CBSP and
CFCM, near Carolina Beach State Park. These sweeps targeted smaller juvenile
crabs, mainly <35 mm carapace width. Catches
at the SPWW site were initially high in 1998-1999, but declined in late
2000-2002. Abundances at the uppermost sites, POWL and CFPM, were intermediate.
These catch patterns suggest that the most important shallow water nursery
habitat may not be occurring near the mouth of the estuary, as has been
suggested for some larger systems such as the Chesapeake Bay and Pamlico Sound,
but rather may be located in the mid estuary.
Although long-term declines have been suggested for blue crab populations
in Pamlico Sound and Core Sound over the past 5 years, 2001 catches of juveniles
did not appreciably differ from 1999 catches at most sites. Highest abundances
were noted in fall 2000 and lowest abundances were noted in 2002. We have
monitored larval recruitment over the past several years and the high larval
catches in 2000 were associated with high densities of larval crabs (zoea and
megalops) in June of that year (R. Barbour, Posey and Alphin, unpublished data),
emphasizing the potential importance of recruitment in determining crab
abundances.
Unlike
sweep nets, seines target larger crabs, generally > 40 mm CW. These crabs
were least abundant at the mid estuary sites (Figure
4.6) and most abundant at
either the upper site (POWL; September of all 3 years) or the lowest site (SPWW;
May-June). Movement towards the lower estuary in winter, and then movement back
into the estuary and eventually into oligohaline waters by larger blue crabs has
been well-documented in Chesapeake Bay and Pamlico Sound. Larger crab abundances
were greatest in 2001, following the high juvenile and larval abundances
observed in 2000.
4.5
Summary and Recommendations
The
Cape Fear River estuary is dominated by opportunistic taxa characteristic of
mesohaline to oligohaline reaches of Mid Atlantic to southeastern U.S.
estuaries. These taxa are quick to recolonize after disturbances and their
dominance suggests the benthic community may be resilient to certain types of
disturbance if that disturbance is short-lived and environmental conditions
return to pre-disturbance levels. There are few long-term trends in diversity,
abundance or species richness that have been consistent over the past 5 years.
Similarly, there has been quick recolonization after disturbance and dominance
by a relatively consistent group of species at each site over this time period.
We interpret these results to indicate that the infauna in the Cape Fear River
have been previously strongly affected by both natural and anthropogenic
disturbances, leading to dominance by a restricted set of opportunistic species
and the paucity of groups such as larger bivalves or larger burrowing worms.
However, the current community includes resilient, opportunistic taxa and does
not appear to be exhibiting continued decline.
The species present are likely to be able to tolerate or at least recover
from water quality changes if these changes are not persistently severe.
There
continues to be a fundamental difference between the upper mainstem Cape Fear
site (NAV) and the Northeast Cape Fear River site (NCF6). NCF6 has lower faunal
abundances relative to NAV, with the only exception in summer 1999 (due to
recruitment and later disappearance of a single polychaete species). NCF6 has
higher diversity than NAV because of relatively similar densities among a suite
of species at this low density site. In contrast, NAV has similar numbers of
species as NCF6, but the community is dominated by only 2 taxa.
Based on these results we have 4 recommendations for future efforts:
1) Uninterrupted seasonal monitoring must be continued at all 4 sites. We now have sufficient data to detect deviations from normal patterns and it is critical to continue this database if we are to detect gradual changes in benthic community patterns.
2)
Inclusion of additional
sites in the Northeast Cape Fear River and upper mainstem Cape Fear River above
the city of Wilmington, as well as an additional site in the lower estuary. One
of the strongest patterns to emerge from this monitoring is the difference
between the NE and mainstem Cape Fear branches. However, this evidence comes
from only one site in each tributary and additional sites are needed to confirm
that patterns are not simply reflecting local conditions around these singular
sampling locations. Another sampling site in the lower estuary will allow us to
observe whether more marine and less opportunistic species begin to appear in
the lower estuary where salinity extremes are not as pronounced. In response to
recent funding, we plan to begin sampling of an additional upper site in each
tributary at locations matching those of ongoing water quality sampling and/or
ongoing monitoring for wetland biota.
3)
Addition of biomass data.
Several sites experienced spikes in recruitment of selected species that
disappeared by subsequent sampling periods. Biomass (size/weight) data would
allow us to better understand and quantify these recruitment fluctuations and
mortality patterns and allow us to factor these fluctuations out of later trend
assessments.
4)
Continuation of blue crab
juvenile monitoring. We currently have funding from the National Science
Foundation to continue sampling into 2003 and plan to seek additional funds or
community assistance to continue that sampling after this project ends.
4.6 Literature Cited
Aschan, M.M. and A.M. Skullerud. 1990. Effects of changes in sewage
pollution on soft-bottom macrofauna communities in the inner Oslofjord, Norway.
Sarsia 75:169-190.
Boesch, D.F.,
R.J. Diaz and R.W. Virnstein. 1976. Effects of tropical storm Agnes on soft-
Hyland, J.L.,
T.J. Herlinger, T.R. Snouts, A.H. Ringwood, R.F. Van Dolah, C.T. Hackney,
G.A.
Nelson, J.S. Rosen and S.A. Kokkinakis. 1996. Environmental quality of estuaries
of the Carolinian Province: 1994. Annual
statistical summary for the 1994 EMAP - Estuaries Demonstration
Project in the Carolinian Province. NOAA Technical
Memorandum NOS ORCA 97. NOAA/NOS, Office of Ocean Resources Conservation and
Assessment, Silver Spring, MD.
102p.
Mallin, M.A.,
M. Posey, M. McIver, S. Ensign, T. Alphin, M. Williams, M. Moser and J.
Mallin, M.A., M. Posey,
M. Moser, L. Leonard, T. Alphin, S. Ensign, M. McIver, G. Shank and J. Merritt.
1999. Environmental
assessment of the lower Cape
Fear River system, 1998-1999. CMSR Report No. 99-01.
Mallin, M.A., M. Posey,
M. Moser, G. Shank, M. McIver, T. Alphin, S. Ensign and J. Merritt.1998.
Environmental assessment of
the lower Cape Fear River system, 1997-1998. CMSR Report No.
98-02.
Posey, M.H., T.D. Alphin and C.M. Powell. 1997. Plant and infaunal
communities associated with a created marsh. Estuaries 20:42-47.
Simboura, N., A. Zenetus, P. Panayotides and A. Makra. 1995. Changes in
benthic community structure along an environmental pollution gradient. Mar.
Poll. Bull. 30:470-474.
van Dolah, R.F.,
D.R. Calder and D.M. Knott. 1984. Effects of dredging and open water disposal on
benthic macroinvertebrates in a South Carolina estuary. Estuaries 7:28-37.
Whitehurst, I. T. and B.I. Lindsey. 1990. The impact of organic
enrichment on the benthic
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