4.0 Benthic Community Patterns in the Lower
Cape Fear River System
Martin Posey and Troy Alphin
4.1 Summary
For the 1999-2000 Cape Fear River report, we
emphasize analysis of site characteristics, annual and seasonal variations in community
structure, variations in species richness and diversity, and effects of multiple
hurricanes on the Cape Fear River benthic community. These analyses are preliminary in
nature, based upon only 4 years of data (3 of which were affected by hurricanes) and only
4 sampling sites. Additional sites will be required to adequately address spatial
patterns. However, the 4 sites sampled provide insights into biotic community structure in
the lower Cape Fear River system. Basic findings for this report are:
The Northeast Cape Fear River and Cape Fear River oligohaline sites
(NCF6 and NAV) are similar in terms of numbers of species present, with low to moderate
relative species richness at both sites. However, they differ in several important
aspects. The NCF6 site is characterized by higher diversity, low faunal densities, and
seasonal variability in abundances. In contrast, the NAV site is characterized by lower
diversity, higher total faunal density, and a tendency towards winter or spring peaks in
faunal abundance and seasonal replacement of dominant species. Such differences indicate
fundamentally different processes controlling these communities.
Lower estuarine sites (M54 and M31) are characterized by moderate
species richness and faunal abundances. They are dominated by taxa typical of other
river-dominated mid Atlantic and southeastern estuaries, though clams continue to be much
less common than reported in many other studies.
Although abundances fluctuated among years at all sites, overall
diversity, number of species and the identity of dominant taxa remained relatively
persistent over the 4 years, with some variations related to seasonality. The only
exception was a gradual trend towards lower species richness over time at M31.
Hurricane effects varied among sites. However, there were few
unequivocal long-term effects of multiple hurricanes on faunal abundance (though, as noted
above, there were some potential effects on species richness).
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. Depending on salinity, taxa dominating the infauna in most estuaries
include small worms (polychaetes and oligochaetes), amphipod crustaceans, clams, and
insect larvae. Benthic animals generally consume detrital, microalgal or planktonic food
sources (with some predatory species present) and are 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, 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 three 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
(depending on the taxa), 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 lifespans
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 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 densities 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 of sediments (such as 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; Index of Biotic Integrity; Ampelisca
toxicity tests; suspension feeder : deposit feeder ratios; deep burrower : shallow
burrower ratios; the Chandler Score, and the BMWP Score Index, indices based on diversity
or number of species) (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 lower Cape Fear
River, supported partially from the basic monitoring program of the Lower Cape Fear River
Program and partially through the Center for Marine Science and contributions from the
Benthic Ecology Laboratory. 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)
establish correlations between benthic abundances and physical measures, 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 1998 and 1999 Cape Fear River reports, we established that there
are strong differences among sites, as expected with salinity differences and estuarine
gradients. We also presented evidence of correlations among faunal abundances and certain
physical factors. Site differences may reflect differential response to and recovery from
hurricane disturbance in 1996 and 1998. This report (1999-2000) focuses on variations in
overall diversity, number of species, total faunal density and changes in relative
abundance of major taxa groups at each site. With 4 years of data, we hope to begin the
process of evaluating significant trends in benthic community health and structure and to
provide a baseline for detecting future changes. With the addition of further monitoring
data, we will develop some testable, predictive ability for estimating community responses
to selected physical perturbations as well as examine functional guild and
species-specific patterns.
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 (see Mallin et al. 1999). 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
for the Lower Cape Fear River Program 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, Hurricane Bonnie in August 1998, and Hurricane Floyd in September 1999,
additional samples were also taken to observe recovery patterns (September 1996, December
1996, February 1997, September 1998, October/November 1998, January 1999, September 1999,
November 1999 and January 2000). 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. Sorting,
identification and QA/QC (LCFRP QA/QC Manual, 1998) is ongoing for some sampling dates.
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 are taken at each sampling location on each sampling date. Grabs are 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 are 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 are 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).
In this chapter, we report patterns of species richness, diversity,
total faunal abundance, and density patterns for major taxa groups. Species richness
refers to the number of species present at a site while diversity refers to both number of
species and their relative abundance. With diversity, a community dominated
proportionately by one species is considered less diverse than a community with a similar
number of species that are all relatively similar in abundance. Disturbance is often
thought to reduce species richness by eliminating taxa not capable of withstanding the
disturbance or of recovering quickly. Diversity also declines with environmental impact
through a combination of effects on species richness and a tendency for impacted areas to
become dominated by a few taxa (i.e. a community having very unequal abundances among
species present). EPA-EMAP guidelines currently suggest diversity ranges indicating
severe, moderate or low impact on a community. For this study, we calculated diversity
based on the Shannon-Weiner Diversity Index (Brower et al. 1999). Both species richness
and diversity can be affected by sample size. Because there were less than 5 grabs taken
at some sites on some dates, we based all richness and diversity measures on the first 3
samples taken at a site on a date and did not include these measures if there were fewer
than 3 reliable samples (as was the case for a few post-storm periods when debris
inhibited sampling). Higher taxonomic categories were used only when they clearly did not
overlap with species-level identifications (e.g. oligochaetes).
4.4 Results and Discussion
A total of 222 taxa have been collected since
Winter 1996. The dominant taxa are described in the 1998-1999 Lower Cape Fear River report
(Mallin et al. 1999) and will be dealt with in more detail in future reports. In general,
less than half of the species were collected at any single site and most species were
relatively rare (Mallin et al. 1999). Below we describe general patterns of diversity and
density for the 4 sites sampled.
Site descriptions
Physical conditions at all 4 sites were described in Mallin et al. (1999;
benthic and epibenthic sections) and additional water quality data is provided in the
present report. General site characteristics 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 (0.67 o/oo) and
higher during summer (5.47 o/oo). 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 3.7.
DO exhibited severe declines immediately after the passage of Hurricanes Fran and Bonnie
(Mallin et al. 1997, 1999, current report). 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 (0.25 o/oo) and
higher during summer (4.03 o/oo). 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 3.9.
DO exhibited declines immediately after the passage of Hurricanes Fran and Bonnie (Mallin
et al. 1997, 1999, current report).
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 the NCF6 and NAV sites. Salinity for the months in which benthos
was sampled averaged 5.2 o/oo but ranged from 0 to 18 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 (Mallin et al.
1997, current report). 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 26.1
o/oo. As with the 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 5.0.
DO exhibited a decline immediately after the passage of Hurricane Bonnie (Chapter
2), but
less so after Hurricane Fran (Mallin et al. 1997). Sediments are predominantly fine sands.
Species Richness and Diversity
At NCF6, NAV, and M54, both diversity and species richness have remained
relatively constant over the 4 years of sampling (Figures
4.1, 4.2, 4.3). Diversity and species
richness values in fall 1999 were similar at all three of these sites compared to those
measured at the same sites in winter 1996. However, as reported in the 1998-1999 LCFRP
report (Mallin et al. 1999), the identity of species present varied considerably between
seasons and years (see Table 7.4). There was some tendency for diversity and species
richness to drop after hurricanes, but this tendency was not strong and varied with sites
and among hurricane events. The relative constancy of these broad community measures at
the 3 sites probably reflects their dominance by relatively opportunistic species that can
tolerate disturbances, including variations in salinity from freshwater to upper
oligohaline, as well as variations in temperature and river flow. In contrast to the other
3 sites, there was a general decline in species richness at M31 over the 4 years of
monitoring (F=2.86; p<0.07; Fig.
4.4; Fig. 7.3). This site is dominated by polychaetes
that may be more severely affected by occasional freshets associated with storms as well
as sediment inputs compared to the fauna dominating other sites. We believe the decline in
species richness at this site reflects a long-term change in the community
characteristics. It may be indicative of cumulative effects of hurricanes in the lower
estuary, interactions between hurricanes and anthropogenic factors (e.g. development,
channelization activity, or agricultural activities), or may reflect other long-term
changes in the estuary.
Comparing richness and diversity among sites, NAV demonstrated lower
diversity than the other oligohaline site sampled, NCF6. The 2 sites did not differ
greatly in species richness, but the NAV site was dominated by a few common taxa, leading
to lower overall diversity. Diversity and richness at the 2 lower estuarine sites were
comparable to the NCF6 station and are similar or slightly less that that reported in
other river-dominated systems for similar salinity and substrate characteristics.
Patterns of Abundance
The NAV site had much higher overall faunal abundances than the NCF6
location. Peak total faunal density at NAV reached almost 275 individuals per grab, with 6
dates of near or greater than 100 individuals per grab (Figure
4.1). Total faunal
abundances at NCF6 never exceeded 100 individuals per grab (Figure
4.2) and were generally
less than 20 individuals per grab. The NAV site was dominated by oligochaetes, with
occasional peaks in polychaete densities (primarily Maranzellaria) (Figure
4.5).
Insect larvae were occasionally common, especially the midges Polypedilum, Procladius,
and Tanypodinae (Table 7.4). The NCF6 site was not clearly dominated by any single taxa,
with the exception of a peak in density of the polychaete Polydora in summer 1999
(Figure 4.6; Table
7.4). Maranzellaria was a common polychaete during spring 1997
and winter 1998. During other sampling periods, dominant fauna included oligochaetes,
insect larvae (especially Polypedilum, Procladius, and Chaoborus) and
the amphipods Gammarus palustris and Gammarus tigrinus.
Seasonal patterns in abundance were apparent at NCF6, with greater
faunal densities occurring in spring or winter for polychaetes, oligochaetes and amphipods
(an exception being the single peak for Polydora in summer 1999) (Figure
4.6; Table 7.4). Insect patterns were more variable and may reflect annual weather conditions.
Seasonal patterns in total faunal densities were more difficult to discern at the NAV site
(Figure 4.5), reflecting differing timing of peaks in abundance for selected taxa among
different years rather than the lack of any temporal variations. Peaks in total faunal
abundance variously occurred in winter (1996, 1998) summer (1997) and fall (1999), often
reflecting single peaks in abundance of specific taxa. Winter 1997, summer 1997, and fall
1999 peaks were due to increases in density of oligochaetes. Winter 1998 was a dramatic
recruitment event of the polychaete Maranzellaria and small increases in density of
insect larvae were apparent throughout fall-winter 1997. One clear pattern is high
variability between years, emphasizing the need for long-term data sets in determining
benthic community trends and cautioning against using data from only 2 or 3 years.
M54 was dominated by a variety of taxa, including polychaetes (Maranzellaria,
Mediomastus and Streblospio), oligochaetes and amphipods (Gammarus,
Lembos, Monoculodes – Table
7.4). As with NAV, seasonal patterns in total
faunal density (Figure 4.3) are obscured by differing periods of peak abundance for
different taxa. Polychaetes generally exhibited highest abundance in winter and spring,
driven especially by Mediomastus spp. and Streblospio, with spring
recruitment peaks of Maranzellaria in 1996, 1998 and 1999. Most of the taxa
dominating at M54 are relatively opportunistic species characteristic of oligohaline to
mesohaline areas and capable of rapid recovery from disturbances.
The most saline estuarine station sampled as part of the basic
monitoring program, M31, is almost completely dominated by polychaetes. This is typical of
the mesohaline to polyhaline regions of many estuaries and reflects the greater diversity
of polychaete worms in more saline waters as well as the loss of oligochaetes and insect
larvae in marine areas. Among the dominant polychaetes at this site are Mediomastus
spp. (M. californiensis and M. ambiseta), Streblospio
benedicti, Nereis falsa, Laeonereis culveri, and Polydora socialis.
Highest densities generally occurred in winter or spring, but there was interannual
variability in patterns with low densities in spring 1999 and relatively higher numbers in
summer 1998. This site may also experience higher salinity intrusions from Snow’s
Cut.
Hurricane Effects
Any discussion of benthic community dynamics over the past 4 years must
consider the potential effects of hurricanes on benthic community structure. Hurricanes
Bertha and Fran hit the Cape Fear estuary in 1996, Hurricane Bonnie in 1998 and Hurricane
Floyd in 1999. There were few consistent relationships between hurricanes and diversity or
species richness, though these measures did decline after some hurricane events at some
sites. Total faunal density declined at all sites after either Hurricane Bertha or
Hurricane Fran in 1996, though this decline did coincide with a normal summer decline in
faunal densities. Total densities declined at NCF6 and M31 after Hurricane Bonnie.
However, there was no decline after Hurricane Bonnie for the other 2 sites reflecting
already low total densities from the summer seasonal low. Responses to Hurricane Floyd
were similar in that declines were observed at M54 and NCF6, but there was little change
for NAV and a spike in abundances at M31. There was little evidence for long-term trends
in abundance, such as would indicate long-term effects of multiple hurricanes, that could
be determined with certainty against background interannual fluctuations.
4.5. Conclusions and Monitoring Recommendations
Our basic conclusions about the types of taxa
dominating the Cape Fear estuary remain unchanged from the 1999 LCFRP report – this
system is dominated by a moderate to low diversity of opportunistic species that are
typical of mid-Atlantic, river-dominated estuaries (Mahooney and Livingston 1982; van
Dolah et al 1984; Holland et al. 1987; Shaffner et al. 1987; Posey et al. 1993; Posey et
al. 1996). One taxonomic group that continues to be significantly underrepresented is
bivalves (clams, oysters and mussels). Diversity and species richness have fluctuated
around relatively stable levels for the past several years, though total densities and the
identity of dominant taxa have changed significantly from year to year and with seasons.
The Northeast Cape Fear River and Cape Fear River stations continue to exhibit differences
in structure, especially with respect to diversity and density patterns, patterns of
seasonality, and responses to disturbance. Community responses to hurricanes appear
generally short-lived at most sites, possibly reflecting the opportunistic nature and
quick recovery potential for most of the dominant taxa observed. The only long-term trend
noted was a gradual decline in species richness at the most saline estuarine station, M31.
As recommended to the LCFRP Technical Advisory Committee, our primary
monitoring recommendation is to include two additional sites in the Northeast Cape Fear
River, two additional sites in the upper mainstem Cape Fear River (above NAV) and one
additional site in the lower estuary. Our data suggests that these regions may exhibit
different responses to disturbances, and possibly to pollutants or other anthropogenic
impacts. However, firm conclusions cannot be made without replicate sites to control for
site-specific differences.
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-bottom macrobenthic communities of the James and York River
estuaries and the lower Chesapeake Bay. Ches. Sci. 17:246-259.
Brower, J.E., J.H. Zar and C.N. von Ende. 1998. Field and Laboratory
Methods for General Ecology. 4th Edition. McGraw Hill, Boston.
Fauchauld, K. and P.A. Jumars. 1979. The diet of worms: a study of
polychaete feeding guilds. Oceanogr. Mar. Biol. Ann. Rev. 17:193-284.
Holland, A.F., A.T. Shaughnessy and M.H. Hiegel. 1987. Long-term
variation in mesohaline Chesapeake Bay macrobenthos: spatial and temporal patterns.
Estuaries 10:227-245.
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.
Mahooney, B.M.S. and R.J. Livingston. 1982. Seasonal fluctuations of
benthic macrofauna in the Apalachicola Estuary, Florida, USA: The role of predation.
Marine Biology 69:207-213.
Mallin, M.A., M.H. Posey, M.L. Moser, G.C. Shank, M.R. McIver, T.D.
Alphin, S.H. Ensign and J.F. Merritt. 1997. Environmental assessment of the lower Cape
Fear River system, 1996-1997. CMSR Report No. 97-01.
Mallin, M.A., M.H. Posey, M.L. Moser, L.A. Leonard, T.D. Alphin,
S.H. Ensign, M.R. McIver, G.C. Shank and J.F. Merritt. 1999. Environmental assessment of
the lower Cape Fear River system, 1998-1999. CMSR Report No. 99-01.
Posey, M.H., T.D. Alphin and C.M. Powell. 1997. Plant and infaunal
communities associated with a created marsh. Estuaries 20:42-47.
Posey, M.H., W. Lindberg, T. Alphin and F. Vose. 1996. Influence of
storm disturbance on an offshore benthic community. Bulletin of Marine Science 59:523-529.
Posey, M.H., C.W Wigand and J.C. Stevenson. 1993. Effects of an
introduced aquatic plant, Hydrilla verticillata, on benthic communities in
the upper Chesapeake Bay. Estuarine, Coastal and Shelf Science 37: 539-555.
Shaffner, L.C., R.J. Diaz, C. R. Olsen and I.L. Larsen. 1987. Faunal
characteristics and sediment accumulation processes in the James River Estuary, Virginia.
Estuarine, Coastal and Shelf Science 25: 211-226.
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 macroinvertebrate communities of a lowland river. Wat. Res.
24:625-630.
