4.0
Benthic
Community Patterns in the Lower
Cape Fear River
System
Benthic
Ecology Laboratory
University
of North Carolina at Wilmington
4.1 Summary
For the 2001 Cape Fear River report, we have emphasized analysis of site characteristics, annual and seasonal variations in community structure (numerically dominant species) at each site, and long-term trends in species richness, diversity and faunal abundance. Basic findings for this report are:
· The Northeast Cape Fear River and Cape Fear River oligohaline sites (NCF6 and NAV) exhibit fundamentally different community structures. The NCF6 site is characterized by moderate number of species, low abundances, and some variability in dominant species among seasons and years. In contrast, the NAV site is characterized by high abundances, domination by relatively few taxa (though similar overall number of taxa to NCF6) and consistent patterns of dominance across seasons and years. Such differences indicate fundamentally different processes controlling these communities.
· There is a general gradient in dominance from oligochaetes and insect larvae in the upper sites, along with the polychaete Marenzellaria on some dates, to exclusive domination by polychaetes in the most downstream, saline site. The mid site, M54, has a mix of these taxa along with peaks in densities of 2 amphipod species.
· NAV and M31 showed persistent patterns of dominant species among years and seasons. There was somewhat more variability in the identity of dominant species at NCF6 and M54, possibly reflecting salinity fluctuations, but even these sites were dominated by a small subset of the 250 total taxa that have been identified from this monitoring effort.
· Relative faunal density and diversity patterns were consistent among years with the exception of 1999. In 1999, there were relatively higher densities at NCF6 and higher diversity at NAV.
· All of the sites are dominated by relatively opportunistic taxa, suggesting possible community resilience to certain short-duration disturbances.
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, depending on the region of the estuary and on salinities. Benthic animals generally consume detrital, benthic microalgal, 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. juvenile fish) and their benthic prey, making this group a crucial element in the trophic dynamics of many (if not most) estuarine systems.
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 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 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 those 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 1999-2000 Cape Fear River report, we focused on details of long-term trends in abundances of higher taxa (polychaetes, bivalves, oligochaetes, amphipods) as well as seasonal changes in diversity and species richness. Additionally, we examined relationships between selected physical factors and patterns of faunal abundance. In this report (2000-2001) we examine patterns of species dominance among sites and over time. Particular attention is given to persistence of patterns over time (i.e. do the same species/taxa dominate at a particular site over time) as well as among-site comparisons of dominance patterns. Additionally, we examine long-term trends from 1996-2000 in diversity, species richness, and total faunal abundance. With the addition of further monitoring data, in future reports we hope to apply existing biotic indices to compare the Cape Fear system to other estuarine systems and to develop some predictive ability for estimating community responses to selected physical perturbations.
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 (Fig. 1.1). These stations span the oligohaline to mesohaline and 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 2000 samples is ongoing and the results from this 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 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).
For the purposes of this report, we will concentrate on dominant taxa, defined as any taxa that comprises greater than 5% of the individuals collected at a given site on a specific date. Most species collected in this monitoring effort are 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 site over a specified time period. Per sample species richness and per sample diversity are used 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 diversity per grab sample at a site over a year of sampling.
4.4 Results and Discussion
A total of 250 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, we describe general patterns of species dominance for the 4 sites, and we discuss patterns of diversity and species richness.
4.4a. Site descriptions
Physical conditions at all 4 sites were described in Mallin et al. (1999a; 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). 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 and Bonnie though less so after Hurricane Floyd (Mallin et al. 1998, 1999b, 2000, current report). Sediments are fine sands to sandy silts, with large detritus 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). 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 (Mallin et al. 1999b, 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 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. 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 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. 1998; 1999). Sediments are predominantly fine sands, with a flocculent layer overlying the substrate surface containing larger detritus.
4.4b Community Patterns
In the following discussion we will define characteristics of the dominant fauna at the 4 monitoring sites. For this report, deep-burrowing refers to taxa burrowing greater than 2 cm below the substrate surface, shallow-dwelling refers to taxa occurring predominantly within the upper 2 cm of the substrate, and tube-dwelling refers to taxa living within permanent or semi-permanent tube structures that reach the sediment surface (in this study most of these organisms feed on detrital material or microalgae on the sediment surface). Deposit feeders consume detrital material and possibly benthic microalgae and may be either non-selective (“gulping” sediment) or selective (feeding only on certain particle types). Suspension feeders are relying predominantly on planktonic food sources but may also consume some resuspended materials.
The NCF6 site is generally characterized by greater diversity and lower faunal abundances than the other oligohaline site, NAV. This is primarily due to the presence of a variety of co-dominants that are present with the dominant species varying among seasons and years and the lack of a single, persistent dominant taxon (Tables 4.1-4.5). The taxa dominating most often at this site are the polychaete worm Marenzellaria, oligochaetes, and various insect larvae (Polypedilum, Procladius, chironomids, Cladotanytarsus). Other polychaete worms, including Polydora, Streblospio and Boccardiella, were common on some dates. However, these polychaetes are generally found in higher salinity areas and their occurrence at high densities at this site most likely reflects higher salinity at the time of a recruitment event. Among-year variability was apparent at this site, with Marenzellaria dominating during all seasons in 1997, but relatively uncommon during all seasons in 1999. Similarly, mesohaline polychaetes (Mediomastus, Polydora, Boccardiella) were numerically dominant in 1999 but not in most other years. This reflects the position of this site at the extreme salinity tolerance for several estuarine taxa and indicates the sensitivity of this site to potential salinity changes over time. Marenzellaria, Polydora, Streblopsio and Boccardiella are closely related tube-dwelling polychaete worms. They live near the sediment surface where they deposit feed and possibly facultatively suspension feed. Numerous studies have indicated that they are susceptible to fish, shrimp and crab predation and thus may be important forage taxa. Most of these species have planktonic larval dispersal. Insect larvae are generally deposit feeders, grazers or predators and are also important forage species for certain fish. Many insect larvae are indicative of low salinity to freshwater areas and may be sensitive to salt intrusion.
The NAV site continues to be fundamentally different from the NCF6 site by having much higher faunal densities and lower diversity in most years. This site shows consistent domination by oligochaetes (oligochaetes sp. and the oligochaete genera Tubificoides in particular) (Tables 4.1-4.5). The only exceptions are domination by the polychaete worm Marenzellaria during 2 winter periods (1998, 2000). Oligochaetes are common in oligohaline to freshwater habitats and are generally considered to be deeper-burrowing deposit feeders. Their importance as food for fish, crabs and shrimp is uncertain, but some studies suggest that juvenile spot and juvenile shrimp may consume them. Pulses of Marenzellaria in winter (Jan-March) may be particularly important at this site for larval fish because of the uncertain accessibility of oligochaetes as prey.
Marker 54 (M54) exhibited low faunal densities relative to the NAV and M31 sites, as is typical of sites located in the turbidity maximum zone of estuaries. The fauna at this site varied from more mesohaline taxa, such as the polychaete worms Mediomastus, Eteone and Streblospio, to more oligohaline taxa such as oligochaetes, insect larvae (Polypedilum) and the amphipod Gammarus palustris. Among-year similarities include domination by Mediomastus during at least one season of each year, appearance of amphipod pulses in most years, and a reduced importance of oligochaetes relative to the upstream NAV site. Polychaetes were generally common at this site in most years (Mediomastus, Streblospio, Marenzellaria), though they were more numerically dominant in some years relative to others (e.g. 1999). When the amphipod pulses occurred, they were generally in winter and spring. The dominant amphipod taxa, Gammarus palustris and Corophium, have been shown to be important prey for benthic-feeding fish and decapods and their abundance at this time corresponds with larval/juvenile fish influx into the estuary. Mediomastus is a deep-burrowing, deposit-feeding polychaete that usually shows little susceptibility to predation by epibenthic fish and decapods. All 3 polychaetes, Mediomastus, Streblospio and Marenzellaria, are considered opportunistic taxa that can quickly recolonize an area. As such, they are resilient to disturbances.
Marker 31 (M31) was characterized by intermediate total faunal densities but relatively high per sample species richness and diversity. As with NAV, this site showed strong consistency in faunal dominance patterns among years (Tables 4.1-4.5). The community was dominated by 3 polychaete taxa, Mediomastus, Streblospio and Marenzellaria. Streblospio and Mediomastus were both dominant during at least one season each year. Marenzellaria was dominant only during winter or spring, possibly reflecting lower salinities at this site during those time periods.
Although 250 taxa were collected from the 4 sites monitored in the program, these sites were numerically dominated by relatively few species. The 2 uppermost sites were variously dominated by oligochaetes, 3 polychaetes, and 3 insect taxa larvae. The downstream 2 sites were dominated by 3 polychaetes, with 2 amphipod taxa and 1 insect larvae species also occasionally common at M54. The dominant taxa are generally considered to be opportunistic in nature, adapted to quickly recolonize after disturbance, and they are common in a variety of mid-estuarine, river-dominated systems, especially with moderate anthropogenic impacts. The dominance of these taxa would suggest that the community should recover quickly from certain disturbances. However, the degree of resiliency will vary depending on the type of disturbance and speed with which environmental conditions return to pre-disturbance levels.
4.4c Long-term trends in Species Richness, Diversity and Faunal Density
Because of some differences in sample effort among years, due primarily to hurricanes, we have examined diversity, species richness and faunal density trends on a per sample basis. Relative faunal density among sites was consistent for all years except 1999. In 1996, 1997, 1998 and 2000, NCF6 had lowest faunal density, M54 had the second lowest density, while highest densities were found in either NAV or M31 (Figure 4.1). Faunal densities at NAV were especially high in 1997, reflecting large numbers of oligochaetes at this site. 1999 differed from other years in that NCF6 had relatively higher densities. This was due exclusively to a one-time influx of the polychaete Boccardiella at this site in summer 1999 (Table 4.4). This polychaete did not persist and was uncommon by the following fall. Such occasional recruitment spikes followed by mortality are not uncommon for taxa with planktonic larvae such as Boccardiella.
Like faunal density, relative diversity between sites among years was also consistent except for 1999. Diversity as calculated here is dependent on 2 factors: the number of species and their relative abundance. A site may have higher diversity compared to another site, even if numbers of species are the same, if the abundances of those species are similar to each other rather than having only a few common taxa and many rare taxa. Diversity in the sites sampled was lowest at NAV for all years sampled except 1999. Highest diversity was generally found at the 2 lower estuarine sites, M54 and M31 (Figure 4.2). Higher diversity at more saline sites is commonly observed in estuarine systems because of the increasing prevalence of marine forms. However, this diversity pattern was not paralleled by the species richness patterns (Fig 4.3), especially for NAV and M54, indicating that for these 2 sites diversity was driven by relative abundance among species (equitability). NAV had low equitability, being dominated by only 1-3 taxa, while M54 had high equitability, with several common taxa of similar relative abundance. Species richness tended to be consistent among years at a site, with the notable exception of 1998 when there were fewer species caught per sample at all sites.
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. Both NAV and M31 showed persistence in dominant taxa from 1996-2000. There was greater variation in dominant taxa at NCF6 and M54, but these sites were still dominated by relatively few of the 250 taxa sampled and the variation in dominant taxa at these sites may reflect variations in salinity. There is a general gradient in dominant species from oligochaetes and insect larvae in the upper sites, along with the polychaete Marenzellaria on some dates, to domination by polychaetes in the lowest, most saline site. The mid site, M54, had a mix of these taxa along with spikes in densities of 2 amphipod taxa.
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 of 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 3 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 2 more sites in the Northeast Cape Fear River, 2 more sites in the mainstem Cape Fear River above the city of Wilmington, and 1 more 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.
3) Addition of biomass data. Several sites experienced peaks 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.
4.6 Literature Cited
Boesh, 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.
Mallin, M.A., M.H. Posey, M.R. McIver, S.H. Ensign, T.D. Alphin, M.S. Williams, M.L. Moser and J.F. Merritt. 2000. Environmental Assessment of the Lower Cape Fear River System, 1999-2000. CMS Report No. 00-01, Center for Marine Science, University of North Carolina at Wilmington, Wilmington, N.C.
Mallin, M.A., M. Posey, M. Moser, L. Leonard, T. Alphin, S. Ensign, M. McIver, G. Shank and J. Merritt. 1999a. Environmental assessment of the lower Cape Fear River system, 1998-1999. CMSR Report No. 99-01.
Mallin, M.A., M.H. Posey, G.C. Shank, M.R. McIver, S.H. Ensign and T.D. Alphin. 1999b. Hurricane effects on water quality and benthos in the Cape Fear Watershed: Natural and anthropogenic impacts. Ecological Applications 9:350-362.
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. 1999b. Environmental Assessment of the Lower Cape Fear River System, 1998-1999. CMSR Report No. 99-01, Center for Marine Science Research, University of North Carolina at Wilmington, Wilmington, N.C.
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.
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.
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