5.0 Epibenthic Sampling in the Cape Fear Estuary

Troy Alphin and Martin Posey

5.1  Introduction
   Funding for the epibenthic portion of the Cape Fear River Program was interrupted for the 1999 sampling year and did not resume until June of 2000. The Benthic Ecology Lab of CMS collected quarterly samples on a gratis basis, but was not able to process any of these samples for the current report. We plan to present these data in the next annual report. In this report we present data from related projects examining epibenthos, along with a brief overview of some of the common epi-benthic groups. We present information from related projects for two reasons: 1) members of either the Advisory Board or other program participants expressed interest in these associated projects, and 2) these data help illustrate not only the types of organisms found as part of the epi-benthos but their usefulness in describing and characterizing estuarine sites. It is important to note that the data presented here are part of related projects supported by sources other than the Lower Cape Fear River Program and should not be reproduced or transmitted without permission.

What are Epibenthos?
    These are organisms that live on or just above the substrate (in this case river bottom). Included in this group are organisms such as snails, some bivalves, amphipods, some insect larvae, as well as those organisms that travel vertically in the water column such as shrimp, juvenile fish, and crabs. While most benthic organisms tend to be sedentary, (at most moving only a few meters throughout the course of their entire lives), many of the epi-benthic organisms are highly motile (some moving vertically in the water column on a daily basis). While benthic organisms tend to reflect the general condition of a particular area, epi-benthos provide information on timing and movement of fisheries species and the organisms they feed on. Many juvenile or postlarvae of commercially or recreationally important species are included as well.

How do we collect epi-benthos?
    An epi-benthic sled was used to collect samples as part of the lower Cape Fear River monitoring program. The sled has a rectangular opening (0.5 m wide X 0.3 m high) with a 1 mm mesh net (2 m long) attached to the frame. A sample consists of a single one-minute tow at constant speed (~1200 rpm). In order to standardize catches by the volume of water sampled a General Oceanics mechanical flow meter was mounted in the mouth of the net to allow calculation of the water volume sampled. Four stations within the Cape Fear River estuary are sampled seasonally for epi-benthic organisms (Fig. 5.1). All sites are located below the state port of Wilmington: the northernmost site is SI2 (located just below the port), SI3 is located near Marker 54, SA2 south of Marker 41, and C8 near Marker 31. These sites correspond to areas previously sampled as part of a project funded by the U.S. Army Corps of Engineers, providing prior background information (Mallin et. al. 1999). Two replicate trawls are collected from each of two depth zones (channel ~40ft. depth and river margin <10 ft. depth), because some of the taxa in question are highly motile and may travel vertically between shallow and deeper water habitats. Replicate tows were collected on separate days, at least 24 hours apart, but no more than five days apart and in most cases only one to two days apart.

5.2  Species Composition of the Cape Fear Epi-benthic Community
   In this section of the report we will briefly discuss some of the common groups found within the Cape Fear River system. We will also provide some general life history information as well as visual representations and pictures where possible. This background information will assist in understanding dynamics of the epibenthic community presented in future reports.

Amphipods:
Gammarid amphipods are among the most common taxa found within the epibenthos. These organisms range in size from 0.5 to 15 mm. Although there are predator species, most of the species found in the Cape Fear River system are facultative or obligate detrivores or herbivores. This group is an important food resource for many juvenile fish in estuarine systems, especially in the spring of the year. Amphipods are more abundant during winter (periods of low predation) with dramatic declines in the spring and into summer coinciding with the arrival of juvenile fish into the system. Amphipods are brooding organisms, meaning that they retain their eggs within a brood chamber on their underside, until the eggs hatch. It is this characteristic that has enabled this group of organisms (and related groups) to quickly colonize areas and to take advantage of patchy food resources.

There are several species of mysid shrimps that occur in the Cape Fear estuary, Neomysis americana is the most numerous. These small organisms show a very strong seasonal pattern with highest abundances in the summer and fall of the year when they reach mean densities of 300-700 per sample and peak at over 2000 in a single sample, but may be completely absent from samples during other times of the year. The value of these organisms as a food resource for fisheries species has been well documented. Mysids are highly motile and can migrate vertically although they are primarily a bottom dwelling group (Gosner, 1979). Like amphipods, mysids are brooders and retain their eggs within a brood chamber. Mysids tend to be very fragile and almost completely transparent and so although they may occur in high numbers it is easy to overlook them. The densities of mysids reported for the Cape Fear River is comparable to those of similar estuarine systems in the southeast (Ogburn et. al 1988).

Isopods:

This is another member of the group peracarida (brooding crustaceans). Isopods and amphipods are the two largest orders (having 10,000 and 5,500 species respectively) of peracarids. Although somewhat similar, isopods differ from amphipods in that amphipods are compressed laterally (side to side) and isopods are dorsoventrally flattened (top to bottom). Most estuarine, freshwater, and terrestrial isopods range in size from 5 to 15 mm (although there are deep sea forms that reach lengths of 40 cm). In general, marine and estuarine isopods exhibit a wide variety of feeding types, including forms that are fish parasites, scavengers, omnivores (feeding on both plants and animals), deposit feeders (ingesting sediment along with any organic material that might be contained there in) and detrivores that mechanically break down detritial material. In the Cape Fear estuary isopods were present at all sites and during most seasons. Although mean abundance tended to be low overall there were several periods of higher abundance but these varied both spatially (among sites) and temporally (among seasons). Even with these considerations this group could provide an important prey resource for juvenile fish, especially those that are more benthic oriented.

Decapod crustaceans make up the largest group (~ 35 taxa) of organisms collected in the epi-benthic samples. Several species of the genus Penaeus (including P. setiferus and P. aztecus), a commercially important group, have been collected from the Cape Fear Estuary, as well as numerous small shrimps and crabs. Also included in this group is one of the most commercially important species for North Carolina, the blue crab (Callinectes sapidus). This species in particular has been the target of a number of studies in the Cape Fear Estuary. Larval blue crabs move into the lower estuary and settle in late summer and throughout the fall of the year. These larvae settle out of the water column at 1-3 mm carapace width, when they encounter suitable habitats. The focus for much of the work conducted in the Cape Fear is distribution of juveniles (5-30 mm carapace width) up the estuarine gradient. Related and intermingled with this topic is the evaluation of habitats as suitable forage and refuge areas for juveniles.
    An examination of juvenile blue crab habitats along the estuarine gradient has been in progress for the last two years (Figure 5.2) (funded by NC Sea Grant and currently funded by the National Science Foundation). Regular updates on this work have been presented at the monthly Technical Committee meetings of the lower Cape Fear River Monitoring Program. These data were collected from the shallow river margins in 0.5-1 m deep water using a standard sweep net. Ten replicate sweeps (each sweep is five meters long) were collected at each site once per month. Although this sampling targeted juvenile blue crabs, a number of other taxa including small fish and shrimp were collected. Juvenile blue crabs are listed both as total C. sapidus as well as by size class (carapace width) as follows; j0= <12mm, j13=13-25mm, j25= >25mm but <40mm, and j40=>40mm. Other combined listings include flatfish (flounder, hog chokers, and tonguefish), the Leiostomas xanthurus/ Micropogonias undulatus group, and goby (because there were some juveniles in this group that made identification difficult we combined several species) (Tables 5.1- 5.8).
    Juvenile blue crabs (<25 mm carapace width) show an interesting trend with a portion of the population utilizing low salinity (<16 ppt) regions of the estuary (Mallin et. al. 1999). Although there is evidence of continuous larval inputs at some low level, periodic pulses of recruits into the system are most likely due to large-scale wind or current events. We have also found early juveniles at all sites along the estuarine gradient indicating that some portion of the new recruits are by-passing the lower estuary and moving directly into the mesohaline and oligohaline regions. This has implications for the importance of these areas as both refuge and forage habitats on these early life history stages.

5.3  Juvenile Fish:

    In addition to sweep net sampling we have also been collecting data on small fish use of shallow water areas within the Cape Fear River, using monthly seine nets at several locations within the river. At each site two (40 ft) seines were pulled 10 m in a purse seine fashion to shore. What we present here is brief species listing and total counts for the common taxa collected from this sampling (Table 5.9)
    Juvenile fish catches in the epibenthic sleds indicates that several species of the bottom oriented fishes such as Micropogonias undulatus (croaker) and Leiostomus xanthurus (spot) move into the system during late winter and early spring (Mallin et al 1999). This coincides with a time of high abundance for some of the benthic and epibenthic organisms, a ready food resource. While these results tend to follow a predictable pattern, it does emphasize the importance of timing on these groups. If disturbances to the system impact the availability of benthic and epibenthic resources during a critical season juvenile fish may respond through reduced abundances or shifts in distribution.
    The above-mentioned groups represent some of the most common taxa found in the epi-benthic sampling but in no way represent a complete listing of the taxa encountered. A total of 150 taxa have been identified from the epi-benthic samples (Mallin et. al 1999) so far and additional species are collected with each new season. A more complete listing species and abundances and standard errors can be found in the 1999 annual report (Mallin et. al. 1999).

5.4  Literature Cited

Gosner, K.L. 1979. A field guide to the Atlantic seashore: Invertebrates and Seaweeds of the Atlantic Coast from the Bay of Fundy to Cape Hatteras. P329.

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 Rive system. CMSR Report # 99-01. p142-166.

Ogburn, M.O., D.M. Allen, and W.K. Michener. 1988. Fishes, shrimps, and crabs of North Inlet estuary, S.C.: a four year seine and trawl survey. Baruch Institute Tech. Report 88-1. University of South Carolina, Columbia.

 

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