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Benthic Community Structure

Stellwagen Bank has high rates of water-column primary production and surprisingly high microalgal production rates within the benthos (Cahoon et al. 1993). It is therefore likely that both suspension-feeding and deposit-feeding benthic invertebrates play an important role in marine food web food production. Because many bottom-dwelling invertebrates are sedentary and live within the sediment, where pollutants tend to accumulate, these benthic communities may be especially susceptible to deterioration as a result of anthropogenic inputs. Thus, benthic communities of Stellwagen Bank likely represent an important and potentially fragile component of this marine sanctuary's biological resources.

The benthic communities of Stellwagen Bank have not been well characterized, despite the fact that the benthic community structure of both Massachusetts Bay and Cape Cod Bay has been well documented (e.g., Young and Rhoads 1971; Gilbert et al. 1976; Michael et al. 1978; Blake et al. 1993; Hyland and Costa 1994). There is an extensive database on benthic community structure for the northeastern United States, collected by the National Marine Fisheries Service between 1953 and 1975 (Theroux 1994). This database includes benthic samples from Stellwagen Bank. Unfortunately, at the present time, the database is not available for use (Theroux, personal communication).

Due to the lack of information, benthic community structure of Stellwagen Bank has been inferred from published data on the community structure of Georges Bank (Pett and McKay, 1990). OCS Sale No. 42 (U.S. DOI 1976), OCS Sale No. 52 (U.S. DOI 1981), and OCS Sale No. 82 (U.S. DOI 1983a) provide reviews of the scientific literature concerning the invertebrates of Georges Bank (Pett and McKay, 1990).

In an attempt to further characterize the benthic community structure of the Stellwagen Bank National Marine Sanctuary, data from studies done in Massachusetts Bay have been reviewed. In these studies, samples were taken from stations within or near the boundaries of the marine sanctuary and subsequently analyzed. Data from samples taken within the sanctuary boundaries came from two sources: the reports of Gilbert et al. (1976), and Blake et al. (1993). Sample locations are shown in Figure 5. Stations one, two and three were sampled by Blake et al. (1993). The remaining stations were sampled by Gilbert et al. (1976).

Basic attributes of benthic communities include species diversity, faunal abundance, and species composition. Species diversity at each location is shown in Figure 6. In order to correct for differences in sample sizes between the two studies, species diversity was calculated using the Sanders - Hulbert rarefaction procedure (Sanders 1968; Hulbert 1971). Diversity is expressed as the number of species expected from a random draw of 500 individuals from a sample.

Figure 5: Locations of sampling stations. Stations 1, 2 and 3 (solid points) were sampled by Blake et al. 1993. The remaining stations were sampled by Gilbert et al. 1976. Stellwagen Bank and the boundary of the Stellwagen Bank national marine sanctuary are outlined.

It is difficult to partition the various processes controlling species diversity in these samples. Differences in the soft-sediment habitat, or temporal changes between the dates of the two studies may also have had an effect on species diversity. The basic pattern, however, shows that species diversity is depth-related. Species diversity tended to increase offshore and with greater distance from Stellwagen Bank. This confirms the observation made by Sanders (1968) that benthic species diversity generally increased with depth.

Figure 6: Species diversity at the sampling sites. Species diversity was calculated using Hulbert's (1971) expected species index with a random sample size of 500 individuals.

Faunal abundances are shown in Figure 7. Because the samples of Blake et al. (1993) were processed with a 0.3 mm sieve, whereas the samples of Gilbert et al. (1976) were processed with a 0.5 mm sieve, the Blake et al. (1993) study will show higher abundances than the study conducted by Gilbert et al. (1976). Comparisons among samples from the two studies are therefore not valid. A trend observed nonetheless is the gradual decrease in faunal abundance offshore. This is probably related to increasing depth and subsequently a decreasing food supply to the bottom.

Figure 7: Faunal abundance (number of individuals per 0.1 m2).


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