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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