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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