Date of Award

Spring 2000

Document Type

Legacy Thesis

Degree Name

Bachelor of Science (BS)

Department

Marine Science

College

College of Science

First Advisor

Alan J. Lewitus

Abstract/Description

In the tidal creeks of North Inlet, a high salinity salt marsh estuary near Georgetown, SC, the American oyster (Crassostrea virginica) is an abundant component of the benthic macrofauna that exerts controls on microorganisms by its grazing and nutrient regenerative activities. We examined the effects of oyster activity on microbial food web structure by comparing microbial community composition: a) in water samples collected from tidal creeks with oyster reefs vs. tidal creeks without oyster reefs (removed as part of a large-scale field manipulation study, the NSF CREEK project), and b) in water flowing into vs. out of oysters contained in flumes. Among several microbial groups quantified in the in situ samples, the only group found to vary significantly with the presence of oyster reefs was the phototrophic nanoflagellates (pflags), which were 1.25 to 2.25-fold less abundant in creeks with oyster reefs during July and August. Because heterotrophic nanoflagellates (hflags) did not vary with the presence of oyster reefs, we hypothesized that preferential feeding for pflags by oysters was responsible for the reduction in their abundance. The hypothesis was tested in 1999, using flumes with live oysters (biomass approx. 200 gdb/m2) vs. dead oyster shells. Although little oyster grazing was evident in March (presumably due to low water temperatures), a significant reduction in pflags was measured in the outflow from live oysters (e.g. 2-fold decrease in abundance) during two trials. However, oyster effects on other microbial components were not observed in March. In July, a decrease in pflag abundance in the outflow from live oysters again was observed at the lowest flow velocities, but the differences in abundance were more pronounced in July (e.g. 5 times greater reduction in cell concentration per unit time than in March). Also, total chlorophyll a was reduced by live oysters, even though ammonium levels increased in the outflow, indicative of high grazing pressure. Among other microbial groups, feeding on several diatom taxonomic groups also took place. However, as in March, the abundance of hflags, cyanobacteria, or heterotrophic bacteria did not vary with treatment. This study demonstrates preferential feeding by oysters on pflags using naturally occurring microbial assemblages. The differences in pflag abundance in creeks with oyster reefs vs. creeks without oyster reefs suggests that this grazing activity can impact the structure of natural microbial communities.

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