The importance of oyster filtering in moderating aspects of water quality has
received increased attention over the past several years. With population growth and
increasing development in coastal watersheds come increased runoff and pollution of
tidal creeks. It has been suggested that bivalves may play an important role in controlling
phytoplankton levels in shallow coastal areas, with several laboratory studies and models
estimating the potential filtration effects of these organisms. However, few field studies
have been undertaken to quantify these effects. This study examined the influence of
intertidal oyster reefs on chlorophyll a, fecal coliform bacteria and total suspended solid
concentrations under field conditions in a tidal creek estuary. Oyster reefs of varying live
oyster density were sampled during summer 2002, winter 2003 and spring 2003. Water
samples were taken upstream and downstream of each reef as well as over a mudflat
control area on an ebb tide and analyzed for concentrations of these water column
constituents. Summer data showed consistent and statistically significant decreases in
chlorophyll a concentrations as water moved over the reef, usually by 10-25%. Fecal
coliform counts were frequently lower downstream, by up to 45%, but were much more
variable and not statistically different in most cases. Data taken in winter, when
temperatures and oyster feeding rates are lower, show less consistency in upstream vs.
downstream patterns. In spring, chlorophyll a decreases were less frequent than in
summer, but significant fecal coliform decreases were more frequent. Data from this
study indicate that feeding by oysters and changes in water flow caused by the presence
of reefs may both play a role in reducing particulate loads in the water column
