Effects of intertidal oyster reefs on water quality in a tidal creek ecosystem

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