Abstract

Estuarine bacterioplankton function within strong physical and chemical gradients shaped by the mixing of freshwater and seawater and altered by autochthonous biological activity. Such environmental gradients, when recreated in microcosm experiments, cause shifts in the phylogenetic composition of bacterioplankton communities. In many estuaries, however, residence time can be too short relative to bacterial growth rate for such a shift to occur. In the Columbia River estuary, Oregon, where water residence time averages 1 to 2 d for most of the year, free-living bacterioplankton are a mixture of freshwater and marine phylotypes, but particleattached bacteria trapped in estuarine turbidity maxima, which are thought to have a residence time of 2-4 weeks, include many unique estuarine phylotypes. Similarly, in the Plum Island Sound estuary, Massachusetts, water residence time during the spring season is short and the bacterioplankton community is a mixture of freshwater and marine phylotypes, but in summer and fall when water residence time is long, a unique estuarine bacterioplankton community develops.

In Chesapeake Bay, long water residence time and salinity stratification contributes to the formation of anoxia in bottom waters during the summer. Phylogenetic analysis of bacterioplankton across the oxycline revealed significant overlap in community composition in oxic and anoxic waters, suggesting that periodic mixing events homogenize communities across the seasonal oxygen gradient and interrupt the development of a unique community in anoxic waters. These results suggest that bacterial communities in systems with short residence times are prevented from fully maturing into communities that are well-adapted to local environmental conditions. This stunted development may limit the ability of bacterioplankton communities to carry out critical ecological functions such as organic matter consumption and nutrient cycling..

Light refreshments are served in the Interaction Area (4th floor of the Oceanography/Physics Building) at 4:00 p.m.

All seminars begin at 3:00 p.m. and are held in room 200 of the Oceanography/Physics building.