Influence of the benthos on growth of planktonic estuarine bacteria

Estuarine planktonic bacteria were incubated (bacterial bioassays) in water previously held in close contact with benthic sediments to assess the potential importance of benthic dissolved organic matter for planktonic bacterial production. The study was conducted using water and sediments collected from along a fresh to salt water gradient in the Parker River and Plum Island Sound Estuary in northern Massachusetts, USA. Initial concentrations of dissolved inorganic nutrients were elevated relative to ambient estuarine water while initial concentrations of dissolved organic carbon were higher during a June bioassay and lower during a December bioassay relative to ambient water. Specific growth rates of planktonic bacteria were calculated from changes in cell numbers during the exponential growth phase. Growth rates were very high, ranging from about 3.6 to 6.5 d-'. Rates at individual sites were generally higher in June than in December and higher in low salinity waters than in high salinity waters. These rates are substantially higher than bacterial growth rates typically found in the water column of Plum Island Sound (1.5 to 65x, depending on method) but had the same spatial pattern as is commonly found in the estuary. Growth rates were comparable to those observed during experimental additions of marsh macrophyte leachate to incubations. Growth rates were linearly related to initial dissolved organic carbon (DOC) concentration in the bioassays, suggesting that substrate may limit bacterial growth. Stable isotopic compositions (6I3C and 6I5N) of microbes grown in the bioassay indicated utilization of benthic derived DOC even during times when the net f l u of DOC was from overlying water into sediments. Further, the 6I5N of bioassay microbes suggested that microbes utilized benthic derived inorganic N to supplement an organic diet deficient in organic nitrogen. It appears that growth rates were elevated as a result of dissolved organic matter exchange with bottom sediments. These results suggest that there were gross fluxes of organic matter across the sedimentwater interface that were not apparent from net changes in DOC concentration in the overlying water pool. This study indicates that benthic systems must be seen not only as sites of inorganic nutrient remineralization important in support of planktonic primary producers, but also as sltes of dissolved organic matter generation and nutrient remineralization important in support of bacterioplankton production.