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 pote ntial importance of benthic dissolved organic matter for planktonic bacteri al production. The study was conducted using water and sediments collected from along a fresh to salt water gradient in the Parker River and Plum Isla nd Sound Estuary in northern Massachusetts, USA. Initial concentrations of dissolved inorganic nutrients were elevated relative to ambient estuarine w ater while initial concentrations of dissolved organic carbon were higher d uring a June bioassay and lower during a December bioassay relative to ambi ent water. Specific growth rates of planktonic bacteria were calculated fro m changes in cell numbers during the exponential growth phase. Growth rates were very high, ranging from about 3.6 to 6.5 d(-1). Rates at individual s ites were generally higher in June than in December and higher in low salin ity waters than in high salinity waters. These rates are substantially high er than bacterial growth rates typically found in the water column of Plum Island Sound (1.5 to 65 x, depending on method) but had the same spatial pa ttern as is commonly found in the estuary. Growth rates were comparable to those observed during experimental additions of marsh macrophyte leachate t o incubations. Growth rates were linearly related to initial dissolved orga nic carbon (DOC) concentration in the bioassays, suggesting that substrate may limit bacterial growth. Stable isotopic compositions (delta(13)C and de lta(15)N) Of microbes grown in the bioassay indicated utilization of benthi c derived DOC even during times when the net flux of DOC was from overlying water into sediments. Further, the delta(15)N of bioassay microbes suggest ed that microbes utilized benthic derived inorganic N to supplement an orga nic diet deficient in organic nitrogen. It appears that growth rates were e levated as a result of dissolved organic matter exchange with bottom sedime nts. These results suggest that there were gross fluxes of organic matter a cross the sediment-water 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 remin eralization important in support of planktonic primary producers, but also as sites of dissolved organic matter generation and nutrient remineralizati on important in support of bacterioplankton production.