SOURCES OF DISSOLVED ORGANIC-CARBON SUPPORTING PLANKTONIC BACTERIAL PRODUCTION IN THE TIDAL FRESH-WATER HUDSON RIVER

Planktonic bacterial production in the tidal freshwater Hudson River i s a major component of secondary productivity and is uncoupled from pl anktonic primary productivity. There are several major sources of allo chthonous dissolved organic carbon (DOC) whose potential contribution to heterotrophic bacterial growth was examined with bioassays. Supply of DOC from the upper Hudson drainage basin and a large tributary in t he mid-Hudson together comprise 70 kT DOC/year, which is the bulk of t he DOC load to the tidal freshwater Hudson River. Two contrasting tida l wetlands contribute DOC to the main-stem river but were only a few p ercent of the tributary load even during summer low-flow conditions. T he quantity of DOC released from fine sediments was intermediate to th e other two loadings considered. Bacterial growth in bioassays receivi ng water from the sources varied, but differences in thymidine incorpo ration between reference and DOC sources were small, usually less than 2 nmol/L/h. Similarity in thymidine incorporation suggests that all s ources of DOC were capable of supporting bacterial growth at approxima tely equal rates. Seasonal shifts in carbon availability were clear in several cases, for example, greater growth on wetland-derived DOC at times of peak plant productivity. Seasonal differences in tributary DO C bioavailability were not large despite the well-known seasonality of tributary inputs. Activities of a suite of extracellular enzymes were used as a biologically based characterization of DOC from the various sources. Shifts in allocation among enzymes were apparent, indicating Chat there are biologically relevant differences in composition among the sources. Fluorescence characteristics and absorbance per unit car bon also varied among sources, providing an independent confirmation o f compositional differences among sources. The absence of large differ ences in bacterial productivity among sources suggests that growth is supported by a wide range of DOG, and the relative importance of the s ources is probably related to the quantitative differences in inputs. Efforts to classify carbon supplies to ecosystems must recognize that organism plasticity in carbon use and physical mixing processes will b oth act to homogenize what might initially appear to be quite distinct ive carbon inputs.