BACTERIAL METABOLISM OF ORGANIC-CARBON IN THE TIDAL FRESH-WATER HUDSON ESTUARY

Planktonic bacterial respiration (estimated from rates of O2 consumption) was the largest fraction of total planktonic respiration at a site in the tidal freshwater Hudson River, New York, USA. No simple relationship between respiration and bacterial production was found, indicating considerable variability in bacterial growth efficiencies. Mean growth efficiency was 22 % but values ranged from < 10 to > 50 %. Non-phytoplanktonic carbon is known to be important in supporting the high rates of planktonic bacterial productivity observed in the Hudson. Experimental additions of particulate detritus derived from the most common submerged macrophyte (Vallisneria americana) and wetland plant (Typha angustifolia) to Hudson water did not result in increases in bacterial productivity. In contrast, additions of dissolved organic carbon (DOC) derived from these same plants consistently yielded large increases in bacterial production to rates several-fold greater than rates measured in the field. Growth efficiencies for DOC were 37 and 63 % for T. angustifolia and V. americana respectively. These measures of respiration, production and growth efficiency allow us to estimate the amount of allochthonous carbon necessary to support bacterial productivity. Required carbon inputs range from 140 to 320 mug C 1-1 d-1 depending on the assumed conversion efficiency. These inputs are 3 to 6x net carbon fixation by phytoplankton. The bacterial assimilation of dissolved organic carbon results in DOC turnover times of 2 to 3 wk which are less than the residence time of water in the mid-Hudson during most of the year. The lack of local or downstream depletion of DOC indicates that there must be new sources of DOC within this region of the Hudson. Bacterial metabolism of terrestrially-derived organic carbon has the potential to alter the quantity and composition of material supplied to the ocean.