FATE OF METHANE IN THE HUDSON RIVER AND ESTUARY

Methane (CH4) concentrations and oxidation rates were measured through out the Hudson River Estuary in March and August of 1991. Methane conc entrations ranged from 50 to 940 nM and were supersaturated with respe ct to the atmosphere along the entire length of the river, with genera lly higher CH4 values in the lower, saline portion of the estuary. A s easonally averaged diffusive flux to the atmosphere from the Hudson Ri ver was estimated to be 5.6 mg CH4 m-2 d-1, corresponding to an annual flux of 0.76 x 10(9) g CH4. The Hudson River Estuary also releases ap proximately 0.2 x 10(9) g CH4 annually to nearshore marine waters. Dif fusive flux across the air/river interface was the dominant removal me chanism for Hudson River CH4 in March. In August, CH4 oxidation was th e dominant CH4 sink in freshwater and brackish (<6 parts per thousand) sections of the river, removing up to 70% of ambient CH4 per day comp ared to maximum daily removal rates of 13% in March. Thus methane oxid ation can play a major role in limiting releases of CH4 to the atmosph ere from rivers and other freshwater environments. Methane oxidation a ctivity decreased rapidly as salinity increased, with less than 2% of ambient CH4 being oxidized per day at salinities greater than 25 parts per thousand. Addition of NaCl or seawater to freshwater samples resu lted in comparable inhibition of methanotrophic activity. Budget calcu lations showed that flux to the atmosphere and CH4 oxidation in March removed less water column CH4 than was supplied to the Hudson River ov er its entire length. In August, however, CH4 removal approximately eq ualed CH4 supply with the result that there was no net accumulation of CH4 over the length of the river.