Nitrogen fixation in the Rhode River estuary of Chesapeake Bay

The distribution, seasonal variation, origin, and significance of biological nitrogen fixation has been determined for a Chesapeake Bay estuary using the acetylene reduction technique. Studies over a 15-month period have shown that nitrogen fixation occurs predominantly in the surface intertidal (marsh) and subtidal sediments. Negligible activity was found in surface waters. A marked seasonal variation in nitrogen fixation was observed for intertidal sediments incubated at a standard 20 degrees C. Average rates of about 37 and 12 ng N/g dry sediment per hour were observed in the late fall months of 1972 and 1973, respectively, and less than or equal to 5 ng N/g dry sediment per hour during other seaons. Peaks of activity were lowered considerably when samples were incubated at ambient temperatures (in situ). Activity in the subtidal sediments was low (less than or equal to 6.8 ng N/g dry sediment per hour but showed a similar (approximately twofold) seasonal variation in nitrogen fixation potential. Light-dark and substrate addition experiments suggest that heterotrophic bacteria are the principal agents for nitrogen fixation in sediments. Integrated estimates of nitrogen fixation in the estuary indicate that biological fixation probably accounts for less than 5% of the total influx of nitrogen into the system. Rates of activity in the intertidal sediments are insufficient to account for the high productivity of marine angiosperms found in the marsh.