Nitrogen Fixation and Denitrification in the Intertidal and Subtidal Environments of Tomales Bay, California

Abstract

We have examined contemporaneous nitrogen fixation and denitrification at two intertidal and three subtidal sites in Tomales Bay, California over a seasonal cycle. At subtidal sites, denitrification dominates, whereas nitrogen fixation dominates the intertidal sites. Potential rates of denitrification in intertidal sites fluctuate and can be up to one-half the nitrogen-fixation rate; therefore, the net input of fixed nitrogen to intertidal systems varies. Intertidal and subtidal sites appear to play quantitatively distinct roles in the internal, microbially mediated nitrogen cycle in Tomales Bay. However, the limited areal extent of intertidal regions (5% of total sedimentary area) minimizes their contribution of fixed nitrogen to the bay ecosystem. This intertidal input by nitrogen fixation amounts to only 3% of the nitrogen lost during subtidal denitrification.