Primary Production and Nitrogen Cycling in an Estuarine Environment

The concentrations of NO3-N and NH4-N in the interstitial waters of the Eden estuary mature salt marsh, developing marsh, and mud flat region are higher than those in the overlying water at high tide. The developing marsh and mud flat region become dominated by a profuse growth of Enteromorpha (mainly E. prolifera) which begins in May, increases to a maximum in August and declines in September–October. At the time of maximum biomass standing crop values of 3.48 g N m-2 and 22.17 g C m-2 are obtained. At the end of the growth season, Enteromorpha becomes buried in the sand, heterotrophic bacterial populations including ammonifying bacteria build up and anaerobic conditions develop in the upper layers of the sediment. Thereafter increased populations of NH4+-oxidising bacteria develop in November–December as increased oxygenation of the upper sediment layers occurs as a result of turbulence in autumn and winter. Numbers of denitrifying bacteria are highest in winter. Biological N2-fixation also occurs in the estuarine sediments, with annual inputs of combined nitrogen ranging from 0.03 to 2.8 g N m-2 ann-1. The overall data suggest that input of nitrogen by decaying algae such as Enteromorpha and by biological N2-fixation are important in contributing new nitrogen to the ecosystem and that there is a close interdependence between nitrogen input, carbon input and nitrogen cycling within such estuarine sediments