AN ANNUAL NITROGEN BUDGET FOR A SEAGRASS ZOSTERA-MARINA POPULATION

The nitrogen dynamics of an eelgrass Zostera marina L. population were . assessed during an annual cycle by using measurements of seasonal ch anges in eelgrass biomass, production, losses and nitrogen content of different plant tissues. Estimated nitrogen uptake and reclamation (in ternal recycling) were compared to incorporation and theoretical requi rements to assess the role of different nitrogen sources and recycling and to determine periods of potential nitrogen limitation. Maximum ee lgrass biomass was 700 g dry wt m-2, annual production was 2388 g dry wt m-2, and total nitrogen incorporation was 34.5 g N m-2 yr-1. Estima ted nitrogen requirements exceeded actual incorporation from June to S eptember. Although eelgrass growth was moderately stimulated by fertil ization of the sediment, the eelgrass population did not appear to be seriously nitrogen limited. Nitrogen uptake from the external media (4 9 % from water column and 51 % from sediment) supplied 73 % of annual incorporation while internally reclaimed nitrogen accounted for 27 %. Reclamation provided the main contribution to incorporation into newly formed tissue in May and June, supporting high growth rates when exte rnal nitrogen availability was low. Externally recycled nitrogen in th e sediment could potentially increase total recycling to about 50 % of plant nitrogen incorporation.