Nitrogen cycling in two temperate Zostera marina beds:seasonal variation

Abstract

Exchange of dissolved inorganic N (DIN) between 2 eelgrass vegetated sediments and the water column along with denitrification, plant uptake and loss of N were measured monthly throughout a full year. The eelgrass beds acted as strong sinks for DIN in the spring and summer months. In autumn the beds acted as DIN sources, whereas during the winter months the beds had reestablished their sink capacity, although exchange rates were much lower than during the spring. The seasonal variation in DIN exchange between the water column and eelgrass beds was mainly controlled by the balance between benthic N-mineralization and plant N-uptake, while denitrification was of minor importance. Although plants and probably other associated primary producers were the dominant N sink, there was no accumulation of N in the living biomass on an annual scale. Model calculation of eelgrass decomposition furthermore suggested that most of the N bound to sloughed plant material was liberated to the environment within 1 yr. Only during the spring summer period was more nitrogen taken up by the beds than was released via decomposition of plant material. Therefore only in that period could the beds be considered as N sinks. The uptake of DIN by the eelgrass beds was significantly higher than the DIN uptake at unvegetated sites during the spring and summer period, whereas outside this period there was no significant difference between DIN fluxes measured at vegetated and unvegetated sites. Denitrification activity in the eelgrass vegetated sediments was similar to the activity in unvegetated sediments. It is suggested that the presence of eelgrass will not alter estuarine N-retention on an annual scale. However, during the spring and summer period when eelgrass beds are superior DIN sinks compared to unvegetated sediments, the temporal retention of N in the eelgrass biomass and detritus pool may reduce nutrient availability for other phototrophic organisms. The presence of eelgrass in N-limited areas may therefore reduce pelagic primary production.