Limnol. Oceanogr., 44(8), 1999, 1978–1992

The initial benthic degradation of senescent Zostera marina leaves was studied in controlled flowthrough microcosm chambers for 33 d. Sediment chambers without added eelgrass leaves served as control chambers. The inflowing artificial seawater and ouflowing seawater were analyzed for dissolved organic carbon and nitrogen (DOC and DON), total acid-hydrolyzable amino acids (THAA), dissolved free amino acids (DFAA), urea, NH , NO , 1 2 4 3 SCO2, and O2 during the 33 d of incubation. Sediment profiles of particulate organic carbon and nitrogen, sediment acid-hydrolyzable amino acids, DON, DFAA, urea, NH and the turnover rate of urea were measured at four 4 different times during the 33 d of incubation. There was an immediate increase in carbon oxidation and the effluxes of DOC, DON, and NH after the addition of eelgrass leaves to the sediment surface. During the course of 4 incubation, 24.3% of the DON efflux was identified as acid-hydrolyzable amino acids, dissolved free amino acids, and urea in the chambers with eelgrass addition, whereas these compounds accounted for 33.8% of the DON efflux in the control chambers. There were indications of a stimulated bacterial growth on the eelgrass leaves during the first 7 d after leaf addition that was measured as an increase in acid-hydrolyzable amino acids. Further, there was a gradual increase in acid-hydrolyzable amino acids in the sediment throughout the incubation that could only be explained as bacterial growth (and/or protein synthesis). Most of the nitrogen for microbial growth was mobilized from the indigenous particulate organic nitrogen pool, whereas it could be inferred that the energy source for bacterial growth was mainly supplied from the added eelgrass leaves. Most of the nitrogen mineralized within the sediment was incorporated into the microbial biomass with a resultant low efflux of inorganic nitrogen from the sediment to the water column. Shallow marine sediments are areas with an intense biogeochemical activity. The sources of organic matter to these sediments are benthic and pelagic primary producers and terrestrial organic matter. The quantity and quality of organic matter introduced into the sediment has been found to have an overall determining effect on benthic mineralization (e.g., Blackburn and Henriksen 1983; Lomstein et al. 1989; Therkildsen and Lomstein 1994; Sloth et al. 1995). The eelgrass Zostera marina was one of the dominating primary producers in Danish coastal waters before the onset of eutrophication. Z. marina is characterized by a high molar C : N ratio (16.4–24.3; Enrı́quez et al. 1993). Further, the plant contains structural plant components (e.g., cellulose, hemicellulose, and phenolic constituents) that are not easily degraded by benthic microorganisms (Boulton and Boon 1991; Buchsbaum et al. 1991). Fenchel et al. (1998) suggested that the large pool of dead organic matter in sediments may reflect the low rate at which 1 Present address: Research Center Foulum, Post box 50, DK8830 Tjele, Denmark. 2 Corresponding author.