Seasonal variation in sediment urea turnover in a shallow estuary

A seasonal study of sediment urea turnover rates was carried out in a shallow Danish estuary. Turnover rates decreased with sediment depth (0 to 16 cm) and were within the range 1.2 to 424.8 nrnol N cm-3 d-'. Integrated (0 to 16 cm depth) turnover rates varied between 1.5 and 16.9 nun01 N m-' d-' with a maximum in July and a minimum in January. Urea turnover followed first-order rate kinetics, as the urea turnover rate constant, k,,,,, was independent of the urea concentration, and the urea turnover rates were positively related to the latter. The urea turnover rate and rate constant were positively related to temperature. Urea production was stimulated by availability of high quality organic material (low C/N) and temperature. Urea production rates and urea concentrations in the sediment reached a maximum in July, when C-mineralization, temperature, and the quality of organic material were maximal, and there was a high benthic macrofaunal biomass. Secondary maxima in urea concentrations matched major phytoplankton sedimentation events in spring and autumn. Urea-N accounted for 7 to 55 % of the total N pool (urea + NH,+ + NO,-+ NO,') in the sed~ment surface (0 to 1 cm). The NH,+ pool was positively related to the urea turnover rate (CO to 16 cm). Urea turnover accounted for a major part of the NH,+ production in the sediment.