Dissolved organic nitrogen: an important source of nitrogen for the microphytobenthos in sandy sediment

To test the hypothesis that dissolved organic nitrogen (DON) is important for sustaining primary production by the microphytobenthos (MPB) in nitrogen-limited conditions, we measured the uptake of (15)N-labelled urea, the amino acids glycine (GLY) and glutamic acid (GLU), and nitrate and ammonium under simulated in situ light and temperature conditions. Microphytobenthic primary production and chlorophyll a (chl a) were also measured. The MPB was dominated by diatoms attached to sand grains, with cyanobacteria making up similar to 30% of the biomass. Activities of the hydrolytic ectoenzymes leucine aminopeptidase (AMA), alkaline phosphatase (APA) and beta-glucosidase (GLA) in filter-fractionated sediment showed that the microbenthic community was strongly deficient in nitrogen (N), with the bacterial fraction (< 1 mu m) also limited in phosphorus. Uptake of DON (urea + GLU + GLY) accounted for similar to 50 to 65% of the uptake of 15N-labelled substrates, with a higher proportion of DON uptake at low substrate concentrations (<= 2 mu M). Except for nitrate, the kinetics fitted a linear model. The calculated relative preference index (RPI), based on pore water concentrations, suggested that the order of preference of the microbenthic community was NH(4)(+) > urea > GLU > NO(3)(-)> GLY. Using a prokaryotic inhibitor (chloramphenicol), and theoretical calculations of algal uptake based on C: chl a ratios, it was estimated that the 'algal' uptake of nitrogen accounted for similar to 55 to 90% of DON uptake. Uptake rates were, however, estimated to cover only 26 to 50% of the nitrogen demand of the MPB, suggesting that pore water concentrations of nitrogen were not sufficient to meet the microalgal demand in early summer and that, in sandy sediments of microtidal waters, the MPB may often be severely limited in nitrogen.