Fate of microphytobenthos nitrogen in subtropical subtidal sediments: A 15N pulse-chase study

Microphytobenthos (MPB) are an important nitrogen (N) sink in coastal systems, but little is known about the fate of this N after it has been assimilated. We used an in situ N-15 pulse-chase experiment in subtidal sands to follow the assimilation, trophic transfer, transformation, and flux pathways of MPB-N over 33 d. Throughout the study MPB dominated N-15 uptake, on average representing only 18.1% of the biomass but 63.9% of the N-15 within 0-2 cm sediment. Following assimilation, N-15 was rapidly transferred to deeper sediment, with 32.1% below 2 cm and 16.5% below 5 cm after 60 h. In contrast to MPB, bacteria represented 39.5% of sediment biomass but accounted for only up to 27.3% of assimilated N-15. Foraminifera accumulated and stored N-15 more than bacteria; their contribution to the N-15 remaining in 0-2 cm sediment at the end of the study was more than double their biomass contribution. Thirty-three days after the N-15 was assimilated by MPB 27% remained in the sediment, 16.5% had been effluxed as NO3-, 20.8% had been effluxed as NH4+, 20.7% had been effluxed as N-2 and 15.1% was missing. Most (12.6%) of N-15 label that was missing at the end of the study was probably lost as dissolved organic N (DON) fluxes. Of the N-15 remaining in 0-2 cm sediment, 80.4% was in MPB, 2.7% in bacteria, 1% in foraminifera and the remaining 15.9% was uncharacterized. Overall there was little benthic trophic transfer with most of the MPB-assimilated N remineralized over 33 d.