Philip Riekenberg, Joanne Oakes, Bradley Eyre

Sediment microbial communities are an important sink for both organic and inorganic nitrogen (N), with microphytobenthos (MPB) biomass having the largest contribution to short-term N-assimilation and retention. Coastal waters are increasingly subject to anthropogenic nutrient enrichment, but the effect of this nutrient enrichment on microbial assimilation, processing, and fate of MPB-derived N (MPB-N) remains poorly characterized. In this study, an MPB community was labeled in situ with a pulse of 15NH4+-N. Laboratory core incubations of this labeled sediment under different nutrient concentrations (NH4+ and PO43-: ambient, 2× ambient, 5× ambient, and 10× ambient) were used to investigate changes in the processing and flux pathways of the 15N-labeled MPB-N across 10.5 d under nutrient enrichment. Initial production of MPB-N was stimulated by nutrient addition, with higher 15N incorporation into MPB in the nutrient amended treatments (71-93%) than in the ambient treatment (38%). After 10.5 d, the nutrient amended treatments had increased turnover of MPB-N out of MPB biomass into an uncharacterized pool of sediment ON (45-75%). Increased turnover of MPB-N likely resulted from a decoupling between EPS production and bacterial remineralization as inorganic nutrients were preferentially used as an N source. This decoupling decreased the efflux of MPB-N via DON in the amended (3.9-5.2%) versus the ambient treatment (10.9%). Exports of MPB-N to the water column were relatively small, accounting for a maximum of 14% of 15N exported from the sediment, and were dominated by export of DON and N2 (denitrification). Overall, there was considerable retention of MPB-N over 10.5 d, but increased nutrient loading shifted N from MPB biomass into other sediment ON.