Fate of intertidal microphytobenthos nitrogen under enhanced nutrient availability: Evidence for reduced nitrogen retention revealed through 15N-labeling

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Authors

Philip Riekenberg, Joanne Oakes, Bradley Eyre

Abstract

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.

DOI

https://doi.org/10.31223/osf.io/j4kde

Subjects

Biogeochemistry, Earth Sciences, Physical Sciences and Mathematics

Keywords

15N, amino acid, denitrification, flood, intertidal, pulse-chase

Dates

Published: 2020-05-25 21:17

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License

CC BY Attribution 4.0 International

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