Effect of settled diatom-aggregates on benthic nitrogen cycling

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Abstract

The marine sediment hosts a mosaic of microhabitats. Recently it has been demonstrated that the settlement of phycodetrital aggregates can induce local changes in the benthic O 2 distribution due to confined enrichment of organic material and alteration of the diffusional transport. Here, we show how this microscale O 2 shift substantially affects benthic nitrogen cycling. In sediment incubations, the settlement of diatom-aggregates markedly enhanced benthic O 2 and NO - 3 consumption and stimulated NO - 2 and NH + 4 production. Oxygen microprofiles revealed the rapid development of anoxic niches within and underneath the aggregates. During 120 h following the settling of the aggregates, denitrification of NO - 3 from the overlying water increased from 13.5 μmol m −2 h −1 to 24.3 μmol m −2 h −1, as quantified by 15N enrichment experiment. Simultaneously, N 2 production from coupled nitrification-denitrification decreased from 33.4 μmol m −2 h −1 to 25.9 μmol m −2 h −1, probably due to temporary inhibition of the benthic nitrifying community. The two effects were of similar magnitude and left the total N 2 production almost unaltered. At the aggregate surface, nitrification was, conversely, very efficient in oxidizing NH + 4 liberated by mineralization of the aggregates. The produced NO - 3 was preferentially released into the overlying water and only a minor fraction contributed to denitrification activity. Overall, our data indicate that the abrupt change in O 2 microdistribution caused by aggregates stimulates denitrification of NO - 3 from the overlying water, and loosens the coupling between benthic nitrification and denitrification both in time and space. The study contributes to expanding the conceptual and quantitative understanding of how nitrogen cycling is regulated in dynamic benthic environments.

Original languageEnglish
JournalLimnology and Oceanography
Volume63
Issue number1
Pages (from-to)431-444
ISSN0024-3590
DOIs
Publication statusPublished - Jan 2018

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