Effects of fluctuating hypoxia on benthic oxygen consumption in the Black Sea (Crimean Shelf)


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alic [ at ] nerc.ac.uk

Abstract

The outer Western Crimean Shelf of the Black Sea is a natural laboratory to investigate effects of stable oxic vs. varying hypoxic conditions on seafloor biogeochemical processes and benthic community structure. Bottom water oxygen concentrations varied between normoxic (175 μmol O2 L−1) and hypoxic (< 63 μmol O2 L−1) or even anoxic/sulfidic conditions within a few kilometres distance. Variations in oxygen concentrations between 160 and 10 μmol L−1 even occurred within hours close to the chemocline at 134 m water depth. Total oxygen uptake, including diffusive as well as fauna-mediated oxygen consumption, decreased from > 15 mmol m−2 d−1 in the oxic zone to < 9 mmol m−2 d−1 in the hypoxic zone, correlating with changes in macrobenthos composition. Benthic diffusive oxygen uptake rates, comprising microbial respiration plus reoxidation of inorganic products, were around 4.5 mmol m−2 d−1, but declined to 1.3 mmol m−2 d−1 at oxygen concentrations below 20 μmol L−1. Measurements and modelling of pore water profiles indicated that reoxidation of reduced compounds played only a minor role in the diffusive oxygen uptake, leaving the major fraction to aerobic degradation of organic carbon. Remineralization efficiency decreased from 100% in the oxic zone, to 50% in the oxic-hypoxic, to 10% in the hypoxic-anoxic zone. Overall the faunal remineralization rate was more important, but also more influenced by fluctuating oxygen concentrations than microbial and geochemical oxidation processes.



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Eprint ID
37968
DOI 10.5194/bgd-12-6445-2015

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Lichtschlag, A. , Donis, D. , Janssen, F. , Jessen, G. L. , Holtappels, M. , Wenzhöfer, F. , Mazlumyan, S. , Sergeeva, N. , Waldmann, C. and Boetius, A. (2015): Effects of fluctuating hypoxia on benthic oxygen consumption in the Black Sea (Crimean Shelf) , Biogeosciences Discussions, 12 , pp. 6445-6488 . doi: 10.5194/bgd-12-6445-2015


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