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

The outer Western Crimean Shelf of the Black Sea is a natural laboratory to investigate e_ects of stable oxic vs. varying hypoxic conditions on seafloor biogeochemical processes and benthic community structure. Bottom water oxygen concentrations varied between normoxic (175 µmolO₂L^-1) and hypoxic (<63 µmolO₂ 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 134m water depth. Total oxygen uptake, including di_usive as well as fauna-mediated oxygen consumption, decreased from >15 mmolm^-2 d^-1 in the oxic zone to <9 mmolm^-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 mmolm^-2 d^-1, but declined to 1.3 mmolm^-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. [ABSTRACT FROM AUTHOR]