Abstract
Abstract. The outer western Crimean shelf of the Black Sea is a natural laboratory to investigate effects of stable oxic versus varying hypoxic conditions on seafloor biogeochemical processes and benthic community structure. Bottom-water oxygen concentrations ranged from normoxic (175 μmol O2 L−1) and hypoxic (< 63 μmol O2 L−1) or even anoxic/sulfidic conditions within a few kilometers' 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 on average in the oxic zone, to 7 mmol m−2 d−1 on average in the hypoxic zone, correlating with changes in macrobenthos composition. Benthic diffusive oxygen uptake rates, comprising respiration of microorganisms and small meiofauna, were similar in oxic and hypoxic zones (on average 4.5 mmol m−2 d−1), but declined to 1.3 mmol m−2 d−1 in bottom waters with oxygen concentrations below 20 μmol L−1. Measurements and modeling of porewater profiles indicated that reoxidation of reduced compounds played only a minor role in diffusive oxygen uptake under the different oxygen conditions, leaving the major fraction to aerobic degradation of organic carbon. Remineralization efficiency decreased from nearly 100 % in the oxic zone, to 50 % in the oxic–hypoxic zone, 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.
Highlights
Hypoxia describes a state of aquatic ecosystems in which low oxygen concentrations affect the physiology, composition and abundance of fauna, altering ecosystem functions including biogeochemical processes and sediment– water exchange rates (Middelburg and Levin, 2009)
To estimate the in situ Total oxygen uptake (TOU) / diffusive oxygen uptake (DOU) ratio for the hypoxic–anoxic zone, in this case we modeled the DOU at these specific conditions based on the volumetric rate and the diffusive boundary layer (DBL) thickness determined by the in situ microsensor profile
With a respiratory quotient of 1 (i.e., 1 mole of oxygen consumed per 1 mole of CO2 produced; Canfield et al, 1993a), the average TOU observed in the oxic zone would be sufficient to remineralize most of the organic carbon input to the seafloor (Table 2), with oxygen fluxes measured in this study as being similar to those previously reported from the same area (Table 4, including references; Grégoire and Friedrich, 2004)
Summary
Hypoxia describes a state of aquatic ecosystems in which low oxygen concentrations affect the physiology, composition and abundance of fauna, altering ecosystem functions including biogeochemical processes and sediment– water exchange rates (Middelburg and Levin, 2009). Sediments underlying a lowoxygen water column often show oxygen penetration depths of only a few millimeters (Archer and Devol, 1992; Glud et al, 2003; Rasmussen and Jørgensen, 1992) This increases the contribution of anaerobic microbial metabolism to organic matter remineralization at the expense of aerobic degradation by microbes and fauna, as reported from the Romanian shelf area of the Black Sea (Thamdrup et al, 2000; Weber et al, 2001), the Neuse River estuary (Baird et al, 2004) and the Kattegat (Pearson and Rosenberg, 1992). These and other studies have indicated, that the degree of oxygenation plays an important role in oxygen uptake, and the frequency and persistency of the low oxygen conditions can shape faunal activity, biogeochemical processes and the functioning of the ecosystem as a whole (Boesch and Rabalais, 1991; Diaz, 2001; Friedrich et al, 2014)
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