Articles | Volume 12, issue 16
Biogeosciences, 12, 5075–5092, 2015
Biogeosciences, 12, 5075–5092, 2015

Research article 27 Aug 2015

Research article | 27 Aug 2015

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

A. Lichtschlag1,a, D. Donis1,b, F. Janssen1,2, G. L. Jessen1, M. Holtappels1,3, F. Wenzhöfer1,2, S. Mazlumyan4,c, N. Sergeeva4,d, C. Waldmann3, and A. Boetius1,2 A. Lichtschlag et al.
  • 1Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany
  • 2HGF MPG Joint Research Group for Deep Sea Ecology and Technology, Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen, 27515 Bremerhaven, Germany
  • 3MARUM – Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, 28359 Bremen, Germany
  • 4A.O. Kovalevsky Institute of Biology of Southern Seas, 2 Nakhimov ave., Sevastopol, 299011, Crimea
  • acurrent address: National Oceanography Center, University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH, UK
  • bcurrent address: F.-A. Forel Institute, University of Geneva, Batelle, Bat. D, 7 Route de Drize, 1227 Carouge, Geneva, Switzerland
  • ccurrent address: Institute of Natural & Technical Systems, Russian Academy of Sciences, Lenin St. 28, Sevastopol, 299011, Crimea
  • dcurrent address: The A.O. Kovalevsky Institute of Marine Biological Research of RAS, Leninsky Ave., 32, 119000, Moscow, Russia

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.

Final-revised paper