High-resolution in situ oxygen microprofiles, porewater and solid phase geochemistry from the Crimean shelf (Black Sea) from Maria S. Merian cruise MSM15/1

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) and hypoxic (< 63 µmol O2/L) or even anoxic/sulfidic conditions within a few kilometers' distance. Variations in oxygen concentrations between 160 and 10 µmol/L 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 on average in the oxic zone, to 7 mmol/m**2/d 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), but declined to 1.3 mmol/m**2/d in bottom waters with oxygen concentrations below 20 µmol/L. 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.

黑海克里米亚西部外陆架是探究稳定有氧与多变低氧环境对海底生物地球化学过程及底栖群落结构影响的天然实验室。该区域内，数公里范围内的底层水氧浓度可覆盖常氧（175 μmol O₂/L）、低氧（<63 μmol O₂/L）甚至缺氧/硫化环境；在水深134米的化学跃层（chemocline）附近，氧浓度甚至可在数小时内于160 μmol O₂/L至10 μmol O₂/L之间波动。总耗氧量涵盖扩散耗氧与生物介导的耗氧过程，从有氧区的平均15毫摩尔每平方米每日降至低氧区的平均7毫摩尔每平方米每日，且与大型底栖生物（macrobenthos）群落组成的变化相关。底栖扩散耗氧速率，即微生物与小型底栖生物（meiofauna）的呼吸作用速率，在有氧区与低氧区内基本持平（平均4.5毫摩尔每平方米每日），但当底层水氧浓度低于20 μmol O₂/L时，该速率会降至1.3毫摩尔每平方米每日。孔隙水剖面（porewater profiles）的测量与模拟结果表明，在不同氧环境下，还原化合物的再氧化对扩散耗氧的贡献仅占极小比例，大部分耗氧过程来自有机碳的好氧降解。再矿化效率从有氧区的近100%，依次降至有氧-低氧过渡区的50%、低氧-缺氧过渡区的10%。总体而言，相较于微生物与地球化学氧化过程，动物介导的再矿化速率不仅更为关键，同时也更易受氧浓度波动的影响。