Benthic community structure near the margin of the oxic zone: A case study on the Black Sea

The impact of ocean deoxygenation on marine ecosystems has recently been recognized as a global problem. The hypoxic layer, a notorious feature of the Black Sea, covers the entire basin at intermediate depths. To understand consequences of the rise of hypoxia in the sea environment, identifying factors that determine the distribution of biological communities at the lower boundary of the oxic zone is important. Continuous measurements of the oxygen profiles throughout the water column and two benthic surveys were conducted in the northeastern Black Sea in 2019–2020. Based on these data and analysis of the oceanographic data archive (1990–2010), two community types that inhabit lower boundaries of the oxic zone were identified: a species-depleted deep-water community (82–117 m) and ephemeral communities in the constant hypoxia zone (128–144 m). Hypoxic event duration was identified as the main environmental factor determining the low diversity of the fauna and benthic community structure in general. The main factor determining the vertical displacement of the multicellular benthos boundary was the duration of the anoxic period. In 2019–2020, this boundary was positioned at a depth of approximately 150 m, which was comparable to the observations from the 1920s and 1930s.