Macrobenthos recovering from eutrophication at the scale of the northwestern continental shelf of the Black Sea

The northwestern continental shelf of the Black Sea has been affected by severe eutrophication and bottom hypoxia since the seventies due to an increase of anthropogenic activities. As a direct consequence, macrozoobenthos has suffered well-established decline in terms of biomass and diversity with modification in the benthic communities assemblage. In this study, we compiled macrozoobenthos data for 138 stations over the shelf from 1995 to 2017 within the framework of the EROS-2000 (1995), SESAME (2008), HYPOX (2010-2011) and EMBLAS (2016-2017) projects. Environmental data are also computed from a coupled physical biogeochemical model. A comparison is made between the 1995 and 2008-2017 period to evidence a potential recovery of the benthic communities in terms of species assemblage and extension in the hypoxic area. The 2008-2017 period is characterized by a significantly higher taxonomic richness compared to the 1995 period. Taxonomic richness is positively correlated with the dissolved oxygen concentration and the extent of the bottom hypoxic area at the scale of the shelf. There is a reduction in bottom hypoxia from 1995 to 2008-2017 due to the decrease of nutrient loads from the rivers and its subsequent eutrophication. A sharp decrease in the total share of the class Polychaeta is observed between both periods and an increase in the total share of epibenthic species (i.e. molluscs, crustaceans) is noticed for the 2008-2017 period. On the path to recovery, decrease of the class Polychaeta can be explained by a reduction in anthropogenic stress on the Black Sea ecosystem leading to a reduction of opportunistic deposit-feeder (oligochaetes and some species of polychaetes) simultaneously with the increase of sensitive epibenthic species. However, development of invasive species and persistence of opportunistic species tend to mitigate the recovery tendency.