Impacts of recent eutrophication and deoxygenation on the sediment biogeochemistry in the Sea of Marmara

Abstract. The biogeochemistry of seafloor sediments can be significantly altered in response to deoxygenation and eutrophication-driven organic carbon production, resulting in increased benthic fluxes of dissolved nutrients (such as ammonia and phosphate) and metals. The Sea of Marmara, which also have faced large-scale mucilage outbreaks in recent years, is undergoing severe eutrophication and deoxygenation but the consequences on sediment biogeochemistry and benthic feedback have not been studied so far. This study aims to understand the impacts of deoxygenation and coastal eutrophication on sedimentary biogeochemical processes in the Marmara Sea, which experiences varying degrees of anthropogenic pressure along with natural inputs from the adjacent Black Sea via Bosphorus surface inflows. Multicore-obtained undisturbed sediment core samples indicate that oxic respiration no longer plays a significant role in Marmara sediments but denitrification, metal reduction and sulfate respiration are prevalent. The deep-water sediments become more reducing in the Eastern Marmara, for instance cores from İzmit Bay reveal important biogeochemical processes such as anaerobic oxidation of methane (AOM), carbonate precipitation, iron reduction, and low-temperature silicate diagenesis. Furthermore, sediment total organic carbon concentrations in core samples were observed to be nearly twice those in less productive sites with oxic bottom waters, indicating a refractive particulate organic matter fraction in the buried sediments. Calculated diffusive benthic nutrient fluxes show markedly high phosphate and ammonium fluxes into the near-bottom waters of highly eutrophic areas of the Eastern Marmara, which are expected to enhance primary production in the upper halocline during the dry season. On the other hand, these sediments are a sink for nitrate, indicating denitrification and removal of fixed nitrogen. This trend contributes to the accumulation of organic matter as well as shifting N/P ratios and the development of a steep hypoxic zone at halocline boundary depths. As a result, we show that sediments are already influenced by the widespread hypoxia in the Sea of Marmara and benthic-pelagic coupling have started exacerbated the existing eutrophication problem, analogous to the benthic ‘vicious cycle’ observed in the shallower Baltic Sea. We conclude that the Sea of Marmara Sea is now on a clear path towards being included within the list of famous ‘dead zones’ of the Earth oceans, such as the Baltic Sea, Gulf of Mexico or Chesapeake Bay. In order to reverse the ecosystem degradation complex predictive models and decision support tools are needed to guide decision-makers, but they should take into account hypoxia-induced benthic biogeochemical processes and benthic-pelagic coupled cycling of nutrients in the Sea of Marmara.