COASTAL EUTROPHICATION AND HYPOXIA UNDER FOCUS: REDOX DEPENDENT BENTHIC NUTRIENT FLUXES ACROSS SEA BOUNDARIES IN THE NORTHEASTERN MEDITERRANEAN AND MARMARA SEA

2022-2-8
Akçay, İsmail
Seafloor biogeochemistry can dramatically shift in response to deoxygenation and eutrophication-driven organic carbon production. In such conditions, biogeochemical cycling of key nutrients can be highly coupled to oxygen concentrations and metal redox cycles, often leading to the increase in dissolved nutrients and metals concentrations in the deep water column. In this thesis, such biogeochemical feedbacks have been studied for the first time for the Turkish Seas. Porewater and solid-state biogeochemistry were determined in the Northeastern (NE) Mediterranean and Marmara Sea, displaying distinct biogeochemical properties, between March 2018 and July 2019. Sediment porewater diffusive nutrient fluxes were calculated from the obtained core samples in these distinct marine environments. The study results indicated that contribution of porewater diffusive nutrient fluxes to the total nutrient fluxes from sea boundaries surrounding the NE Mediterranean Sea was calculated as 5.8% for phosphate, 12% for nitrate, 19% for ammonium and 22% for reactive silicate, respectively. Porewater nitrate, sulfate and hydrogen sulfide profiles obtained in the Marmara Sea showed that organic matter degradation processes in the upper 20-30 cmbs have occurred by oxic respiration, denitrification and sulfate reduction whilst organic matter decomposition was limited by oxic respiration in the upper sedimentary column in the southern Marmara Sea. The distribution of porewater sulfate, hydrogen sulfide, and major elements throughout the sediment cores obtained specifically from the İzmit Bay, suggested principal biogeochemical and early diagenetic processes such as anaerobic oxidation of methane (AOM), carbonate precipitation, iron reduction, iron sulfide precipitation and low-temperature silicate diagenesis. Furthermore, the biogeochemical cycling of sedimentary phosphorus (P) was studied in the NE Mediterranean, Marmara and Black Seas and the pool of “potentially mobile P” was determined for the studied sites. The study results showed that porewater and sediment biogeochemistry displayed remarkable spatial variability in the studied sites with the maximum concentrations of porewater phosphate, ammonium, reactive silicate, surface sediment organic carbon, nitrogen, phosphorus and total phosphorus measured in the hypoxic Marmara Sea and suboxic/anoxic Black Sea. The decline in the total phosphorus concentrations of all sediment core samples indicated P-mobilization to the overlying water. The pool of “potentially mobile P” varied between 0.023 and 0.148 mol/m2 in the studied sites with the minimum values recorded in the marine environments with highly oxygenated deep waters. The correlation between calculated diffusive nutrient fluxes and the deep waters dissolved oxygen concentrations indicated the redox-dependency on the benthic nutrient dynamics in the NE Mediterranean and Marmara Sea as the lower deep water dissolved oxygen concentration resulted in higher porewater diffusive phosphate, ammonium and reactive Si fluxes. For the ecosystem health of NE Mediterranean and recently deoxygenating Marmara Sea, studying redox dependent benthic nutrient dynamics and organic matter geochemistry are of critical importance to attain Good Environmental Status (GES) for these distinct marine basins.

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Citation Formats
İ. Akçay, “COASTAL EUTROPHICATION AND HYPOXIA UNDER FOCUS: REDOX DEPENDENT BENTHIC NUTRIENT FLUXES ACROSS SEA BOUNDARIES IN THE NORTHEASTERN MEDITERRANEAN AND MARMARA SEA,” Ph.D. - Doctoral Program, Middle East Technical University, 2022.