Diagenesis of Organic Matter in Eelgrass (Zostera marina L.) Vegetated Sediments

In February and July 2014, multidisciplinary geochemical studies of the sediments were carried out at two stations. One of them was located in Voevoda Bight, which has a bottom depth of 4 m, covered by eelgrass (Zostera marina L.); the other one served as a background station, because it is devoid of seagrass and is located in the northern part of Amur Bay (near Rechnoj Island). This station has a bottom depth of 6 m. The chemical composition of sediment pore water was studied along the depth of the core (80 cm) for the following: concentrations of dissolved organic carbon, carbohydrates, humic substance, nutrients (inorganic phosphorus, silicon, ammonium), and the parameters of the carbonate system (pH, total alkalinity—TA, dissolved inorganic carbon—DIC, CO2 partial pressure—pCO2). Contents of organic carbon (OC), fulvic acids, humic acids, and water were measured in the sediment solid phase. It was established that that the OC concentration was more than 5 and 2% in Voevoda Bight and at the background station, respectively. The sum of the fulvic and humic acids was about 40% of the OC content. It was found that the OC content decreased by 50% within 40 cm of the top layer of the sediments in Voevoda Bight. Within the same layer, a sharply increased nutrient concentration, TA, and DIC and decreased pH and sulfate ion concentrations were observed, which were the opposite of the background station. Seasonal variability of the observed parameters was weak, except for the significant decrease in pH and increase in pCO2 observed in summer due to temperature effect on the constants of the carbonate system. The chemical composition of pore water suggests that sulfate reduction was an important process. However, it cannot explain the sharp decrease in OC; therefore, it was suggested that OC is transformed into infauna biomass by the food chain.