The effects of re-oxygenation of central Baltic Sea sediment; a laboratory study

The eutrophication of the deep central Baltic Sea, Baltic Proper, has caused deoxygenation severe consequences for benthic life. To overcome such limitations and spreads of anoxic basins, proposed techniques such as re-oxygenation of anoxic bottom waters are proposed. However, in the case of the Baltic Proper, the effects of oxygenation in the short term are unknown. Therefore, this research focuses on understanding the geochemistry changes in bottom waters' and sediments during the transition from anoxic/hypoxic to oxic conditions. Six sediment cores were retrieved from the northern and southern Baltic Proper (triplicate cores for each location), where bottom waters have dissolved O₂ concentrations of ≈0 mg/L. The bottom waters were exchanged with oxygenated Baltic Sea surface waters for 96 hours. The pH, electroconductivity (EC), and metal concentrations in the exchanged water were measured over time in 12 and 24 hours intervals in the experiment cores. The results indicate that the pH of both sites didn't show any significant change in the exchanged bottom waters during the experiment. However, the EC of the bottom waters on average reduced from 15 to 9 µs/m in both sites. In terms of soluble metals/metalloids, As, Ba, Co, Mn, Mo, Rb, Sr, and U were detected in higher concentrations than unoxygenated bottom water from southern Baltic Proper. Manganese and Sr showed the highest released concentrations in both sites in terms of concentration. This study indicates that undesired release of metals from sediment to the water column may occur during re-oxygenation of Baltic Sea bottom waters. The next stage of this research will focus on the metal transfer from the surface sediments to the bottom waters over the 4-day experiments. This will be done via a sequential chemical analysis scheme, in which surface sediment samples from the experiment will be compared to reference cores collected at the same time as the experimental cores were collected.