Spatial and daily variability of oxygen balance and chlorophyll content in the Bay Sivash ecosystem, the world’s largest hypersaline lagoon

Oxygen concentration, salinity, temperature, total suspended solids, chlorophyll a content as well as wind direction and speed were measured in 13 sites of Bay Sivash, the largest hypersaline water body in the world, in August 2021. The oxygen concentration and oxygen saturation of water increased significantly after sunrise between 6.30 and 14.00 h, from 1.2–1.3 mgO L−1 to 5.0–6.0 mgO L−1 and from 25%–28% to 112%–142%, respectively. Between about 11 am and 4 pm, the flow of oxygen was from the lagoon to the atmosphere, averaging 0.1 mgO L−1 h−1. The rest of the time, the flow of oxygen was from the atmosphere to the lagoon, on average 0.13 mgO L−1 h−1. There was a significant negative correlation between salinity and oxygen concentration in the lagoon. Chlorophyll a concentration varied significantly during the daytime negatively depending on the oxygen concentration and temperature. The primary production of phytoplankton averaged 5–7 mgC L−1 h−1. New data indicate that in summer at night, anoxic conditions also can form in places where there are no algal mats.