Daily Rhythms and Oxygen Balance in the Hypersaline Lake Moynaki (Crimea)

Field observations of the diurnal behavior of several parameters (oxygen concentration, chlorophyll fluorescence, photosynthetically active radiation (PAR), wind speed, temperature, suspended matter concentration, and zooplankton abundance) were conducted at three sites in the marine hypersaline lake Moynaki (Crimea). The diurnal course of PAR followed a bell-shaped form, with the maximum at 14:00 on the 15th and 16th of September 2021. The oxygen concentration varied over a wide range from 3.2 to 9.3 mg L⁻¹, demonstrating a clear diurnal rhythm. From sunrise until about 17:30, it increased. Both the maximum and minimum values were marked on the site where there were Ruppia thickets. The daily rhythm of Chlorophyll a concentration was clearly expressed during the observation period, varying from 2.49 to 18.65 µg L⁻¹. A gradual increase in the concentration of chlorophyll a began after 10:00 and lasted until about 2:30–3:00 of the next day. The daily production of oxygen during photosynthesis averaged 27.3 mgO L⁻¹ day⁻¹, and the highest values were noted at the windward site of 37.9 mgO L⁻¹ day⁻¹, and the lowest at the leeward site of 19 mgO L⁻¹ day⁻¹. The total respiration of the community per day was, on average, 15.9 mgO L⁻¹ day⁻¹. It averaged 63% of the primary production created. The contribution of animals to the total oxygen consumption of the community was small, averaging 5%.