Daily oxygen variations in marine fish ponds, Elat, Israel

A computerized data acquisition system (DAS) was deployed in marine fish ponds at Elat, Israel. The DAS recorded dissolved oxygen, pH and temperature in two ponds and in the water inlet to ponds. Wind velocity and solar irradiation were also recorded. During phytoplankton bloom conditions, wide daily variations were observed in dissolved oxygen (between 3 and 16 mg O₂l^-1) and in pH (between 8.2 and 8.8). During phytoplankton crash conditions, the amplitude of these variations decreased and the ponds approached anoxic conditions (∼2 mg O₂l^-1). Operation of a mechanical paddle wheel during crash conditions overrode the metabolically induced fluctuations and brought the oxygen levels close to saturation with the atmosphere. Using the rate of oxygen decrease during the night, community respiration and oxygen exchange rate with the atmosphere were calculated. Respiration at the beginning of the night was higher by a factor of 4 compared to respiration at the end of the night. Primary productivity for the entire pond community was calculated based on the oxygen rate of change during the day, and compared to standard BOD incubations. This comparison suggests that 65% of the primary productivity was by the phytoplankton and 35% by the benthonic algae and the macroalgae. Based on this study we compare the difference between seawater and freshwater pond systems and consider the implications for aquaculture of phytoplankton dynamics, benthonic algae and macroalgae, paddle wheel operation, and computerized data acquisition.