Description of the water quality conditions in a semi-intensively cultured marine fish pond in Eilat, Israel

Monthly sampling showed typically a large diurnal trend in dissolved oxygen and temperature with a slight trend also in dissolved inorganic nitrogen but not in phosphate or silica. There is a high uptake of all these nutrients in the pond as shown by the difference between the input water and pond water. Temperature in the pond decreased to a minimum of 15°C in February and then increased to 33°C in July. In the autumn there was relatively high primary productivity (PP) supported mainly by nutrients in the inflow. PP decreased to a minimum in winter, probably due to decreasing temperature and light, then increased to a summer maximum together with planktonic biomass, and diurnal oxygen variation. The increase was due mainly to increased temperature and nutrient supply from food input. These data were used to explain the incidence of fish mortalities mainly in mid-summer. The importance of frequency of sampling in such a dynamic system is discussed.