Effects of fertiliser and crayfish on plankton and nutrient dynamics in hardwater ponds

Although phosphorus fertilisation can improve productivity in most freshwater ponds, phosphate may become limiting in extremely hard water due to rapid precipitation with calcium. Hence we studied the characteristics of plankton and nutrient dynamics in water containing >400 mg CaCO₃ l^–1in pond and microcosm systems. The field experiment was conducted in eight earthen ponds involving two nutrient ratios (N:P = 1:1 and 20:1) with or without crayfish. Fertilisation significantly increased concentrations of NO₂–N and NO₃–N, but soluble reactive phosphorus was depleted to the level prior to fertilisation within 24 h. The laboratory test showed that after 6 h of fertilisation, 45% phosphorus was precipitated by calcium, 30% phosphorus was assimilated by phytoplankton and only 25% phosphorus remained in water column. The phytoplankton abundance in hardwater ponds was regulated by the abundance of zooplankton population rather than by either crayfish or fertilisation. The presence of crayfish only increased the concentration of total phosphorus. This study suggests that when phytoplankton production is required in crayfish ponds the maintenance of phytoplankton abundance will depend on the effective control of zooplankton rather than fertilisation. Due to the rapid precipitation of phosphorus by calcium in hard water ponds, more frequent phosphorus fertilisation is needed to enhance primary productivity.