Phytoplankton dynamics in shrimp ponds

The productivity, community composition and dynamics of phytoplankton were examined in commercial shrimp = penaeid prawn] ponds in eastern Australia. Nutrient and light regimes were examined concurrently. Analysis of chlorophyll a, algal accessory pigments and bacterial numbers over a 6-month period showed considerable daily variation. Bacterial numbers and nutrient concentrations were not significantly different between the top and bottom of the water column nor between sites within a pond, but there were significant depth differences in chlorophyll a, and the diatom and cyanobacteria indicator pigments, fucoxanthin and zeaxanthin, and site differences in alkalinity and fucoxanthin. Statistical analysis of spatial and temporal variation was used to develop sampling strategies. Our results showed that to accurately reflect changes in phytoplankton throughout the grow-out season, chlorophyll a and the accessory pigments must be sampled for longer than five consecutive days. A suggested alternative to manual sampling is the use of automated loggers. Factors that might be limiting phytoplankton growth and affecting community composition were determined. Light appears to be limiting phytoplankton growth and it is suggested that a shallower pond depth and/or higher turbidity would increase algal productivity. Changes in nitrogen-to-phosphorus ratios and ammonia concentrations coincided with changes in phytoplankton community structure.