Abstract
Experiments using six outdoor experimental ponds equipped with a continuous flow-through system of the same influx nitrogen and phosphorus concentrations and with different fish (goldfish) densities were conducted from Aug., 1993 to Sep., 1994 to investigate the factors influencing the dominance of blue-green algae in summer. In general, blue-green algae predominated in fish ponds, in contrast to green algae in fishless ponds. Blue-green algae started to flourish after the latter half of August, when the PO4-P concentrations decreased to less than 10 μg·l-1. The difference in dominated phytoplankton between fish and fishless ponds could be explained by the reduction of total phosphorus and subsequently the depletion of PO4-P in pond water due to the feces of fish and their growth, the condition of which is advantageous for blue-green algae than others.
