STUDY ON WATER QUALITY IMPROVEMENT BY BIOMANIPULATION

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(1) Influence of fish on dominance of blue-green algaeExperiments 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 for 13 months 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 P0_4-P concentrations decreased to less than 10 ug V^-^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 P0_4-P in pond water due to feces of fish and their growth, the condition of which is advantageous for blue-green algae than others. In addition, the decomposition processes of chlorophyll a were different between in fish ponds and in fishless ponds.(2) In-situ measurement of productivity and respiration in aquatic systemsChanges in productivity and community respiration rates were determined hourly in Lake Himuna and Hiroshima Bay for several days, using a newly developed box incubation and free water method. The design of the box included light and dark box, with simultaneous measurement of temperature, DO and pH of the water in light and dark boxes in which the water was exchanged with outside water at regular intervals. This box method is a capable one for evaluating gross and net production, and respiration in a wide variety of aquatics.(3) Study on water environment in irrigation ponds with or without macrophyteBiomass of macrophyte, which was determined by lake dimensions (surface area and depth), influenced greatly the ratio of phytoplankton to nutrient, the seasonal change pattern of organic pollution and sediments.