Influence of Water Exchange Frequency on Phytoplankton Succession in Inland Fish Culture Ponds

The phytoplankton community is the base of productivity in fish culture ponds, but the community structure keeps changing constantly because of the increase in nutrient concentration, and sometimes such changes may become unfavorable, with a tendency towards the prevalence of cyanobacteria in water bodies, which may affect the health of fish. Water exchange is one of the most common methods for the control of the process, but it is always carried out without any planned frequency. In this study, the influence of four different frequencies of water exchange – non-exchange, once every 14, 7, and 3 days (20% of pond volume)- on phytoplankton succession in twelve earthen carp ponds during 120 days of culture was assessed. Higher exchange reduced ammonia, phosphorus, and chlorophyll-a, improved dissolved oxygen and transparency, and stabilized pH. The total phytoplankton abundance decreased from 18.6 × 10⁶ cells L⁻¹ in non-exchange to 5.7 × 10⁶ cells L⁻¹ when exchanged at a 3-day interval, and the Shannon index increased from 1.84 to 3.02. Cyanobacteria formed over 50% of all cells after about day 38 in the absence of exchange, after day 62 in the case of two-weekly exchange, and transiently just before day 87 in the case of one-weekly exchange, and not at all in the case of three-day exchange. A cyclic scheme for the relationship between exchange, dilution, environment, community structure and pond management is suggested.