Control of Species Composition in Enriched Mass Cultures of Natural Phytoplankton Populations

The changes in species composition of intensively harvested outdoor cultures of enriched natural populations of micro-algae were studied using a variety of nutrient additions and dilution rates. The control of species in such semi-continuous outdoor cultures is not only important for aquaculture systems but is also helpful in identifying the effects of eutrophication on coastal phytoplankton populations. In a series of three experiments in which cells were counted and identified during the sixth to tenth day of the daily 50% harvest, N, P and Si definitely favoured the growth of diatoms. The addition of just N and P or N, P and trace metals stimulated a green micro-algae population. Addition of treated sewage effluent resulted in a 70-80% diatom culture. In another series of experiments the rate of daily dilution was varied and in two experiments in June a clear control of species composition resulted. In 10% daily dilution green micro-algae dominated while at 75% dilution diatoms dominated particularly Skeletonema costatum (now Stephanopyxis costata). The results from experiments in July and August were not as definitive as those in June and the ubiquitous diatom Cylindrotheca closterium prevailed at all dilution rates.