Some aspects of the limnology of a small tropical impoundment and an assessment of two techniques for managing water quality, with special reference to the growth of cyanobacteria

Physical, chemical and biological aspects of the limnology of the Solomon Dam, a small tropical reservoir on an island off the North Queensland coast, were examined regularly over a four year period. Thermal stratification with pronounced chemical differentiation of the epilimnion and hypolimnion was a dominant feature of the reservoir. There was a single annual cold season circulation in June-July, but the catchment character and seasonal rainfall often resulted in extensive flushing and mixing of the reservoir during the monsoon period of January to March. Periods of strong stratification were characterised by an anoxic hypolimnion and subsurface blooms of cyanobacteria, which, when accompanied by extensive drawdown, markedly affected water quality. Circulation usually improved water quality, through oxidation and sedimentation of reduced compounds from the anoxic hypolimnion and redistribution of algal biomass throughout the water column. The composition of the phytoplankton was also related to the stratification characteristics of the reservoir. Prolonged stratification increased the sedimentation of diatoms, and promoted the development of buoyant cyanobacterial species in the shallow epilimnion, and photosynthetic bacteria below the chemocline. The dominant cyanobacterium, Cylindrospermopsis raciborskit (shown to be toxic in mouse bioassay tests), formed blooms in the late "dry" season, when chlorophyll concentrations reached 90 mg/m³ (>200 mg/m). Large nutrient pulses, generally associated with disruption of the hypolimnion by turbulent mixing, often favoured the development of flagellate algae, particularly Trachelomonas. An algicide (copper sulphate) and artificial aeration were tested separately as water quality control techniques, in trials during the final two years of the study, 1983-84. The hydraulic properties of the storage and catchment mitigated against the effectiveness of copper treatment in regulating water quality. Artificial aeration maintained a completely oxygenated water column and prevented the transfer of iron and manganese from the sediments into the waterbody. Homogenisation of the water column damped the previously observed fluctuations in a range of biological and chemical characteristics of the reservoir. In particular the high biomass concentrations characteristic of algal blooms were ameliorated by circulation and dilution throughout the water column, although, there was a considerable increase in the annual mean standing crop, attributable to greater nutrient availability resulting from the increase in mixed depth. Consistent mixing maintained a diatom (Synedra ulna) dominated flora, which had previously been restricted by stratification. Diatom abundance eventually became regulated by silica availability and the seasonal decline in the ratio of mixed depth to photic depth favoured their replacement by Chlorophyceae. Periodicity of the zooplankton was affected by flushing and by the thermal tolerance of key species. Artificial aeration through its suppression of cyanobacterial growth, probably contributed to the increased abundance and diversity of the cladoceran fauna in 1983-84