Ecological and ecophysiological impacts of ferric dosing in reservoirs

The use of iron salts in the control of eutrophication of surface water res ervoirs can only be successful if there are no environmental effects. We ex amined the acute and chronic effects upon Daphnia longispina, the dominant member of the zooplankton. Laboratory tests established that, although diss olved iron had no measurable effect, the presence of particulate iron cause d mortalities in acute tests (48 hours) and reduced number of broods per fe male and numbers of young hatching in chronic tests (21 days). At 11.48 mg Fe l(-1), 50% mortalities occurred in the population, whilst 4.49 mg Fe l(- 1) caused reductions in reproduction and increased mortality rates over 21 day tests. In inert china clay, similar results were observed, suggesting t hat it was the particulate nature of the ferric sulphate that caused the ef fects, rather than any toxic properties. A safe level of 1.69 mg Fe l(-1) w as calculated. The area of the filter of the third thoracic limb of the dap hnids, from both the laboratory cultures and the south arm of Rutland Water (which received ferric-dosed inlet water) increased in size in the presenc e of ferric sulphate and china clay in chronic tests. This is a response to low food concentrations. Abnormal feeding behaviour was also observed in f erric sulphate. Thoracic beat rate, decreased in ferric sulphate but was un affected in china clay. This suggested that the feeding rate depression was a response to the chemical nature of the ferric sulphate, rather than its particulate nature. The number of times that particles were rejected from t he food groove increased in both ferric sulphate and china clay, in respons e to large clumps of these substances collecting there.