Lake restoration: capabilities and needs

Abstract

The use of iron salts in the control of eutrophication of surface water reservoirs can only be successful if there are no environmental effects. We examined the acute and chronic effects upon Daphnia longispina, the dominant member of the zooplankton. Laboratory tests established that, although dissolved iron had no measurable effect, the presence of particulate iron caused mortalities in acute tests (48 hours) and reduced number of broods per female and numbers of young hatching in chronic tests (21 days). At 11.48 mg Fe 1⁻¹, 50% mortalities occurred in the population, whilst 4.49 mg Fe 1⁻¹ caused reductions in reproduction and increased mortality rates over 21 day tests. In inert china clay, similar results were observed, suggesting that it was the particulate nature of the ferric sulphate that caused the effects, rather than any toxic properties. A safe level of 1.69 mg Fe 1⁻¹ was calculated. The area of the filter of the third thoracic limb of the daphnids, from both the laboratory cultures and the south arm of Rutland Water (which received ferric-dosed inlet water) increased in size in the presence of ferric sulphate and china clay in chronic tests. This is a response to low food concentrations. Abnormal feeding behaviour was also observed in ferric sulphate. Thoracic beat rate, decreased in ferric sulphate but was unaffected 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 the food groove increased in both ferric sulphate and china clay, in response to large clumps of these substances collecting there.