Toxicological, behavioural and morphological studies on Daphnia longispina O.F. Muller in relation to ferric toxicity

Rutland Water in Leicestershire, UK, has been dosed with ferric sulphate for eutrophication control through phosphorus inactivation, since 1990. Iron concentrations between 1990 and 1994 were generally <0.5mg Fe 1-1 (maximum 17.5mg Fe 1-1 recorded). Examination of the long-term data (collected since 1980) showed that phophorus has declined in the water column since 1990. Iron and phosphorus have accumulated in the sediments around the pumped inlet through which iron was added. Algal biomass (measured by chlorophyll a) has declined since 1990 although cyanobacterial blooms have still occurred.;Laboratory studies established that growth of the Chlorophyte Chlorella vulgaris was inhibited at concentrations >50mg Fe 1-1 and cellular aggregation occurred at concentrations >150mg Fe 1-1. When the Cladoceran Daphnia longispina was exposed to concentrations >11mg Fe 1-1 over 48 hours, significant deaths occurred. 30 second exposure to concentrations >0.5mg Fe 1-1 caused a reduction in feeding rate. Exposure to >3mg Fe 1-1 over 21 days resulted in a reduction in population growth rate. Over this time-span the filtering area of daphnid thoracic limbs increased significantly in concentrations of iron >9mg Fe 1-1. A safe limit of 1.69mg Fe 1-1 was determined from toxicity tests, below which field populations would suffer no harmful effects.;There was no evidence of any impact of ferric dosing on daphnid numbers in the reservoir. However, the filtering area of the third thoracic limb in daphnids from around the inlet were significantly greater than in daphnids elsewhere in the reservoir, which may have been a consequence of long-term exposure to sub-lethal concentrations of iron. The observed decline in the size of daphnids in the reservoir since 1980, suggests predation by fish has been a significant force in the reservoir.;The success and implications of ferric dosing for eutrophication control in Rutland water, and elsewhere are discussed, and future strategies considered.