Ecotoxicological impact of iron III sulphate on chironomid cultures and profundal reservoir communities

Ferric sulphate dosing has been used as a eutrophication control measure in water supply reservoirs. The dosing results in precipitation of available phosphorus to a layer overlying the natural sediments. Stream sites contaminated with iron from mine drainage exhibit impoverished fish and benthic invertebrate communities. This study investigated the potential impact of prolonged exposure to sedimented iron precipitates on chironomids (Diptera, Chironomidae), as representative of benthic invertebrates.;The distribution of chironomid taxa in relation to ferric dosing at Rutland Water, Leicestershire has been examined. Areas of impoverished chironomid communities coincided with areas having increased sediment iron levels. Procladius (Tanypodinae) and the tribe Tanytarsini, dominant elsewhere in the reservoir, were largely absent from these areas.;A laboratory investigation of the effects of sedimented ferric precipitates on Chironomus riparius (Meigen) has been performed. This species is easily cultured and as a non-selective benthic detritivore was a suitable test organism. Simple static-with-replacement tests of various duration were used due to the flocculant nature of the precipitates and to avoid loss of small early instar larvea.;Precipitation of iron was rapid and virtually complete after addition of ferric sulphate to filtered reservoir water. High larval mortality resulted from initial, temporary, depression of pH due to addition. Depression of pH is unlikely to be a major problem in a large well-buffered reservoir. Retardation of larval growth and development, and delayed adult emergence were related to increasing target iron concentration. Iron uptake was dominated by precipitate ingestion and little iron was internally absorbed. Reduced energy intake over time due to dilution of food by ingested precipitates is suggested to be the main mode of action. This may be a factor of the reduced chironomid diversity at iron-contaminated field sites.