Abstract

The pollution of water courses by single pollution incidents and intermittent events has become increasingly important in the UK as legal regulation has reduced loadings from continuous discharges. Ammonia is a common pollutant which frequently causes damage to freshwater ecosystems during pollution episodes originating from industrial, sewage and farm discharges. A field experiment designed to simulate the effects of some of the components of farm waste effluent (increased ammonia and reduced dissolved oxygen concentration) on the crayfish Austropotamobius pa/lipes (Lereboullet) was carried out on a second order stream in South Wales. Ammonia concentrations were raised to between 5 and 7 mg l^-1 by dosing with ammonium chloride and the toxicant maintained in the un-ionised state by the simultaneous addition of sodium hydroxide to raise pH to approximately pH 9. Dissolved oxygen concentrations were reduced using sodium sulphite. During the simulation (24 hours in duration) the mortality of A. pallipes placed in the downstream treatment reaches of the dosing points were compared with those of A. pallipes maintained in an untreated upstream reference reach. Significant A. pallipes mortalities were recorded under conditions of elevated ammonia levels and elevated ammonia / low dissolved oxygen levels.

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