Lethal and sub-lethal toxicity of field simulated farm waste episodes to several freshwater invertebrate species

Four field experiments designed to simulate the effects of the individual components of farm waste effluent (increased ammonia, reduced dissolved oxygen concentrations) on the invertebrate fauna of receiving waters were carried out on two second order streams in South Wales. In two studies the dissolved oxygen concentration was reduced to 2 and 1 mg 1−1 by the oxidation of sodium sulphite, and in a further two studies ammonia concentrations were raised to 5 and 7 mg 1−1 by dosing with ammonium chloride and the toxicant maintained in the unionized state by the simultaneous addition of sodium hydroxide to raise pH to approx. pH 9. During each simulation (6–24 h in duration) the response of several invertebrate species placed in the downstream treatment zone(s) was compared with that of animals maintained in an upstream untreated reference zone. Significant invertebrate mortalities were recorded only under conditions of reduced dissolved oxygen. The feeding rate of Gammarus pulex was significantly reduced during dosing with ammonia but recovered post-exposure. Parasitism of G. pulex with Pomphorhynchus laevis resulted in a reduced feeding rate and an increased mortality in comparison to uninfected animals. The disruption of precopula in G. pulex was a sensitive and rapid indicator of stress under both reduced dissolved oxygen and increased ammonia concentrations.