EFFECTS OF THE PRESENCE AND ABSENCE OF VARIOUS FRACTIONS OF DISSOLVED ORGANIC-MATTER ON THE TOXICITY OF FENVALERATE TO DAPHNIA-MAGNA

Toxicity of fenvalerate to Daphnia magna, in the presence of particulate or dissolved organic matter, was investigated. Dissolved organic matter (DOM) was isolated from Jack Lake, Ontario (JL-DOM), and separated into four size fractions. A sample of fibrils from Lake St. George, Ontario, and two model substances, pectin and polygalacturonic acid, represented DOM of algal origin. Chlamydomonas reinhardii was included as a source of particulate organic matter for comparison. Daphnia magna (less-than-or-equal-to 48 and less-than-or-equal-to 24 h old) was exposed to treatments of fenvalerate plus these sources of DOM. The end point of the static 48-h bioassays was the inability of daphnids to swim during a 10-s period after gentle prodding. Results from treatments with JL-DOM (10 x natural concentrations) and fibrils (4 mg/L) were analyzed independently. The EC50 of fenvalerate plus >0.2 mum JL-DOM (0.22 mug/L) was significantly lower than that of the >10-kD-molecular-weight fraction (1.99 mug/L), but neither was different from fenvalerate alone (0.91 mug/L). There were no significant differences between treatments including lake fibrils, pectin or polygalacturonic acid, or between age groups. No bioassays using Daphnia magna and algal fibrils as a source of DOM have been conducted previously. An adhesion phenomenon occurred between Daphnia magna and the DOM (JL-DOM and fibrils) in the presence of fenvalerate. DOM and fibrils aggregated on the spine and antennae of the daphnids, which impaired the ability of the organisms to swim. Adhesion increased as the concentration of fenvalerate increased but was absent in controls. This phenomenon may confound the results of bioassays that use immobility as an end point. Adhesion could be an important sublethal effect of fenvalerate, affecting daphnid survival and its role as a food source in aquatic systems.