A POND MESOCOSM STUDY TO DETERMINE DIRECT AND INDIRECT EFFECTS OF LINDANE ON A NATURAL ZOOPLANKTON COMMUNITY

We assessed the threshold concentrations for toxic effects by lindane on a zooplankton community using mesocosms in a natural pond. Enclosur es (1000 litres) were treated to give mean concentrations of 0, 2, 6, 12, 18, 24, and 50 mu g litre(-1) over a test period of 39 days. Larva e of the phantom midge Chaoborus flavicans showed a high susceptibilit y to lindane and were virtually eliminated from enclosures with greate r than or equal to 12 mu g litre(-1) 4 days after application. Populat ions of cyclopoid nauplii (Eucyclops serrulatus) were reduced at a lin dane concentration of 2 mu g litre(-1) and showed high mortality after 11 days exposure to greater than or equal to 6 g litre(-1), No lindan e-induced effects could be seen in the cladoceran species Daphnia long ispina and Chydorus sphaericus. Rotifer species were probably not dire ctly affected by lindane; however loss of the principal predator Chaob orus was accompanied by an increase in the numbers of Asplanchna priod onta. A simultaneous decrease in Keratella quadrata possibly reflected predation since its decline showed no dose-response relationship with lindane and it is among the major prey of Asplanchna. Such complex se condary effects, occurring through trophic interactions between a pred ator and two co-occurring prey, would not have been predicted from sin gle-species toxicity tests. These, and effects on cyclopoid population s that occurred predominantly through the mortality of nauplii, demons trate the value of toxicological studies on natural communities. We co nclude that a combination of laboratory and field investigations, coup led with mono-species and community assessments, provide the best rout e for understanding the effects of toxicants such as lindane for regul atory purposes. Copyright (C) 1996 Elsevier Science Ltd