FOSSIL RECORD OF CLADOCERAN AND ALGAL RESPONSES TO FISHERY MANAGEMENT-PRACTICES

1. We hypothesized that the fishery management practices of toxaphene application and trout stocking would affect non-target organisms in la kes. Because these practices were rarely monitored in the past, cladoc eran and algal assemblages were quantified in sediment cores from two lakes treated 30+ years ago to determine the long-term response of org anisms near the base of the food chain. 2. Chydorids were remarkably r esistant over the short term (a few years) in both the oligotrophic an d eutrophic lakes despite toxaphene treatments that extirpated native fish and other invertebrates. In the oligotrophic lake (Annette Lake), six chydorid taxa were less abundant in the years following treatment , although no loss of species richness was detected. In the eutrophic lake (Chatwin Lake), the dominant Chydorus cf. sphaericus declined coi ncident with toxaphene treatment, but longer-term declines of all taxa were probably related to food web or other changes rather than to tox aphene toxicity. Cause and effect coupling was complicated by the fact that many chydorids were present at low concentrations in some pretre atment samples. 3. The algal communities (as fossil pigments) responde d to treatment differently in the two lakes. In the oligotrophic lake, planktonic diatoms, dinoflagellates and chlorophytes were replaced as dominants by deep-water or benthic blooming cryptophytes, chrysophyte s and cyanobacteria. This shift occurred along with increases in large daphnids and the 'grazing indicator', pheophorbide a. While both lake s appear to have had enhanced pigment preservation following treatment , the eutrophic lake encountered few long-term changes in its fossil p igment assemblage. Redundancy analysis estimated that the presence or absence of stocked trout explained much of the variation in the algal assemblages, particularly in the oligotrophic lake. 4. Toxaphene remai ned elevated in profundal sediments from these lakes 30 and 35 years a fter treatment.