Forest harvest impacts on water quality and aquatic biota on the Boreal Plain: introduction to the TROLS lake program

Eleven headwater lakes in Alberta's Boreal Plain were monitored for nutrien ts and plankton 2 years before and 2 years after variable watershed harvest ing (harvesting mean 15%, range 0-35%). After harvesting, variations in ann ual precipitation resulted in lake water residence times that differed by a n order of magnitude from one year to the next. During the first posttreatm ent year, total phosphorus concentrations increased (overall 40%) in most l akes; however, response was most consistent in lakes that were shallow and the water column mixed or weakly thermally stratified. Chlorophyll a, cyano bacteria (Aphanizomenon-Anabaena), and cyanotoxins (microcystin-LR) increas ed after harvesting, primarily in shallow lakes. Zooplankton abundance and biomass decreased after harvesting, particularly in stratified lakes where edible phytoplankton biomass declined. In the weakly or nonstratified lakes , declines in zooplankton biomass were associated with higher cyanobacteria l biomass and cyanotoxins. Posttreatment change in total phosphorus concent ration was strongly related to weather (greatest response in a wet year) an d relative drainage basin size (drainage basin area to lake volume, r(2) = 0,78, P < < 0,01). There was no evidence that buffer strip width (20, 100, and 200 m) influenced lake response. These results suggest that activities within the entire watershed should be the focus of catchment-lake interacti ons.