THE EFFECTS OF CLIMATIC WARMING ON THE PROPERTIES OF BOREAL LAKES ANDSTREAMS AT THE EXPERIMENTAL LAKES AREA, NORTHWESTERN ONTARIO

A period of prolonged warmer, drier-than-normal weather in northwester n Ontario during the 1970s and 1980s resulted in severe forest fires t hat caused dramatic changes to lake and stream catchments. The changed interactions of weather with catchments and hydrological processes ca used unexpected changes in physical, chemical, and biological processe s in lakes and streams. Permanent first-order streams became ephemeral . Flows at spring melt were lower, and chemical exports from catchment s were reduced. Although catchments burned by forest fire had slightly higher flows and chemical exports than unburned basins in the years f ollowing fires, chemical exports generally declined due to lower strea mflow. Decreased exports of silica indicated lower rates of weathering . Base cation exports also decreased, as did the ratio of base cations to strong acid anions in streams. Changes in lakes included warmer te mperatures, clearer waters, deeper thermoclines and euphotic zones, hi gher alkalinities, and higher concentrations of base cations and nitro gen, but lower concentrations of dissolved organic C, silica, and P. T he increase in alkalinity, was caused by increases in the ratio of bas e cations to strong acid anions, resulting from the interaction of inc reased water retention, microbial sulfate reduction, and exchanges of cations between water and sediments. Declines in chlorophyll and incre ases in phytoplankton biomass were observed, but there was no detectab le effect on areal phytoplankton production. Summer subthermocline hab itats for cold stenotherms were reduced slightly in extent as the resu lts of thermocline deepening and lower hypolimnetic oxygen. There is c onsiderable potential for interaction between climatic change and othe r human perturbations affecting boreal lakes, including acidification, increased incident UV radiation, eutrophication, and overharvesting.