Crustacean Plankton and the Eutrophication of Lakes in the Okanagan Valley, British Columbia

Abstract

In early September 1969 and late August 1971, the vertical and horizontal distribution of temperature, Secchi disc visibility, some chemical components, and zooplankton abundance in lakes Okanagan, Skaha, and Osoyoos were investigated. In addition, lakes Kalamalka and Wood were sampled only in late August 1971. Four different basins could be distinguished in Lake Okanagan: the Penticton-Summerland basin, the Peachland-Kelowna basin, a basin north of Kelowna, and one formed by the northern end of the lake. Total dissolved solids, calcium, and electrical conductivity within the lake decreased gradually from north to south. Macroelement values within lakes Okanagan, Skaha, and Osoyoos were apparently not substantially different but Kalamalka and Wood lakes were about twice higher in sodium, potassium, magnesium, and about 1.5 higher in total dissolved solids. Four species of copepods and nine species of cladocerans were found with Cyclops bicuspidatus thomasi and Diaptomus ashlandi dominant in all five lakes. Diaphanosoma leuchtenbergianum and Daphnia longiremis were the most common cladocerans in all five lakes. Also, Daphnia thorata was common in all except Wood Lake. Eighty-nine percent of the total plankters in Lake Okanagan were located in the upper 50-m layer. Relatively little horizontal variation was found within the lake but two areas, near Kelowna and in the northern end showed in both years of investigation greater abundance of plankton than the remaining areas. Crustaceans were usually less abundant inshore than offshore. Volume of settled net plankton varied from 8 mm3/cm2 in Okanagan Lake to 31 mm3/cm2 in Wood Lake. No substantial changes in species composition of zooplankton, Secchi disc visibility, and dissolved oxygen in the hypolimnion were found relative to the data of 1935 reported by Rawson. However, in 1969 and 1971 net plankton volumes were about five and three times larger, respectively, than in 1935. Calculations of total phosphorus load from two independent sources of data provided similar results and indicated that lakes Okanagan and Kalamalka are still in the oligotrophic range but close to mesotrophy. Lake Wood is moderately eutrophic and lakes Skaha and Osoyoos showed a marked degree of eutrophy, higher than in Lake Washington, U.S.A. before sewage diversion, or in Lake Erie. The estimates for lakes Wood, Skaha, and Osoyoos were supported by the values of the hypolimnetic areal deficits characteristic for highly eutrophic lakes. The trend of changes in phosphorus load during the past century is presented and predictions are made about the load in 1990 in situations with and without phosphorus removal from sewage.