A Survey of the Algae and Aquatic Macroinvertebrates of Weso Lake, Bayfield County, Wisconsin, with Specific Reference to the Drummond Wastewater Treatment System

Abstract

The algae, aquatic macroinvertebrates, and water quality of Weso Lake, Bayfield County, Wisconsin, were investigated from May 1979 to September 1980. Objectives were: (1) to determine the taxonomic composition and relative abundance of the algae and aquatic macroinvertebrates, (2) to monitor the physicochemical characteristics of the lake water, and (3) to use laboratory bioassays to define the algal growth potential of Weso Lake water with effluent and nutrient additions. The data provide an information base for the long term evaluation of the impacts on Weso Lake of effluent drainage from the Drummond bog wastewater treatment system. Background water quality values were generally typical of an acidic, dimictic, brown-water lake. Significantly higher chloride and conductivity levels in Weso Lake in 1980 were attributed to effluent drainage from the Drummond system. Significant reductions in true and apparent color, and in concentrations of total and soluble reactive phosphorus, and total Kjeldahl nitrogen, were also noted during 1980. The mid-summer surface phytoplankton counts were dominated by green algae, particularly Sphaerocystis sp. Melosira sp. and Cryptomonas spp. were the most abundant taxa in other seasons. The mean surface phytoplankton densities for the summer periods (May-September) were 1927 cells ml^-1 in 1979 and 2969 cells ml^-1 in 1980. The average chlorophyll a con­ centrations for the corresponding periods were 7.1 and 6.8 mg. 1^-1. Algal aufwuchs densities on Sphagnum were variable, ranging from 3230 to 29,400 cells mg^-l host dry weight. Eunotia spp. and Anabaena spp. dominated the algal aufwuchs. Benthic macroinvertebrate density and number of taxa were significantly different with depth and year. The deep profundal fauna was generally restricted to Chaoborus sp. Chironomus plumosus (Linnaeus) 1758 dominated the macroinvertebrate biomass in the upper profundal and sublittoral zones. Thirty-one taxa of invertebrates, including 14 taxa of chironomids, were collected in the littoral region. The average macroinvertebrate densities for 1979 and 1980 were 8969, 1322, 2214, and 735 m^-2 at 1, 2, 4, and 6 m depths, respectively. Chironomidae and Odonata, primarily Leucorrhinia spp., dominated the macroinvertebrate aufwuchs in sweep net samples, which were collected in the littoral vegetation. Macroinvertebrate aufwuchs numbers and biomass were not significantly different for either cover type or year. Laboratory bioassays using Drummond secondary lagoon effluent diluted with Weso Lake water indicated that the effluent did not stimulate exponential growth of the test alga, Anabaena sp. Nutrient addition bioassay of the secondary effluent indicated that phosphorus was the primary limiting nutrient. Algal nutrient assay showed that lake water, if buffered and fertilized with phosphorus, resulted in expo­nential growth. A discussion of the fish species in Weso Lake is also presented.