Limnol. Oceanogr., 44(3, part 2), 1999, 739–756

The Qu’Appelle Valley drainage system provides water to a third of the population of the Canadian Great Plains, yet is plagued by poor water quality, excess plant growth, and periodic fish kills. Fossil algae (diatoms, pigments) and invertebrates (chironomids) in Pasqua Lake were analyzed by variance partitioning analysis (VPA) to determine the relative importance of climate, resource use, and urbanization as controls of aquatic community composition 1920–1994. From fossil analyses, we identified three distinct biological assemblages in Pasqua Lake. Prior to agriculture (ca. 1776–1890), the lake was naturally eutrophic with abundant cyanobacterial carotenoids (myxoxanthophyll, aphanizophyll), eutrophic diatoms (Stephanodiscus niagarae, Aulacoseira granulata, Fragilaria capucina/bidens), and anoxia-tolerant chironomids (Chironomus). Principal components (PCA) and dissimilarity analyses demonstrated that diatom and chironomid communities did not vary significantly (P . 0.05) before European settlement. Communities changed rapidly during early land settlement (ca. 1890–1930) before forming a distinct assemblage ca. 1930–1960 characterized by elevated algal biomass (inferred as b-carotene), nuisance cyanobacteria, eutrophic Stephanodiscus hantzschii, and low abundance of deep-water zoobenthos. Recent fossil assemblages (1977–1994) were variable and indicated water quality had not improved despite 3-fold reduction in phosphorus from sewage. Comparison of fossil community change and continuous annual records of 83 environmental variables (1890–1994) using VPA captured 71–97% of variance in fossil composition using only 10–14 significant factors. Resource use (cropland area, livestock biomass) and urbanization (nitrogen in sewage) were stronger determinants of algal and chironomid community change than were climatic factors (temperature, evaporation, river discharge). Landscape analysis of inferred changes in past algal abundance (as b-carotene; ca. 1780–1994) indicated that urban impacts declined with distance from point sources and suggested that management strategies will vary with lake position within the catchment. Lake eutrophication remains a widespread problem despite intense research efforts to determine factors that regulate water quality (Schindler 1990; Williamson et al. 1999). Factors regulating eutrophication have been studied extensively in dimictic, soft-water, temperate lakes (e.g., Schindler 1990), but remain poorly understood in polymictic, hardwater, prairie lakes of the northern Great Plains where humans are important agents of environmental change (Allan 1980; Kenney 1990). Prairie lakes lie in large, fertile agricultural catchments that supply high concentrations of nutrients (Peters 1973) and are characterized by low nitrogen : phosphorus (N : P) ratios and surface blooms of nitrogen (N2)-fixing cyanobacteria (Haertel 1976). The inability to