Effects of agriculture, urbanization, and climate on water quality in the northern Great Plains

The Qu'Appelle Valley drainage system provides water to a third of the popu lation of the Canadian Great Plains, yet is plagued by poor water quality, excess plant growth, and periodic fish kills. Fossil algae (diatoms, pigmen ts) and invertebrates (chironomids) in Pasqua Lake were analyzed by varianc e partitioning analysis (VPA) to determine the relative importance of clima te, resource use, and urbanization as controls of aquatic community composi tion 1920-1994. From fossil analyses, we identified three distinct biologic al assemblages in Pasqua Lake. Prior to agriculture (ca. 1776-1890), the la ke was naturally eutrophic with abundant cyanobacterial carotenoids (myxoxa nthophyll, aphanizophyll), eutrophic diatoms (Stephanodiscus niagarae, Aula coseira granulata, Fragilaria capucina/bidens), and anoxia-tolerant chirono mids (Chironomus). Principal components (PCA) and dissimilarity analyses de monstrated that diatom and chironomid communities did not vary significantl y (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 beta-car otene), nuisance cyanobacteria, eutrophic Stephanodiscus hantzschii, and lo w abundance of deep-water zoobenthos. Recent fossil assemblages (1977-1994) were variable and indicated water quality bad 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) usi ng VPA captured 71-97% of variance in fossil composition using only 10-14 s ignificant factors. Resource use (cropland area, livestock biomass) and urb anization (nitrogen in sewage) were stronger determinants of algal and chir onomid community change than were climatic factors (temperature, evaporatio n, river discharge). Landscape analysis of inferred changes in past algal a bundance (as beta-carotene; ca. 1780-1994) indicated that urban impacts dec lined with distance from point sources and suggested that management strate gies will vary with lake position within the catchment.