Effects of sequential depositional basins on lake response to urban and agricultural pollution: a palaeoecological analysis of the Qu'Appelle Valley,Saskatchewan, Canada

1. Palaeolimnological analyses of fossil diatoms and pigments were conducte d in four lakes of the Qu'Appelle Valley, Saskatchewan, Canada, to quantify the effect of upstream depositional basins on lake response to urban and a gricultural human activities. Pasqua, Echo, Mission and Katepwa lakes exhib it similar modern limnological characteristics, lie sequentially downstream from urban point sources of growth-limiting nitrogen (N), yet drain simila rly large areas of farmland (38-40 x 10(3) km(2)). 2. Analyses indicated that all lakes were naturally productive, contained e utrophic diatoms (i.e. Stephanodiscus niagarae, S. hantzchii, S. parvus and Aulacoseira granulata), and supported blooms of colonial (as myxoxanthophy ll) and potentially toxic N-fixing cyanobacteria (aphanizophyll), even prio r to the onset of European settlement (ca. 1890) and urban development ten (ca. 1930). 3. The onset of agricultural practices ca. 1890 had only modest effects on algal communities in the Qu'Appelle lakes, with subtle increases in eutroph ic diatom species (Pasqua, Mission and Katepwa lakes) and 25-50% increases in pigment-inferred algal abundance (Echo, Mission and Katepwa lakes). 4. Despite naturally high production, total algal abundance (beta-carotene) in upstream Pasqua Lake increased by more than 350% after intense urbaniza tion beginning ca. 1930, while eutrophic diatoms became more common and cya nobacteria populations increased ten-fold. Principal components analysis (P CA) explained 64% of diatom variance, and identified three eras correspondi ng to baseline, pre-agricultural communities (1776-1890), an era of high pr oduction (ca. 1925-1960) and recent variable community composition followin g tertiary treatment of urban sewage (ca. 1977-1990). 5. Analyses of three downstream lakes demonstrated that urban impacts follo wing 1930 remained evident in fossil profiles of beta-carotene and myxoxant hophyll, but that large blooms of N-fixing cyanobacteria were restricted to the past 25 years at downstream Mission and Katepwa lakes. Similarly, PCA showed that fossil diatom assemblages exhibited little directional variatio n until the 1970s. 6. Together, these analyses support the hypothesis that upstream lakes were effective at reducing the impacts of point-source urban nutrients on downs tream lakes. In contrast, diffuse agricultural activities had only limited impacts on water quality and these were less well ameliorated by upstream b asins.