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

Abstract

1. Palaeolimnological analyses of fossil diatoms and pigments were conducted in four lakes of the Qu'Appelle Valley, Saskatchewan, Canada, to quantify the effect of upstream depositional basins on lake response to urban and agricultural human activities. Pasqua, Echo, Mission and Katepwa lakes exhibit similar modern limnological characteristics, lie sequentially downstream from urban point sources of growth‐limiting nitrogen (N), yet drain similarly large areas of farmland (38–40 × 103 km2). 2. Analyses indicated that all lakes were naturally productive, contained eutrophic diatoms (i.e. Stephanodiscus niagarae, S. hantzchii, S. parvus and Aulacoseira granulata), and supported blooms of colonial (as myxoxanthophyll) and potentially toxic N‐fixing cyanobacteria (aphanizophyll), even prior to the onset of European settlement (ca. 1890) and urban development (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 eutrophic 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 (β‐carotene) in upstream Pasqua Lake increased by more than 350% after intense urbanization beginning ca. 1930, while eutrophic diatoms became more common and cyanobacteria populations increased ten‐fold. Principal components analysis (PCA) explained 64% of diatom variance, and identified three eras corresponding to baseline, pre‐agricultural communities (1776–1890), an era of high production (ca. 1925–1960) and recent variable community composition following tertiary treatment of urban sewage (ca. 1977–1990). 5. Analyses of three downstream lakes demonstrated that urban impacts following 1930 remained evident in fossil profiles of β‐carotene and myxoxanthophyll, 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 variation 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 downstream lakes. In contrast, diffuse agricultural activities had only limited impacts on water quality and these were less well ameliorated by upstream basins.

Landscape effects of climate, agriculture, and urbanization on benthic invertebrate communities of Canadian prairie lakes

“…Finally, community variability was also maximal since 1980 in Buffalo Pound Lake, a site without direct sewage inputs. Instead, we suggest that recent increases in algal production (Dixit et al 2000) and invertebrate community variation reflect regional climate change, as recorded east (Schindler et al 1996) and west (Vinebrooke et al 1998) of the Qu'Appelle drainage.…”

“…In general, high rates of variation reflect changes in the absolute abundance of deep-water Chironomus and littoral Tanytarsus s.l. and correspond to periods of rapid diatom community change and elevated abundance of colonial cyanobacteria (Hall et al 1999a;Dixit et al 2000;Hall et al unpubl. data).…”

“…1). The Qu'Appelle Valley watershed contains seven major lakes and Hall et al (1999a,b), Dixit et al (2000), Sorrano et al (1999), Graham (1997), and unpublished analyses. na, values not available; *, estimated using N : P ratios.…”

“…Long-term patterns of chironomid community change were summarized using principal components analysis (PCA). All lakes were included in the PCA ordinations to allow quantitative comparison of chironomid changes among sites (Dixit et al 2000). Ordinations were scaled to focus on intersample distances and were centred by species using CANOCO 4.0 for Windows (ter Braak and Smilauer 1998).…”

“…These patterns are particularly evident on a landscape basis, where lowland lakes draining upland agricultural areas are characterized by relatively high conductivity, nutrient content, and algal abundance, along with low water clarity relative to headwater systems (Soranno et al 1999). Paleoecological analyses suggest that these changes in water quality result from a combination of agricultural practices and urbanization (Kitchell and Sanford 1992;Dixit et al 2000) but that climatic influences account for only 5-10% of variance in algal communities during the 20th century (Hall et al 1999a). Unfortunately, little is known of the response of benthic invertebrate communities to such multiple stressors, nor of how their responses might vary along a landscape gradient of point and nonpoint disturbances.…”