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 . 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 Ͼ 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 ␤-carotene), nuisance cyanobacteria, eutrophic Stephanodiscus hantzschii, and low abundance of deep-water zoobenthos. Recent fossil assemblages (1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(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 ␤-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.

We measured nitrogen (N) transport to and storage in nine lakes linked by the Qu'Appelle River, Saskatchewan, Canada, to quantify the unique effects of N on the eutrophication of phosphorus (P)-replete lakes. Stable isotope content (d 15 N) was measured for dissolved N, periphyton, particulate organic matter (POM), and sediment samples collected at 10 stations along Wascana Creek and the Qu'Appelle River, lotic ecosystems that receive wastewaters from the City of Regina and that drain into Pasqua Lake. Urban effluent (d 15 N ,16 6 2%) enriched dissolved N isotope ratios of river water by up to 15% but was not stored in lotic sediments. Instead, urban N increased d 15 N signatures of lotic periphyton and POM by 10-15% and was transported to Pasqua Lake, where sedimentary d 15 N values increased from ,6.5% during the 19th century to 14.0% by the 1990s. This increase was linearly correlated both to the mass of dissolved N released from Regina (r 2 5 0.84, p , 0.0001) and to a 300% increase in the production of Pasqua Lake (as fossil pigments) since ca. 1880 (r 2 5 0.69, p , 0.0001). Similar isotopic enrichment was recorded in five downstream lakes, but not three reference ecosystems, although the degree of downstream enrichment declined rapidly, mainly as a result of sequestration of urban N in lake sediments. Together, these patterns demonstrate that urban N can directly degrade surface waters of P-sufficient lakes, but that these ecosystems can eliminate urban effects through permanent storage of wastewater N in their sediments.

Cyanobacterial blooms are a regular feature of lakes in central North America, but little is known of their importance to the nitrogen (N) cycle and nutrition of aquatic food webs. We hypothesized that N 2 -fixing cyanobacteria constitute a significant source of N to prairie lakes, that fixed N is effectively transferred to primary and secondary consumers, and that the importance of fixed N is structured on a landscape scale due to spatial gradients of lake chemistry and catchment characteristics. These hypotheses were tested using stable isotope analyses and mass budgets in six chained lakes of the northern Great Plains that were sampled biweekly during summers of 1994-2004. Mean annual N isotope signatures of particulate organic matter (d 15 N-POM) were highly correlated to the abundance of N 2 -fixing cyanobacteria (r 2 5 0.75, p , 0.001), but exhibited no marked spatial organization. Instead, the temporal variability of d 15 N-POM was greatest in downstream lakes where N 2 -fixing cyanobacteria were abundant. Furthermore, seasonal declines in d 15 N-POM were correlated with reductions in d 15 N of individual zooplankton taxa (Daphnia spp., Diacyclops thomasi, Leptodiaptomus siciloides, Leptodora kindtii), especially in downstream eastern lakes. N mass balances revealed that the importance of biological N 2 fixation relative to total N inputs (up to 77%) and initial N standing stock (up to 201%) were significantly linearly correlated with landscape position (0.59 # r 2 # 0.83, p # 0.07) and effective drainage area (0.61 # r 2 # 0.98, p # 0.07), as were volumetric and areal estimates of N 2 fixation during summer (r 2 $ 0.87, p # 0.007). These patterns demonstrate that fixed N is spatially structured, highly predictable based on landscape position, and readily assimilated into aquatic food webs.

The annual record of fossil pigments and zooplankton was compared with detailed contemporaneous records from two manipulated lakes from 1940 to 1986. Annually resolved sedimentary records accurately monitored known changes in plankton communities, identified periods of trophic change, and proved a powerful tool for examining long-term, complex interactions. Both Paul and Peter Lakes underwent the same three complete changes in their fish assemblages (trout, cyprinid, bass), and Peter Lake received repeated inputs of lime. Alterations in fish community composition produced long-lived changes in zooplankton communities that cascaded to the microbial level of the food web. Liming, in concert with trophic changes, caused distinctive phytoplankton dynamics in Peter Lake.Paleolimnological data recorded all major plankton dynamics known from coeval limnological data. Specifically, the sediment record showed transitions in cladoceran size structure and species composition, changes in water clarity resulting from both food web and chemical manipulations that affected vertical zonation of primary producers, and changes in absolute abundance of all algal divisions except dinoflagellates. Undegraded Chl a indicated deep blooms and, in conjunction with Chl c, fucoxanthin and P-carotene, indicated metalimnetic chrysophytes. Transient (2-3 yr) nonselective increases in sedimentation corresponded with increases in grazing rates. Isorenieratene indicated overlap of photic and anoxic zones, revealing changes in transparency and conditions for pigment preservation.

Many alpine lakes have low concentrations of ultraviolet (UV) radiation-absorbing dissolved organic matter, yet receive higher UV radiation flux than low-elevation lakes. We tested whether ambient UV radiation affected periphyton development in a small alpine lake in Banff National Park, Canada. After 30 d, total periphytic biomass and chlorophyll accrual on artificial substrates were enhanced -100% (t-test, P < 0.05) by removal of UV radiation (~400 nm). The inhibitory effect of UV radiation was species-specific, significantly suppressing (Bonferroni-adjusted t-test, P < 0.05) Achnanthes minutissima Kiitzing but not other colonists. Although taxa apparently differed in their sensitivity to UV radiation, periphyton communities remained dominated by early successional taxa, especially A. minutissima (75% of total biovolume). In contrast, natural epilithic communities were dominated by cyanobacteria (Anabaena subcylindrica Borge, Calothrix sp.). These findings suggest that ambient UV radiation at alpine sites can suppress periphyton development by inhibiting littoral diatom production during the short ice-free season (July-September).