Browsing by Author "Mezzini, Stefano"

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Open Access
Abrupt changes in the physical and biological structure of endorheic upland lakes due to 8-m lake-level variation during the 20 th century
(Wiley, 2022-03-07) Bjorndahl, Judith A.; Gushulak, Cale A.C.; Mezzini, Stefano; Simpson, Gavin L.; Haig, Heather A.; Leavitt, Peter R; Finlay, Kerri
Climate-induced variation in lake level can affect physicochemical properties of endorheic lakes, but its consequences for phototrophic production and regime shifts are not well understood. Here, we quantified changes in the abundance and community composition of phototrophs in Kenosee and White Bear lakes, two endorheic basins in the parkland Moose Mountain uplands of southeastern Saskatchewan, Canada, which have experienced > 8 m declines in water level since ~ 1900. We hypothesized that lower water levels and warmer temperatures should manifest as increased abundance of phytoplankton, particularly cyanobacteria, and possibly trigger a regime shift to turbid conditions due to evaporative concentration of nutrients and solutes. High-resolution analysis of sedimentary pigments revealed an increase in total phototrophic abundance (as β-carotene) concurrent with lake-level decline beginning ~ 1930, but demonstrated little directional change in cyanobacteria. Instead, significant increases in obligately anaerobic purple sulfur bacteria (as okenone) occurred in both lakes during ~ 1930–1950, coeval with alterations to light environments and declines in lake level. The presence of okenone suggests that climate-induced increases in solute concentrations may have favored the formation of novel bacterial habitats where photic and anoxic zones overlapped. Generalized additive models showed that establishment of this unique habitat was likely preceded by increased temporal variance of sulfur bacteria, but not phytoplankton or cyanobacteria, suggesting that this abrupt change to physical lake structure was unique to deep-water environments. Such climate-induced shifts may become more frequent in the region due to hydrological stress on lake levels due to warming temperatures across the Northern Great Plains.
Open Access
Impacts of a century of land-use change on the eutrophication of large, shallow, prairie Lake Manitoba in relation to adjacent Lake Winnipeg (Manitoba, Canada)
(John Wiley & Sons Ltd., 2023-11-08) Gushulak, Cale A. C.; Mezzini, Stefano; Moir, Katherine E. M; Simpson, Gavin L.; Bunting, Lynda; Wissel, Björn; Engstrom, Daniel R.; Laird, Kathleen R.; Amand, Ann St.; Cumming, Brian F.; Leavitt, Peter R.
1. Evaluation of large lake response to centennial changes in land use and climate can be complicated by high spatial and hydrological complexity within their catchments, particularly in regions of low relief. Furthermore, large lakes can exhibit abrupt changes in structure and function that obscure causes of eutrophication. 2. We provide the first quantification of historical trends in lake production, cyanobacterial abundance, sediment geochemistry and diatom composition since c. 1800 in Lake Manitoba, the 29th largest lake in the world, and compared them to Lake Winnipeg, a morphologically similar, adjacent basin with a 10-fold larger catchment and an abrupt increase in production around 1990. 3. Before 1900, Lake Manitoba was mesotrophic, with low sedimentary concentrations of carbon, phosphorus, nitrogen, cyanobacteria and algal pigments, as well as assemblages of low-light-adapted benthic diatoms. Analysis of pigment time-series with hierarchical generalised additive models revealed that Lake Manitoba eutrophied during 1900–1930 as a consequence of the development of intensive agriculture within its local catchment, but thereafter exhibited stable cyanobacterial densities with limited expansion of N2-fixing cyanobacteria despite persistent eutrophication. 4. Lake Manitoba did not undergo an abrupt change as seen in Lake Winnipeg. 5. These findings suggest that catchment size had little influence on water quality degradation and that nutrient influx from proximal agricultural sources was sufficient to initially degrade these large prairie lakes. The abrupt change in Lake Winnipeg around 1990 required additional intensification of local land use that did not occur in the Lake Manitoba catchment.
Open Access
Impacts of hydrologic management on the eutrophication of shallow lakes in an intensive agricultural landscape (Saskatchewan, Canada)
(Wiley, 2024-05-01) Gushulak, Cale A. C.; Chegoonian, Amir M.; Wolfe, Jared; Gray, Kristen; Mezzini, Stefano; Wissel, Bjoern; Hann, Brenda; Baulch, Helen M.; Finlay, Kerri; Leavitt, Peter R.
1. Hydrologic management of shallow lakes is often undertaken to prevent fluctua- tions in lake level, and to ensure sufficient water volume for economic, domestic, and recreational uses, but there is inconsistent evidence of whether lake-level sta- bilisation through hydrological management promotes or hinders eutrophication. 2. Here we used multi-proxy paleolimnological assessments of water quality (sedi- mentary carbon, nitrogen, total phosphorus, fossil pigments), and zooplankton community ecology (fossil Cladocera assemblages), combined with Landsat- derived estimates of lake surface area in two shallow eutrophic lakes, in the Prairies of southern Saskatchewan, Canada, to quantify how 8 decades of con- trasting hydrological management strategies (continuous or intermittent) affect primary production and phytoplankton composition. 3. Analysis revealed that irregular hydrological management of Pelican Lake led to sharp increases in primary production concomitant with lake-level decline. In contrast, continuously managed Buffalo Pound Lake, a drinking water reservoir for regional cities, exhibited slow, persistent eutrophication over decades despite active regulation of water levels. In both lakes, strong correlations of δ 15N val- ues with pigments from diazotrophic cyanobacteria (canthaxanthin) showed that N2-fixation increased during eutrophication irrespective of the timing of change. Finally, variation in fossil cladoceran density and composition reflected changes in pelagic and littoral habitats (e.g., reduced macrophyte cover) due to changes in both lake level and water quality. 4. Basin comparison shows that while hydrologic management can moderate water quality degradation due to lake-level change, it does not prevent eutrophication when nutrient influx remains high. 5. Given that regional water availability is forecast to decline in coming decades, we anticipate that continued hydrological management will be unavoidable and will be unable to improve water quality unless nutrient influx is also controlled.