Asynchronous onset of eutrophication among shallow prairie lakes of the Northern Great Plains, Alberta, Canada

Abstract

Coherent timing of agricultural expansion, fertilizer application, atmospheric nutrient deposition, and accelerated global warming is expected to promote synchronous fertilization of regional surface waters and coherent development of algal blooms and lake eutrophication. While broad-scale cyanobacterial expansion is evident in global meta-analyses, little is known of whether lakes in discrete catchments within a common lake district also exhibit coherent water quality degradation through anthropogenic forcing. Consequently, the primary goal of this study was to determine whether agricultural development since ca. 1900, accelerated use of fertilizer since 1960, atmospheric deposition of reactive N, or regional climate warming has resulted in coherent patterns of eutrophication of surface waters in southern Alberta, Canada. Unexpectedly, analysis of sedimentary pigments as an index of changes in total algal abundance since ca. 1850 revealed that while total algal abundance (as β-carotene, pheophytin a) increased in nine of 10 lakes over 150 years, the onset of eutrophication varied by a century and was asynchronous across basins. Similarly, analysis of temporal sequences with least-squares regression revealed that the relative abundance of cyanobacteria (echinenone) either decreased or did not change significantly in eight of the lakes since ca. 1850, whereas purple sulfur bacteria (as okenone) increased significantly in seven study sites. These patterns are consistent with the catchment filter hypothesis, which posits that lakes exhibit unique responses to common forcing associated with the influx of mass as water, nutrients, or particles.

Increased variability and sudden ecosystem state change in Lake Winnipeg, Canada, caused by 20thcentury agriculture

“…Limited unique effects of climatic variability on eutrophication of southern Lake Winnipeg during the 20 th century (Fig. ) are consistent with predictions of the Energy‐ mass (E m ) flux framework (Leavitt et al ) and empirical observations from more than 25 agriculturally‐impacted lakes within the Canadian Prairies (Pham et al ; Leavitt et al , Maheaux et al ). Regional fall, winter, and spring mean and minimum temperatures have increased ∼3°C since the late 1800s (Statistics Canada 1871–2006), leading to ∼35 d increase in ice‐free season in southern MB (Hall et al ).…”

“…Strongly enriched (3–4‰) sedimentary δ 15 N values recorded after 1900 are consistent with increased influx of N from agricultural (Anderson and Cabana ; Bunting et al ) or urban sources (Savage et al ; Leavitt et al ). Although it is difficult to distinguish among N sources (Mayer and Wassenaar ), we infer that the City of Winnipeg is the most likely source of enriched N, despite accounting for only 5–10% of TN influx, as similar enrichments have not been recorded in eight cores from the north basin of Lake Winnipeg (Bunting et al ), four cores from adjoining Lake Manitoba (Leavitt et al ), and 25 other lakes within the catchment (Pham et al ; Leavitt et al ; Maheaux et al ), all sites which receive substantial agricultural N, but not urban N. As reviewed elsewhere (Savage et al ; Leavitt et al ), urban wastewater treatment can enrich dissolved N by 10–25‰ due to intense isotopic fractionation during NH 3 volatilization or denitrification of waste N. Consistent with the inferred importance of urban N, changes in fossil δ 15 N were correlated more highly with growth of Winnipeg's population during the 20 th century ( r 2 = 0.82, p < 0.0001) than with those of the main livestock ( r 2 = 0.52–74, p < 0.0001).…”

“…5). As in other prairie lakes (Hall et al 1999;Leavitt et al 2009;Maheaux et al 2016), climatic variability during the 20 th century had limited effects on water quality. Finally, southern Lake Winnipeg experienced a sudden ecosystem state change (Scheffer et al 2001;Scheffer and Carpenter 2003) during the third stage (ca.…”

The legacy of large regime shifts in shallow lakes

“…This highlights the watershed‐specific factors that influence shallow lake ecology, similar to the recent findings of Maheaux et al. (). We posit that it is the regime history of the aquatic ecosystem that defines or provides the context for the modern ecological state (clear or turbid) of the lake.…”

Regional versus local drivers of water quality in the Windermere catchment, Lake District, United Kingdom: The dominant influence of wastewater pollution over the past 200 years

“…In contrast, variation in mass (water, nutrients, suspended matter) influx to lakes arises from urban pollutants such as Wastewater Treatment Works (WwTWs) (Jenny, Normandeau, & Francus, ), short‐term or site‐specific variation in precipitation and inflow regimes (Chen, McGowan, Xu, Zeng, & Yang, ), or basin‐specific changes in agricultural practice (Maheux, Leavitt, & Jackson, ), all of which influence delivery of particulate material (Dearing & Jones, ). Such local disruptions often reduces synchrony of limnological variables among basins (Leavitt et al., ; Maheux et al., ) and impede the effectiveness of generalized mitigation strategies. The development of effective management of aquatic resources would benefit from a hierarchical understanding of the relative importance of regional versus local stressors that threaten freshwaters (Jackson, Woodford, & Weyl, ; Vörösmarty et al., ).…”