Land use changes and salinization: Impacts on lake phosphorus cycling and water quality

Abstract

Over the past few decades, there has been a rapid global increase in urbanization accompanied by the conversion of natural or agricultural land into more impervious land cover. This ongoing acceleration of global urbanization has raised significant concerns regarding the deterioration of water quality in urban lakes, such as worsening eutrophication symptoms. Eutrophication of inland waters, primarily driven by phosphorus (P) enrichment caused by human activities, is characterized by increased primary production that, in the most extreme cases, results in harmful algal blooms. Additionally, anthropogenic salinization has emerged as another stressor affecting the health of urban freshwater ecosystems. Although the ecological ramifications of both P enrichment and salinization on freshwater ecosystems are recognized, their combined impacts on water quality have hitherto been considered separately. The work presented in this thesis is based on an extensive acquisition and analysis of data for a lake currently located along the edge of the Greater Toronto metropolitan area: Lake Wilcox. Before the most recent phase of rapid urban development, the lake’s watershed underwent the conversion of its original forested land cover to agricultural use. Based on the data, I investigated the following questions: (1) How did the successive historical changes in land use/land cover (LULC) impact the water quality and P cycling in the lake?; (2) How has the rapid expansion of imperviousness during urban growth impacted the lake’s eutrophication symptoms, in particular, the oxygenation of the deeper water and the remobilization of P from the bottom sediments?; (3) How effective have agricultural and urban stormwater best management practices been in mitigating the external input of P to Lake Wilcox?; (4) Of the road salt applied in the watershed during winter, how much reaches the lake and how much is retained in the watershed?; and (5) What is the rate of salinization of Lake Wilcox and management intervention could help the lake recover from excessive use of the road salt? To address these research questions for Lake Wilcox, I combined sediment core analyses, statistical data time series tests, and mass balance modeling. I further evaluated the transferability of the findings for Lake Wilcox to other lakes in North America. In this final research activity, I tested the key hypothesis that emerged from my work on Lake Wilcox, namely that the changes in a freshwater lake’s mixing regime caused by salinization exacerbates eutrophication symptoms, even in cases where the external P inputs to the lake are reduced. In chapter 2, a dated sediment core, recent water quality data, and historical records were used to reconstruct changes in P loading to and cycling in Lake Wilcox associated with changes in land use/land cover (LULC) since the 1920s. The lake’s originally forested watershed was cleared for farming and, starting in the 1950s underwent agricultural intensification. Since the 1980’s, urbanization rapidly increased the watershed’s impervious land cover, now accounting for about 60% of the total surface area. The results illustrate the absolute and relative changes in P external and internal loading resulting from the LULC changes and the implementation of various agricultural and urban stormwater management practices. By analyzing the sediment core data, I reconstructed the historical P loading patterns, as well as the response of the lake's P dynamics to the evolving human activities in the watershed. The results of this chapter highlight the large differences in the impact of agricultural versus urban land use on the lake’s P budget and cycling, and on other aspects of the lake’s biogeochemistry. Chapter 3 focuses on the most recent phase of rapid urbanization of Lake Wilcox’ watershed. Of particular interest is to understand why Lake Wilcox remains in an apparent eutrophic state even though external P inputs to the lake have been declining since the 1980s. I analyzed 22 years of water chemistry, land use, and climate data (1996–2018) using principal component analysis (PCA) and multiple linear regression (MLR) to identify the contributions of climate and urbanization to the observed changes in water chemistry. The results show that the progressive salinization of the lake impacts the lake mixing regime by strengthening thermal stratification during summer. A major consequence is a worsening oxygen depletion of the hypolimnion that increases internal P recycling in the lake. My research therefore establishes a novel link between salinization and eutrophication symptoms. Building on the significant increase in salinity presented in the earlier chapters, Chapter 4 delves into a deeper investigation of the road salt management practices in the watershed of Lake Wilcox. I delineate the changes in geochemical water type in the period 2000-2020 while using mass balance calculations for dissolved chloride and sodium to reconstruct the yearly salt loading to the lake and the amounts of salt ions that are retained within the watershed. Results showed that further increase in salinity may eventually inhibit the fall overturn of the lake. They also point to the large salt legacies accumulating in the watershed, likely in soil and groundwater compartments. The fate of these legacies will require further research to determine the long-term risks they pose to water resources and receiving aquatic ecosystems. In chapter 5, I use water chemistry data for several other urban lakes in Ontario, Wisconsin, and Minnesota to analyze how lake salinization intersects with water temperature and lake morphometry to modify lake stratification. The goal is to determine to what extent salinization in these lakes can cause eutrophication-like symptoms such as those seen in Lake Wilcox. Trend analyses of chemical and physical variables are carried out for all the lakes, and the Brunt-Väisälä frequency is used as a measure of the summer stratification intensity. The results consistently indicate that salinity is becoming an increasingly stronger regulator of water density than temperature in urban freshwater lakes experiencing cold winters. Overall, my research demonstrates that rising salinity can have a significant impact on water column stratification of freshwater lakes. This, in turn, can reduce the oxygenation of the hypolimnion and enhance internal P loading from the sediments. These findings thus highlight that the management of salt inputs to urban lakes, including de-icing salt applications in cold and cold-temperate regions, should be taken into consideration to control lake eutrophication symptoms.