Trends, Patterns, and Drivers of Freshwater Aluminium Concentrations: Revisiting the Conceptual Model of Freshwater Acidification
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Aluminium (Al) is toxic to freshwater and terrestrial flora and fauna. Al can cause reduced primary productivity, species mortality and extirpation, reduced bird nesting success, reduced forest health resulting in reduced carbon dioxide uptake, and human neurological and osteological diseases. In Europe and North America, base cation (CB) depletion due to acid deposition is the primary cause of elevated freshwater Al concentrations. Since legislation reducing the emission of precursors to acid deposition was passed, several local scale studies documented freshwater acidification recovery. However, three recent continental and global scale studies on freshwater acidification recovery patterns indicate that acidification recovery may not be as widespread as once thought. Furthermore, the existing model of freshwater acidification does not explain the recovery patterns observed in these studies. Despite the threat of Al to ecosystems, local scale studies often do not focus on this element, resulting in an incomplete understanding of the drivers of freshwater Al concentrations. Additionally, global scale analyses have not examined Al, and global Al trends and concentrations remain unknown. My research focuses on filling these knowledge gaps by examining the trends, concentrations, and drivers of Al at both the regional and global scale. This thesis presents the identification, characterization, and causal attribution of short-term temporal fluctuations of Al and its species in Nova Scotia, Canada; it presents the creation of a global scale database of surface water chemistry and a use case of this database to identify, characterize, and attribute of the cause of long-term increasing Al trends across acidified regions; lastly, it presents how the aforementioned findings are used to update the existing 1983 model of freshwater acidification. Globally, two hotspots are identified where ecosystem health is threatened by increasing Al concentrations: Nova Scotia and Sweden. Contrary to existing conceptualizations, Al is predominantly predicted by freshwater organic carbon concentrations in these hotspots and other chronically acidified (low CB) regions. My research contributes empirically and theoretically to the field by changing and enhancing our understanding of freshwater acidification recovery, and by providing a high quality, openly available, global dataset of acidification-related surface water chemistry.