Ionic aluminium concentrations exceed thresholds for aquatic health in Nova Scotian rivers, even during conditions of high dissolved organic carbon and low flow

• Main Authors: S. M. Sterling, S. MacLeod, L. Rotteveel, K. Hart, T. A. Clair, E. A. Halfyard, N. L. O'Brien
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-10-01
• Series: Hydrology and Earth System Sciences
• Online Access: https://hess.copernicus.org/articles/24/4763/2020/hess-24-4763-2020.pdf

<p>Acid deposition released large amounts of aluminium into streams and lakes during the last century in northern Europe and eastern North America. Elevated aluminium concentrations caused major environmental concern due to aluminium's toxicity to terrestrial and aquatic organisms and led to the extirpation of wild Atlantic salmon populations. Air pollution reduction legislation that began in the 1990s in North America and Europe successfully reduced acid deposition, and the aluminium problem was widely considered solved. However, accumulating evidence indicates that freshwater systems still show delays in recovery from acidification, with poorly understood implications for aluminium concentrations. Here, we investigate spatial and temporal patterns of labile cationic forms of aluminium (<span class="inline-formula">Al_i</span>) from 2015 to 2018 in 10 catchments in Nova Scotia, Canada; this region was one of the hardest hit by acid deposition, although it was not considered to have an aluminium problem due to its high dissolved organic carbon (DOC) concentrations that were expected to reduce <span class="inline-formula">Al_i</span> concentrations. Surprisingly, our results show the widespread and frequent occurrences of <span class="inline-formula">Al_i</span> concentrations that exceed toxic thresholds in all sampled rivers despite high DOC concentrations. Generalized linear mixed model results reveal that DOC, instead of being inversely related to <span class="inline-formula">Al_i</span>, is the strongest predictor (positive) of <span class="inline-formula">Al_i</span> concentrations, suggesting that the recruitment properties of DOC in soils outweigh its protective properties in streams. Lastly, we find that, contrary to the common conceptualization that high <span class="inline-formula">Al_i</span> levels are associated with storm flow, high <span class="inline-formula">Al_i</span> concentrations are found during base flow. Our results demonstrate that elevated <span class="inline-formula">Al_i</span> concentrations in Nova Scotia continue to pose a threat to aquatic organisms, such as the biologically, economically, and culturally significant Atlantic salmon (<i>Salmo salar</i>).</p>