Abstract. 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 (Ali) 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 Ali concentrations. Surprisingly, our results show the widespread
and frequent occurrences of Ali 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 Ali, is the strongest predictor (positive) of
Ali 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 Ali levels are
associated with storm flow, high Ali concentrations are found during
base flow. Our results demonstrate that elevated Ali concentrations in
Nova Scotia continue to pose a threat to aquatic organisms, such as the
biologically, economically, and culturally significant Atlantic salmon
(Salmo salar).
Reactivity continuum of dissolved organic carbon decomposition in lake water