Five Aluminum Seasonality Regimes Identified in Chronically Acidified Rivers of Nova Scotia.

Despite reductions in acidic deposition, high freshwater Al concentrations continue to threaten acidified ecosystems across the northern hemisphere. Seasonally elevated Al concentrations may pose a particular threat to freshwater organisms. Despite this threat, there is a lack of understanding about the timing and drivers of seasonal Al fluctuations. Here, we address this knowledge gap by identifying seasonal patterns of Al and their drivers in 16 rivers across Nova Scotia, Canada. We identify five distinct Al regimes with different timing of seasonally elevated Al concentrations. Regimes are distinguished by correlation strength and direction between Al and base cations, total organic carbon, turbidity, and discharge. Most notably, regimes are distinguished by a gradient of Al-base cation decoupling as Ca and Mg concentration approaches 1.4 mg L-1 and 0.6 mg L-1, respectively. Seasonally elevated Al concentrations exceeded the 0.1-0.2 mg L-1 World Health Organization drinking water guidelines in all regimes, and inorganic monomeric Al is projected to exceed the 15 µg L-1 threshold for aquatic health in most rivers. This research highlights the complexity of seasonal Al dynamics and the importance of understanding seasonal variation of Al to quantify the impact of Al on human health, water treatment, and aquatic organisms.

Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America.

Climate warming is projected to affect forest water yields but the effects are expected to vary. We investigated how forest type and age affect water yield resilience to climate warming. To answer this question, we examined the variability in historical water yields at long-term experimental catchments across Canada and the United States over 5-year cool and warm periods. Using the theoretical framework of the Budyko curve, we calculated the effects of climate warming on the annual partitioning of precipitation (P) into evapotranspiration (ET) and water yield. Deviation (d) was defined as a catchment's change in actual ET divided by P [AET/P; evaporative index (EI)] coincident with a shift from a cool to a warm period - a positive d indicates an upward shift in EI and smaller than expected water yields, and a negative d indicates a downward shift in EI and larger than expected water yields. Elasticity was defined as the ratio of interannual variation in potential ET divided by P (PET/P; dryness index) to interannual variation in the EI - high elasticity indicates low d despite large range in drying index (i.e., resilient water yields), low elasticity indicates high d despite small range in drying index (i.e., nonresilient water yields). Although the data needed to fully evaluate ecosystems based on these metrics are limited, we were able to identify some characteristics of response among forest types. Alpine sites showed the greatest sensitivity to climate warming with any warming leading to increased water yields. Conifer forests included catchments with lowest elasticity and stable to larger water yields. Deciduous forests included catchments with intermediate elasticity and stable to smaller water yields. Mixed coniferous/deciduous forests included catchments with highest elasticity and stable water yields. Forest type appeared to influence the resilience of catchment water yields to climate warming, with conifer and deciduous catchments more susceptible to climate warming than the more diverse mixed forest catchments.