The influences of acid rain and geology on eastern brook trout populations in the White Mountains National Forest

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brook trout , White Mountain National Forest (N.H. and Me.)
Heisler, Ryan E.
Since the mid-1900s, Eastern Brook Trout (EBT) populations in the northeastern United States have been negatively impacted by acid rain through chronic and episodic acidification. The negative effects of acidification on fish are exacerbated when high concentrations of inorganic monomeric aluminum (IMAl), a common but potentially toxic cation, are mobilized from soil during precipitation events. However, the biological availability of IMAl is directly related to water's pH and concentrations of dissolved fluoride, organic carbon, and silicon anions, which have been shown to "buffer" aquatic life from the toxicity of IMAl. This study was conducted on wild populations of Eastern Brook Trout present in the White Mountains National Forest of New Hampshire, a region impacted in the past by chronic and episodic acidification. The White Mountains region offers a unique opportunity to document the potential influence of high fluoride concentrations on IMAl toxicity because much of the national forest contains a layer Jurassic granite bedrock, a contributor of fluoride to soils and waterways. This study’s objective was to determine if the concentrations of IMAl and fluoride explain observed variability in Eastern Brook Trout density, biomass, and relative weight. In order to address this objective, we established a sampling network of 15 sites across the White Mountains’ region that varied in the amount of underlying Jurassic Granite bedrock. We collected water chemistry data during baseflows and stormflows at each site twice during the course of the study. Habitat assessments were conducted during baseflows in order to assess the influence of water chemistry and habitat on EBT populations. EBT populations were sampled twice over the course of the study using three pass depletion backpack electro-fishing, and total and age 1+ density and biomass were calculated for each site. Using multiple linear regression, water chemistry, specifically IMAl and fluoride concentrations, were found to explain more variability in Eastern Brook Trout population density and biomass than habitat metrics. We found stormflow and baseflow concentrations of IMAl and fluoride helped explain EBT density and biomass but had minimal predicted impacts on the population metrics from a management perspective. The directional relationships between IMAl and EBT population metrics conflicted with previously published studies, which we believe is at least partially explained by the fluoride concentrations attributed to underlying bedrock. We found a significant interaction between stormflow IMAl and fluoride when explaining density and biomass variability, suggesting a context-dependent relationship in the presence of the other. High concentrations of IMAl and fluoride, surpassing previously documented lethal thresholds, were recorded at locations associated with a high percentage of underlying Jurassic granite. We conclude that the effects of IMAl and fluoride on EBT populations are influenced by underlying geology and therefore spatially dependent within the White Mountains National Forest.