Hydrologic and Landscape Drivers of Seepage Lake ANC Status in Northern Wisconsin, USA

Abstract

In some seepage lakes, acid neutralising capacity (ANC) is regulated by precipitation chemistry and in-lake biogeochemical processes with little influence from groundwater and catchment runoff. However, additional environmental contexts, for example, landscape position and changing precipitation dynamics, may also contribute to variability in ANC across lakes over time. We used a mixed effects model to assess the influence of changing precipitation patterns and the chemistry of hydrologic inputs on ANC in seven northern Wisconsin, USA seepage lakes from 1984 to 2018. We observed differential ANC responses across lakes to the concomitant changes in precipitation acidity and post-drought oxidative acidity pulses from adjacent wetland soils. Although all study lakes lie within a 4 km² area, mean lake ANC values ranged from 32.8 ± 25.2 μeq L⁻¹ to 71.9 ± 26.8 μeq L⁻¹ and were inversely related to lake landscape position. Lakes with higher mean ANC also showed lower relative variability over time, suggesting that these lakes were better buffered than those with lower ANC. Mixed effects modelling explained 50% of the ANC variability across all lakes, while model explanatory power ranged from 39% to 71% when ANC was assessed within individual lakes. The range of modelled ANC accuracy highlights the complex nature of biogeochemical regulation in individual seepage lakes, despite their shared geographic setting. Our study demonstrates the utility of mixed effects modelling for repeated measures analysis, emphasising the necessity of long-term studies to evaluate seepage lake ANC responses to changing hydrologic and biogeochemical inputs.