Regional application of an integrated biogeochemical model to northern new england and maine

An integrated biogeochemical model (PnET-BGC) was applied to 60 Direct/Delayed Response Program (DDRP) lake watersheds in northern New England and Maine to investigate processes regulating spatial and temporal patterns in lake SO₄^2- concentrations, past changes in the acid-base status of soil and surface waters, and their response to future emission control scenarios. Model simulations indicate that watershed elevation is an important factor influencing inputs of atmospheric S deposition to the watersheds and thus spatial patterns in lake SO₄^2- concentrations. Wetland S retention, vegetation composition, and thickness of surficial deposits were also found to influence lake SO₄^2- concentrations. Model simulations also suggest that decreases in lake SO₄^2- concentrations observed during 1984-2001 were mostly a result of decreases in atmospheric S deposition. Decreases in lake SO₄^2- concentrations were coupled with a near-stoichiometric decline in base cation concentrations. Comparison of atmospheric deposition of base cations and simulated drainage loss during this period indicates that decreases in atmospheric base cation deposition were partially responsible for decreases in lake base cation concentrations. Decreases in drainage loss of base cations were mainly a result of declines in net loss from soil exchange sites associated with decreases in SO₄^2- leaching. Model forecasts generally suggest further decreases in SO₄^2- concentrations and increases in acid neutralizing capacity (ANC) in lake water and soil percent base saturation (BS) at most of the DDRP lake watersheds under three future emission control scenarios, with the more aggressive control scenarios resulting in faster recovery from acidic deposition. Under an aggressive control scenario, surface water ANC is simulated to increase at a median rate of 0.30 μeq·L^-1·yr^-1 in northern New England and 0.22 μeq·L^-1·yr ^-1 in Maine. However, although marked improvement is expected, recovery to background conditions will probably not occur before 2050.