Using Stream Chemistry to Evaluate Experimental Acidification and Natural Recovery in the Paired Catchments at the Bear Brook Watershed in Maine

Acidic deposition from the burning of fossil fuels has been studied extensively for more than 40 years. The amounts of sulfate (SO42-) and dissolved inorganic nitrogen (DIN: NO3- + NH4+) in deposition has fluctuated through time due to increased use of fossil fuels and emissions control legislation. Even though biologically-relevant chemistry in many surface waters has improved due in part to the Clean Air Act Amendments of 1990 (CAAA), questions still remain on how the processes of acidification and recovery affect biogeochemical cycles. The Bear Brook Watershed in Maine (BBWM) project provides a long-term record of deposition and catchment processing of SO42- and DIN in two adjacent ~10 ha catchments. West Bear has been experimentally acidified with 1800 eq/ha/yr dry ammonium sulfate ((NH4)2SO4) since 1989. East Bear serves as the reference catchment where chemistry reflects ecosystem changes including SO42- emission reductions mandated by the CAAA. Long term trends in surface water flux of the base cations Ca2+ and Mg2+, SO42-, DIN, aluminum (Al), and hydrogen (H+) were contrasted under scenarios of experimental acidification and natural recovery at BBWM. A major question I addressed was how the West Bear chemical response evolved during the experimental acidification. The responses determined by stream and precipitation chemistry included changes through time, and differences between catchments, for base cations, Al, H+, DIN and SO42-. Annual and monthly fluxes were calculated at BBWM for 1989 through 2003. Mass balances for SO42- and NO3- were also estimated. Long-term trends in annual export were determined using the Seasonal Kendall Tau test. From 1989-2003, East Bear stream chemistry revealed significant (p\u3c0.05), base cation, and A1 export. West Bear fluxes of these solutes showed little change after 1995; therefore, the data were split into two time periods and analyzed separately for trends. From the start of the treatment in 1989 until 1995, West Bear stream chemistry revealed significant increases in SO42-, Al, and H+ export. The only significant trend from 1996-2003 in West Bear was a decline in base cation export. Mass balance estimates for DIN showed high retention in both the reference (94%) and treated (84%) catchments. East Bear averaged 100 eq/ha/yr net export of SO42- when dry deposition was estimated as 130% of wet. West Bear has retained an average 900 eq/ha/yr SO42-, or about half of the experimental treatment, since 1995. West Bear has been resilient to the long-term experimental treatment contrary to MAGIC model predictions that stream concentration and mass export of SO42- would continue to increase above 1995 levels. SO42- flux leveled off after the initial rapid increase from the start of the treatment, suggesting a new steady-state was reached. West Bear\u27s resiliency indicates an unexpectedly high capacity for SO42- retention in the acidified catchment that has consequences for continued net export of SO42- under scenarios of lower acidic deposition