The Bear Brook Watershed, Maine (BBWM), USA

The Bear Brook Watershed Manipulation project in Maine is a paired calibrated watershed study funded by Be U.S. EPA. The research program is evaluating whole ecosystem response to elevated inputs of acidifying chemicals. The consists of a 2.5 year calibration period (1987-1989), nine years of chemical additions of (NH4)(2)SO4 (N-15- and S-34-enriched for several years) to West Bear watershed (1989-1998), followed by a recovery period. The other watershed, East Bear, serves as a reference. Dosing is in six equal treatments/yr of 1800 eq SO4 and NH4/ha/yr, a 200% increase over 1988 loading (wet pins dry) for SO4 and 300% for N (wet NO3 + NH4). The experimental and reference watersheds are forested with mixed hard- and softwoods, and have thin acidic soils, areas of 10.2 and 10.7 ha, and relief of 210 m. Thin till of variable composition is underlain by metasedimentary pelitic rocks and calc-silicate gneiss intruded by granite dikes and sills. For Be period 1987-1995, precipitation averaged 1.4 m/yr, had a mean pH of 4.5, with SO4, NO3, and NH4 concentrations of 26, 14, and 7 mu eq/L, respectively. The nearly perrenial streams draining each watershed have discharges ranging from 0 (East Bear stops flowing for one to two months per year) to 150 L/sec. Prior to manipulation, East Bear and West Bear had a volume weighted annual mean pH of approximately 5.4, alkalinity = 0 to 4 mu eq/L total base cations = 184 mu eq/L (seasalt corrected = 118 mu eq/L) and SO4 = 100 to 111 mu eq/L. Nitrate ranged from 0 to 30 mu eq/L with an annual mean of 6 to 25 mu eq/L; dissolved organic carbon (DOC) ranged from 1 to 7 mg/L but was typically less than 3. Episodic acidification occurred at high discharge and was caused by dilution of cations slightly increased DOG, significantly higher NO3, and the sea-salt effect. Depressions in pH were accompanied by increases in inorganic Al. The West Bear catchment responded to Be chemical addition; with increased export of base cations, Al, SO4, NO3, and decreased pH, ANC, and WC. Silica remained relatively constant. Neutralization of the acidifying chemicals occurred dominantly by cation desorption and mobilization of Al.