摘要：

The Bear Brook Watershed Manipulation project in Maine is a paired calibrated watershed study funded by the U. S. EPA. The research program is evaluating whole ecosystem response to elevated inputs of acidifying chemicals. The project consists of a 2.5 year calibration period (1987–1989), nine years of chemical additions of (NH 4 ) 2 SO 4 ( 15 N- and 34 S-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 SO 4 and NH 4 /ha/yr, a 200% increase over 1988 loading (wet plus dry) for SO 4 and 300% for N (wet NO 3 + NH 4 ). 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 cale-silicate gneiss intruded by granite dikes and sills. For the period 1987–1995, precipitation averaged 1.4 m/yr, had a mean pH of 4.5, with SO 4 , NO 3 , and NH 4 concentrations of 26, 14, and 7 μeq/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 weighied annual mean pH of approximately 5.4, alkalinity = 0 to 4 μeq/L, total base cations = 184 μeq/L (sea-salt corrected = 118 μeq/L), and SO 4 = 100 to 111.μeq/L. Nitrate ranged from 0 to 30 μeq/L with an annual mean of 6 to 25 μeq/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 DOC, significantly higher NO 3 , and the sea-salt effect. Depressions in pH were accompanied by increases in inorganic Al. The West Bear catchment responded to the chemical additions with increased export of base cations, Al, SO 4 , NO 3 , and decreased pH, ANC, and DOC. Silica remained relatively constant. Neutralization of the acidifying chemicals occurred dominantly by cation desorption and mobilization of Al.

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