Changes in streamwater chemistry after 20 years from forested watersheds in New Hampshire, USA

Long-term patterns of streamwater chemistry provide valuable evidence of th e effects of environmental change on ecosystem biogeochemistry. Observation s from old-growth forests may be particularly valuable, because patterns sh ould not be influenced by forest succession. Water samples were collected b iweekly from four streams in, and near, the old-growth forest watershed of the Bowl Research Natural Area in the White Mountains of New Hampshire from May 1973 through October 1974, and from June 1994 through June 1997. Avera ge NO3- concentrations, which ranged from 40.8 to 46.1 mu equiv..L-1 in 197 3-1974, declined significantly to averages of 14.9-20.1 mu equiv..L-1 durin g 1994-1997. Concentrations of the base cations, Ca2+ and Mg2+, also declin ed in stream water between the two sampling periods. The northeastern Unite d States, including the study area, has been subjected to elevated atmosphe ric deposition of sulfur and nitrogen for more than 40 years. This observat ion has led to the concern that mature forest ecosystems may exhibit N satu ration and depletion of Ca2+ from exchangeable soil pools. While the Bowl e xhibits a pattern of elevated concentrations of NO3- throughout the year, s uggestive of conditions of N saturation, concentrations have declined marke dly over the last 20 years. Concentrations of Ca2+ have also declined sugge sting possible depletion from the exchangeable soil pool, but the acid neut ralizing capacity of stream water has remained constant or increased, indic ating resistance to additional acidification.