ECOSYSTEM DISTURBANCE AND CHANGES IN STREAM CHEMISTRY IN AN OLD-GROWTH FORESTED WATERSHED IN OLYMPIC NATIONAL PARK, WASHINGTON

Old-growth temperate rain forests in coastal Washington are subjected to natural disturbances including wind throw, landslides, mass wasting events and stream debris flows. More recently there has been concern that air pollution from Asia may be impacting stream chemistry in these ecosystems. Our research objective was to determine time trends in stream chemistry in a coastal old-growth temperate rainforest over a 16-year period from 1985 to 2001 in West Twin Creek watershed (58ha) in the Hoh River Valley, Olympic National Park, Washington. Windthrow and a massive debris flow in 1999 did not seem to influence stream chemistry as much as increased atmospheric nitrate deposition between 1993 and 1996. This resulted in an increase in stream nitrate concentrations and a decline in pH; nitrate concentrations returned to near pre-1993 levels by 1997. Alkalinity decreased throughout the study period and that attributed to biological sources decreased strongly. Calcium, bicarbonate and sulfate were the dominant ions in the stream derived from the weathering of uplifted marine sediments in the watershed. There was also a considerable amount of nitrogen in these sediments, particularly in siltstone, and as much as 3250 kg N/ha may be contained in rocks in the upper 20 cm of unweathered parent material. The presence of this large pool of nitrogen may predispose the forest and aquatic systems to sensitivity to atmospheric nitrogen inputs.