Chemical flux in an acid-stressed stream

The acidity of rain and snow falling on widely separated areas of the world has been increasing during the past 30 yr (refs 1–3). Acid rainfall consists of a dilute solution of sulphuric and nitric acids due to the oxidation and hydrolysis of airborne sulphur and nitrogen4 and frequently has a pH of <4.0. Recent studies have shown that acid rain alters the chemistry and biology of streams and lakes in large regions of the world1,3. Results from reconnaissance studies in the field5–9 and physiological studies in the laboratory9 show that diversity and numbers of aquatic organisms of all major trophic levels are affected by low pH (high acidity). The quantitative effects of such acidification on biogeochemistry and biological function in natural streams have received little attention. In contrast, much is known about aquatic ecosystems affected by acid mine drainage10–13. However, waters receiving acid mine drainage show effects caused by metal contamination and deposition of iron oxide particulates, as well as acidification. In our study, we experimentally acidified a third-order section of a small mountain stream in the Hubbard Brook Experimental Forest, West Thornton, New Hampshire, USA. Our aim was to measure the effects of increased acidity on chemical and biological export in the stream. It was found that experimental stream acidification to pH 4 did alter the chemical and biological flux. The most significant inorganic component affected by the experiment was aluminium. A significant net flux of carbon and nitrogen occurred in the biologically bound forms but not in dissolved substances. Net flux of phosphorus was significant in biologically bound forms. The increased loss of nutrients in the particulate organic fraction was also important, particularly if scaled tb the total stream ecosystem.