Effects of stress from mine drainage on ecosystem functions in Rocky Mountain streams

Research in this dissertation tested a hypothesis that relates biodiversity, community biomass, and ecosystem function to a gradient of stress. Biodiversity was predicted to have a low threshold of response to stress, while biomass and function were predicted to be stable or increase under low to moderate stress, and decrease only under high stress. This hypothesis was tested by examination of biological communities and ecosystem functions in mountain streams under stress from mine drainage. Mine drainage presents both chemical (low pH, dissolved metals) and physical (deposition of metal oxides) stresses on stream biota.; Stress from mine drainage reduced the diversity of all communities that were examined. Low pH and high concentrations of dissolved metals such as zinc were closely related to decreases in diversity in stressed streams compared to pristine streams. The biomass and function of communities were usually high in streams with only chemical stress, but were usually depressed in streams with physical stress from metal oxide deposition. Functions had different thresholds of response to chemical stress. Primary production and fungal activity on decomposing litter were high in even the most stressed sites. Two ecosystem functions were especially sensitive to chemical stresses. Litter breakdown by shredding invertebrates was sensitive to stress from dissolved zinc; and nitrification was completely inhibited at low pH and high concentrations of zinc.; Physical stress from the deposition of metal oxides usually decreased biomass and function of stream communities even at low to moderate stress, contrary to the hypothesis. The effect of physical stress was complicated by the fact that the type of metal oxide (iron versus aluminum) had different effects on biomass and function of primary producers. Algal biomass and production could be high at sites with deposition of iron oxides, but was usually low at sites with deposition of aluminum oxides.; Overall, chemical stresses (low pH, elevated zinc) did produce the hypothesized trends in ecosystems under stress. Physical stress (deposition of metal oxides), however, usually decreased biomass and function at even low stress, contrary to the original hypothesis. Thus, ecosystem response to stress can differ depending on the nature of the stress (chemical or physical).