Effects of Thermal Additions From the Yellowstone Geyser Basins on the Benthic Algae of the Firehole River

Abstract

During the spring and summer of 1971 and 1972 quantitative measurements of temperature, water chemistry, and standing crop of benthic algal mats were made at a series of stations along the Firehold River as it flows through the main geyser basins of Yellowstone National Park. The river is an ideal study area for assessing the long—term effects of thermal additions, since it has been subject to heating effects for hundreds of years. Hot springs and geysers in the geyser basins add to the river heated and nutrient—enriched water of about one—third the volume of river water. As the river flowed through the thermal areas, the temperature gradually increased to a maximum daily midsummer temperature 12 degrees higher than the unheated upper portion of the river (15 degrees to 27 degrees C), and the river began cooling when the thermal additions to the river ceased. The river was coldest in early June, when maximum input from melting snow occurred, and remained relatively warm in the heated areas throughout the winter. Alkalinity, pH, conductivity, phosphate, and chloride also increased markedly; nitrate and ammonium concentrations appeared not to be influenced by the additions of hot spring water. The quantity of chlorophyll per unit area extracted from epilithic algal mats also increased with the amount of thermal water input to 20 times that in the unheated control areas. The apparent growth rates of benthic algae calculated from periodic quantitative sampling of cleaned rocks were over fivefold greater in the warmer than in the cooler portion of the river. The effect of temperature on light—dependent incorporation of (14)CO_2 by epilithic, epipelic, and epiphytic algal communities at different stations was measured. The temperature optimum for CO_2 fixation paralleled the midsummer temperature of the habitat although it averaged about 3 degrees higher at each station. The algae retained their midsummer temperature optima even during the early June period of cold water, suggesting that these optima are controlled by the mid—summer maximum temperature. Photosynthesis with natural populations was measured in the presence of increased concentrations of the algal nutrients to determine whether the increased algal growth rate in the warmer parts of the river might be due to nutrient enrichment rather than to temperature. The results suggest that higher temperatures from the additions of thermal water increase the algal standing crop through increase in growth rate of the algae, and that the algae in the heated portion of the river are optimally adapted to the temperatures of their habitats.