The Separation of Thermal and Chemical Effects in Evaluating Geothermal Influences on Aquatic Biota

In this study a field based, stream microcosm system to experimentally separate the effects of the thermal and chemical components of geothermal fluids on the benthic community was developed. Using this system in the separate and combined influences of these components on the numerical abundance of bacteria, the standing crop and productivity of algae, and the density, standing stock, and community structure of macroinvertebrates were evaluated. This microcosm study was designed on the basis of the results from an in-stream study which was a natural comparison of benthic communities in geothermally influenced and uninfluenced stream segments.Additionally, a new type of field experiment to determine the upper lethal temperature thresholds for three insect species found in a geothermally influenced stream was developed. This heat shock experiment simulated acute thermal effects on organisms that drift into the lethal temperature zones that seasonally occur in geothermal streams. It also mimicked the effects of heated effluents on aquatic organisms that are drawn into the cooling water intake of a power plant, or entrained in the discharge plume.Although the in-stream study demonstrated that a moderate addition of geothermal fluids into a non-geothermal stream could substantially alter the structure of the benthic community, the analysis did not establish the cause of the alteration. The stream microcosm study demonstrated that the thermal component of those geothermal fluids had greater influence than the chemical component in determining the structure of the benthic community