Geothermal Effects on Stream Benthos: Separate Influences of Thermal and Chemical Components on Periphyton and Macroinvertebrates

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Abstract
Stream microcosms were used in situ to evaluate the separate effects of the thermal and chemical components of geothermal fluids from natural hot springs (which simulate accidental releases from geothermal energy developments) on benthic microorganisms and macroinvertebrates of Big Sulphur Creek, a third-order stream at The Geysers, Sonoma Co., California, USA. The thermal component of geothermal fluids had greater influence than the chemical component in determining benthic community features in Big Sulphur Creek. A natural input of geothermal fluids to Big Sulphur Creek, which elevated both temperature (by 7.5°) and chemical levels (e.g. boron by 10 times, NO3-N by 4 times, PO4-P by 2 times), increased benthic chlorophyll a by 40 times, bacterial numbers by 10 times, and macroinvertebrate density by 2 times. The stream microcosms showed that heating of nongeothermal water by 8° (i.e. thermal exposure alone) similarly increased chlorophyll a by 40 times and bacterial numbers by 2 times, but macroinvertebrate density declined by one fourth. Cooling of geothermal water by 14.5° (i.e. chemical exposure alone) reduced algal productivity but increased macroinvertebrate density by 70 times. Naturally occurring geochemical concentrations apparently can be tolerated by many aquatic organisms in Big Sulphur Creek, but changes in water temperature significantly alter benthic community structure.