Quantifying linkages between riparian shading, water temperature, and energetics of smallmouth bass and crayfishes in Ozark streams

Shading by riparian vegetation is an important factor influencing stream temperatures and may play a significant role in moderating effects of extreme summer temperatures on biota of coolwater streams in the Ozarks. The goals of this study were to quantify the influence of riparian shading on summer water temperatures in Ozark and Ozark border streams, to determine how water temperatures over the temperature range of 18–30°C affect growth scope of smallmouth bass and five species of Missouri crayfish, and to relate riparian shading and its effects on stream temperatures to growth conditions for these species during summer. A U.S.G.S. stream temperature model was tested in and adapted for northern Ozark and Ozark border streams and was used to develop relations between riparian shading and stream temperatures under the warmest conditions experienced during an average year. Relations between water temperature and maximum daily consumption and specific daily metabolic demand, the two primary components of growth scope, were developed for smallmouth bass and five species of crayfish with varying distributions in Missouri. Results indicated that riparian shading plays significant roles in reducing daily temperature fluctuation in northern Ozark and Ozark border streams and in reducing the rate of stream warming downstream from conduit springs. Results from laboratory studies indicated that adult smallmouth bass and three Ozark crayfishes had optimum temperatures for growth (OTG) of 22°C; growth scope declined rapidly for these organisms at higher temperatures. OTGs were higher for young-of-the-year smallmouth bass (29°C) and the crayfishes O. virilis and O. luteus (26°C). Riparian shading alone is insufficient to provide optimum thermal habitat for species with OTGs of 22°C during the warmest conditions experienced in an average year; best growth conditions for these species would occur in maximally-shaded streams near springs. However, reduction in water temperature fluctuation with increased riparian shading is potentially important in keeping stream temperatures from substantially exceeding OTGs for all species. Relations between riparian shading and water temperatures in Ozark and Ozark border streams have important implications for growth of smallmouth bass and crayfishes, as well as for fish and crayfish population dynamics and community structure. Results of this study were used to make recommendations for riparian management in Ozark and Ozark border streams with temperature-based objectives.