Microthermal gradients and ecological implications in Dorset rivers

Microthermal gradients, involving significant variations in temperature ove r distances of a few centimetres to a few metres, were investigated in the water column and substratum of the River Frome and Bere Stream in Dorset, U K, which are groundwater dominated streams fed by chalk aquifers. In many o f the sections surveyed, strong lateral contrasts of up to c. 7 degrees C w ere evident as a consequence of solar heating of shallow channel margin zon es and thin surface layers isolated by floating vegetation from the main bo dy of flow. Shading by instream, emergent and riparian vegetation, and by r iver banks also caused significant microthermal gradients in the water colu mn. Detailed logging of temperatures in the substratum at selected sites re vealed damping of variation with increasing depth below the bed surface, se asonal reversal in bed temperature gradients and considerable local variati on in the substratum temperature profiles of a pool-riffle sequence. The la tter did not conform to the pattern expected from advective heat transfer a ssociated with downwelling of water at the riffle head and upwelling at the tail, and measurements of interstitial flow velocities and particle size s uggested more complex flow circulation and heat transfer. There was some ev idence that the microthermal gradients identified were of ecological signif icance. Copyright (C) 1999 John Wiley & Sons, Ltd.