Microthermal gradients and ecological implications in Dorset rivers

Microthermal gradients, involving significant variations in temperature over 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 of the sections surveyed. strong lateral contrasts of up to c. 7°C were evident as a consequence of solar heating of shallow channel margin zones and thin surface layers isolated by floating vegetatio- n from the main body of flow. Shading by instream, emergent and riparian vegetation, and by river banks also caused significant microthermal gradients in the water column. Detailed logging of temperatures in the substratum at selected sites revealed damping of variation with increasing depth below the bed surface, seasonal reversal in bed temperatur- e gradients and considerable local variation in the substratun temperature profiles of a pool-riffle sequence. The latter did not conform to the pattern expected from advective heat transfer associated with downwelling of water at the riffle head and upwelling at the tail, and measurements of interstitial flow velocities and particle size suggested more complex flow circulation and heat transfer. There was some evidence that the microthermal gradients identified were of ecological significance.