Microthermal gradients and ecological implications in Dorset rivers

Abstract

Microthermal gradients, involving signi®cant 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, UK, which are groundwater dominated streams fed by chalk aquifers. In many of the sections surveyed, strong lateral contrasts of up to c. 7 8C were evident as a consequence of solar heating of shallow channel margin zones and thin surface layers isolated by¯oating vegetation from the main body of¯ow. Shading by instream, emergent and riparian vegetation, and by river banks also caused signi®cant 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 temperature gradients and considerable local variation in the substratum temperature pro®les of a pool-rie sequence. The latter did not conform to the pattern expected from advective heat transfer associated with downwelling of water at the rie head and upwelling at the tail, and measurements of interstitial¯ow velocities and particle size suggested more complex¯ow circulation and heat transfer. There was some evidence that the microthermal gradients identi®ed were of ecological signi®cance.

Predicting water temperatures using a deterministic model: Application on Miramichi River catchments (New Brunswick, Canada)

“…The consideration of streambed heat flux in 1998 at Catamaran Brook, where limited data were available, showed that an improvement of less than 0.1°C could be achieved with the consideration of streambed fluxes. Groundwater and geothermal contributions have been observed to modify the thermal regime of rivers as reported in other studies (Clark et al, 1999). Heat flux at the sediment water interface has also been shown to be a contributing factor (Comer and Grenney, 1977;Hondzo and Stefan, 1994).…”

Factors influencing stream temperatures in small streams: substrate effects and a shading experiment

“…The influence of substrate type on diel temperature fluctuations is very dramatic in WS 3 because of the homogeneity of each reach type and the volume of water exposed to each substrate type. Most streams have high heterogeneity of substrate conditions, which may explain why the influence of specific substrates on in-stream temperature dynamics is not broadly recognized (i.e., Clark et al 1999).…”

“…Dampening of subsurface diurnal water temperatures has been documented as a function of depth into subsurface gravels (Ringler and Hall 1975;Evans et al 1998;Malcolm et al 2002). Influences of hyporheic exchange flows on surface stream temperatures have generally been observed only at microscales, such as upwelling locations or within small side channels (White et al 1987;Evans et al 1998;Clark et al 1999), creating important thermal refugia for biota.…”

Experimental investigations into processes controlling stream and hyporheic temperatures, Fryxell Basin, Antarctica

“…Another factor explaining the coincidence of the temperature and net shortwave radiation peaks is the shallowness of Fryxell Basin streams, typically less than 0.3 m. Vugts reported the rapid response of a small stream to meteorological conditions [53], and a study by Clark et al showed that water in shallow areas along the channel margins reached maximum and minimum daily temperatures an average of 180 and 100 min earlier, respectively, than water in the main channel [9]. Shallow depths may play a role in localized areas of heat gain in Von Guerard Stream.…”