Heat exchanges and temperatures within a salmon spawning stream in the Cairngorms, Scotland: Seasonal and sub-seasonal dynamics

Stream temperatures are often used to predict salmonid embryo development; but there are very few medium-term studies of the heat exchanges determining water column and bed temperatures. Furthermore, no research exists on the energy balance for sub-arctic Scottish rivers. This paper reports results of a hydrometeorological study of a Cairngorm stream (Girnock bum, northeast Scotland) over the salmon spawning-hatch season (late October 2001 to mid-April 2002) that aims: (1) to characterize seasonal and sub-seasonal stream energy budget and thermal dynamics; and (2) to explain these variations with respect to meteorological and hydrological factors. In terms of average energy flux contributions, sensible heat (38.7%), the bed heat flux (37.0%) and friction at the stream bed and banks (24.3%) are heat sources, while latent heat (73.1 %) and net radiation (26.9%) are heat sinks. All energy losses and 38.7% of heat gains occur at the air-water interface; and 61.3% of energy gains (including friction) take place at the water-channel bed interface. Typically, temperatures increase (+ 1.97degreesC) and show dampening of thermal response from the water column to depth in the stream bed. The most salient findings include: (1) the stream bed (atmosphere) is the dominant energy source (sink) for heating (cooling) channel water, which may be attributed to inferred heat advection by groundwater up-welling into the bed of this upland stream; (2) sensible heat is the primary atmospheric energy source due to limited net radiation; (3) friction at the stream bed and banks is an important heat source. Energy budget terms and temperatures exhibit (sub-)seasonal changes in response to meteorological and hydrological conditions; a schematic diagram is presented to summarize these results. This paper clearly illustrates the need for further medium- to long-term empirical stream energy balance research to characterize heat flux dynamics and, thus, understand and predict water temperature variations over time-scales of relevance to biological studies. Copyright (C) 2004 John Wiley Sons, Ltd.