Oxygen dynamics and evaluation of the single station diel oxygen model across contrasting geologies

In aquatic ecosystems, the single station diel oxygen model assumes constant ecosystem respiration and aeration rate (notwithstanding temperature effects) over the course of a single night. This assumption was tested for four small streams representing two geologies (Chalk and Greensand) over a one year period, by examining the behaviour of the night-time dissolved oxygen (DO) saturation deficit at points where change in DO is zero. This was then used as a proxy for the ratio of aerobic ecosystem respiration (R) to the aeration rate constant (k) and compared with the corresponding ratio (the regression quotient) obtained from night-time regression analysis (Hornberger and Kelly, 1975). For two streams (one Chalk and one Greensand), the regression quotient persistently underestimated the observed DO deficit. These two streams showed similar timing patterns of oxygen dynamics with the point of minimum DO occurring relatively quickly after sunset in spring and early summer, although the two Chalk streams were more similar to one another in terms of DO magnitudes. Therefore, characterisation of a Chalk stream is not just dependent on geology. Comparisons between different streams using the single station model on the presumption that it is equally appropriate in all cases may lead to misleading conclusions.