Seasonality of epCO(2) at different scales along an integrated river continuum within the Dee Basin, NE Scotland

Spatial and temporal variations of free CO2 concentrations in surface waters are mainly controlled by dynamic processes encompassing terrestrial inputs and in-stream biotic cycling. Free CO2 can be expressed as 'excess partial pressure of CO2' (epCO(2)), indicating supersaturation or under-saturation with respect to atmospheric CO,. Seasonal patterns of epCO(2) at sites draining nested upland catchments between 3.40 and 1837 km(2) were assessed within the River Dee basin in NE Scotland. EpCO(2) values ranged from 0.14 at the lowermost site on the mainstem in autumn to 12.7 on a major tributary during spring.

A seasonality index was derived to describe contrasting winter/spring maxima and summer/autumn minima as annual mean epCO(2) values could not clearly distinguish between different sites. Seasonal differences tended to increase downstream as progressive changes in physicochemical conditions enhanced the influence Of autotrophic communities. However, perturbations to this continuum occurred as CO2 inputs from high DOC, heterotrophic tributaries and land-use changes between open moorland and forest affected downstream continuity. Major tributaries showed reduced differences between seasons compared to the mainstem. Smaller headwaters indicated a lack of seasonality as high connectivity of responsive, peaty soils enabled continual inputs of terrestrially derived free CO2 to streams concomitant with limited autotrophic CO2 removal, maintaining epCO(2) > 1 throughout.

Seasonality of epCO(2) was mainly driven by the ability of in-stream biota to consume CO2 during optimal conditions in summer/autumn. This was confirmed by multiple linear regression analysis, which indicated that, apart from catchment area, baseflow and biotic activity indicators were the best predictors of epCO(2) seasonality characteristics at any particular stage of the river system. Copyright (C) 2009 John Wiley & Sons, Ltd.