The significance of dissolved carbon dioxide in major lowland rivers entering the North Sea

Data on excess carbon dioxide partial pressures (EPCO2) in 15 major rivers and tributaries entering the North Sea are presented and compared with basin-wide information for the Humber region. Large variations are observed with values ranging from less than a tenth to over a hundred times saturation. There is a marked geographical change with median concentrations typically increasing from north (EpCO(2) < 1) to the south (EpCO(2) about 10) as the influence of industrial and urban pollution increases. This is most clearly indicated with median values being strongly and linearly correlated with pollution indicators such as dissolved organic carbon, ammonium, nitrite and (to a lesser degree) soluble reactive phosphorus and nitrate. The distribution of excess partial pressures changes from one of high skew for the cleaner rivers towards a normal distribution for the more polluted rivers. The broad features are explained in terms of the relationship between the biological effects of photosynthesis and respiration across the region. However, the detailed patterns observed are complex for each river: the data scatter is high and there is little relationship for EpCO(2) with the nutrients, flow and temperature. Broadly, the lowest concentrations are observed during the spring to autumn period when the rivers experience the highest algal activity. The majority of the rivers have CO2 concentrations above the atmospheric equilibrium and act as local sources to the atmosphere. Estimates of the rates of degassing of CO2 for one of the rivers, the Swale, show losses generally > 100 mmol/m(2)/day but reaching over 1000 mmol/m(2)/day at times. This loss is compensated for by tributary inputs and in-stream generation such as respiration of labile organic compounds derived from effluents, thus explaining the good correlation between dissolved organic carbon and EpCO(2). (C) 1998 Elsevier Science B.V.