Distributions and seasonal variability of pH and alkalinity in the Tweed Estuary, UK

Monthly axial profiles of the Tweed Estuary were carried out between July 1996 and August 1997 as part of the UK Natural Environment Research Council Land Ocean Interaction Study. During these surveys, master variables and a range of chemical constituents, including pH and total alkalinity, were measured between the mouth of the estuary, at Berwick-upon-Tweed, and the freshwater tidal reach, at Union Bridge, approximately 10 km up-estuary. Alkalinity and pH showed clear seasonal variations in values within the lower salinity region of the estuary, and these could be related largely to changes in river flows, and hence to variations in the ratios of the source waters. River waters at high flows were dominated by poorly buffered surficial runoff, and pH and alkalinity values were low; conversely at low flows, groundwaters rich in weathered bedrock ions dominated the river water, and pH and alkalinity values were significantly higher. Biological production and respiration also had an effect on this seasonal variability, although this was secondary. The behaviour of pH and alkalinity within the estuary appeared generally conservative, although apparent non-conservative distributions were observed in the freshwater tidal reach. In a number of cases this behaviour could be attributed to biological activity. Estimated values of the excess partial pressures of CO2 within the fresh-water tidal reach were low (< 4), reflecting the relatively pristine character of the Tweed. During the spring and summer, CO2 was significantly undersaturated with respect to the atmosphere; extensive production by bed-anchored macroalgae was probably the reason for this, although the effects of water column algae cannot be discounted. In winter, production was minimal and respiration more important. As a result, partial pressures within the water column increased to a maximum of approximately four times atmospheric. Only infrequently was the CO2 in equilibrium between the river and atmosphere, suggesting that the tidal reach of the Tweed is a dynamic environment with respect to carbon cycling processes.