R.J.M. Howland

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.

Concentrations of suspended chlorophyll in the tidal Yorkshire Ouse and Humber Estuary

Measurements of salinity, suspended particulate matter, chlorophyll a and phaeopigments in the tidal reaches of the Yorkshire Ouse and Humber Estuary, UK, are presented for the period March 1994–March 1995. Chlorophyll a concentrations were<3 μg l−1 throughout the tidal Ouse and Humber during March and April 1994. Generally, concentrations were also low in the non-tidal Yorkshire Ouse (<4 μg l−1). Increases in chlorophyll a in the non-tidal river during May–August corresponded with increased concentrations in the tidal river, with highest concentrations occurring at the lowest observed salinities. Large declines in chlorophyll a concentrations, from 60 to<13 μg l−1, occurred along a transect from the non-tidal river to the tidal river during summer, which probably resulted from a combination of net negative growth rates and mortality in the extremely turbid receiving waters. Chlorophyll a concentration decreased to<10 μg l−1 (but usually<5 μg l−1) in the non-tidal river during autumn and was less than 1 μg l−1 throughout much of the winter. Concentrations were also at a minimum within the tidal river and Humber. However, a noticeable feature was a local maximum of chlorophyll a at the interface between fresh and brackish waters, which was associated with the zone of higher turbidity.

Concentrations of dissolved nutrients in the tidal Yorkshire Ouse River and Humber Estuary

Abstract Measurements of dissolved nitrate, silicate, phosphate, nitrite and ammonium in the tidal reaches of the Yorkshire Ouse and Humber Estuary, UK, are presented for the period March 1994 to March 1995. The data and their spatial trends are consistent with independent measurements derived from both freshwater and coastal measurements. Nitrate was typically less than about 550 μM. Concentrations were relatively low in the outer Humber and coastal zone and increased strongly up-estuary, reaching a mid-estuarine peak before decreasing to the lower concentrations that characterized the non-tidal river. The nitrate peak was well defined during summer and early autumn, when a strong maximum in turbidity occurred. Silicate distributions were similar to nitrate but with mid-estuarine maxima in the range 90–260 μM. These maxima were closely associated with the high turbidity zone during summer and autumn. Phosphate was dominated by freshwater inputs during spring through late summer, but exhibited a mid-estuarine peak between 5 and 10 μM and located several km up-estuary of the freshwater-saltwater interface (FSI) during the rest of the year. A pronounced peak in nitrite, between 9 and 12 μM and associated with the high turbidity zone, was observed up-estuary of the FSI during summer and autumn followed by a sharp drop to low concentrations. This nitrite depletion zone was located down-estuary of the high turbidity zone. Twin, mid-estuarine ammonium maxima were observed during winter. The up-estuary maxima (∼35 μM) were located about 30-km up-estuary of the FSI and the higher turbidity zone. Nutrient distributions are discussed in terms of physical and local, source-sink processes, and the various inputs from freshwater tributaries and wastewater discharges.