Rainfall events and their influence on nutrient distributions in the Ythan estuary (Scotland)

For a 34-day period in Autumn 1994, automatic water samplers were installed at the gauging station on the River Ythan and on a catamaran mooring in the estuary. Water samples were analysed for total oxidized nitrogen (TON), soluble reactive phosphorus (SRP) and silicate. There were two significant rainfall events during the experiment. On both occasions, TON and silicate concentrations in river water at the gauging station decreased during the periods of increased river flow; in contrast, those of SRP increased. Total oxidized nitrogen/salinity plots indicated that changes in TON concentrations in river water determined distributions within the estuary and that the response was rapid (<1 day). Relative to the short time scale (1-2 days) of changes in freshwater composition, the flushing of the estuary was slow (5-12 days). The relatively slow flushing of the estuary resulted in a delayed response within the estuary to the observed increase in TON freshwater concentration which followed the end of each rainfall event. The flow-related concentration changes for silicate in freshwater were too small to be detected in the estuary. Under the low river flow conditions experienced during the experiment, phosphorus inputs to the estuary were dominated by those from a sewage treatment facility (STW) located below the sampling/gauging point on the river. Consequently, SRP concentrations in the estuary were higher than anticipated on the basis of results from the gauging station. A simple model was used to demonstrate that the dilution effect of increased river flow on phosphorus input from the STW was more important in controlling estuarine SRP concentrations than the increase in SRP observed at the gauging station. During periods of higher river flow, SRP concentrations in the estuary showed a decrease rather than the increase anticipated on the basis of the gauging station results. These results identify a requirement for more sophisticated modelling and highlight some of the practical difficulties associated with testing these. (C) 1997 Academic Press Limited.