Seasonal variability of salinity, temperature, turbidity and suspended chlorophyll in the Tweed Estuary

Results are presented from a campaign of measurements that were undertaken to examine seasonal variability in physical and chemical fluxes and process es within the Tweed Estuary during the period September 1996-August 1997, T he study utilised monthly surveys, each of approximately 1 week duration. T his article interprets a subset of the salinity, temperature, turbidity [su spended particulate matter (SPM) levels] and chlorophyll a data. Measuremen ts discussed here were obtained throughout the estuary during high-speed tr ansects that covered the region between the tidal river and the coastal zon e. Longitudinal distributions of surface salinity depended strongly on fres hwater runoff. During high runoff the surface salinity was low and the fres hwater-saltwater interface (FSI) was located close to the mouth. The revers e was true at times of low runoff. Salinity stratification was generally st rong. During the surveys, river runoff temperatures ranged from approximate ly 2 to 18 degrees C and coastal waters (similar to 33 salinity) from appro ximately 6 to 15 degrees C. Turbidity was low throughout the campaign (SPM < 30 mg l(-1)). Because of rapid flushing times (one or two tides), turbidi ty tended to mix conservatively between river and coastal waters. Higher co astal turbidity was associated with stronger wind events, and higher fluvia l turbidity with spate events. Suspended chlorophyll a levels were usually low throughout the estuary (typically <2 mu g l(-1)) and showed large spati al variability. Because of the rapid flushing of the estuary, it is hypothe sised that it was not possible for several algal cell divisions to occur be fore algae were hushed to the coastal zone. A 'bloom' occurred during the M ay 1997 survey, when chlorophyll a levels reached 14 mu g l(-1). Higher chl orophyll a concentrations at that time occurred at very low salinities, ind icating that these waters and algae were largely fluvially derived, and may have resulted from increasing springtime solar irradiation. (C) 2000 Elsev ier Science B.V. All rights reserved.