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Estuary Chemistry

Results

Silicate, Phosphate and Nitrate concentrations calculated from water samples taken from the Fal estuary on-board the Bill Conway and Winnie the Pooh vessels were plotted against salinity, here used as an index for conservative mixing.

From figure 1 we can see that the silicate concentrations follow the theoretical dilution line quite well but being slightly below it throughout the estuary. The silicate concentrations drop slightly further below the TDL further up the estuary.

Figure 2 shows the change in Phosphate concentration with salinity. There is an increase in Phosphate higher up the estuary between salinities 5 - 12.5. It then begins to decrease as salinity increases further. All the points in this case remain above the TDL throughout the salinity range.

Nitrate concentration is shown in figure 3. The points follow the TDL fairly closely. At lower salinities from the Winnie the Pooh data the points are just above the TDL whereas the points from Conway are focused at the higher salinity end and sit below the TDL.

Discussion

The slightly lower silicate concentration in the upper estuary could be due to the severe lack of rainfall in the area during the week leading up to the date of sample collection. This may have led to reduced runoff from dissolved siliceous rock and so reduced silicate input to the upper estuary. It is reasonable to say that silicate still follows the TDL within the limits of error and so shows conservative behaviour in the Fal estuary.

The estuarine mixing diagram for phosphate shows it to behave non-conservatively in the Fal estuary with addition of phosphate to the upper estuary. A possible cause of this phosphate increase could be due to external factors affecting the phosphate input at these sampling locations. The two sampling locations furthest up the estuary; stations N & O were located very close to a ferry docking area and just below a multiple river convergent zone. Both of which could have contributed to a larger phosphate input through increased sewage runoff.

Nitrate shows conservative behaviour in the Fal estuary with only very slight addition in the upper estuary at lower salinities. This could be due to increases in remineralization or from other factors similar to that contributing to the addition of phosphate.