The aim of collecting data was to investigate the change in nitrate, phosphate, silicate, oxygen and chlorophyll concentrations at 8 stations along the estuary. At stations 33, 34, 35 and 36, samples were collected at three different depths using a set of Niskin bottles attached to a CTD rosette. There were fired acoustically from the sorfware at selected depths estimated from the flurometer readings. At stations N, O, P and Q, Samples were collected at the water surface using a horizontal Nisking bottle and closed using a hand messenger. 6 plastic sample bottles were rinsed and filled for each set of water samples collected. The chemical methods are as used by Johnson & Petty, 1983 for Nitrate, as used by Parsons et al, 1984 for Silicate and phosphate and as used by Grasshoff et al, 1999 For dissolved oxygen.

The estuarine mixing diagram shows that the nitrate concentration decreases as the salinity increases (fig. 20). At the top of the estuary, where the salinity is 0, its concentration is 305.00μmol/L whereas at the bottom of the estuary, where salinity is 35, the nitrate concentration is significantly lower, at 0.25μmol/L. The data points follow its theoretical dilution line (TDL) between the two endmembers, representing the conservative behaviour of nitrate (Loder and Reichard 1981).

The Silicon concentration decreases as the salinity increases on the estuarine mixing diagram (Figure 21). In fresh water the silicon concentration is 110mmol/L and decreases to 0mmol/L in marine water. Silicon concentrations portray conservative behaviour as the data follows the TDL (Loder and Reichard 1981).

Figure 20. Nitrate estuarine mixing diagram. Samples collected on boats Bill Conway and Winnie the Pooh the 8th July 2017 from 8:00 UTC to 14:00 UTC. The samples were collected via Niskin bottles attached to a rosette on Bill Conway, and a horizontal Niskin bottle on Winnie the Pooh, from the river Allen to Black Rock.

Figure 21. Silicon estuarine mixing diagram. Samples collected on boat Bill Conway and Winnie the Pooh the 8th July 2017 from 8:00 UTC to 14:00 UTC. The samples were collected via Niskin bottles attached to a rosette on Bill Conway, and a horizontal Niskin bottle on Winnie the Pooh, from the river Allen to Black Rock.

Figure 22. Phosphate mixing diagram. Samples collected on boat Bill Conway and Winnie the Pooh the 8th July 2017 from 8:00 UTC to 14:00 UTC. The samples were collected via Niskin bottles attached to a rosette on Bill Conway, and a horizontal Niskin bottle on Winnie the Pooh, from the river Allen to Black Rock.

The estuarine mixing diagram of phosphate concentration (fig. 22) decreases with increasing salinity, however does not follow the TDL. Phosphate concentration is 1.6μmol/L in freshwater and 0.3μmol/L in full marine water. Phosphate is presented to have non-conservative behaviour as the plotted concentrations are significantly above the TDL. This indicates an addition of phosphate in the estuary. This could be due to the presence of a sewage treatment outlet (Designatedsites.naturalengland.org.uk, 2017) next to the estuary stations sampled by Winnie the Pooh, as well as a scrap yard slightly further up the estuary.

Disclaimer: The views and opinions expressed are those of the contributors and do not reflect the views and opinions of the University of Southampton

Nitrate and Silicate both appear to follow the Theoretical Dilution Line, which suggests they behave conservatively - the change is concentration is due to mixing of silicate rich fresh water and silicate deprived seawater (Boyle et al. 1974). There are many tributaries joining to the estuary which effect the assumptions of the TDL, but the results suggest the only factor effecting these concentrations is mixing. Phosphate however seems to behave non-conservatively, with the data being above the TDL, suggesting input of phosphate. This could be due to the sewage outlet recorded as 30 yards away from the first station on Winnie the Pooh, or the scrapyard also recorded as near the first station. These may be polluting the river with phosphate, causing higher concentrations than expected. Collecting samples from further up the river may confirm this theory, but data collection was limited by tide. Combining this data with data from other groups studying Falmouth estuary over a longer time period will give a much better indication as to the behaviour of the nutrients.

Boyle, E., Collier, R., Dengler, A.T., Edmond, J.M., Ng, A.C. and Stallard, R.F., 1974. On the chemical mass-balance in estuaries. Geochimica et Cosmochimica Acta, 38(11), pp.1719-1728.

Environment agency. 2017. South West Water fined for pollution near shellfish beds. [ONLINE] Available at: https://www.gov.uk/government/news/south-wet-water-fined-for-sewage-pollution-near-shellfish-beds . [Accessed 7 July 2017]

Loder, T.C. and Reichard, R.P., 1981. The dynamics of conservative mixing in estuaries. Estuaries and Coasts, 4(1), pp.64-69.