The views and opinions expressed in this website are not necessarily those expressed by the National Oceanography Centre or the University of Southampton. The opinions and views expressed are those of the authors of the website

Figure 4.6 indicates that nitrate behaves non-conservatively in the Fal estuary, as it does not plot along the theoretical dilution line. This is due to removal of nitrate from the water. It is not possible to confirm any of the causes of removal but a possible cause is uptake by phytoplankton (Domingues et al., 2011). Concentrations of nitrate are higher the higher further up the estuary. This is partly due to land runoff, as agricultural runoff has a high concentration of nutrients, including nitrate. As well as this, closer to the mouth of the estuary, the freshwater is diluted in the seawater, and organisms such as phytoplankton use the nutrient for growth.

Figure 4.7 indicates how phosphate concentrations vary across the Fal estuary. It does not plot along the theoretical dilution line; therefore it did not behave conservatively during our period of study. There was variation in the concentrations of phosphate along the whole theoretical dilution line, especially around the river end members. Both removal and addition processes are implied by the position of the samples along the theoretical dilution line: addition could be due to pulse events such as soil disturbance or continuous events such as sewage or other waste disposal; removal could be due to settling to the sediments or use by organisms such as phytoplankton (Smith and Longmore, 1980).

Silicon, behaves relatively non-conservatively in the Fal estuary (Fig 4.8). Silicon is often correlated to phytoplankton population fluctuations, such as diatoms during the spring and autumn bloom where there is removal of silicon from the estuaries, as they require silicic acid to build external skeletal material (Paasche, 1973). During the period of study silicon concentrations did vary from the theoretical dilution line. This suggests that the concentration of silicon in the estuary is not purely determined by mixing processes but also by removal processes such as use by phytoplankton.

Estuarine mixing diagrams can be used to determine whether a chemical constituent is acting conservatively or non-conservatively in the Fal estuary. For the following, the river end members were provided at the lab because low salinities were not recorded during the survey.

REFERENCES

Domingues, R., Barbosa, A., Sommer, U. and Galvão, H. (2011). Ammonium, nitrate and phytoplankton interactions in a freshwater tidal estuarine zone: potential effects of cultural eutrophication. Aquatic Sciences, 73(3), pp.331-343.

Paasche, E. (1973). Silicon and the ecology of marine plankton diatoms. II. Silicate-uptake kinetics in five diatom species. Mar. Biol., 19(3), pp.262-269.

Smith, J. and Longmore, A. (1980). Behaviour of phosphate in estuarine water. Nature, 287(5782), pp.532-534.

Figure 4.6 Estuarine Mixing Diagram showing Nitrate in the Fal Estuary with a Theoretical Dilution Line,  hover over image to see zoomed samples

Figure 4.7 Estuarine Mixing Diagram showing Phosphate in the Fal Estuary with a Theoretical Dilution Line, hover over image to see zoomed samples

Figure 4.8 Estuarine Mixing Diagram showing Silicon in the Fal Estuary with a Theoretical Dilution Line, hover over image to see zoomed samples

Chemisty

Nitrate

Phosphate

Silicon

Standards

Figure 4.3 This is the Nitrate Standard for the known sample concentrations, calculated in the lab.

Figure 4.5 This is the Silicon Standard for the known sample concentrations, calculated in the lab.            

Figure 4.4 This is the Phosphate  Standard for the known sample concentrations, calculated in the lab.