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A Mixed Reducing Reagent was made from 10 parts Metol-Sulphite solution, 6 parts Oxalic Acid solution, 6 parts Sulphuric Acid (50%) solution, and 8 parts MQ water added in that order. This was added to each of the samples, including the endmembers. A set of Calibration Standards was also produced. Spectrophometric Determination (the Hitachi U1800 Instrument) was used to analyse the absorbance capacity of the resultant mixtures.

The fresher samples (closer to A49) had higher Si concentrations and the more saline samples (clsoer to G59)had lower concentrations. The more saline samples seemed to have more stable concentrations throughout the water column compared to the more variable fresher samples.

Water samples were taken from the Niskin bottles and separated into sample bottles for lab analysis of nitrate, phosphate and silicate concentration. The water was filtered through a syringe with filter paper to remove any plankton, and stored in glass and plastic bottles to measure nutrient concentrations the following day. Silicate samples were stored in plastic bottles since glass contains silicon that could interfere with the results.

Nitrate samples were filtered once again in the lab, to ensure that all plankton had been removed, and run through a peristaltic pump which mixed the reagents (1% Sulphanilamide and 0.1% NEDH) together. The Cadmium reduced the nitrate to nitrite, which reacted with the reagents to produce a copper containing azo dye. This was then run through a UV spectrometer, which detected the concentration of the blue coloured solution. These results were transferred onto a chart recorder which printed a series of peaks, used to determine the nitrate concentration within the samples.

In general, the fresher water samples have higher nitrogen concentrations compared to the more saline samples. The shallower samples also showed higher concentrations of Nitrate compared to deeper samples from the same stations. The increase in depths correlates to the deepening of the river channel and was not a result of sampling preference.  

The standards and reagent were made and 3ml of the reagent was added to 30ml of the water sample. The samples were then left to sit for an hour before the phosphate concentrations of the samples and the standards were measured in the spectrophotometer.

There is no clear correlation between station location or salinity and phosphate concentration, with depth seeming to have more of an effect.

Figure 12: Nitrate concentrations of water samples taken from 8 stations on 27/6/16 from the Fal estuary. At each station, 2 or 3 depths were sampled. Station A49 was closest to the river mouth and G59 had the highest salinity.

Figure 11: Phosphate concentrations of water samples taken from 8 stations on 27/6/16 between 08:00-16:00 UTC along the Fal estuary. At each station 2 or 3 depths were sampled. Station A49 was closest to the river mouth and G59 had the highest salinity.

Figure 10: Silicon concentration at 8 stations along the Fal estuary on 27/06/16, with 2 to 3 depths sampled at each station. Station A49 is the freshest station sampled to G59, the more saline station.

Figure 13: Estuarine mixing diagrams (EMD) for silicon (a), nitrate (b) and phosphate (c) in the Fal estuary from 27/06/2016. The seawater ends of the EMD have been expanded to better observe the mixing behaviour of the nutrients within the estuary. Theoretical dilution lines (TDL) are included, which connect the Truro river endmember to the Fal seawater endmember. Note that the water endmember samples for the Truro, Kenwyn and Allen rivers were taken on 26/06/2016. The second Truro site is marked with a dashed blue line on each figure, to signify the known sewage input site.

(b)

(c)

The Truro river endmember was found at a salinity of 0.4, where the Kenwyn and Allen rivers merged into the Truro. A sample was taken at this point in the Truro river and was used to form the TDL on the EMD. Each nutrient’s concentration decreases with increasing salinity (Figure 13), demonstrating that the river water has a higher concentration of nitrate, phosphate and silicate than the seawater. This is likely due to the weathering and erosion of rock and the runoff of water from land, which transfers the nutrients to the river.

The Kenwyn river had a low silicon and phosphate concentration and a high flow rate, whereas the Allen river had a high silicon and phosphate concentration and a low flow rate, as seen in Figures 13a and 13c. This may be due to differing geological composition of the land near both rivers. Another sample was taken from a second site in the Truro river with a salinity of 7.1 (noted with a dashed blue circle), where there was a known sewage input. This explains why there was an extremely high phosphate concentration at this site (Figure 13c).

Although a large portion of the estuary was sampled, the majority of the samples were at the higher salinity ranges with only the river endmembers and two other samples taken further towards the head of the river. This makes the data difficult to interpret and is less useful in terms of seeing trends of conservative vs. non-conservative behaviour. For example, each nutrient seems to show conservative behaviour on the original EMD, yet the expanded versions show differing mixing behaviour. Silicon is usually observed to show conservative behaviour, with the concentration mainly controlled by estuarine mixing. Although there is some negative deviation from the TDL (Figure 13a), possibly due to the uptake of nutrients by phytoplankton, the silicon concentration in the Fal estuary is mainly conservative. Phosphate also follows a similar pattern, with most concentrations lying on or close to the TDL (Figure 13c). Nitrate, however, shows mostly non-conservative behaviour, with a larger negative deviation from the TDL (Figure 13b). This deviation could be due to nitrate uptake by phytoplankton or a short estuarine residence time.