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Plymouth Field Course 2019

Group 7

On Tuesday the 9th of July 2019, the samples collected on the ribs in the upper and lower estuary of the river Tamar, were analysed for chlorophyll, nitrate, phosphate and silicate concentrations.

The Chlorophyll samples were put into a spectrophotometer and the chlorophyll concentration recorded in µg/L.

The nitrate and phosphate samples were analysed using a QuAAtro39 Autoanalyser, which gave out readings for the concentration of nitrate and phosphate in µmol/L.

The silicate samples and the calibration samples were prepared and analysed using a U-1500 Spectrometer, which allowed for the silicate concentrations to be recorded in µmol/L.

The samples collected using the zooplankton net were observed under a microscope and each species was counted.

T/S analysis for the lower Tamar estuary

Temperature and salinity profiles were compiled from the data collected by the group sampling the lower estuary; using a YSI 600 QS Data Sondethis from the Saltash Pontoon down into Plymouth Sound, stations C- J. This was for the purpose of seeing how the profiles differ down the course of the lower estuary and to identify the locations and depths of significant areas of change. IOutlined below are individual T-S plots for each station.

C - Saltash Pontoon

Here (Figure 1), the temperature changes as expected, albeit not a lot. The surfaces waters are marginally warmer than they are at depth, (0.3°C), with the decay being exponential as expected. The salinity is lower in the surface waters as the less dense riverine water overlays the denser seawater. Again, there is not a significant change shown, only 0.6 PSU. This is likely to be due to it being high tide during the time of the sampling meaning there is a large seawater input and lower riverine input. There has also been very little rain recently, thus rendering the river input reduced further.

D - Lynher Confluence

At this point, the river Lynher meets the Tamar. Therefore it was predicted that the salinity would be lower here due to a larger riverine input. However, this is not the case. Salinity has increased since the previous station, and increases down the water column. Again, the tidal state could be reducing the effect of the extra freshwater input. The temperature range is larger than before, but follows the same decay shape.

E - Looking Glass Point

Surface salinity is similar to that of the Lynher Confluence station, but is increased at depth. This fits with this location being further downstream. There is a broader temperature range here too, indicative of a larger saline input. There is some variability in the layers, but the overall trend is clear.

F - Torpoint Ferry

Salinity was further increased with a very smooth logarithmic increase down the water column; the surface waters were now more saline than before. The waters were still cooler, (although not by much). Both profiles show stability down the water column.

G - St John’s Lake

The profiles here do not fit the nice stable curves shown more or less up to this point. The surface waters are slightly less saline than the Torpoint Ferry location, but at depth it is the most saline location so far. However, salinity decreases linearly to begin with, then drops away to a maxima of 35 PSU. Surface temperature is considerably lower, and decreases linearly for the first four metres, after that there is little change. It is worth noting that the maximum depth of the estuary in this location is only 7.7m.

H - Cremyll

Located at the mouth of the estuary, it is marginally cooler than the previous location and there is only a very small change in salinity over the whole water column. (Less than 0.2PSU). On this profile there is a notable point. Between 5 and 8 metres, both the temperature and the salinity spike sharply. This is due to one reading and is therefore likely to be an anomaly in the results. Disregarding this point would produce much smoother, predictable curves.

I - Western Channel of the Breakwater

The surface water is the warmest so far, however, deeper than 1m it reduces drastically giving a profile similar to that of previous locations when the surface result is ignored. It is not significantly colder in Plymouth sound than in the lower estuary. Salinity increases with depth in layers, therefore showing some evidence of stratification. The end salinity reached is higher than that of the previous stations, as it’s at the very bottom of the estuary, this is not surprising.

J - Eastern Channel of the Breakwater

This station is located further away from the mouth of the estuary and produces a very stable profile. Temperature decreases by 1°C in the first four metres of water, then sees less change. (The depth of the location was only 16m). The range in salinity is rather small with a change of less than 0.3PSU between the surface and the seafloor. Like temperature, the majority of the change is seen in the first four metres of water. It is the most saline location measured, and is the furthest away from the river input.

Nutrients

NO3 TDL

Conservative behaviour

This graph shows how nitrate concentrations change with increased salinity within the Tamar estuary from a riverine endpoint at salinity 0 to a seaward endpoint of 34.96. By illustrating the graph with a theoretical dilution line (TDL) it can be seen that conservative mixing occurs as there is very little deviation away from the TDL. There is a strong negative correlation between nitrate concentrations and increasing salinities.

Silicate TDL

Conservative behaviour

This graph (figure 10) shows the silicate concentrations and how they change throughout increasing salinities within the Tamar estuary, from a riverine endpoint at salinity 0 to a seaward endpoint of 34.96. By plotting the data alongside a TDL connecting the endmembers it can be observed that silicate behaves conservatively within the estuary, as there is very little deviation from the TDL. There is a strong negative correlation between silicate concentrations and increasing salinities.

The riverine endmember at a value of approximately 56 μmol/l was taken a week after a previous endmember was taken, which had a value of approximately 70 μmol/l. This decrease over the week was due to a lack of rainfall, high temperatures and sunny weather, meaning there was very little addition from weathering of sediments. Looking at the graph, if a TDL was drawn connecting the seawater endmember with a riverine endmember of 70 μmol/l, it would appear that there may be a small amount of removal between salinities of approximately 9 and 20.

Phosphate TDL

Non-conservative behaviour

This graph (figure 12) shows how the phosphate concentrations change through the estuary and differing salinities within the Tamar estuary, from a riverine endpoint at a salinity of 0 to a seaward endpoint of 34.96. By plotting a TDL between the endmembers it can be observed that phosphate displays non-conservative behaviour and there is a clear addition of phosphate into the estuary. Addition of phosphate is a common occurrence in the Tamar estuary, and the results are as expected from the procedures undertaken. The addition may be due to agricultural inputs, or due to the large influence of tidal activity within the estuary causing rapid mixing of fresh and seawater, leading to desorption from the fresh water particles.

 As salinity increases, phosphate remains relatively constant, however falls rapidly at salinities of over 30.

Chlorophyll

Methods

Three filters were prepared onboard the RIBs at each station by filtering 50ml of sample water through a syringe prepared with a filter at the end. The filters were then removed and preserved in acetone for lab analysis. This involved removing 90% of the acetone then adding the sample with the filter into a fluorometer, which displays the value. Averages were then taken for each station from the three values.

Results

The overall trend of chlorophyll appears to be a decrease in concentration as salinity increases down the estuary (figure 13). This is by no means a strong correlation, for example the first station A0 has a visibly low chlorophyll concentration. This may be as the values for A0 were 0.02, 0.37 and 0.42 µg/l, suggesting the first result was an anomaly and should be removed from further data processing. Comparing with the phytoplankton counts, Station A0 was observed to contain a high proportion of Chaetoceros diatoms, which may not be shown here in the chlorophyll concentration. The highest chlorophyll concentrations appear around salinities 12 to 18 which correlates with where the removal of silicate would be with the older endmember (see Silicate TDL section). Chaetoceros forms silicate frustules, meaning removal must occur from the water.

Figure 1 - a temperature-salinity diagram of Saltash Pontoon (Station C), the 1st station of the lower estuary data collection

Figure 2 showing a temperature-salinity diagram of Lynher Confluence (Station D), the 2nd station of the lower estuary data collection

Figure 3 showing a temperature-salinity diagram of Looking Glass Point (Station E), the 3rd station of the lower estuary data collection

Figure 4 showing a temperature-salinity diagram of Torpoint Ferry (Station F), the 4th station of the lower estuary data collection

Figure 5 showing a temperature-salinity diagram of St John's Lake (Station G), the 5th station of the lower estuary data collection

Figure 6 showing a temperature-salinity diagram of Cremyll (Station H), the 6th station of the lower estuary data collection

Figure 7 showing a temperature-salinity diagram of the Western Channel of the Breakwater (Station I), the penultimate station of the lower estuary data collection

Figure 8 showing a temperature-salinity diagram of the Eastern Channel of the Breakwater (Station J), the final station of the lower estuary data collection

Figure 9 showing an estuarine mixing diagram depicting the relationship between nitrate concentration and salinity from bottle data and salinity probe, along the course of the River Tamar. Error bars represent 5% error bars. Riverine and seawater endmembers are used to plot a theoretical dilution line (TDL).

Figure 10 showing an estuarine mixing diagram depicting the relationship between silicate concentration and salinity from bottle data and salinity probe, along the course of the River Tamar. Error bars represent 5% error bars. Riverine and seawater endmembers are used to plot a theoretical dilution lines (TDL)

Figure 11 showing the calibration curve for determining silicate concentration.

Figure 12 showing an estuarine mixing diagram depicting the relationship between phosphate concentration and salinity from bottle data and salinity probe, along the course of the River Tamar. Error bars represent 5% error bars. Riverine and seawater endmembers are used to plot a theoretical dilution lines (TDL)

Figure 13 is a line graph showing how chlorophyll concentration and silicate concentration change with increasing salinity along the course of the River Tamar.