Falmouth Field Course 2017

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Estuarine Chemistry

Introduction

On 05/07/17 we obtained values for nitrate, phosphate, silicon, chlorophyll and dissolved oxygen concentrations from survey samples collected at stations 9 to 13 on 04/07/17. For the individual lab methods see the ‘Lab Protocol’ tab. This aimed to give us an understanding of how nutrients changed with space and time along the Fal estuary as well as provide evidence for conclusions developed within the  plankton lab analysis. For all mixing diagrams the riverine endmember was collected at 50° 15.146’ N, 05° 02.335’W and the seawater endmember used our highest salinity, obtained at Station 17.

Phosphate:

Figure 2 shows non-conservative behaviour as most of points are above the theoretical dilution line. At ~20psu there is a clear addition of Phosphate which could be the consequence of anthropogenic inputs which greatly influences nutrient distribution through sewage outfalls or soil disturbance (Jickells et al., 2014). Phosphate concentration decreases with increasing salinity. Greater numbers of phytoplankton were observed in water samples from the lower estuary as seen on the ‘Estuarine, Biology’ of this website. This increase in phytoplankton could possibly be the cause behind the sharp reduction of phosphate concentration.

Silicon:

Silicate shows non-conservative behaviour as all points are below the theoretical dilution line showing removal of silicate in the estuary. This could be due to the uptake of silicate by diatoms such as Rhizosolenia setigera, Guinardia delicatula and Chaetoceros. In addition to this, the primary source of dissolved silicon is from silicate rock weathering however in the days prior to our survey there was below average rainfall. Dissolved silicon is an essential nutrient for phytoplankton growth (Smyth, et al., 2010).

Nitrate:

Nitrate shows non-conservative behaviour with points deviating above and below the theoretical dilution line. The addition of nitrate is seen at the lowest salinities at the head of the estuary suggesting inputs of nitrate to the system from the River Fal. Addition of nitrate could be due to potential sewage outfall which increases during summer months as the number of tourists increases (Hall et al., 2001). Nitrate shows removal at higher salinities at the mouth of the estuary suggesting uptake of nitrate by organisms.

References

C.M Hall., 2001, ‘Trends in ocean and coastal tourism: the end of the last frontier?’, Ocean & Coastal Management, 44(9-10), pp.601-618.

T.D Jickells, J.E Andrews, D.J Parker, S Suratman, A.A Aziz, Y.Y Hee, 2014, ‘Nutreint transport through estuaries: The importance of the estuarine geography, Estuarine, Coastal and Shelf Science, 215, pp. 215-229

Smyth, T. J. et al., 2010. 'A broad spatio-temporal view of the Western English Channel observatory', Journal of Plankton Research, 32(5), pp. 585-601.

Figure 1.  Nitrate Mixing Diagram for the Fal Estuary

Figure 3. Silicon mixing diagram for the Fal Estuary.

Figure 2. Phosphate mixing diagram for the Fal Estuary

Chlorophyll:

The chlorophyll results from the investigation were highly inconclusive, with no clear correlation across the stations. The station which correlated most closely with predictions was station 12, which decreased in chlorophyll concentrations from 0.3 ug/l to 0.078 ug/l. This was predicted, as it is more beneficial for chlorophyll to be present close to the surface as this is where the greatest amount of irradiance is available for photosynthetic use. As light attenuates down the water column, phytoplankton should also decrease. However, stations 9, 10 and 11 show almost no change in concentrations, only ranging from 0.01 to 0.05 ug/l. Station 13 is even more unexpected, as it shows chlorophyll increasing rather than decreasing with depth. This might be due to nutrient limitation in the surface waters at this particular station, or it could just be anomalous data.

Figure 4. Chlorophyll depth profiles for stations 9 to 13.

Dissolved Oxygen:

The oxygen saturation data collected by the CTD on Conway was only a single measurement at each station. This is due to group 7 choosing to fire the bottles at the same singular depth at each station. Consequently, there are no results for analysing the relationship between oxygen saturation and depth. The graph shows that oxygen saturation values stay around the 100% value, ranging from a minimum of 91.5% at station 13 to a maximum of 104% at station 12. There is a spatial correlation for oxygen saturation along the length of the estuary. Station 9 of Figure 5 is located at the mouth of the estuary and 13 the furthest riverward, showing how the dissolved oxygen decreases as the water moves towards the sea. This is likely due to the water being relatively older nearer the mouth meaning it has been exposed to more bacterial respiration, which may be consequence of the increased dead organic matter resulting from the nutrient inputs.

Figure 5.  Dissolved oxygen percentages for stations 9 to 13.

Limitations

As a result of engine and hydraulic failure at the start of our afternoon surveying session we could only collect data from a single station (13) with the CTD. Therefore we also used the results from Group 7 stations (surveyed the same morning) in order to provide enough information to develop conclusions about the region.

During the chemistry lab the phosphate blanks containing MQ water and mixed reagents were giving readings that were too high and shifted absorption values of all other cores. One solution would be to standardise using group 3 and 4’s blank standard calibration.

Summary

Analysis of previous studies suggests that there is a considerable anthropogenic input into the Fal estuary. The nutrients Nitrate and Phosphate exhibited non-conservative behaviour with addition at low salinities. This is likely due to the increasing agricultural runoff and presence of sewage outlets, which itself led to the closure of a nearby shellfish farm. The fact eutrophication was not obviously present in this region suggests that the estuary may have a short residence time, also explaining the ambiguous chlorophyll data shown in Figure 4, which would otherwise display a stronger relationship to the nutrient mixing diagrams. Nitrate, phosphate and silicate concentrations all showed removal from the water column closer to the estuarine end member where the biological activity was at its highest.