Falmouth Field Course 2017

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PHYSICS

Figure 1.  Temperature Profiles

Salinity

Figure 2 shows the data collected from the vertical profiles and demonstates the change in salinity (PSU) with increasing depth (m) for each of the 6 stations. There are many salinity spikes due to the short-term discrepancy between the temperature and conductivity sensors (Mensah, et al., 2009) this adds uncertainty into the data which makes it difficult to read. However ignoring the various spikes, a trend can be deduced; very few sample stations experience a change in salinity with depth. Salinity increases with depth very slighty with station C42 and C43 having the lowest of the high salinities. It is difficult to see which trends are real.

Density

Our calculated density profile show a positive correlation between density and depth, as expected. Through the water column the temperature decreases, pressure increases and, in stratified estuarine systems, salinity also increases (see Figure X and X). It also shows a high fluctuation in density between 10 and 12m at Site 39, however this is likely a result of salt spikes within the CTD dataset rather than genuine changes in the environment. Density results ranged between 1024.97-1026.73kgm-3, fairly typical of near shore ocean conditions. With the lowest result found in surface waters at Site C43. This point was closest to the estuary, and clearly shows a thin freshwater layer, suggesting stratification. Additionally the site was very shallow (around 8m) and therefore warm more quickly and was less subject to pressure influences. Sites C40-42 all located close together were fairly similar, with higher densities found further offshore. For example C42, while also very shallow, was situated further around the headland from the estuary and any freshwater input would have been far more diluted. The highest density was found at C39 which was furthest offshore. This deeper water would have had virtually no influence from the Fal estuary waters and would also have been the coldest. These relatively deeper regions showed no defined pycnocline, instead gradually increasing with depth.

PAR

PAR readings exponentially decreased with depth. As an indicator of light attenuation these results are as expected. C40 had the highest PAR values, while C38 had the lowest. This is unusual due to the fairly similar environment however was likely just a result of the C40 readings being taken later in the day which had lighter conditions. C39 had a fairly irregular curve compared to the other results potentially due to scattering within the shallow part of the water column.

Fluorescence

Fluorescence is not a direct measure of chlorophyll but acts as a proxy for chlorophyll. For stations C38-C41 (the offshore stations) fluorescence increases gradually with depth in the top 20m between 0.05-0.10v. Station C39, the furthest offshore station displays the largest chlorophyll maximum at 20m. Stations C38 &C40 present smaller maxima at ~25m. C42 shows no maxima, C43 shows a very shallow, surface maxima ~4m. In all cases, once the maximum is reached it declines with increasing depth. At site C39 at ~55m there is a large increase in fluorescence which is unlikely to be a true reading at this depth and is likely to be an instrument error. The chlorophyll maximum at the majority of the offshore stations occurs between 20 – 30m. This depth correlates with the presence of phytoplankton at high concentrations in the pycnocline where nutrient rich waters below are accessable as well as sunlight at the surface.

Transmission

Transmission is a measure of turbulence thus how easily light is transmitted. In the corresponding graph, transmission can be seen to increase in the surface water from site C43 at the mouth of the estuary to the furthest offshore site C39. Transmission in the surface increases with distance offshore as the water is clearer due to any suspended sediment having sunk. Sites C43 & C42 at the mouth of the estuary display a range of surface transmission values between ~0.8 – 4.5 v. When water from the river Fal meets estuarine water, velocity slows and sediment is ‘dumped’. Small, less dense particles remain suspended and are transported out offshore on the ebbing tide. As total dissolved solids (TDS) increase with depth so does the turbidity and therefore a decrease in transmission is recorded, this is clearly visible at site C40.

Figure 3.  Density Profiles

Figure 2.  Salinity Profiles

Temperature

Figure 1 shows the data collected from the vertical profiles and demonstates the change in temperature (°C) with increasing depth (m) for each of the 6 stations. All sample stations display an overall decrease in temperature from the surface measurements to the deepest measurement of 65.3580m which was recorded at station C39. The greatest surface temperature, 16.8382°C was recorded at station C39, while station C41 had the lowest surface temperature of 15.2727°C. Station C39 shows the most dramatic decrease in temperature, from 16.8382°C to 12.9926°C, whereas station C42 had the most vertically linear trend, with a small decrease from 15.4518°C to 15.2902°C.

 Temperature decreases with depth as we had hypothesised as increased solar radiation during the summer months inputs buoyancy potential into the surface water causing stratification. The denser underlying water body absorbs less heat energy then the overlying water body as irradiance decreases exponentially with depth. Additionally the extent of stratification increased with distance from the shore. Of the warmest three surface temperatures, Station C38, C39 and C43, two were situated the furthest from land. This is because deeper water was less mixing between stratified layers so heat energy has more time to accumulate. The inland stations had lower recorded temperatures due to greater mixing between stratified layers and so less time for heat accumulation.  

Figure 4. Par Profiles

Figure 5. Fluorescence Profiles

Figure 6. Transmission Profiles

Richardson Number

>1.00 - High density gradient = Low shear = Stratified

<0.25 - Low density gradient = High shear = Well mixed

Station C42 and C43 show all Ri values are less than 0.25, this suggests that these stations are well mixed and therefore have a low density gradient with high shears. This is because these stations are further inshore and hence are more affected by tidal mixing.

Station C38 shows Ri values greater than 1, this suggests laminar floor and hence stratification with low shears, this is to be expected further offshore as the water column deepens and therefore tidal mixing has a reduced effect on the water column.

Stations C40 and C41 both show a halocline between 10 and 20m as Ri is greater than 1 and hence there is laminar flow at these depths but turbulent flow is seen at all other depths.

Station 39 shows no depths that have laminar flow however at depths between 10 and 20m they are in the transition stage between laminar and turbulent flow.

References

Mensah, V., Le Menn, M., & Morel, Y. (2009). Thermal mass correction for the evaluation of salinity. Journal of Atmospheric and Oceanic Technology, 26, 665-72. Retrieved July 12, 2017

Figure 7. Scatter graph of Ri values with depth