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Plymouth Field Course 2019
Group 7
Figure 1 - Velocity contour plot for Station 23
Station 23 was sampled at the marker on this ADCP contour plot, at 13:03 UTC. This clearly shows two zones of more turbid water at approximately 12.5m and 26m depth. Looking at the TS profile for this station, there is a double thermocline from the seasonal heating. The deep chlorophyll maximum (DCM) appears at around 28m, visible from the fluorescence profile and the chlorophyll bottle analysis. We chose to sample these depths using the zooplankton nets in order to investigate further.
Figure 2-Velocity contour plot for Station 22
The two zones of intense backscatter were also identified here at Station 23, at 1-12m and 26-38m depth.
Temperature, Salinity, PAR and fluorescence profiles
T-S profiles along the onshore-offshore transect show an increasingly pronounced thermocline from Station 18 to 23 (Figures 1 - 7), illustrating increasing stratification as distance from shore increased; thermoclines at Station 22 and 23 were particularly pronounced. Coinciding with the base of the thermocline in these stations a prominent deep chlorophyll maximum was visible, indicated by fluorescence values (Figures 12 and 13). Transmissivity at these stations was also dramatically decreased at this depth, and at Station 23 the density of cells causing attenuation of light appears to limit the growth of photosynthetic cells below this depth; the DCM ends abruptly. High surface irradiance of around 4800 µmol photons/m2/s (approx. 1043 W m-2) meant that 1% light was reached at 28m, just below the fluorescence maximum at 26.7m. Although this is typically far higher than the mean compensation irradiance for the North Atlantic (15 µmol photons/m2/s, (Siegel et al., 2002) the abrupt end of the DCM suggests self-shading causing growth stress (Saito & Tsuda, 2003).
The thermocline also became increasingly pronounced throughout the day; severe weather in the weeks before this study may have prevented development of the development prior to this - as such, Station 19 at L4 buoy at 09:23:41 UTC exhibits only a weak thermocline (Figure 2) whilst by Station 24 at 15:10:45 UTC the thermocline had deepened slightly, and elevated surface water temperatures in the upper few metres indicate the continual heating throughout the day (Figure 7). (Smyth et al. (2010)).
An island effect was indeed observed at Eddystone Rocks between station 20 taken up-current of the rocks and station 21 taken down-current. Up current of Eddystone rocks, a slight thermocline was observed to have begun development. However, at Station 21 downcurrent of the rocks, an abrupt thermocline is observed down to 7.5m below which the temperature is almost completey uniform (Figure 4). This may be due to shear resulting from the rocks on the sea bottom mixing the overlying water column, leading to a more homogenous temperature in deeper waters. Figure 3 shows a weakening of the thermocline at station 20.
A strong halocline was not observed at any of the stations, suggesting any density differences in the water column are mainly driven by temperature. The lack of a great salinity effect at any of the stations may be because of low freshwater input at stations this far offshore.
ADCP Velocity/Backscatter