GROUP
Plymouth Field Course 2019
4
Disclaimer: the views and opinions on this website are not necessarily expressed by The National Oceanography Centre or the University of Southampton. They are completely representative of the authors of this website.
OFF-SHORE
CTD Depth Profiles
Figure 1:
In Figure 1, a clear thermocline was seen between 10m and 20m where high stratification occurred between a mixed top layer and homogenous bottom layer. This stratification limited vertical transfer of nutrients, such as nitrate, throughout the water column as seen by its low concentration in the surface layer compared to depth. Chlorophyll concentration increased with depth towards the thermocline, at which point it remained fairly constant (between 0.8-1.4 µg/L) down to 50m. Oxygen saturation appeared to be inversely proportional to temperature, also showing a clear thermocline that prevents mixing between 10 and 20m.
TO NOTE: Hover over graphs to see figure captions
Figure 2:
In Figure 2, temperature decreased steadily with depth, showing a minor thermocline between 5m and 20m, again restricting the vertical transfer of nutrients across this boundary, but not to the same extent as that of figure 1. The spike in chlorophyll concentration between 20m and 25m water depth occurred simultaneously with a decrease in nitrate concentration. This was a result of nitrate intake by autotrophic phytoplankton that were clearly more abundant at this depth. As depth increased to roughly 55m, chlorophyll concentration decreased steadily and due to a lack of phytoplankton at this depth, nitrate concentration increased. Similarly to figure 1, oxygen saturation showed the inverse of temperature, gradually increasing with depth until the thermocline where it remained fairly constant, around 252-253 µmol/L, to the deepest point.
Figure 3:
In figure 3, surface temperatures were between 16.5oC and 17oC and decreased with depth to 28m where a clear thermocline was seen. At this depth, nitrate concentration was at its highest, resulting in a large spike in chlorophyll concentration to over 2.0 µg/L, indicating the presence of a deep chlorophyll maximum (DCM). Unfortunately, the niskin bottle at depth did not seal properly when fired, causing the sample to be contaminated and not representative of the true in situ conditions and therefore nitrate concentration could not be measured. In the surface layer, oxygen was most likely being used up by aerobic respiration of zooplankton, since the DCM provides them with a plentiful food supply and results in an increase in their abundance.
Figure 4:
Figure 4 exhibited the highest stratification in temperature out of all the CTD profiles with a minor thermocline around 20m of 14.5oC and a major thermocline at 30m where temperature suddenly dropped from 14.5 to 12.8oC. Chlorophyll concentration across the whole profile was less than half of that at station 27, likely due to the fact that it was further from the estuary and therefore nutrients had been further diluted before reaching the station. Despite the lower overall chlorophyll concentration, a clear DCM was seen around 25m where the chlorophyll concentration doubled and then halved as depth increased past the thermocline. As expected, the nitrate concentrations in the surface mixed layer and at the DCM were extremely low due to the high rate of uptake by phytoplankton, and increased greatly with depth as the presence of phytoplankton diminished. Oxygen saturation followed the same patterns as all the other stations; low in the surface mixed layer and increasing as temperature decreased towards the thermocline, after which the oxygen saturation remained constant with depth.