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Chlorophyll- Figure 1

At all stations the fluorescence and chlorophyll concentrations start low and increase to a sharp peak at a certain depth, before rapidly declining and maintaining a constant concentration. Peak chlorophyll and fluorescence indicate the location of the Deep Chlorophyll Maximum (DCM); this peak point fluctuated between 19m and 24m. At station 34 the peak fluorescence was found at 19m but the peak chlorophyll was found at 24m, 5m below the thermocline.


The thermocline was located from the temperature recordings in the CTD data as the point at which there is a rapid decrease in temperature in a very small change in depth. For each of the stations except station 34, the thermocline can be found at the point at which the fluorescence and chlorophyll concentrations peak. This implies that at each of these stations, the deep chlorophyll maximum sits on the thermocline. There are a number of reasons as to why station 34 may be showing differing results- the unexpected result is from the chlorophyll concentration, which may have been caused by human error in data collection/processing or internal waves in the water column. The fluorescence and temperature were both measured at the same time by the CTD, whereas the chlorophyll samples were collected afterwards in a net. Internal waves occur throughout the water column, so therefore it is possible that in the time between the CTD analysis and the sample collection internal waves pushed the phytoplankton deeper in the water column (Muacho et al., 2013).


Nutrients- Figure 2

All nutrient concentrations increase with depth to around 25-30m. Stations 29, 31, and 33 had some deeper samples taken where we can see that nutrient levels remained high once below around 30m.  The DCM moved upwards during the day between 25m and 20m. The DCM is situated at the top of the nutricline and the bottom of the surface mixed layer (see CTD profiles for further detail).

The theory behind this is that nutrients accumulated in the winter are used up by phytoplankton in the upper layers during the spring, when light levels begin to increase. During the summer, phytoplankton have to move deeper into the water column where there is still enough light but there is also access to the deeper, mixed layers of nutrient rich water (peak seen just below the DCM). This causes there to be low nutrient levels above the DCM, but peak nutrient levels just below the DCM as this is the phytoplankton’s lower limit (Huisman et al., 2006).

Figure 2. Profiles of multiple different nutrient concentrations against depth for each of 6 stations sampled at E1 observatory buoy.


Oxygen- Figure 3

The oxygen saturation data from the E1 time series shows that most measurements from all stations were taken between 15 - 30 m.  Between these depths oxygen saturation went from around 110% to 85% as depth increased from 8 to 30 m.  The super-saturation (>100%) is an indicator of photosynthetic activity, therefore suggesting the presence of phytoplankton between these depths.  This is further supported by results for the Deep Chlorophyll Maximum (DCM), which appeared to be within this range, as indicated by peak fluorescence around 0.12 mg m-3 at depths around 20-25m between 1000 and 1500 UTC (see fluorescence graphs in the physical section).  This was also the depth of the observed thermocline where temperatures between 15-18 ᵒC from the surface to 20 m dropped to 13 ᵒC at 20 m in the mixed layer below this depth (observed in measurements to 70 m). Deeper measurements taken at stations 31, 33 and 34 indicate that oxygen saturation was around 85-88% within the mixed layer at depths between 45-53 m, which correlates with lower temperatures (13 ᵒC) (see temperature graphs in the physical section).


Chemistry

Huisman, J., Pham Thi, N.N., Karl, D.M., Sommeijer, B., 2006. “Reduced Mixing Generates Oscillations and Chaos in the Oceanic Deep Chlorophyll Maximum”, Nature, 439(7074) pp.332.

Muacho, S. et al, 2013, ‘Effect of internal waves on near-surface chlorophyll concentration and primary production in the Nazare Canyon (west of the Iberian Peninsula), Deep Sea Research Part I: Oceanographic Research Papers, volume 81


Figure 3. O2 % saturation vs Depth for all sample stations at E1 observatory buoy.


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Niskin bottles from

CTD deployments were

sampled to calculate nutrient

concentrations at different depths.


Figure 1. Chlorophyll concentrations against depth for all E1 observatory buoy stations on Monday 8th July 2019