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CTD
Data regarding temperature, salinity, chlorophyll and depth was collected using a
CTD at the 4 different stations. The CTD was slowly lowered over the side of the
vessel to a maximum depth of 58m. For stations 4,5 and 7 the CTD was lowered once,
and the down profile recorded. For station 6, the CTD was lowered three times for
each station, creating a ‘yoyo’. The data collected clearly showed the chlorophyll
maximum and thermocline. In some cases, e.g. Station 4, there was a sharp thermocline
which suggested a not very mixed surface layer which resulted in a less smooth profile.
Temperature
For all stations, (Figure 1, Figure 2) temperature decreased with depth, with all
stations plateauing at a depth of ~30-35m where they reached ~12.5°C. At station
4, the surface temperature was cooler (~15.6°C) than that shown for station 7 (~16°C),
this lower temperature difference is consistent between the stations in the surface
waters, up to a depth of ~11m. This is likely because the measurements taken at station
7 (Figure 1) were taken later in the day, meaning there was more exposure to sunlight,
which in turn causes heating. This would also have been affected by the reduced cloud
cover experienced in the afternoon. At 11-25m there is a steady decline which is
consistent for all the stations, although there are slight fluctuations in temperature
shown in the Figure 1 which may be due to small local differences caused by wind
at the surface or currents.
Salinity
For all stations the salinity increases with depth, with the surface salinity remaining
consistent for all stations at ~35.35. All stations show a general increase within
the first 30m, reaching ~35.47. Below this, changes are relatively small therefore
the main halocline is likely to be located at around 5-30m. The increase in salinity
could be explained by evaporation. Surface waters experience winds and the greatest
intensity from the sun, the resultant evaporation of the fresh water leaves a higher
concentration of salt in the remaining water. Values for salinity at the lowest depths
are similar across all stations with the exception of station 4 where the final salinity
reading was 35.36. This sudden drop in salinity could suggest a deep freshwater input
or perhaps a lack of mixing between the surface waters and deeper waters. However,
it is hard to compare and draw conclusions since the other two stations were not
measured at this depth.
At station 6 (Figure 2) the CTD was lowered 5 times, and each time recorded slightly
different data. Although they all have similar curves between 0-30m, they gave slightly
different values. Despite this, by 35m all measurements for station six had plateaued
near 12.5°C. The data for station six match the general trend set by the other three
stations.
Figure 1a: Temperature-depth profiles for stations 4, 5 and 7
Figure 1b: Temperature-depth profiles for Station 6
Figure 2a: Salinity-depth Profile for Station 4, 5 & 7
Figure 2b: Salinity-depth Profiles for Station 6
For all stations the salinity increases with depth. The surface salinity remained
consistent for all stations at ~35.35 (PSU), with the exception of station 6a1 (figure
2b), the first leg of the ‘yoyo’ which had a slightly lower salinity of ~35.30. Nevertheless,
by 5m the salinity profile for 6a1 matched the other stations’, which were all fairly
similar. A general increase in salinity within the first 30m, after which all stations
plateaued at ~35.47.
Chlorophyll
For all stations the chlorophyll concentration at the surface is near zero. Between
0m and 20m depth there is a slow increase, reaching ~0.5µgL-1. After this point there
is a steep increase for all stations, with the peak being reached at ~25m. Chlorophyll
maximum is due to an increase in nutrients which have been used up in the surface
waters. At this depth, Station 4 shows a peak of ~3.8µgL-1, which is slightly higher
than that shown for Station 5, ~3.4µgL-1. After this peak the concentrations for
both stations 4 and 5 returned to a near zero value, with station 4 reaching near
zero at ~30m, while station 5’s concentration remains higher to a greater depth of
~34m. Station 7 shows a very different profile, with a steeper increase in chlorophyll
at 15-20m and a significantly lower peak of ~1.2µgL-1. Station 7 also returns to
a near zero concentration at a shallower depth of ~27m. This difference shows that
chlorophyll concentration in the chlorophyll maximum decreases throughout the day,
possibly due to a change in temperature or nutrient concentrations.
The five data sets for station 6 all have ~0 µgL-1 at the surface and drop to this
again by ~35m with the exception of 6a1. However, all five stations peak at different
chlorophyll values, ranging from ~1 µgL-1 at station 6b1 to ~3.5 µgL-1 at station
6a2. Just like the concentration of chlorophyll, the depth of the chlorophyll maximum
is inconsistent. Stations 6a, 6a2 and 6b all peak at a depth of roughly 20m, station
6b1 peaks slightly lower at around 25m whereas station 6a1 peaks just below 30m.
This much variation in the chlorophyll depth profiles measured in the same spot at
similar times suggests some sort of error in taking the measurements or analysis.
A slight drift from the original measurement would not explain the severe concentration
changes.
Figure 3a: Chlorophyll-depth Profiles for Stations 4, 5 & 7
Figure 3b: Chlorophyll-depth Profiles for Station 6
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