Group 2 Plymouth Field Course

ADCP

CTD

PHYSICS

Fig 1 - Depth profile of temperature, salinity, irradiance, dissolved organic matter, nitrite, silicate, nitrate, phosphate, chlorophyll a and dissolved oxygen saturation for all stations; a=C1, b=C3, c=C4, d=C5, e=C6 and f=C7

a

b

c

d

e

f

On the Callista, a CTD rosette was deployed at every station. The rosette contained sensors for temperature, salinity, conductivity, dissolved organic matter, fluorescence, turbidity, chlorophyll and had 12 Niskin bottles attached to it. The CTD transmits data in live time, meaning the data can be quickly analysed on the down cast to determine what depths to sample with the Niskin bottles on the up cast. Here the CTD data for the six stations has been presented

To test the hypothesis mentioned in the chlorophyll section on the chemistry page, we plotted multiple parameters alongside each other for every offshore station. Some stations showed clear stratification. C3 had a surface layer to ~10m, where temperature was high, and salinity low. It can be seen that nutrients is depleted in this layer before being regenerated under the thermocline. Interestingly, phosphate reverses this trend. This could be because phosphate is not limiting in the surface layers, whereas nitrite, silicate and nitrate are, and then phosphate becomes limiting under the thermocline. There is not a coherent deep chlorophyll maximum, with chlorophyll high above the thermocline, lower on the thermocline and then increasing slightly under the thermocline. This is surprising as the dissolved O2 does increase below the thermocline, implying photosynthesis is taking place. Further chlorophyll samples are necessary to identify the presence or absence of a deep chlorophyll maximum.

Station C5 however does show a deep chlorophyll maximum. In the surface layers, nutrients is limiting and thus chlorophyll is low. This is again reversed below the thermocline when nutrients replenish and chlorophyll increases in line with dissolved oxygen. This is more expected than the chlorophyll data for C3.

Fig 2- Richardson Number with depth for each station

The views and opinions expressed are those of the individual and not representative of the University of Southampton or the National Oceanography Centre.

Fig 3 - Backscatter and velocity ADCP transects for the Callista, stations C3 (a and b) and C7 (c and d)

The ADCP data was first viewed using Winriver II, which allowed us to see the velocity and magnitude of the current, determined by four beams of acoustic pulses in different directions beneath the boat, which bounced (were backscattered) off particulates in the water column at different depths, doppler shifting them according to the current’s movement speed, and other information such as the backscatter, the intensity of the acoustic pulse returned.

By obtaining a measure of shear - an increase in which tends to destabilise the water column - from the difference in current speed and direction measured by the ADCP between layers in the water column, and a measure of the stabilising buoyancy, the buoyancy-frequency - an increase in which indicates a more stable or stratified water column - from the density measurements taken by the CTD at the same depths. These measurements were combined in the Richardson number, Ri, which is the unitless ratio of buoyancy-frequency to the square of shear magnitude, such that a higher richardson number - above 1 - indicates a more stratified water column, a lower one - below 0.25 - indicates a more turbulent and well-mixed water column, and values in-between are indeterminate, and are likely to be similar to conditions either side of them. The data for all of the stations show very turbulent conditions, since most of the Richardson numbers are between 0 and 0.25, which is not surprising for a macrotidal estuary with strong offshore winds.

By looking at the velocity and backscatter contour plots, it is possible to tell how fast the water is moving and how much suspended material there is at different depths in the water column respectively. By comparing this with other information, such as the richardson numbers above, current directions from the ADCP data, and other information such as fluorimetry or salinity data from the CTD at each station, it is possible to infer certain information about the water column.

C3

This transect was taken from inshore, to the East of Plymouth Sound, at 11:30. There was a lot of backscatter lower in the water column of such an intensity that it prevented the ADCP from measuring current speeds below around 13m, which could be due to a large amount of sediment stirred up by the high-energy environment, especially exposed to the South-Westerly winds hitting the area, an idea backed up by the low Richardson numbers throughout the water column, indicating turbulent conditions, and the higher current speeds and interference backscatter seen at the surface, potentially from waves.

C7

This transect was taken from further offshore, to the West of Plymouth Sound, at around 1500. This also shows backscatter at depth from suspended sediment, but not nearly to the extent of that seen at C3, possibly because it is a more open region of ocean, further away from sources of suspended sediment, even though it is just as turbulent, as seen from the low Richardson numbers. There is a layer of high backscatter at the surface, which could have been due to the presence of a large amount of plankton at the surface, but this could just be interference again.