Falmouth Group 8

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General Offshore Physics

Transects-Figure 1-12

Transect 1-2 (langragian measurements) Backscatter: Surface backscatter can be ignored as it is most likely the wake from the R.V. Calista. Backscatter visible as a distinct layer from 10-25m is most likely due to a plankton layer. The plankton layer decreases with depth. On the right hand side backscatter anomalies are visible.   

Velocity and direction: Between the surface and 14m there is a westerly flow, below 14m the flow directions are random, with low flow speeds (0.00 to 0.14 ms-1).

Transect 2-3 (Langrangian measurements) Backscatter: Backscatter is visible between the two currents travelling in opposite directions, generating a thin but distinct layer of backscatter visible at 20m (could also be plankton layer), possibly generating a shear force and a turbulent layer. At depths of >30m there is clear water with no visible backscatter.  

Velocity and direction: From surface to 20m the flow direction is west.  At depths of greater than 20m the current is flowing in an opposite direction (easterly) to the current directly above it. The Easterly current is faster at ( 0.375 ms-1)

Transect 3-4 (Langrangian measurements) Backscatter: At depths of 10-20m there is a distinct layer of backscatter which is most likely a plankton layer.

Velocity and direction: No stratification visible, random directional flow with no variation.

Transect 4-5 (Langranian measurements) (5 is on one side of The Bellows) Backscatter: A backscatter ‘patch’ is present at a depth of 10-20m (left side), in the shape of a wake. However it is 150m wide and it is classed as an unknown anomaly. There are vertical anomalies visible.

Velocity and direction: From surface to 22.86m the current is traveling in a southeast direction at speeds of 0.125 to 0.375 m s-1, and moving in a more easterly direction in shallower water.  As the water mass moves into shallower depths, the water column is condensed. Between 22.86 m and 34.29 m the current is traveling in a different direction (only on the left hand side), the current speed slows to 0.007 to 0.125 m s-1 ;clear stratification in the direction of the flow. From depth 34.29 to the seabed the flow increases once more.

Transect 5-6 (Lagranian measurements) (6 is on the other side of The Bellows travelling towards The Bizzies) Backscatter: Coming off of the Bellows there are high levels of backscatter. As the surface water transect goes from shallower 20m water to  deeper 50m water the layer of high backscatter sinks from the surface to 20m (seen previously in transect 4-5)

Velocity and direction: The shallow water flow direction is easterly to southeast as it moves off the shelf. The deeper water has a north-easterly direction but moves more slowly at 0.1m s -1. Shear is visible on the shelf at speeds of 0.4 ms-1. There is not much return of data from depths greater than 30m due to backscatter of phytoplankton.

Stratification Front-Figure 13

We moved from stratified to mixed waters at about 12:00 UTC and remained there until 12:45 UTC. The temperature decreased from approximately 14°C to 12.8 °C. The fluorescence increased from approximately 0.35 volts to 0.6 volts. Fluorescence is an indicator of chlorophyll, therefore there was more chlorophyll present in the mixed surface waters due to higher nutrient availability. Salinity remains constant throughout the survey.

Click graphs to enlarge

Light Attenuation Coefficient-Figure 14

When the K values from the two methods are compared, it can be seen that they follow a very similar trend, and values from both are strongly correlated with each other. These fluctuations are also not restricted to a single direction, and both increases and decrease are seen between the two. The K values taken from the light probe can generally be seen as more accurate, as these are not as open to the human error encountered with the Secchi disk. Readings for this replied entirely on the eyesight of the spotter, and the ability to judge distance on a moving line, and although close, cannot be taken with the same accuracy as a dedicated probe. A large attenuation coefficient means light can penetrate further through the water column, and this is more typical of the more turbid waters seen in coastal areas. More open water is typically clearer, and so light is allowed to pass further through the water column without attenuation.