Introduction


The aim of the offshore data collection was to determine the location, structure and length of the front. The objective was to take samples from specific sites that were identified to be in or on either side of the front.

Station 1

Station 2

Station 3

Station 4

Station 5

Physical Data

CTD: results


For Stations 1 to 4 (Figures 1 to 4), the rapid increase in salinity, typically around 35.3, coincided with a chlorophyll maximum.  The chlorophyll maximum also overlaps with the steepest temperature change at each station. A strong fall in temperature from the surface and between 15 and 25m was observed at all stations.

Stations 1 and 4 (Figure 1 and 4) had the greatest chlorophyll maximum, 3.8µg/l and 4.6µg/l, respectively. Following these maximums, both of these stations saw dramatic falls in chlorophyll to values around 1.4µg/l.

The chlorophyll maximum did not coincide with a rapid change in salinity at Station 5 (Figure 5). The temperature at this station increases gradually with depth as opposed to the other stations which exhibited sharp changes in temperature.

ADCP: results


At Station 1 (Figure 6a), there was a large band of strong backscatter at 16m measuring 75dB. There is also strong backscatter at the seabed in Stations 1, 2 and 4 (Figures 6a, 7a and 9a) and at the surface at Stations 1 to 3 (Figures 6a, 7a and 8a). The ship’s track suggests that currents in the area flow generally south west.

For all Stations bar 1, the ship’s track showed that the currents in the area generally flowed north. At Station 1 (Figure 6b), the current flow was in a south west direction.


CTD: discussion


The steep changes in temperature and salinity at each station, reflects the thermocline and halocline. The chlorophyll maximum at the thermocline and halocline which implies that the phytoplankton live along the boundary of the two water bodies [1]. When this occurs, it indicates the presence of a front. Stations 1 and 4 have profiles that reflect what would be observed at a front. It is assumed that Stations 2, 3 and 5 are located on the landward side of the front.

Station 5 is less stratified that the other stations. This means that nutrients are more likely to occur throughout the water column and as such, phytoplankton will utilise them. The lack of nutrient data means this cannot be confirmed.

ADCP: discussion


The strong backscatter band near the surface at all of the stations represents zooplankton.  There was also some backscatter near the seabed, most likely from re-suspended sediment. The large area of high backscatter near the surface is likely to be due to bubble entrainment from ship wakes.

At Station 2, the zooplankton band is less well defined due to the relative shallowness of the water column. At Station 3 the zooplankton band was of a greater intensity suggesting greater abundances. Station 4 showed a more diffuse zooplankton layer at approximately 17m. The discrete area of re-suspension near the seabed may suggest a change in sediment type from small to large grains.

The zooplankton layer was less well defined at Station 5 than at other stations, suggesting a low abundance. There was little surface entrainment or re-suspension of sediments near the seabed, suggesting slower flows.

Conclusion


Two of the stations observed trends associated with a front. There was generally strong backscatter observed at all stations due to zooplankton blooms.


Chemical Data

Richardson number


For Station 1 (Figure 10), the Richardson number was determined by using CTD and ADCP data. The following equation (gx(ρ1- ρ2)*depth interval)/(mean density*(δu)^2) was used. At this station the surface layer had a Ri<0.25 indicating mixing was occurring due to turbulent flow and or weak density structure. From 20m to depth there was a low Ri number indicating turbulent mixing in the water column. From the Figure, a Ri>1 was observed at 17m, indicating laminar flow and the presence of the thermocline.

Nutrients


The nutrient minimum at Station 4 (Figure 11) correspond to a chlorophyll maximum at the same depth (Figure 4). We can therefore consider that the depletion of silicon is, at least, in part due to the uptake by diatoms.

The general trend of phosphate (PO43-) concentration (Figure 12) is due to the scavenging of the nutrient from particulates falling in the water column.

Due to a combination of sparse sampling and analytical issues, the data from nitrate (NO3-) cannot be used to show any patterns and therefore, draw any conclusions.

Oxygen Saturation


The observed trend at the surface is due to the close proximity to the Water-Air interface.

Conclusion


The nutrient concentrations are lower at the surface due to the utilisation by phytoplankton and the oxygen saturation is highest at the surface because of interaction with the air.

Biological Data

Phytoplankton


Diatoms were the dominant group in 4 of 6 samples. No diatoms were found in samples at 1.48m at Station 1 (Figure 14) or 34.11m Station 5 (Figure 14). The greatest abundance of phytoplankton was found at the surface of Station 5, while the lowest abundance was found at 34m at Station 5 (Figure 14).

Zooplankton


Among the zooplankton, Copepods were the dominant group in 6/7 of the samples. A large number of Hydromedusae were found were Copepods were not dominant. The other zooplankton groups were found in more variable abundances. The greatest number of zooplankton were found in samples 3 and 4, from Stations 2 and 3 (Figures 16 and 17) respectively, while the lowest abundance of zooplankton was found in sample 1 from Station 1 (Figure 15).

Discussion


The low abundances of both phytoplankton and zooplankton found at Station 1 may be a result of the highly stratified water column limiting growth to deeper waters. The high abundances of plankton at the surface of Station 5 may be due to increased mixing between layers of the water column. The high abundance of hydromedusae at Station 4 may have been due to sampling error during the counting and identification stage, or a localised patch of the zooplankton.

Conclusion


Station 5 had the highest diatom values of all stations at the surface. Copepods were the most dominant zooplankton observed at the majority of stations.  

References


[1] Fernàndez, E., Cabal, J., Acuna, J.L., Bode, A,. Botas, A. and García-Soto, C. (1993) Plankton Distribution Across a Slope Current-Induced Front in the Southern Bay of Biscay. Journal of Plankton Research, 15(6),  619-641.

Fig 1

Fig 2

Fig 5

Fig 4

Fig 3

Fig 6a

Fig 7a

Fig 8a

Fig 9a

Fig 6b

Fig 10

Fig 11

Fig 12

Fig 14

Fig 15

Fig 17

Fig 16

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