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Physics






















PAR/Irradiance

Overall, the irradiance measurements across the time series show a very similar trend. Light is absorbed most rapidly between 20 and 40m at each station. The depth of the euphotic zone ranges from 30 to 45m at the 1% light level, and 50 to 70m at the 0.1% light level. Station 34 shows a rapid decline in irradiance between 5 and 10m. This is probably the result of a sinking coccolithophore bloom. The coccolithophores of the phytoplankton can in certain conditions reflect light. This would cause a pause in the irradiance profile. This is supported by the transistometer data, which shows a layer of high disturbance at the same depth range.


Fluorescence

Peak fluorescence up to 0.132 mg m-3 was seen around 20 – 22 m at all stations throughout the time series.  This is an indicator of the Deep Chlorophyll Maximum (DCM) level and was used as a determining factor when choosing sampling depths for nutrient, chlorophyll, oxygen and phytoplankton analysis.

There was in increase in fluorescence values through depth from the surface to 20 m from 0.04 to 0.132 mg m-3, correlating with increasing phytoplankton levels.  Below the DCM


Temperature

Temperature profiles were taken at each station from 1000 to 1500 UTC.  Near surface results from 0-10 m depth were between 16 - 18 °C.  The temperature then decreased until it reached a depth of 20 m where the temperature was around 14.5 °C, from there it then decreased more slowly for a further 2 m by 1.5 °C.  There was a constant temperature around 13 °C until the measured depth of the profile was reached at 72 m.  This profile therefore indicated stratification with a pronounced thermocline around 20 m, separating a surface mixed layer with a temperature range from 17.8 °C to around 13 °C and a deeper mixed layer with a relatively constant temperature around 13 °C.


CDOM

The coloured dissolved organic matter concentration is consistently lowest in the surface 10 m, at approximately 0.023-0.024 ppb. As depth increases there is a rapid increase to a peak concentration at between 20-23 m which occurs at every measurement throughout the day. The peak is up to approximately 0.036 ppb and is sustained in the water column below 23 m at all stations. This peak layer of CDOM coincides with the depth of the deep chlorophyll maximum as would be expected from an area of increased phytoplankton productivity.






































Turbidity

Throughout the day the pattern of turbidity remains largely constant at each station down through the water column. The surface 10-15m has a distinctively higher turbidity of approximately 0.20-0.22 NTU suggesting a higher concentration of particulate matter in the upper layers above the thermocline. Below this point the turbidity drops down to between 0.16 and 0.18 NTU in the region of water 15-20m in depth. Beyond 20m the turbidity remains at the lower value. This isolation of turbid water at the surface is likely due to the inability of the water to mix below the thermocline. The high turbidity region correlates very closely with a reduction in beam transmission in the surface 15m at all stations as more particulate matter scatters the down-welling light. The general turbidity of the water, however, was still consistently very low even in the surface waters, as supported by visual observations from the vessel of high water clarity and light penetration.


Beam transmission

Beam transmission follows a very close relationship with turbidity; remaining lower and more scattered in the surface water at approximately 4.5%. It then increases below the first 15-20m, beyond the deep chlorophyll maximum, to approximately 4.9%. This pattern is most likely due to the high particulate matter and abundance of phytoplankton which absorb and scatter light in the surface 20m. Below this there is an observed lack of chlorophyll and irradiance and hence reduction in particulate matter and phytoplankton, which supports the increased beam transmission below the thermocline. Notably, at station 34, there is spike in decreased transmission at approximately 12-14m, during which time irradiance has a distinct peak. This supports a possible theory of a high localised concentration of coccolithophore tests which can focus light similar to a lens.





Physical parameters

at E1 were investigated

using CTD deployments and

ADCP data across the time period.


(Clockwise) Figure 1: Depth of the 1% and 0.1% irradiance levels against offshore stations. Figure 2: ADCP ship trace at E1 buoy showing average current velocity. Figure 3: flow magnitude profile of ship trace.



ADCP transect

The backscatter plot for the E1 time series tracked all stations continuously from 1000 UTC to 1500 UTC.  From this data, high backscatter values (red and yellow) were observed at the surface and to depths of 10 m throughout the time series.  This suggested a high level of plankton in the water column.  Additional high backscatter levels (yellow) were seen just above 20 m depth between stations 30 and 31, and around station 33, which correlates to plankton observations on and above the Deep Chlorophyll Maximum (DCM).   Unusually, there was an apparent void of plankton in and below the DCM, indicated by limited backscatter return (blue to purple) between 20 to 30 m.  The backscatter was then fairly consistent between depths of 30 m and 70 m.  This data was observed in real time during the time series and used to determine the depths for zooplankton trawls and seawater samples for phytoplankton analysis.


Two large peaks seen during the time series from 30 to >60 m correspond to significant ship re-positioning (red).



Figure 2: CTD profiles of Turbidity and Beam Transmission from 6 E1 stations


Figure 1: CTD profiles showing temperature, salinity, fluorescence, CDOM, PAR from 6 E1 stations