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Offshore Nutrients

Method

A Rosette with 6 NISKIN bottles and a CTD was deployed at L4 buoy on the 8th of July 2019. The bottles were fired at different depths throughout the water column and where interesting features were observed during the CTD descent. The water samples taken at each depth were filtered and transferred to bottles labelled with the corresponding date, station number and depth.

They were later analysed by an auto-analyser (QuAAtro39 from Seal Analytical) following standard colorimetric protocol (Grasshoff et al., 1983).

Results

All nutrient figures (No2, total NO3, NO3 – NO2, PO4, and Silicon) show a general increase in trend with depth. At the surface low concentrations were recorded. This would suggest a removal of nutrients occurring in the surface layer of the water column by the phytoplankton species. The increasing trend with depth is thought to be due to deep water mixing, regeneration, and remineralisation of nutrients. The NO2 sample taken at 12:00 UTC(+1) stands out as the concentration remains at a similar level, 0.033 µmol/L and 0.035 µmol/L until 30m and only increases visibly (0.065 µmol/L) at 48m. Similarly, total NO3 sample taken at 12:00 UTC(+1) shows a similar occurrence with the total concentration of nitrate not changing until 30m and increases rapidly at the bottom.

Silicon measurements from the second sample onwards were contaminated due to the emergency use of a Van Dorn Bottle and a leeching of silicon in the filtration tube, leading to inaccurate measurements apart from the first NISKIN cast. Therefore, the silicon concentration profile is made using the data taken form the station E1 that was conducted on the same day by group 6 on RV Callista. All the samples show the same behaviour with the concentration increasing at depths greater than 30m.

In comparison to the station E1 data collected by group 6, NO2 and PO4 concentration shows similar behaviour to the station L4 data. Total NO3 concentration and NO3 – NO2 concentration differ greatly between L4 and E1. In E1, they show a rapid increase in concentration between 20-30m, followed by a more constant concentration. While, at station L4, the concentration of PO4 and NO3 – NO2 follow a more linear trend.