At each station, four niskin bottles were fired- depths varied slightly and were dependent on where the DCM had been observed. Niskin samples were used for silicate, phosphate, nitrate/nitrite, O2, chlorophyll, bacterial analysis using DAPI staining (first station only), cytosine, and phytoplankton.

O2

Oxygen was always the first sample to be taken, due to air contamination happening as soon as the niskin bottles had been opened. Jannah connected a tube to the bottle tap, rinsed the glass sample bottle 3 times and filled to the brim slowly to ensure no air bubbles were present. 2x10ml pipettes of reactant was added; first, manganous chloride, and second, alkaline iodide. This caused the oxygen to react and precipitate out of solution. Bottles were then sealed and stored under seawater to prevent any interaction with the atmosphere.

Silicate, Phosphate, Nitrate/Nitrite

A syringe and filter were washed out with water from each niskin bottle, then 12ml of seawater was filtered through the syringe into a sample bottle, labelled, and stored in the fridge for analysis in the lab. Samples were auto-analysed in a QuAAtro39 machine in order to get concentrations for silicate, phosphate and nitrate/nitrite. Standard protocol was followed from Grasshoff et al., 1983 to add reagents to create colorimetric complexes; Nitrite and total nitrite (nitrite and reduced nitrate) formed purple complexes, phosphate and silicate formed blue complexes. These were measured at the appropriate wavelengths.

(Grasshoff, et al 1983)

Chlorophyll

Liam filtered 3x50ml samples from each niskin bottle simultaneously on a filtering rig, then collected the filter paper and stored these in acetone. These bottles were labelled and stored in a freezer in the lab overnight, with the freezer acting as a sonicator to break the cells on the filter paper and allow the chlorophyll to be dissolved in the acetone. Each sample of chlorophyll dissolved in acetone was run through a fluorometer and a result was collected in µg/L, which was then converted to total chlorophyll concentration using the same method used for the estuarine samples.

Bacterial Analysis

At the first station (29, 10:00 GMT) a sample was taken from each niskin bottle to be DAPI stained and studied by Liam and Owen using some of the Marine Biological Association’s microscope kit. Further details for this are in the bacterial analysis write-up.

Phytoplankton

A 200ml sample was taken from each bottle and put into a bottle containing Lugols iodine in order to fix the phytoplankton. In the lab, 1ml of each sample was put into a Sedgewick-Rafter plate and 5 rows of 20 cells were counted. These counts were then scaled up to get a value of phytoplankton per m3 seawater.

Cytosine

A 50ml sample was taken from each bottle and put into a frozen bottle containing glutaraldehyde and shaken before being returned to the freezer to bring ashore. Samples were passed through a flow cytometer to measure the size and number of cells and photos were taken of those above a certain size; the machine was programmed to do this by the lab tech.

Coccolithophores

At the last station of the day (34, 15:00 GMT) a 200ml subsample of the 1L zooplankton sample was taken to filter for coccolithophores, due to there being a slight milky colour observed below the surface and an unusual irradiance curve for the same depth. The sample was filtered using the same filtering rig used for chlorophyll, but with a 20µm filter paper- however the filtering system didn’t work due to a problem with the vacuum or the filter being clogged, so the sample wasn’t collected.

Introduction

Methodology

The group went offshore on Callista on 8th July 2019 to station E1 which is located at 50.026N 004.225W, 33km from the coast. A time-series of data was collected hourly between 10:00 UTC until 15:00 UTC. Wind speed was 5m/s in an easterly direction at Plymouth, visibility was good, and the sea state was calm.

The summer of 2019 arrived much later than 2018, with poor weather throughout May and June, meaning a much lower level of solar input and a less well developed thermocline. However, it was still expected that at E1 there would be a thermocline developing and that this would be influencing the location of the Deep Chlorophyll Maximum.

All opinions expressed are of our own, and not of the University of Southampton

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