Offshore

University of Southampton OES Undergraduate Falmouth Field Course 2016 - Group 3 databank and initial findings.

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At each of the six stations investigated during the offshore survey a CTD deployed off the stern of the R.V. Callista so that depth profiles could be obtained. Five of the six stations were sampled through the collection of water in Niskin bottles (mounted onto the CTD) at specific, chosen depths and through the deployment of a zooplankton net, and a secchi disc. The purpose of the survey was to enhance our understanding of the spatial and depth structure of the offshore environment in connection with the Fal Estuary. Our focus was around the location of the front, between the well mixed, shallow, tidally controlled area, and the stratified, deeper, temperature controlled area.

Methodology

CTD, Secchi Disc and Zooplankton sampling:

Discrepancies between data sets from Callista and Conway may be due to difference in manufacturers of the CTDs used (Seabird and OSI, respectively).

The CTD was deployed at each station. It was attached to a rosette along with six, operating Niskin bottles, a fluorometer, and a transmissometer. A maximum of five bottles were used. A single bottle was fired at the chosen depths.

A closing zooplankton net (diameter: 50cm; mesh: 200 microns) was deployed at five of the six stations at various depths. This technique was different to the trawling used aboard the Conway.

The deck hands were also responsible for deploying the closing zooplankton net which had a diameter of 50cm and had 200 micron mesh. It was deployed to varying depths at each station dependant on the CTD profile and then hauled up the specified distance. The sample was retrieved and formalin was added.

ADCP:

ADCP profiles were taken both at, and between stations each of the six stations. CTD profiles were also generated at all stations, however water samples were only collected from five of the six stations.

Bio and Chem analysis:

A temperature and salinity profile was obtained as a CTD was lowered through the water column providing an idea of water column structure from which we were able to choose the depths to sample. Water samples were collected in Niskin bottles at varying depths. All containers were rinsed with water from the Niskin bottle which their sample originated from.

Dissolved oxygen:

- Put tube connected to Niskin bottle output to bottom of bottle and fill to top, minimising the exposure of the sample to the atmospheric oxygen

- Pipette 1ml of manganous chloride to precipitate out the oxygen present

- Then pipette 1ml of alkaline iodine to neutralise the reagents in the solution and prevent further change.

- Place the lid in the glass bottle. Do this slowly so air bubbles are not added to the sample.

- Store in a bucket containing sea water deep enough to cover the bottle.

Silicon:

- Rinse syringe with sample

- Use to fill bottle with sample

Nitrate and Phosphate:

- Rinse syringe with sample

- Use to fill glass bottle with water sample