Home Geophysics Onshore Offshore Estuary Summary

Offshore


For the offshore portion of the Plymouth Investigation we looked at changes in the water column over a time series.

The original intention was to go out to the L4 station but the weather conditions were too rough for safe operation.

To fulfil our aim of obtaining some offshore data with stratified waters  we went to

Oxygen Sampling Method


Collecting Samples


Oxygen samples were drawn from Niskin bottles attached to a CTD deployed at station ‘Z’ on 10/07/2018, hourly. Samples were drawn quickly to avoid contamination from atmospheric oxygen and sample temperature is taken. Tubing is slipped onto the Niskin valve and a volumetric flask is rinsed 3 times. With seawater from the Niskin still flowing, the tubing linked to the Niskin valve is placed at the bottom of the volumetric flask. The flask is placed upside down as water flows, and is overflowed twice to prevent the influx of bubbles into the flask. The tubing is then carefully removed from the flask.

After drawing the sample, 1ml of Manganous Chloride solution is added to the sample flask, followed by 1ml of sodium-hydroxide-sodium iodide solution. There must be no air bubbles added to the flask before the addition of these reagents.

A stopper is placed on the flask to avoid the trapping of air, and the samples are shaken to to disperse the precipitate, and is repeated once the precipitate has settled to the bottom of the flask.


Analysing Samples


An autotitrator is used in order to analyse the samples for their oxygen concentration. Clear water was added to the water bath and a UV light turned on. 1ml of sulfuric acid is added to the labelled sample flask, and stir bar added to break up the precipitate and release the dissolved oxygen when the flask is placed in the water bath.

Once the flask shows no trace of precipitate, a titration is set up using a Metrohm Dosimat. Sodium thiosulphate is added at increasingly slower rates as the UV detector measures transmission I UV light through the sample. A reading is taken during titration when there is no further change in absorption detected by the UV detector and all iodide has been consumed, which can be seen when the titration curve plateaus.

Working out the Oxygen Saturation Percentage


Using the pre-prepared oxygen excel document, the constants needed to calculate oxygen saturation were already inputted. The bottle numbers on each flask were related to a specific location of each sample; and the CTD data provided the temperature and salinity at these locations. The weight of the O2 bottles (ml) and amount of Sodium Thiosulphate (ml) used for each sample during the experiment were then inputted to calculate the oxygen saturation percentage.


(The 100% value was made using Temperature, Salinity and density)



Chlorophyll Sampling Method


Nutrients Sampling Method


For every Niskin bottle fired a nutrient sample was taken in order to study silicon and phosphate. The water samples for each bottle was filtered and placed in a tube whilst on the boat.


In the lab standards were made up for both silicon and phosphate and the samples were placed in an auto-analyser.


Phytoplankton Sampling Method


On the Falcon Spirit our Principal Scientific Officer identified the 4 sample sites where a sample would be taken at depth and then identified the appropriate depths at which to take a phytoplankton sample we took a dark brown glass bottle filled with a measured amount of Lugols iodine the sample was taken directly from the Niskin bottles and stored in a cool chamber until the next day when the lab work was taken out.

We took a 1ml sample of the phytoplankton solution that was preserved with Lugols iodine and placed it into an indented slide then placed a covering slide on top careful to avoid the presence of bubbles. Then we counted 100 squares and noted any phytoplankton species identified by genus with use of a key.


Zooplankton Sampling Method


Zooplankton was sampled via Plankton net. This was equipped with a flow meter and bottle. The flow meter provides a measurement of current so the amount of water that has passed through the net can be calculated. The bottle collects the zooplankton when the net is removed from the water.

The net would be deployed over the side of the boat by hand, and as the boat was anchored, the flow of water was provided by the tidal current. The net was left in the water for approximately 10 minutes at a time. Once the net is retrieved, formaldehyde is added as a preservative and the sample stored in the fridge.

For processing in the lab, a 10ml aliquot is taken, then a sample from this is pippetted under a microscope and the preserved fauna counted and identified using a guide.

Introduction

What We Did:

Offshore Biology Offshore Coastal and Physical