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GEOPHYSICS

TAMAR SAMPLING

OFFSHORE

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Lab Protocol

Home Geophysics Tamar Sampling Offshore

Offshore

INTRODUCTION

A tidal front is formed during summer, when tidal mixing is seen to be dominant (Sun and Cho, 2010), and there is a clear transition between well-mixed and stratified waters (Lodar et al., 1993). Tidal mixing occurs in the benthic region, whereby high velocities caused by tidal movements, lead to friction and shear upon the seabed, ultimately leading to turbulent flow. It is with this turbulent flow that a bottom well-mixed layer is created, both in deeper waters and within a shallower estuary (Zhou, Liu and Li, 1999). A surface mixed layer can also be seen in both instances, whereby wind movements also causes turbulence, in the buoyant, heated surface layer of the ocean. However, the extent that these two processes have upon the main water column, is related heavily to water depth. In areas further out to sea, where greater depths are present, these two mixed layers do not intercept. This leaves a stagnant, and therefore stratified layer of water in-between those that are mixed. This stratified thermocline can be observed with temperature and salinity values. However, as depths become shallower, this stratified layer becomes smaller, eventually leading to the mixed layers intercepting, meaning two-layer helicoidal flow causes the column to be fully well mixed. In this case, there would be no stratification. The boundary between the well-mixed waters and stratified waters is called the tidal mixing front (O'Donnell, 1993). This can be distinguished from a sudden spike in temperature, whereby surface heat flux within the well-mixed waters is transferred throughout the column, meaning temperature mid-column and below is much higher than that of the stratified areas (Sun and Cho, 2010). As these mixing processes are vital to fuelling biology and supplying nutrients into these systems, the location of a mixing front is useful in order to understand the effect of mixing upon abiotic and biotic factors, and how they interlink within this dynamic system (Lodar et al., 1993).

MATERIAL

CTD rosette – Seabird 911:

In order to collect data for temperature, salinity, depth, dissolved organic matter, fluorescence (chlorophyll proxy) and irradiance, a CTD rosette was deployed at each station. Using a hydraulic A-frame the CTD equipped, with 6 10L NISKIN bottles, was deployed from the Callista. The CTD was then able to take a continuous profile of the water column on the way down and up. The NISKIN bottles were tripped while sampling on the way up, at sites of interest determined from the data observed while the CTD was lowered to the seafloor.

NISKIN bottles:

Samples obtained from the NISKIN bottles on the CTD were analysed to determine nutrient concentration (phosphate, nitrite, silicate and nitrate), chlorophyll-a, plankton analysis, and dissolved oxygen concentration in the lab.

Dissolved oxygen samples: These samples were taken from the NISKIN bottle first, using tubing, attached to the bottle, extending to the bottom of the sample bottle. The lid was kept closed until the NISKIN bottle was reached. The bottle and lid were rinsed three times, before being filled to the top, and overflowed three times the flasks volume, to make sure the sample collected was untouched. After being taken to the wet lab, 1ml of Manganous chloride solution was added, followed by 1ml of alkaline iodine solution. The sample was then stored in cold water.

o Plankton samples: 100ml was collected from each NISKIN bottle, and the contents added into bottle with Lugol’s solution. Once the lid was replaced, samples were shaken and put into cold storage..

o Dapi & flow cytometry samples: Tubing was not used for this samples collection. A 60ml bottle used was rinsed three times with water from the NISKIN. The sample was killed with formalin, in preparation for staining in the lab later on.

All 4 samples were taken all in the same profile, at station 40.

o Chlorophyll: A filtration rank was used to prepare these samples. The filtration rig was set up with 3 new filters, and 3 50ml samples were collected from the NISKIN bottle using a measuring cylinder. The water was added to each filter. After checking the filters for damage, each was lifted, placed into a 7ml tube containing 90% acetone, and submerged. The tube number and total volume filtered were recorded. A new acetone tube was used for each filtration and the method was repeated. The samples were stored in the fridge.

O Nutrient samples: The syringe was rinsed three times using water from the NISKIN bottle, and the filter attached and rinsed once, to check for leaks and to flush it. The syringe was then filled to 50ml, with the filter still attached and taken to the wet lab. From there it was transferred into a tube, which was rinsed with filtered seawater three times, and then a 12ml sample was collected, and stored in the fridge.

ADCP: An acoustic doppler current profiler, integrated into the ship’s system, was used to take a profile of the water column along the transect. This allows for potential features, such as plankton, in the water column to be identified from the back scatter, assisting in helping to decide where to sample.

Vertical zooplankton close net: The net used had a mesh size of 200 µm, and 0.5m in diameter. To determine the zooplankton species and number present at a number of stations, a net was deployed to vertically sample the water column between specific depths. Samples collected for lab analysis under microscope. Sample depths were chosen based on feed back from the ADCP and CTD.

Formaldehyde was added to each sample for preservation. Once in the lab, 5ml of each sample was pipetted into a bogorov chamber twice, viewed under a microscope, and the number of species counted. This was done twice per sample, and the number of zooplankton per metre cubed was calculated.

RESULT

We divided the result section into three parts, click on the buttons below to understand more!

BIOLOGICAL

CHEMICAL

PHYSICAL

In July the water column off the coast of Plymouth is usually highly stratified. However, this year there has been a breakdown of thermocline due to inclement weather conditions. The cooler weather with and increased wave frequency has contributed to the delay in

development in the stratification in the water column.

The recent weather has been much warmer and calmer, creating good conditions for the development of the thermocline, so the goal of the survey on board the R.V. Callista was to locate the thermocline.

Data was collected at multiple stations along the route (35-40). Multiple parameters were recorded, using a variety of instruments. In order to gain a full view of the water column. Temperature, salinity, nutrient concentrations (Silicate and Phosphate), and plankton were all analysed along the path.