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Falmouth 2015 - Group 7

Offshore

Methodology

Photograph 1b - Joe preparing the CTD rosette, taken by Matt

Physical

Biological

Chemical

An ADCP transect was started on leaving the pier and stopped on arrival at each station.

At each station the CTD rosette, with a fluorometer attached, was lowered to around 5m depth above the seafloor, enabling collection of data for temperature, fluorescense, salinity, PAR/irradiance, turbidity, CDOM and beam transmission. A secchi depth was also obtained by the same member of the group at each station using a sechhi disk.

Based on the peaks in fluorescense throughout the water column the depths of water collection were decided and the CTD was raised slowly and stopped at these depths where a Niskin bottle was fired from the onboard computer. The CTD was recovered and wet lab analysis was carried out. This involved the collection of dissolved oxygen samples, a 100ml sample to be analysed in the lab for chlorophyll , a 120ml nutrient sample and quantitative analysis of phytoplankton. For dissolved oxygen, glass bottles were filled in a continued flow, in order to avoid formation of air bubbles and they were analysed using the Winkler method, where 1ml of Manganese sulphate and 1ml of a alkaline solution are added to each sample collected, resulting in the formation of a precipitate, that is used to determine the amount of dissolved oxygen in the water. Then the remaining water taken from the Niskin bottles was analysed for the following parameters:

Chlorophyll - Three 50ml samples from each depth,were filtered using a GFF in order to obtain the filter paper. This produced three replicate samples per depth. The samples were then placed in plastic tubes filled with 6ml of acetone in order to preserve them before being analysed in the laboratory. The determination of chlorophyll was carried out by finding the absorbance using a 10-AU Fluorometer. The result was then converted to mg/l using the equation: Concentration=(ml of acetone/ml of seawater) x absorbance.

Oxygen- Samples were collected at all stations at three depths for station 63 and at surface and bottom for the remaining stations. After the collection, 1 ml of manganese sulphate and 1ml of an alkaline solution were added to each sample, resulting in a formation of precipitate, and they were stored in a basket with seawater to prevent exchange with the atmosphere and preserved in the laboratory until later analysis. This further analysis involved adding 1ml of sulphuric acid to the glass bottle during constant stirring using a magnetic stirrer and titrating the solution with sodium triosulphate. The total volume of the titrate used in this process and the weight of the bottle was therefore used to determine the concentration of dissolved oxygen in the samples.

Nutrients - Samples were filtered and placed in glass bottles (phosphate and nitrate) or plastic bottles (silicon).

For the nitrate analysis, all the previously filtered samples were filtered again and then injected to a FIA-system (Flow Injection Analysis System) that converts the nitrate into nitrite. The nitrite/nitrate concentration was recorded in peaks on a track and the height of the peaks converted to concentration in µmol/L by comparing the height of the peaks of the samples with the peaks of the 4 standards taken between the measurements via a calibration curve.

For the silicate analysis 6 standards and 3 blanks were taken with 15 ml of each sample and 2 random replicates and stored in plastic tubes. Then, 6ml of Ammonium Molybdate and 9ml of a Mixed Reducing Reagent, containing MQ water, metal-sulphite, oxalic acid and sulphuric acid solutions,  were added to the samples. After 2 hours, the absorbance was taken using a Hitachi U1500 Spectrophotometer at 810mn and converted to the amount of silicate in µmol/l via the calibration equation produced from the standards and blanks.

For the phosphate analysis 7 standards and 3 blanks were taken with 30 ml of each sample and 2 random replicates and stored in plastic tubes.Then, 3ml of a Mixed Reducing Reagent, containing ammonium molybdate, sulphuric acid, ascorbic acid and potassium antimony tartrate solutions, was added to the samples. After 2 hours, the absorbance was taken using a Hitachi U1800 Spectrophotometer at 882mn and converted to the amount of phosphate in µmol/l via the calibration equation produced from the standards and blanks.

When leaving each station another ADCP transect was started.

The station locations are as follows:

Top

Map A - The offshore stations taken from GoogleEarth

Phytoplankton - 100ml of unfiltered seawater was added to a glass bottle containing a pre-determined amount of lugol, in order to facilitate the quantitative analysis of the phytoplankton community in the laboratory.

Zooplankton- A 200μm mesh  zooplankton net of 50cm diameter was lowered spanning the chlorophyll maxima and was closed with a messenger weight. The net was recovered and hosed from top to bottom to collect the zooplankton and it was transferred to a sample bottle and preserved with approximately 100ml of 10% formalin solution, and later quantitatively analysed in the laboratory.

Lab methodology

Oxygen

Oxygen samples were collected at all stations and depths in which the Niskin bottles were fired. They were collected from the Niskin bottles using a plastic tube to flow into glass stoppered bottles. After the collection, 1 ml of manganese sulphate and 1ml of an alkaline solution were added to each sample, resulting in formation of a precipitate, and they were stored in a basket with seawater to prevent exchange with the atmosphere and preserved for 24h at the laboratory until further analysis. This laboratory analysis consisted of adding 1ml of sulphuric acid to a glass bottle during constant mixing using a magnetic stirrer and titrating the solution with sodium triosulphate. The total volume of the titrate used in this process and the weight of the bottle is therefore used to determine the concentration of dissolved oxygen in the samples.

Chlorophyll

Three 50ml replicate samples were taken to determine the chlorophyll for each depth and then filtered using a GFF, which was then stored in plastic tubes containing 0.6ml of acetone and frozen overnight. At the laboratory, the determination of chlorophyll was carried out by finding the absorbance using a 10-AU Fluorometer. The result was then converted to mg/l using the equation: Concentration=(ml of acetone/ml of seawater) x absorbance.

Nutrients

Two samples of seawater at each depth and station, one for silicate and other for nitrate and phosphate determination, were taken and filtrated. The silicate samples were stored in plastic bottles and the nitrate/phosphate samples were stored in dark glass bottles and then analysed in the laboratory.

Nitrate

For the nitrate analysis, all the previously filtered samples were filtered again and then injected to a FIA-system (Flow Injection Analysis System) that converts the nitrate into nitrite. The nitrite/nitrate concentration was recorded in peaks on a track and the height of the peaks converted to concentration in µmol/L by comparing the height of the peaks of the samples with the peaks of the 4 standards taken between the measurements via a calibration curve.

Silicate

For the silicate analysis 6 standards and 3 blanks were taken with 15 ml of each sample and 6 random replicates and stored in plastic tube. Then, 6ml of Ammonium Molybdate and 9ml of a Mixed Reducing Reagent, containing MQ water, metal-sulphite, oxalic acid and sulphuric acid solutions,  were added to the samples. After 2 hours, the absorbance was taken using a Hitachi U1500 Spectrophotometer at 810mn and converted to the amount of silicate in µmol/l via the calibration equation produced from the standards and blanks.

Phosphate

For the phosphate analysis 7 standards and 3 blanks were taken with 30 ml of each sample and 5 random replicates and stored in plastic tubes.Then, 3ml of a Mixed Reducing Reagent, containing ammonium molybdate, sulphuric acid, ascorbic acid and potassium antimony tartrate solutions, was added to the samples. After 2 hours, the absorbance was taken using a Hitachi U1800 Spectrophotometer at 882mn and converted to the amount of phosphate in µmol/l via the calibration equation produced from the standards and blanks.

Phytoplankton

100 ml of unfiltered seawater from Niskin bottles was collected and stored in dark glass bottles containing lugol to facilitate the identification and quantification of the different species. The samples were incubated overnight in measuring cylinders to sedimentation of cells and 90 ml was pumped out at the laboratory. 1ml of sample was taken out of the 10ml remaining after the pumping and identified using guides while they were counted over 5 columns of a Sedgewick Rafter using GXML1500 Microscope.

Zooplankton

A zooplankton net with a mesh of 200µm was lowered to a depth below the peak of fluorescence and towed upwards at a constant velocity. When recovered, the value of the flow was noted from the flowmeter and 1L of sample was stored in plastic bottles and preserved with 100ml of formaldehyde 10%. 10ml of these samples were later quantitatively analysed in the laboratory using a Bogorov chamber in two steps of 5ml analysis with a Nikon SMZ800 microscope and identification guides. The number of zooplankton per m³ in seawater was then calculated from the equation: Number of zooplankton per m³ seawater = (number of zooplankton/volume of seawater sampled).