The methods used reflected those previously completed on the Callista for the off
shore work. Samples were collected using a small rosette with a CTD and Niskin water
collection bottles and a sechhi disk was also used at each station to calculate the
light penetration depth - the same as for offshore research. Samples collected were
then processed in the wet lab, preparing samples for dissolved oxygen detection,
nutrient concentration samples (120ml), a 100ml chlorophyll sample filtered and a
sample for phytoplankton analysis stored.
Glass bottles were filled in a continued flow to analyse oxygen, 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 was then used to determine the
amount of dissolved oxygen in the water. The remaining water taken from the Niskin
bottles for the other analyses. Chlorophyll was ascertained through three 50ml samples,
from each depth, were filtered 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. Nutrient samples were filtered and placed in glass bottles (phosphate
and nitrate) or plastic bottles (silicon). For phytoplankton, 100ml of unfiltered
seawater was added to a glass bottle containing a pre-determined amount of lugol
iodine, in order to facilitate the quantitative analysis of the phytoplankton community
in the laboratory.
A zooplankton net was used at three pre-decided stations. The net itself had a 50cm
diameter opening and 200µm mesh. A flow meter was attached to the entrance of the
net and the net was then trawled for five minutes at the surface of the water. The
collection bottle at the bottom of the net was then removed and transferred to a
sample bottle. This was then preserved with a solution of 10% formalin. These samples
were later quantitatively analysed in the laboratory.
Finally an ADCP transect was taken across the sample station and the data for this
stored on the computer.
The latitude and longitude of the stations are as listed below:
Oxygen
Oxygen samples were collected at all stations (61, 62, 63, 64, 65) at three depths
for station 63 and at surface and bottom for the remaining stations. They were collected
from the Niskin bottles by 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 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.
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 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.
Phosphate
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.
Phytoplankton
100 ml of unfiltered seawater from Niskin bottles was collected at 4 stations (61,
63, 64, 65). The station 61 sample was collected at middle depth, while the remaining
samples were collected at the surface. They were 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 the
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 horizontally towed at the surface for
5 minutes 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).
