FALMOUTH FIELDCOURSE

Group 5

The views expressed are those of the individuals concerned and do not express the views of the University of Southampton or those of the National Oceanography Centre Southampton.

Silicates

The levels of silicon measured show an increase with depth. There appears to be a sharp increase at all sites occurring at around 20 meters. This may be due to the lack of phytoplankton in this area, causing less nutrients to be used, therefore increasing the absorption. The levels are very similar at all sites.

Nitrates

The concentration of nitrate increases sharply with depth, however, at a certain point (between 15 to 30 meters) the concentration begins to decrease. This may be due to the nutrients being recycled by organisms causing the concentration to be reduced. All sites follow the same pattern however the decrease occurs at different depths for each area. This may be due to their distance from the shore.

Phosphates

The concentration of phosphate measured offshore generally shows a fairly linear increase. However, there is an anomaly at site C8 with a sharp increase (3.5µg/l) occurring at around 22 meter. This station was located the furthest out to sea and therefore the water column would be more stratified potentially causing an increase in the phosphate.

Dissolved Oxygen

All stations have high oxygen saturations near the surface due to the atmosphere. Oxygen decreases with depth as organisms respire. The rate of this change is affected by deep chlorophyll maxima being present at all three stations at around 20m. Best illustrated by station C8 where the DCM is located around 22m oxygen saturation decreases from 104.8% to 90.1% between depths 22-25m. This is likely caused by there being no net primary production below the DCM therefore respiration means O2 is removed, decreasing saturation.

References:

Morris AW, Riley JP (1963) Anal. Chim. Acta 110, 123.

Ruzicka J, Hansen EH (1988) Flow injection analysis, 2nd edition. John Wiley & Sons.    

Click below for more data from offshore

Click below to see our chemical data from the estuary and the pontoon at King Harry Ferry

Nutrients

Silicon concentration was calculated using a calibration curve and the absorbance measured by a spectrometer.

The technique for measuring the nitrate concentration uses the conventional azo dye chemistry of Morris and Riley (1963). This produces a pink fluid that absorbs at 543nm. This is implemented as a flow injection method. For this the nitrate must be reduced to nitrite, which is done with the use of a cadmium column that causing the reduction reaction. The flow injection method involved the flow of a sample along narrow tubes that allow mixing to occur. This mixing is highly reproducible (Ruzika and Hansen, 1988). Reagents are added to the sample producing a pink colour. The pink colour is then measured in a flow through 1cm path length cell in a spectrophotometer. The signals produced is plotted onto a chart recorder.

Phosphate was measured by putting 30ml of seawater into 3 ml of reducing agent. This was left for 2 hours to allow the reaction to occur. The absorbance was measured using a spectrometer.

Above: Magnetic spinner used in dissolved oxygen processing (Photo credit - Eleanor Spencer)

Oxygen samples were taken from every bottle at each station, ensuring the sample was first to be extracted, minimising contamination and change to the sample. The water was collected into a glass flask, making sure no bubbles were present. Two chemicals were added to the sample collected, Manganous chloride (1ml) and alkaline iodine (1ml). This precipitated out the oxygen and neutralised the reagents. Once this was completed the flasks were placed into water to preserve them further.

In the lab the sample was mixed using a magnetic spinner. It was then titrated against thiosulfate until the sample the sample became clear, this was made more accurate by using a chart reader to aid observations of clarity.