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University of Southampton Falmouth Field Course 2013    26th June - 6th July 2013    Group 9

26th June 2013 - Offshore Sampling
Falmouth Tides (UTC): HW 07:20 5.1m
LW 13:50 0.3m
Cloud cover: 100% - 0%
Sea State: Flat - Slight
Air Temperature: 14.9°C

Chemical Results

Chlorophyll

Chlorophyll (figure OC.1) gives an indication of abundance of phytoplankton in the water column (Fennel et. al. 2003). At station one (G9) the chlorophyll maximum occurs at a depth of 27 metres (m), just above the thermocline. At station two (G9), the chlorophyll maximum cannot be calculated as the sample taken at 16.4m was lost and therefore a reading was not possible. The Niskin bottle at 32.5 metres did not close properly when fired. However, the unclosed Niskin bottle sample is still shown as it may provide some indication of chlorophyll at depth. Station one (G11), shows an opposite profile to that of station one (G9), the chlorophyll minimum at station one (G11) is within 1m of the thermocline, with maximum chlorophyll at the surface.

Figure OC.1: Chlorophyll concentrations across station one and two (G9) and station one (G11)

Dissolved Oxygen Saturation (%)

Dissolved Oxygen (O2) (figure C.2) can be used in combination with chlorophyll measurements to determine respiration and photosynthesis relationships. Station one (G9) shows the greatest decline and range in O2 saturation percentage from the sea surface to 41m. At station two (G9) there is little variation in O2 saturation percentage, with an increase at the thermocline. Station one (G11) shows the smallest variation throughout the water column in the amount of O2 saturation percentage. The general trend in dissolved oxygen saturation at all sites is that percentage of saturation decreases with depth. This shows that photosynthesis of phytoplankton communities decreases with depth and respiration dominates, with the exception of station two (G9) where the increase in dissolved oxygen saturation percentage at the thermocline indicates photosynthesis exceeds respiration.

Figure OC.2: Dissolved Oxygen Saturation (%)

Silicon (Si)

Silicon (figure OC.3) is primarily used by diatoms to synthesise their siliceous frustules (Hecky et. al., 1973). Station one (G9) shows an increase in silicon with depth from 15 metres, with surface water showing a slight increase. Station two (G9) shows maximum silicon concentrations at the thermocline, with minimum values at surface and depth. Station one (G11) shows a similar profile to that of station two (G9), with less variation.

Figure OC.3: Dissolved Silicon Concentration

Phosphate (PO43-)

Phosphate (figure OC.4) is required as part of the Redfield ratio (C:N:P 106:16:1) by photoplankton for growth and energy production (Libes, 1992). The station one (G9) PO43- profile shows a decrease with depth, from the sea surface to 27 metres (past the thermocline), then increasing with depth to 41m. PO43- at station two (G9) also shows a decrease with depth, then increasing from the thermocline. Here it is important to note that at the deepest measurement the Niskin bottle ‘misfired’ and that the PO4 concentration may be similar to that of the surface value due to this error.

Figure OC.4: Phosphate Concentration

Nitrate (NO3-)

Nitrate (figure OC.5) is also required as part of the Redfield ratio in phytoplankton uptake. At station one (G9) we see an increase in NO3- from the sea surface to the thermocline, then a decrease to 27 metres, then following another increase at depth. At station two (G9) we see increased surface values of NO3-, decreasing with depth. Here we see another increase with depth. It is important to note that the sample at depth was contaminated with surface waters as the Niskin bottle misfired, and that this sample may be a representation of surface waters.

Figure OC.5: Nitrate Concentration

References

Fennel, K., Boss, E. (2003). Subsurface maxima of phytoplankton and chlorophyll: Steady-state solutions from a simple model. Limnology and Oceanography. 48 (4), 1521-1534.

Hecky, R. E., Mopper, K., Kilham, P., Degens, E. T., (1973). The Amino Acid and Sugar Composition of Diatom Cell-Walls. Marine Biology. 19, 323-331.

Libes, S. M., (1992). An introduction to Marine Biogeochemistry John Wiley & Sons Inc. 142 - 154

Parsons, T. R., Maita, Y., Lalli, C., (1984). A manual of chemical and biological methods for seawater analysis. Pergamon. 173

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