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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 Discussion

At station one (G9) and station two (G9) the chlorophyll maximum occurs where phytoplankton counts are highest. For station two (G9) however, as the Niskin bottle top was open, it is likely that phytoplankton were captured throughout the water column, giving this measurement an inaccurate value. At station one (G11), the chlorophyll maximum exists at the surface of the water column, whilst maximum phytoplankton abundance exists at depth (21m) (Fennel et. al. 2003). This may be because of inaccurate measurements, calibration or phytoplankton counts, as it is unlikely that the chlorophyll maximum and phytoplankton maximum depths would differ.

Using the Sverdrup’s critical depth model (Smetacek et. al. 1990) it is understood that respiration is constant with depth, unlike production. Dissolved O2 saturation is used to show that if more Oxygen is consumed, by respiring phytoplankton or by microorganisms utilising organic matter, than produced (Bierman Jr., V. J. et. al. 1994). Results show photosynthesis of phytoplankton communities decreases with depth and respiration dominates at all stations, with the exception of station two (G9) where the increase in dissolved oxygen saturation percentage at the thermocline indicates photosynthesis exceeds respiration. This can indicate that zooplankton communities are grazing upon phytoplankton communities throughout the water column, where O2 saturation percentage is decreasing.

Station one (G9), where dissolved Silicon (Si) decreases with depth after the thermocline, shows that the upper water column is depleted in Si due to utilisation of diatoms and some dinoflaggelates that utilise Si. Below the thermocline, Si concentrations increase due to the remineralisation of the deceased sinking siliceous phytoplankton (Yool, Tyrrell, 2002). Stations two (G9) and one (G11) show Si increases towards the thermocline. According to Yool, Tyyrell (2002), Si uptake may be limited by phosphate, giving a possible explanation as to why Si increases with depth. As phosphate is utilised in the surface waters, like all other essential nutrients, it becomes a limiting factor. This means that the dissolved Si cannot be utilised until the thermocline; where waters become increasingly rich in nutrients.

By comparing the essential nutrients nitrogen (in the form of NO3-) and phosphorus (in the form of PO43-) across all the stations it could be suggested that the limiting nutrient at station one (G9) and station one (G11) is nitrate and at station two (G9) is phosphate (Libes, 1992). This is because phosphate concentrations are relatively high in the surface waters, more so in station one (G11) due to its locality where concentrations are increased due to fresh water input, and nitrate concentrations are close to 0 µmol/L. At station two (G9) NO3- concentrations are relatively high with relatively low PO43- concentrations, suggesting PO43- is limiting phytoplankton growth.
   

References

Bierman Jr., V.J., Hinz, S. C., Wiseman Jr., W. J., Rabalais, N. N., Turner, R. E. (1994). A Preliminary Mass Balance Model of Primary Productivity and Dissolved Oxygen in the Mississippi River Plume/Inner Gulf Shelf Region. Estuaries. 17 (4), 886-899.

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

Smetacek, V., Passow, U., (1990). Spring bloom initiation and Sverdrup's critical-depth model. Limnology and Oceanography. 35, 228-234.

Yool, A., Tyrrell, T., (2002). Role of diatoms in regulating the ocean’s silicon cycle. Global biogeochemical cycles, Vol 17, No. 4, 1-21.  

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