University of Southampton OES Undergraduate Falmouth Field Course 2016 - Group 3 databank and initial findings.
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Station 33:
The concentration of silicon and nitrate in the surface waters (0.48µM/L and 0.41µM/L respectively) is greater than that of phosphate (0.02µM/L) but all three nutrients show a peak in concentration at around 35m depth. The greatest change is seen in nitrate concentration, with concentration increasing by 1.0µM/L from 15m to 35m depth. The concentration of all three nutrients at the maximum depth sampled is greater than at the surface. The greatest difference between surface and maximum depth concentration is seen in silicon, which shows an increase of 0.55µM/L.
Station 34:
The concentration of phosphate (0.08 µM/L) is lower in the surface layer compared to silicon and nitrate values (0.35 µM/L and 0.78 µM/L respectively). Phosphate concentration remains relatively low, increasing to only 0.19µM/L by maximum depth sampled. Silicon shows a trough (decreasing by 0.04 µM/L) at 20m depth, then increases to a peak of 1.07µM/L at 40m. Nitrate has the highest peak of 1.52µM/L found at 20m depth. All nutrients have greater concentrations at maximum depth sampled compared to the surface. Nitrate shows the greatest difference between surface and maximum depth with an increase in value by 0.28µM/L.
Station 36:
Nitrate concentration (0.02µM/L) is lower than silicon and phosphate within the surface layer (0.47µM/L and 0.09µM/L respectively). Nitrate and silicon concentrations continue to increase with depth, the greatest concentration is seen at the maximum depth sampled (1.57µM/L and 1.18µM/L respectively). The greatest peak of phosphate at 14m depth with a value of 0.95µM/L. All nutrients show greater values at the maximum depth sampled compared to the surface. The greatest difference between surface and maximum depth concentration is seen in nitrate with an increase in value by 1.55µM/L.
Station 37:
Phosphate concentration (0.05µM/L) is lower than nitrate and silicon in the surface water (0.07µM/L and 0.42µM/L respectively). Phosphate concentration remains relatively low, increasing to only 0.20µM/L by maximum depth sampled. Nitrate and silicon both have peaks at 18m (0.28µM/L and 0.55µM/L respectively) and troughs at 32m depth (0.06µM/L and 0.50µM/L respectively). All nutrients show greater values at the maximum depth sampled compared to the surface. Nitrate shows the greatest difference between surface and maximum depth with an increase in value by 0.83µM/L.
Station 38:
The surface Niskin bottle failed to fire, resulting in only two depths of data points available. Therefore, the patterns of each concentration is unclear. Nitrate concentration appears to be greater than phosphate and silicon from 10m to the maximum depth sampled.
Chlorophyll:
For each station average chlorophyll increases in the surface layer (0-15m). The greatest average chlorophyll is seen at station 34 at ~10m with a value of 47 µg/L. The lowest average chlorophyll concentrations for each station are found between 30-40m depth, with the exception for station 33 which had the lowest concentration at the maximum depth sampled. Station 38 has an unclear pattern of chlorophyll as the surface Niskin bottle failed to fire and this site had shallower water, therefore, below 20m samples were not possible.
Oxygen saturation (%):
Oxygen saturation decreases slightly from the surface water to the maximum depth sampled for station 36 and 37 (12% and 10% respectively). Station 33 and 34 show troughs within the depth layer of 10-20m, with station 34 having the lowest trough value of oxygen saturation at 31%. All stations have lower values of oxygen at the surface compared to the maximum depth sampled. Station 38 has an unclear pattern of oxygen saturation as the surface Niskin bottle failed to fire and this site had shallower water, therefore, below 20m samples were not possible.
References
Arrigo, K. (2005). Marine microorganisms and global nutrient cycles. Nature, 437(7057), pp.349-355.
Holligan, P. (1981). Biological Implications of Fronts on the Northwest European Continental Shelf. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 302(1472), pp.547-562.
Ikeda, T. (1985). Metabolic rates of epipelagic marine zooplankton as a function of body mass and temperature. Mar. Biol. 85(1), pp. 1-11.
Simpson, J. H. and Sharples, J.: Introduction to the Physical and Biological Oceanography of Shelf Seas, Cambridge University Press, Cambridge, UK, 2012.
Tilman, D. Kilham, S. and Kilham, P. (1982). Phytoplankton Community Ecology: The Role of Limiting Nutrients. Annu. Rev. Ecol. Syst. 13(1), pp.349-372.
Offshore sampling - Chemical Data
At Station 33 a chlorophyll maximum is visible at around 15m depth also. The chlorophyll maximum occurs at the same depth as the thermocline because in this region cool, nutrient rich waters from depth are mixing with warmer waters from the surface where photosynthetic organisms are present. At this depth light is still present and the input of nutrients allows rapid growth and reproduction through photosynthesis, therefore, an increase in chlorophyll is present [Simpson and Sharples, 2012]. The percentage of dissolved oxygen is lowest from 15m to 40m depth due to the consumption of oxygen by zooplankton through photosynthesis. Numbers of zooplankton are great in this area because of the presence of photosynthetic organisms at the chlorophyll maximum that they use as a food source. Sinking aggregates of ´marine snow´ also contribute to the low dissolved oxygen levels as remineralisation occurs through bacterial decomposition and oxygen is used up in respiration.
The amount of chlorophyll seen at Station 34 is the greatest across all sites and is expected as tidal mixing fronts provide optimal conditions for primary production due to the mixing of the warm water body with a cooler, nutrients rich one [Holligan, 1981].
At Stations 34 and 37 the concentration of phosphate in the surface layer is shown to have the lowest values, compared to nitrate and silicon concentration, whilst at Station 36 it is the concentration of nitrate that is shown to be the lowest. Phytoplankton use up different nutrients depending on their needs (e.g. diatoms need silicon to build their frustules) and so the presence of different phytoplankton at different stations could be the reason for this [Tilman, Kilham and Kilham, 1982]. All nutrients show a general trend of increase with depth. This is because nutrients are used up at a slower rate at depth (because photosynthesis is limited at the lower light levels) and remineralisation that occurs recycles sinking particles and puts nutrients back into the water at depth.
In the water column at Station 36 the percentage of dissolved oxygen is constant. The sample of zooplankton, collected from 10m to the surface, shows there to be few zooplankton present in the water column. Therefore, little oxygen consumption is occurring and there are few variations in percentage [Ikeda, 1985]. This station was also positioned close to the front but the Stratification Index calculated (2.753) shows the amount of mixing to be less than at Station 34.
The increased nutrient concentrations observed from 30m to 40m depth at Station 37 are possibly due to remineralisation; recycling sinking particles are regenerating nutrients at this depth [Arrigo, 2005]. The zooplankton sample collected at 25m depth demonstrated an increased zooplankton population. This is due to high phytoplankton concentrations at thermocline, and results in oxygen depletion observed from 20-30m depth.
No conclusions could be taken from Station 38, due to inappropriate sample collection, since the Niskin bottle failed at the surface. Therefore, there were not enough data points for the generation of the depth profile for nutrients, oxygen and phytoplankton.
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Offshore sampling - Discussion