Falmouth Group 13 - 2017

Offshore Chemistry

The depth profile shows silicon depletion at all stations in the surface layers due to uptake by diatoms in building their silicon frustules. Silicon increases slightly with depth and increases significantly below the DCM where there is no uptake by phytoplankton.  At the depth of the DCM, silicon concentrations are higher than in the surface layers but significantly lower than at depth, due to phytoplankton uptake. Despite this, there are still sufficient silicon levels present to support phytoplankton growth.

Phosphate is low in the surface waters at all stations. At station 27, phosphate increases significantly at the depth of the DCM (approximately 32m). At all other stations, phosphate is relatively constant with depth but increases below the DCM. This is due to phytoplankton utilising phosphate between the surface waters and the DCM but not below this depth, as there is insufficient light for phytoplankton growth.


Nitrate shows less consistent patterns with depth than the other nutrients. Stations 25, 27, and 29 show little variation with depth, whereas station 26 shows a significantly different trend – nitrate levels are low in the surface and at the DCM between 20-30m, possibly due to consumption by phytoplankton. There is a smaller DCM at around 11m, corresponding with high nitrate concentrations. This suggests there could be an input of nitrate here which supports this phytoplankton growth, but it is possible that phytoplankton levels are not yet large enough to deplete the nitrate. It is worth noting that station 26 is the closest station to land and is therefore the most likely station to have terrestrial inputs of nitrogen, potentially explaining the significantly higher nitrate concentrations displayed in the depth profile.

Oxygen saturation is approximately 55% at all stations in the surface waters and remains constant throughout the water column, with the exception of station 25, which shows a strong oxygen saturation peak at 30m. These low surface values suggest that a process was occurring to deplete the oxygen levels. There are high CDOM signals present, potentially due to a period of intense warmth from the previous week, which may have triggered harmful algal blooms. When these algal blooms then degrade, aerobic microbes increase the biological oxygen demand and deplete oxygen in the surface waters. On the day sampling took place (8/7/2017), the sea state was very calm with almost no waves (sea state 0). This would mean that physical process such as wave action, bubble entrainment and turbulence, which would usually incorporate oxygen into the surface layers, had minimal effect on oxygen concentrations. As a result, diffusion was the only process by which oxygen entered surface water layers, since there was very little biological growth in the surface to produce this oxygen.


Overall, the chlorophyll concentrations obtained via the acetone extraction method closely follow the trends of the fluorometer data. Low chlorophyll concentrations are displayed at the depths where there was low fluorescence; high chlorophyll concentrations are shown where there were peaks in fluorescence. DCMs were present at all stations, supported by sufficient light for photosynthesis and the supply of new nutrients brought up from depth. Values for 1% light depths (shown on the offshore physics page) indicate that there was insufficient light to support phytoplankton growth below the DCM, providing an explanation for the low chlorophyll levels displayed on the depth profiles for each station.


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