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Falmouth 2015 - Group 1

Figure 19: Chlorophyll Concentration against depth (Click to Enlarge)

Figure 16: Nitrate Concentration against depth (Click to Enlarge)

Figure 20: Oxygen Concentration against depth (Click to Enlarge)

Figure 18: Phosphate Concentration against depth (Click to Enlarge)

Figure 17: Silicon Concentration against depth (Click to Enlarge)

Nitrate

Each station shows lowest nitrate concentration levels at the surface, with station one reaching a minimum concentration of 0.2µmols per litre; this could indicate nitrate uptake by plankton in this region and/or a reduction in nitrate run off due a prior period of dry weather.  Concentration increases with depth fairly rapidly for station two which appears to relate to the position of the thermocline, whereas concentration levels at station four remain constant and only begin to increase below 23 meters depth. Station one exhibits a concentration increase of nitrate to 0.57µmols per litre at 22 meters and then experiences a return decline to similar levels as seen at the surface at 32 meters. Following this the concentration increases with depth at a high rate to 1.27µmols per litre, overall indicating the presence of a double thermocline at this station.


Silicon

Silicon concentration across all three stations shows an increase with depth below 20 meters which could reflect the remineralisation of silicon with depth; however at station one between 7.2-22 meters depth, concentration decreased slightly before increasing steadily down to 45 meters. This decrease nearer the surface could be attributed to the uptake of silicon by some species of phytoplankton such as diatoms. Over the same depth range the concentration at station four increases only slightly, before rapidly increasing to 4.33µmols at 28 meters. The steep increase in concentration of silicon could be due to a heightened riverine silicon flux into the Falmouth estuary or turbidity in the area resulting in a decrease in light attenuation and therefore a reduced capacity of phytoplankton abundance.  Station two, furthest offshore, only shows a marginal increase with depth in silicon concentration. This may suggest that phytoplankton are persisting to a greater depth, utilising the silicon, coinciding with the supposition that the euphotic zone extends to a greater depth further offshore.



Phosphate

Station one concentrations increased slightly above 22 meters followed by an escalation in concentration from 0.16µmols per litre at 32 meters to 0.2µmols per litre at 39 meters depth; which could again suggest remineralisation in this area. Station two shows a rapid increase in phosphate concentration in the first 19.5 meters to 0.7 µmols per litre, with an equally rapid decline in concentration to 0.25µmols per litre at 45 meters. The decrease may be due to uptake by zooplankton below the thermocline, however the frequency of data prevents a valid conclusion being drawn. Station four showed an opposing trend to station two with an initial concentration decrease from 0.10 µmols per litre to 0.09 µmols per litre, followed by a concentration increase; however it is realistic to consider that this occured over a smaller scale with low variation between the concentrations. Station four may support the proposed uptake above the thermocline and remineralisation below the thermocline of phosphate.



Chlorphyll

Stations two and four follow a similar trend with concentrations of chlorophyll increasing from the shallowest values down to 19.5 meters for station two, and 23 meters for station four. Using chlorophyll as a proxy for phytoplankton abundance, all station data implies the organisms increase in number from the surface to thermocline and subsequently decrease below the thermocline, as indicated by the decrease in chlorophyll concentration. At station one there is an overall increase in concentration levels, however from 22 meters depth there is a steeper increase in concentration from 0.56 µmols/L to 2.26 µmols/L at 32 meters, before decreasing to its final concentration of 1.13µmols/L at 39 meters. Station one displays a two-step thermocline which is reflected in the chlorophyll data as the concentration of chlorophyll increases at two locations in the water column before declining below the secondary thermocline, therefore implying that phytoplankton numbers may be increased on the thermoclines in a deep chlorophyll maxima.



Oxygen

Stations one and four show very close covariation in initial increase and subsequent decrease in concentration to a depth of 28 meters. The initial increase in oxygen concentration could be resulting from phytoplankton in the surface layers releasing oxygen through photosynthesis to the surrounding environment. At station one, concentrations continue to decline to 243.39 µmols per litre at 32 meters. The decrease in oxygen concentration could be attributed to an overall reduction in dissolved oxygen with depth and the requirement for organisms to increase their uptake to support metabolic functions. Station one then displays a second concentration increase to 257.065µmols per litre at 39 meters; this potentially indicates the base of the two-step thermocline. Whilst station two shows a significantly higher initial oxygen concentration than the other two stations at 272.8 µmols per litre, it decreases in the surface layer with depth. This may potentially indicate lower levels of primary production further offshore and therefore more oxygen, in theory, available in the surrounding environment at the very least in the surface waters. However this is challenged by the chlorophyll data which shows an increase in concentration in the surface layers implying higher levels of primary production, thus concluding further investigation is required to prove this trend.



Chemical Overview

The initial findings outlined above are speculated solely from the data obtained working offshore. In many cases there is not sufficient evidence to draw up valid reasons for apparent trends and analysing very small concentration changes may not be evident of a trend but simply a minor variation as the water column is mixed via diffusion and larger- processes such as waves. Additionally, sampling error may be amplified when analysing nutrients offshore. For example, oxygen concentrations can become increasingly inaccurate with greater periods of exposure to the atmosphere. When sampling at station 2, furthest offshore, the boat was situated above a double-swell convergence which caused the boat to increase its pitch and roll angle. The working of oxygen samples into appropriate containers therefore increased in difficulty during this period and the chance of error likely increased.

Chemical

Samples acquired from the RV Callista and processed in the lab.

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Disclaimer: The views expressed here are not associated with those of the National Oceanography Centre Southampton or the University of Southampton.