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Falmouth 2017 - Group 12 7TITLE

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Offshore

Results

Temperature and chlorophyll

The vertical temperature contours in the surface as well as an overall smaller temperature change throughout the depths are indicating a more mixed water column in the beginning of the time series. Sunny weather and little wind would have caused energy uptake of the surface waters throughout the day resulting in an increase in surface temperature seen in Figure 1. This would also be a factor explaining the stratification of water column with time forming a distinct thermocline around 18 meters in the end of the time series. The tide moving from low tide in the morning to high tide in the afternoon could explain the mixing event indicated around 13:00 UTC. It would be expected to see a peak in the tidal current around the low and high tide in the English Channel (Simon Boxall – personal comms) causing more turbulence and mixing. This could suggest why the water column is observed to be more mixed around low tide as well as the mixing event observed prior to the high tide, both seen in Figure 1. For Terramare no ADCP data was obtained and further discussion around the effect of the tidal flow is therefore difficult.

The mixing event prior to the high tide resulted in cold water being mixed up to the thermocline bringing nutrients up from the deeper waters. This could have resulted in a phytoplankton bloom explaining the chlorophyll peak occurring at the same depth and time shown in Figure 2.

Nutrients and chlorophyll

The aggregation of phytoplankton around or just below the thermocline is common during the summer months with high light availability. The deep chlorophyll maximum is related to the thermocline as the surface water is depleted in nutrients such as phosphates, nitrates and silicates (Ross & Sharples, 2007). Phytoplankton will therefore accumulate at depth where nutrients are available. This is clearly seen in the results indicating a deep chlorophyll maximum around 30 meters shown in Figure 2. The plots of phosphate, nitrate and silicate concentrations shown in Figure x to y all show peaks around 13:00 UTC which can be related to the mixing event previously discussed drawing nutrients up from deeper waters.

The phosphate levels are low in the surface waters as expected throughout the time series due to phytoplankton uptake of most of the bioavailable phosphate as well as stratification inhibiting nutrient rich waters mixing up from the deep. The increase around 14:00 UTC could be due to CDOM dissolving back into the water. The chlorophyll levels remain low in the surface water, but at the DCM the chlorophyll concentration increases with the phosphate concentration.

For nitrate the surface water levels are increasing in the surface waters and decreasing around the depth of the deep chlorophyll maximum in the beginning of the time series. The levels in the surface waters are then dropping around 13:00 UTC when the levels at 30 meters are peaking. This could be due to the peak in light availability at 12:00 UTC causing increase in uptake by phytoplankton in the surface layers. The increase in chlorophyll levels from 11:00-14:00 UTC could be related to this drop. However, the nitrate levels are increasing from 13:00-14:00 UTC and more investigation is therefore needed to explain this. At 30 meters the chlorophyll levels follow overall the increase in nitrate.

For the surface waters the silicon levels follow the chlorophyll as expected.

For the surface waters the silicon levels follow the chlorophyll as expected. At 30 meters the silicon levels show the same trend as for nitrate with a decline before increasing from around 11:00-14:00 UTC following the chlorophyll levels. However, the silicon is peaking later than nitrate where the chlorophyll is lowest. This could indicate that silicate and chlorophyll is not directly linked. However, further investigation is needed to explain this.

Oxygen – station 11, yo-yo

The surface layer of the water column at all “stations”/data collection times was supersaturated due to several factors. Direct contact with the air incorporates oxygen into the surface waters. Passing boats and tidal movements would also aerate the water, thus adding more oxygen to the surface layer. Phytoplankton populations in the surface layers help drive up the dissolved oxygen saturation in the shallow surface waters through photosynthesis. However, as shown in Figure 2, the chlorophyll level in the surface layers is overall low and would therefore have little effect on the surface oxygen levels. The oxygen saturation peaks at all stations occurring just below the thermocline observed at each station, suggesting oxygen being trapped in the isopycnals.

Biological discussion

Offshore zooplankton

The offshore zooplankton samples show that species richness increases with depth. The pie charts of the surface waters (2 m) show less species than the charts which show the species at 30 m (which w the depth of the DCM). The species richness at 2 m is between 6 and 7 for both mesh sizes compared with 30 m that has a species richness between 10 and 10.5 for both of the mesh sizes. This can be due to factors such as food availability, temperature and even factors such as UV radiation and predation that will cause zooplankton to aggregate at the DCM (Williamson, et al., 2011). The species evenness is lower on the 100 micrometer mesh size compared to the 200 micrometer mesh (averaging between 0.4-0.5 and between 0.5-0.7 respectively). This means the lower size spectrum of the zooplankton community is more strongly dominated by a few species than the larger size zooplankton, which show a more even spread of the individuals across the different species.

Offshore phytoplankton

The offshore diversity of phytoplankton is higher at 30 m than it is at the surface (2 m) the species richness at the surface is on average 5.4 and the species richness at 30 m is 7.8. This is due to the stratification of the water column as further discussed chemical section.

These higher salinities were dominated by Rhizosolenia spp (the abundance ranges from 33% to 92% between stations) (this included R.alata and R.setigera). Rhizosolenia is an example of a large diatom, there is a slower predation rate due to the size of the diatom (Barton, et al., 2012) when compared with the other phytoplankton species that were present at the offshore stations. Rhizosolenia spp. has a more prominent dominance in surface waters.

Limitations zooplankton

A large limitation on our data was that the counting process was carried out by 10 separate people, half with limited experience in identifying species under a microscope and in our zooplankton samples we experienced a large phytoplankton bloom especially at 30m which meant that the phytoplankton and algae in our samples impaired our ability to count a lot of the zooplankton. Another human error factor related to the amount of zooplankton, in some of our 10ml samples there were over 500 copepod species to count therefore the accuracy of the counting was questionable.

Limitations phytoplankton

A large limitation on our data was that the counting process was carried out by 10 separate people, half with limited experience in identifying species under a microscope. Also, many of the phytoplankton counts had a total bellow 20, and some even bellow 10 per ml which translates to below 100.000 per litre. These counts are so small that the ratios between species and the dominant species cannot be determined in a reliable way. This, resulted in the lack of ability to determine trends and the difficulty to fully describe the biological patterns along the Fal estuary.

Discussion - Offshore results