Weather Conditions

Lab method

Laboratory methods to determine phosphate and silicon levels were in accordance with the procedures described by Maita and Lalli (1894). The method involved adding reagents and measuring colour intensity, and determining unknown concentrations of samples by means of a calibration curve from working standards of dissolved phosphate/silicon solutions.

Dissolved oxygen levels were determined by titration with sodium thiosulphate using the method described by Grasshoff, Kremling and Ehrhardt (1999).

Chlorophyll levels were determined using a 10 AV Fluorometer followed by data conversion to seawater concentrations.

Water samples from each station were analysed for nitrate using a flow injection system as described in Johnson and Petty (1983).

Results/Discussion

The chemical data shows that in general dissolved silicon increases with depth (Figure 7.), as anticipated due to it being stripped from the upper water column by diatoms for frustule development. Below the thermocline at Station 4 dissolved silicon increases more rapidly to 3.98µmolL-1, reflecting the colder nutrient rich water that lies below. Station 3 is somewhat of an anomaly in that silicon was observed to peak at 3.62 µmolL-1 around the thermocline depth of 10m. This is possibly due to the more windy conditions seen at the station, which almost rendered it impossible to sample, mixing the water down to the thermocline whilst bringing up some dissolved silicon from the layer below.

The profile for phosphate is more similar to what was expected in that at all the stations the concentration increases from the surface to the thermocline (Figure 8.) and more rapidly to the bottom. The stratification of the water mass, with limited mixing between the two layers causes this decrease as phytoplankton remove the phosphate from the upper layer. Where the water is well mixed towards the shore, phosphate is seen to decrease linearly with depth.

Figure 9. shows generally similar trends to phosphate with an increase to the thermocline and a further more rapid increase towards the seabed. Station 4 decreases much more rapidly to 2.68µmolL-1 at 20m ,in relation to the well mixed water at Stations 1&2, (which decreases to 4.92µmolL-1 and 5.89µ molL-1 at 12.2m and 10.3m respectively) due to increased numbers of phytoplankton offshore. Station 3, as with silicon, shows the opposite pattern to the other stations where it is seen to decrease with depth. Again, this may be due to intense mixing by the wind.

Oxygen saturation (Figure 10.) generally decreases with depth, this is likely to be due to the highest amount of light being at the surface and thus the maximum photosynthetic rate of phytoplankton, and thus the highest oxygen is likely to be at the surface. The highest surface chlorophyll is seen at Station 4 with a reading of 113.4%, decreasing to 94.4% at depth, with a more marked decrease below the thermocline. Station 3 again is an anomaly with the opposite pattern to all the other stations; it shows an increase in dissolved oxygen with depth, again perhaps due to intense mixing by the wind before we began our measurements. There is also an increase below the thermocline, possibly attributable to tidal mixing.

Stations 1 & 2 show fairly uniform chlorophyll concentrations with depth, shown in (Figure 3.) which was to be expected given the well mixed water column. Fluorescence peaks at the thermoclines indicated by the CTD suggest high phytoplankton abundance. The data presented in this figure contrasts with that indicated by the CTD readings, with chlorophyll concentrations at stations 3 and 4 being highest at the surface and decreasing with depth.  Possible explanations to this inconsistency derive from procedural errors in the laboratory, primarily from phytoplankton in the sampling bottles sinking to the bottom of the containers.  This means that the chlorophyll values obtained via the acetone method will potentially be lower than that expected from the initial fluorescence readings from the thermosalinograph and CTD.  

The biological data shows that the phytoplankton community structure is affected by a series of physical, chemical and biological properties such as nutrient availability, temperature, light availability, water stability, parasitism and grazing (Suthers and Rissik, 2009). The phytoplankton communities identified across the four stations sampled offshore comprised of both diatoms and dinoflagellates. The dominant genus present across stations 1-3 in terms of cell count was Chaetoceros, a colonial genus of diatom that form long linked chains of individual cells. The presence of diatoms indicate sufficient dissolved silicon concentrations for diatom fustule formation (See Figure 11). Station 4 had a significantly high cell count for diatom Nitzschia, which has not been reported as present for the other three stations which were in the vicinity. This can be attributed to human error as Nitzschia is at first hardly visible under the microscope, and can be overlooked during manual cell count and identification. Present in lower numbers across all stations were Coscinodiscus, Ceratium and Rhizosolenia. The plankton communities present at the surface did not differ significantly from those at depth.

The manual phytoplankton cell counts do not directly relate to chlorophyll level measurements recorded with the fluorometer, which can also possibly be attributed to human error.

Zooplankton abundance was highest at surface waters, presumably as they are attracted to the high phytoplankton abundance in the upper water column. Figure 12 shows station 2 had higher zooplankton abundance than Station 4 at both surface and deeper waters, which may be due to the positioning of Station 2 which was more sheltered compared to Station 4, allowing zooplankton to maintain position within the water column.  Copepods are usually the dominant organism in zooplankton communities (Todd, Laverack & Boxshall 1991), as confirmed across all stations. Ctenophores and Appendicularians were also present at each station. Echinoderm larvae was present in high numbers in surface samples at both stations. The low number of zooplankton at Station 4 may explain the high presence of phytoplankton, due to lack of grazing.

At Stations 3 and 4, further offshore stratification was observed in the water column with a thermocline of approximately 2°C. The upper layer of the water column proved to be the area where most phytoplankton were found, with maximum attenuation observed at the thermocline. The nutrients were highest in the lowest layer of the water, not having been taken up by phytoplankton, which strip out the nutrients from the upper water column. Stations 1 and 2 were less than 20m deep and as such they were well mixed throughout the water column and phytoplankton decreased linearly with depth.  

Figure 8. Phosphate  (umol/L) against depth

Figure 7. Silicon against depth

Figure 9. Nitrate against depth

Figure 10. Oxygen against depth

Figure 11. Phytoplankton cell count per litre of seawater including species/genus breakdown of phytoplankton communities at each station at the surface (S) and below 10m (D). The detail above each bar is an enlarged view of the topmost section of that bar, but is not to scale.

Figure 12. Zooplankton species present at Stations 2 and 4 at surface and below surface.

Phytoplankton: order clockwise from top left:

Chaeteros, Guinardia , Nitzchia, Rhizosolenia

Zooplankton: order clockwise from top left: Copepod, Hydromedusae, Appendicularia, Echinoderm larva

Introduction

In the summer months strong thermal stratification can be regularly observed and is well-studied, (Harris, 2010), (Smyth et al, 2010), in the western English Channel.  This can result in the development of tidal fronts, where tidal mixing causes the thermocline to emerge at the surface. This front presents ideal conditions for phytoplankton growth where nutrients are being transported to the surface by mixing. The higher zenith in the summer months means there is surface heating which creates a warm, stable layer above the thermocline. Between the two layers there is a density barrier which means there is likely to be high Richardson numbers and only sporadic mixing. This barrier causes the phytoplankton to be found just below the thermocline where nutrients are most abundant and irradiance is optimal for growth.

As such the purpose of the investigation was to establish the relative effects and importance of mixing and stratification on the structure and properties of phytoplankton communities. The investigation was conducted on the 2nd July 2013 onboard the research vessel Callista. While the survey was taken the weather conditions were unfavourable for working, with winds gusting in excess of 30kt causing the boat to pitch and roll significantly. This limited any sampling to the sheltered areas within Falmouth Bay. In this shallower coastal water there is likely to be strong mixing due to the wind and tides, resulting in colder water closer to shore.

The tidal front was tracked using 4 stations to see where the chlorophyll levels changed, as seen below in figure 1.

Figure 1: A map to show the locations of the four offshore stations . Hover over image to enlarge.

Disclaimer: Views and opinions are not representative of the University of Southampton or the National Oceanography Centre Southampton

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