Disclaimer: the views and opinions expressed are those of Group 9 members and not necessarily those of the University of Southampton, National Oceanography Centre or Falmouth Marine School

NITRATE (Fig 2) - At Station 13 Nitrate concentration increases with depth. The greatest increase is from 28m – 67m. This is due to surface depletion of nitrate due to presence of primary producers in surface waters. As the depth increases, remineralisation of biological nitrate increases adding more chemical nitrate to the water column. Station 14 shows no substantial change in nitrate concentration from the depths 4 - 24m and a decrease from 24 - 61m. This means that the population of primary producers at this station are not nitrate limited because the surface waters were not depleted of nitrate.(Smyth et al., 2009).  Station 15 shows a depletion in nitrate at 22m and an increase in nitrate concentration from 22m – 32m. At 22m there was also an increase in chlorophyll indicating the presence of primary producers which deplete nitrate concentration. From 22m to 32m the nitrate would remineralise back into the water column.

CHLOROPHYLL (Fig 5)- The chlorophyll presented similar distribution at all sampled stations, with lower concentrations at the surface and at deep waters and with a sudden increase between 20 and 40 meters depth, area where the thermocline and pycnocline are located. The highest values for chlorophyll in each station were 3.05, 6.12 and 4.71 µg/L for Stations 13, 14 and 15 respectively.  Station 14 presented the biggest variation in concentration between depths, with the lowest value been 0.33 µg/L at 60.6m.

The Chlorophyll maximum values are found near the pycnocline and thermocline, suggesting an increase on the phytoplankton concentration in these regions in despite of the surface and sea bottom. According to Sharples et al. (2001), the growth of phytoplankton requires sufficient supplies of light and nutrients and since the development of continuous fluorescence measurements, the thermocline has been observed to be a region of enhanced chlorophyll concentration, as well as the region of the maximum density gradient (pycnocline). The low concentrations of chlorophyll at surface indicate the removal of nutrients from the surface waters during the spring bloom, limiting phytoplankton growth in the surface during summer. The decrease in chlorophyll concentration below the thermocline can be due to light limit conditions.  Near the thermocline, some mixing is occurring and enhancing the nutrient supply to the euphotic zone, originating areas of high primary productivity (Pennington et al., 2006).

Figure 2 - A graph showing the change in nitrate with depth at the stations, with some points averaged.

O2 (Fig 3) - At Station 13 the O2 concentration decreases with depth, although an intermediate depth value isn’t present to analyse. A lower concentration of O2 in surface waters is due to the presence of primary producers using O2 for photosynthesis in the euphotic zone. As depth increases there is less utilisation of O2 so mixing increases the concentration. Station 14 shows a decrease in O2 concentration from surface waters to 24m and then and increase to the deeper depth of 60m. At 24m there was also an increased chlorophyll concentration indicating a higher abundance of primary producers. The presence of these primary producers could be responsible for the decrease in O2 concentration at this depth. Mixing in the water column reforms the O2 concentration at deeper depths. (Borges &. Frankignoulle, 2003). At Station 15 a decrease in O2 concentration from surface waters to 22m is present and then and increase at the depth of 32m. At 22m there was also an increased chlorophyll concentration indicating a higher abundance of primary producers. Once again, the presence of these primary producers could be responsible for the decrease in O2 concentration at this depth and mixing in the water column reforms the O2 concentration at deeper depths.

Figure 3 - A graph showing the change in Oxygen saturation with depth at the stations, with some points averaged.

SILICON (Fig 4) - The silicon concentrations were higher at the first 10 meters in all of the three stations, with maximum values of 2.05, 1.96 and 1.68 µmol/L on Stations 13, 14 and 15 respectively. The concentration decreases with the increasing depth and a peak in concentration can be seen between 20 and 40 meters, where the thermocline is located.  The biggest peak was at the farthest offshore Station 13, where the silicon concentration decreased from 2.0 µmol/L at 22.4m depth to 0.38 µmol/L at 27.9m. Below the thermocline, the silicon concentration continued to decrease until it reaches its minimum on deeper waters, with concentrations of 0.24 (Station 13), 0.75 (Station 15) and 0.61 µmol/L at 66.9, 60.6 and 32.3 m depth respectively.

According to Sarmiento et al. (2004) the ocean’s biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. This statement means that we would expect to see a vertical profile with low silicon concentration values at the upper water column, with a subsequent increase in concentration with depth. However, at the sampled stations of this survey the silicon concentration had the highest value on the surface, presenting a decrease in concentration with the increasing depth. The peak in silicon concentration at depths ranging from 20 to 40m suggests an increase in the biological uptake associated with a well defined and highly stratified seasonal thermocline. The peak area with low values of silicon coincides with the Chlorophyll maximum values, indicating the presence of diatoms which incorporate dissolved silicon in their frustules (Bell, 1994). The observed decrease of concentration below the thermocline can show that there is still some stratification preventing mixing between waters situated above and below the thermocline.

Figure 4 - A graph showing the change in Silicon with depth at the stations, with some points averaged.

Bell, R.G. (1994). Behaviour of Dissolved Silica, and Estuarine/coastal Mixing and Exchange Processes at Tairua Harbour, New Zealand. New Zealand Journal of Marine and Freshwater Research. 28 (1). p.55–68.

Borges A. V., M. Frankignoulle.. (2003). Distribution of surface carbon dioxide and air-sea exchange in the English Channel and adjacent areas. Distribution of surface carbon dioxide and air-sea exchange in the English Channel and adjacent areas. 108 (C5), 1-11.

Pennington, J.T., Mahoney, K.L, Kuwahara, V.S., Kolver, D.D., Calienes, R.& Chavez, F.P. (2006) Primary production in the eastern tropical Pacific: A review. Progress in Oceanography. 69. p.285-317.

Sarmiento, J.L., Gruber, N., Brzezinski, M.A. & Dunne J.P. (2004). High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature. 427(1). p.56-60.

Sharples, J., Mark Moore, C., Rippeth, T.P., Holligan, P.M., Hydes, D.J., Fisher, N. R. & Simpson, J.H. (2001) Phytoplankton distribution and survival in the thermocline. Limnology and Oceanography. 46 (3). p.486-496.

Smyth T, J., J. R. Fishwick, L. AL-Moosawi, D. G. Cummings, C. Harris, V. Kitidis, A. Rees, V. Martinez-Vincente and E. M. S. Woodward. (2009). A broad spatio-temporal view of the western English Channel observatory. Journal of Plankton Research. 32 (5), 585-601.

Figure 5 -  A graph showing the change in chlorophyll with depth at the stations, with some points averaged.

Figure 6 - A graph showing the change in phosphate with depth at the stations, with some points averaged.

PHOSPHATE (Fig 6) - Overall there is an increase in the phosphate with depth. Stations 13 and 15 show an overall increase whereas Station 15 shows a maximum of 0.43 µmol/L

at around 24.5m. Station 13 shows the expected trend of reducing in surface waters due to primary producers abundance before the increase with depth. Phosphate is used by organisms within the upper water column for growth and would be remineralised at lower depths through biological breakdown. The remineralisation account for the increase shown in Stations 13 and 15. Station 15 has a higher initial concentration of PO4 due to increased riverine input. The station then follows the expected increase with depth.  Station 14 shows an unexpected increase in the PO4 concentration between the depths of 4 and 24.5m. This can be due to being located at the front two water increasing the PO4 concentration beyond the expected. The deep chlorophyll maximum at Station 14 results in the decreasing PO4 from 24m.

AIM -To identify a thermocline (if present) and compare the nutrient variation with depth at inshore and offshore stations.

On Wednesday 24th June 2015, Group 9 took part in a scientific research trip on the RV Callista to locations off the coast of Falmouth, UK. The trip sampled three depth profiles and net catches, at Stations 13, 14 and 15. It took approximately 5 hours between 07:00 UTC and 12:00 UTC. The first station was the furthest offshore with the subsequent stations successively inshore. Conditions were ideal for sampling with minimal swell and waves, and clear skies with very light winds. The only noticeable wave motion came from close passing ships. The only issue was failure of the plankton net closing at the correct depth.

Surveying occurred on a neap tide.

Table 1 - A table showing the general metadata from Callista

To attain the data ,four pieces of equipment were deployed. A CTD rosette with Niskin bottles, an ADCP and a plankton net.

To obtain the chemical data, a CTD rosette with niskin bottles was used. The samples were then preserved with 10% Formalin for analysis in the lab.

CHEMICAL ANALYSIS METHODOLOGY - Standard methodology was used for nutrient analysis.

Note that in some cases we refer here to manuals for methods rather than the original method paper that may have undergone some modifications.

Manual chlorophyll, dissolved Phosphate and Silicon - Parsons T. R. Maita Y. and Lalli C. (1984) “ A manual of chemical and biological methods for seawater analysis” 173 p. Pergamon.

Dissolved oxygen - Grasshoff, K., K. Kremling, and M. Ehrhardt. (1999). Methods of seawater analysis. 3rd ed. Wiley-VCH.

Nitrate by Flow injection analysis - Johnson K. and Petty R.L.(1983)  “Determination of nitrate and nitrite in seawater by flow injection analysis”.  Limnology and Oceanography 28 1260-1266.