Group 2 Plymouth Field Course
CHEMISTRY
The observed behaviour of nitrate varied throughout the offshore stations. No nitrate was measured along the entire depth profile at station C6 while concentrations were 0 𝜇mol/L until 10m at station C3 where it then increased to 0.148 𝜇mol/L at 20m. At station C5, the nitrate concentration was 0.557 𝜇mol/L at 5m and decreased to 0𝜇mol/L by 15m where it then increased to 0.310 𝜇mol/L at 20m, decreased back to 0 𝜇mol/L at 25m and increased to a maximum concentration of 0.875 𝜇mol/L at 40m. Similarly at C7, there was no nitrate in the surface waters but the concentration then increased to a maximum concentration of 0.526 𝜇mol/L at 15m, was depleted by 25m and increased to 0.1750 𝜇mol/L at 45m. The increase in concentration below the thermocline may be due bacterial nitrification (Ward et. al., 1982). Only one nutrient sample was collected at both C1 and C4 at 20 m and 10 m, respectively, with concentrations of 0 𝜇mol/L at both stations
Nitrite concentrations were depleted in the surface waters and then were marginally
regenerated with depth. At station C3, the concentration increased from 0 𝜇mol/L
at 10m to 0.033 𝜇mol/L at 20m. For stations C5 and C7, nitrite concentrations increased
from 0 𝜇mol/L around 25m to 0.016 𝜇mol/L at 40m and to 0.013 𝜇mol/L at 45m, respectively,
while at C6, it increased from 0 𝜇mol/L at 20m to 0.013 𝜇mol/L at 45m. At C1 and
C4, concentrations were 0 𝜇mol/L. This general trend of increasing concentrations
below the thermocline may be due to uptake by phytoplankton in the surface waters
and the bacterial oxidation of NH4+ and reduction of NO3-
The observed trends in phosphate behaviour varied throughout the 6 stations. For stations C6 and C7, concentrations increased linearly with depth from 0.055 ��mol/L and 0.059 𝜇mol/L at 10m to 0.165 𝜇mol/L and 0.211𝜇mol/L at 45m, respectively. While stations C5 also showed an increasing trend at depth, concentrations reached at maximum of 0.161 𝜇mol/L at 20m. Conversely, at station C3, the concentration was highest at 5m with concentration 0.2 𝜇mol/L decreasing to 0.112 𝜇mol/L at 10m and increasing slightly to 0.127 𝜇mol/L at 20m. This increase near the thermocline may be due to microbial or zooplankton regeneration of dissolved phosphate (Valiela, 2015). The C1 sample at 20m was similar to those for C6 and C7 while that at C4 was similar to that at the sample depth for C3.
The oxygen samples were also prepared from the water collected in the Niskin bottles. They were collected first as oxygen gas in the atmosphere can easily contaminate the samples. Two reagents are added to each sample sequentially which trap the oxygen and create a precipitate. The samples are then stored submerged in water to prevent any addition or removal. The next day, 1000 µl of sulfuric acid was added to cause the reagent to go from solid back to liquid. The sample is placed in an incubation chamber in an endpoint detector. There is a magnet at the bottom of the detector and a magnetic mixer is added to the sample to speed to process. A light is shone through the sample and detected by a sensor on the other side. Then sodium thiosulfate is titrated into the sample, which causes it to slowly turn a lighter colour; the more the solution is lightened, the more light can pass through it so more the sensor picks up. The sodium thiosulfate has a normality of 0.22. A potentiometric recorder records the amount of light measured and once it plateaus the amount of titrate added is recorded. The amount of oxygen in the sample can then be calculated by working back from this.
At all stations, silicate was depleted in the surface waters and at stations C1, C3, C4, C6 and C7, remained relatively low with depth ranging from 0 𝜇mol/L to 0.6𝜇mol/L. This may be due to uptake by siliceous phytoplankton such as diatoms that require silicate in suitable concentrations for skeletal formation (Leadbeater, 2015). Conversely at C5, concentrations remained low between 0.028 mmol/L and 0.669 mmol/L until 25m where it then increased significantly to 5.523 𝜇mol/L.This value however may be the result of a sample contamination as it is very high compared to any of the other stations.
The chlorophyll samples were collected on the Callista from the Niskin bottles. 100 ml at a time were passed through a filter which collected the chlorophyll on its surface and three repeats were done for each depth. These filters were placed in a tube of 90% acetone and left overnight, allowing the acetone to strip the chlorophyll off the filter. The next day they were analysed in the lab using a fluorometer. First a blank solution of 90% acetone was used to get a baseline. The acetone is placed in the machine sample by sample and the filters are discarded. The mass of chlorophyll per litre of acetone is recorded for each one. This is converted to the mass per litre of seawater by multiplying by the volume of acetone divided by the volume of seawater filtered.
Fig 1 -
Fig 2 -
Fig 3 -
Fig 4 -
Fig 5 -
Fig 6 A -
Fig 6 B -
Nitrate, nitrite, phosphate and silicate samples were collected on the R/V Callista from Niskin bottles. Water samples were collected from the bottles at each station, filtered through a GF/F filter with diameter of 25mm and pore size of 0.7 𝜇m, stored in plastic tubes and refrigerated until lab analysis the following the day. The nutrient concentrations were quantified using a nutrient autoanalyzer designed for nutrient analysis of seawater samples.
NITRATE
NITRITE
PHOSPHATE
SILICATE
The day the offshore data was collected, there were very large easterly winds, generating a lot of waves, which in turn caused surface mixing. The graph shows this as the chlorophyll does not show a clear deep chlorophyll maximum, which is expected at this time of year (need reference). The graph does show that for most stations, the deepest sample had the highest amount of chlorophyll. This is expected in summer months as there are very few nutrients left in surface waters after spring and early summer blooms, meaning most phytoplankton activity occurs deeper. The surface water is expected to be stratified during summer. The depletion of nutrients in surface waters cause phytoplankton to sink to the deeper, mixed layer, at the bottom of the euphotic zone (Steele and Yentsch, 1960).
To test this hypothesis, we plotted multiple parameters alongside each other for every offshore station. Some stations showed clear stratification. C3 had a surface layer to ~10m, where temperature was high, and salinity low. It can be seen that nutrients is depleted in this layer before being regenerated under the thermocline. Interestingly, phosphate reverses this trend. This could be because phosphate is not limiting in the surface layers, whereas nitrite, silicate and nitrate are, and then phosphate becomes limiting under the thermocline. There is not a coherent deep chlorophyll maximum, with chlorophyll high above the thermocline, lower on the thermocline and then increasing slightly under the thermocline. This is surprising as the dissolved O2 does increase below the thermocline, implying photosynthesis is taking place. Further chlorophyll samples are necessary to identify the presence or absence of a deep chlorophyll maximum.
Station C5 however does show a deep chlorophyll maximum. In the surface layers, nutrients is limiting and thus chlorophyll is low. This is again reversed below the thermocline when nutrients replenish and chlorophyll increases in line with dissolved oxygen. This is more expected than the chlorophyll data for C3.
References
Steele, J.H. and Yentsch, C.S. (1960) “The Vertical Distribution of Chlorophyll”,
Journal of the Marine Biological Association of the United Kingdom, 39, 217-
Leadbeater, Barry S.C. “Biogeochemical cycling of silicon in seawater.” The Choanoflagellates: evolution, biology and ecology, Cambridge University Press, 2015, pp. 102–104.
Valiela, I., 2015. Nutrient Cycles in Ecosystems. In Marine Ecological Processes
(pp. 529-
Ward, B.B., Olson, R.J. and Perry, M.J., 1982. Microbial nitrification rates in the
primary nitrite maximum off southern California. Deep Sea Research Part A. Oceanographic
Research Papers, 29(2), pp.247-
The views and opinions expressed are those of the individual and not representative of the University of Southampton or the National Oceanography Centre.
Dissolved oxygen concentration varies relatively largely with location. For station
C5 the relatively high oxygen concentration corresponds to a relatively high density
of phytoplankton (as shown in Figure 6). C3 has high light levels (which can be seen
on the physics page) but variable phytoplankton density levels and low dissolved
oxygen concentration oxygen at 10m, although at the surface it is higher. On the
other hand, station C4 which is not far from C3 and is also close to the shoreline
has a very high dissolved oxygen concentration despite lower irradiance levels (see
Figure 6) and low phytoplankton density. This could possibly be due to the presence
of photosynthetic plants such as algae or a kelp forest (which from our side scan
data is known to be present further west along the coastline). Dissolved oxygen concentration
behaviour varies with depth from station to station. At stations C3 and C5, concentration
increases with depth, whereas at C6 and C7 – further offshore -
At stations C5 and C7 the water goes from supersaturated just above 20m to increasingly undersaturated with depth. This could be due to increased salinity with depth (as more saline water holds exponentially less dissolved oxygen) (Davis, 1979). This is confirmed by the ADCP data, that at these stations the water becomes more saline with depth (see physics page). On the other hand, C5 (and to some extent C3) which is not as far offshore has increasing dissolved oxygen saturation percentage with depth, probably due to decreasing temperature and pressure (Davis, 1979). However, both are still undersaturated.
