Aim:

Groups 3 went offshore from Plymouth marina to E1 scientific bouy collecting water samples and performing plankton trawls. The aim of this investigation was to assess the nutrient distribution within the water column in open water in comparison to the estuary system. We also aimed to investigate how these conditions changed with depth and along a transect of stations. By sampling at E1 we also have access to previous environmental data.

Method:

Upon arrival at E1 data was collected using a CTD rosette and ADCP system along with a plankton trawl, using a close net system (See Vessels for more details)

The water samples collected through the niskin bottles on the CTD rosette were processed in order to test for silicate, nitrate, phosphate, oxygen and chlorophyll. The plankton trawl samples had formaldehyde added to them in order to preserve the zooplankton in the sample.

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Figure 8: Station C35 (E1)   50° 02.651’ N   004° 22.792’ W    ---   09/07/13  10:03 UTC T/S profile

There is a very high sea surface temperature of 20 °C at 1.5m below the surface, this high temperature quickly dropped off reaching a steady temperature of 11°C at a depth of 48m down to the seafloor. The salinity fluctuates through the water column with a slight stratification visible at the surface where the salinity starts at 35.5 and then decreases down to 35.0 at 2m water depth where it remains down to the seafloor at 69m. The peak in the salinity at around 26m is a CTD error and therefore inadmissible for this data graph.

Figure 9: Station C36   50° 7.664’ N  004° 23.079’ W   ---   09/07.18  11:38 UTC T/S profile

The temperature is high at the sea surface at 19.5° C but rapidly drops off, losing a degree roughly every 6m until reaching a constant temperature of 12.5° C at 32m. The salinity is remaining relatively constant throughout the water column with only a variation of 0.9 salinity units from 34.9 to 35.8. This lack in variation in salinity is showing that this station is a well-mixed water column.

Figure 10: Station C37   50° 11.006’ N  004° 22.776’ W   ---   09/07/18  12:36 UTC T/S profile

This figure is showing how the temperature and salinity of the water column changes with increasing depth. The temperature decreases rapidly within the first 20m of the water column from 19.7°C down to 15°C, before the temperature change begins to slow down levelling off at a temperature of 12°C at 60m. The salinity is fluctuating throughout the water column between 35.4 and 35.0, which is not a significant change. There is also a possibility of CTD error where it records a higher salinity value than the real salinity in the middle of the water column. However due to the salinity range not being great, this figure is showing a well-mixed water column.

Figure 11: Station C38  50° 14.98’ N  004° 22.26’ W   ---   09/07/18  13:40 UTC  T/S profile

This graph is showing the relationship between the temperature and salinity within the water column with increasing depth. The temperature is showing a steady decrease with increasing depth, from a high 18.8 °C at the surface, it increases slightly to 19.0°C after 0.5m before its steady decrease down to 14°C at 49m. The salinity is showing an overall higher, although variable, salinity in the upper metres of the water column decreasing from 35.5 down to 34.9 at 22m, where the salinity stays constant with only small fluctuations down to the seafloor at a depth of 50m

Figure 12: Station C39  50°20.416' N  004°20.421' W     ---   09/07/18  15:26 UTC  T/S profile

This graph  shows the temperature and salinity profiles over depth for station C40. Between around 2m and 4m the temperature drops rapidly from around 20oC to 17.5oC, it then drops a further 2oC to 15.5oC over the rest of the depth profile to 18m. The salinity increases sharply from 34.7 PSU at 2m to 35.35 PSU at 3m. between 3 and 18m the salinity deviates above and below a general trend of 35.3 PSU.

Figure 13: Station C41  50°21'292' N  004° 20.514’ W    ---   09/07/18  15:38 UTC  T/S profile

This shows the temperature and salinity profiles over depth for station C41. Between around 1.8m and 4m the temperature drops from 19oC to 17oC it then drops to 16.5oC between 4m and 12m. The salinity increases from 34.95 to 35.25 PSU between 1.8 and 2m, it then deviates around 35.25 PSU for the rest of the depth profile.

Figure 15: Station E1 (C35) 50° 02.651’ N 004° 22.792’ W    ---   09/07/18  10:03 UTC Nutrient profile

Each of the nutrients as well as the Chlorophyll scales are different indicating various concentrations through the water column. There is a prominent chlorophyll peak at 22m which corresponds with a dip for both phosphate and nitrate of 0.04 µmol/L and 0.2 µmol/L respectively, silicon is also showing a low concentration at 0.35 µmol/L. After this chlorophyll peak the concentration decreases dramatically down to 10.48 µmol/L at 41m before levelling out to 3.48 µmol/L at 69m. After the dip of nutrient concentration all three nutrients increase up to 0.9, 1.6 and 0.26 µmol/L for Silicon, Nitrate and Phosphate respectively.

Figure 16: Station C36; 50° 07.664’ N 004° 23.079’ W   ---   09/07.18   11:38 UTC Nutrient profile

Each of the three nutrients shown in this figure are on separate scales, with Phosphate on the smallest scale therefore showing the smallest concentrations compared to Silicon which is on the largest scale. From the surface there is a steep increase in the chlorophyll concentration with a stagnation of the concentrations of the nutrients until 24.73m. At this depth there is a clear peak of chlorophyll, at 24.73m, up to 32 µg/L. This peak corresponds to a nutrient low with all three nutrients having their lowest concentrations of 0.01, 0.05 and 0.8µmol/L for Phosphate, nitrate and silicon respectively at the same depth of 24.73m. After this depth there is a steep decrease in the chlorophyll concentration paired with an increase in all nutrient concentrations. Both nitrate and silicon increase continually up to a depth of 69m, whereas after reaching a depth of 41.8m the phosphate concentration decreases down to a concentration of 0.10µmol/L at the lowest depth recorded, of 65m.

Figure 17: Station C37; 50° 11.005’ N 004° 22.776’ W   ---   09/07/18   12:36 UTC Nutrient profile

This figure is showing Nitrate as having the largest scale and therefore concentration through the water column, compared to the other two nutrients. Both Nitrate and Phosphate increase in their concentrations from the surface down to the first depth recorded of 21.3m, for nitrate the concentration decreases down to 0.6 µmol/L, whereas the phosphate concentration continues to increase up to 0.104 µmol/L at 38.8m from 0.06µmol/L at 21.3m. The silicon concentration decreases from the surface from 0.1 µmol/L to 0.05 µmol/L before increasing through the other recorded depths ending at its highest concentration of 1.3 µmol/L at 59m.

Figure 18: Station C38; 50° 14.98’ N 004° 22.26’ W   ---   09/07/18   13:40 UTC Nutrient Profile

Phosphate is shown on a smaller scale than the other two, indicating smaller concentrations are found within the water column at the recorded depths. All three nutrients are showing an increase in their concentrations from the surface sample to the seafloor sample at 49.8m. They are all showing a constant increase, however this data is not completely accurate to the concentrations through the water column due to a misfire by bottle 4 on the CTD, meaning we are lacking the data to see the fluctuations of the nutrient concentrations in the middle of the water column.

Figure 19: Station C39; 50° 18.68’ N 004° 21.84’ W   ---   09/07/18   14:41 UTC Nutrient Profile

Like shown at station C38 both the Phosphate and Nitrate concentrations increase at from the first recorded depth from 0.03 to 0.1 µmol/L and 0.1 to 1.3µmol/L respectively from 4m to 17m. However, Silicon decreases in its concentration down to 0.08 to 0.02 µmol/L before increasing steadily up to 0.9 µmol/L at 33m. Like Silicon, the Phosphate concentration continues to increase in concentration, however after reaching a peak at 17m, decreases down to 0.65 µmol/L at the highest depth of 33m, this depth is still higher than its initial concentration at the surface, but is lower than the concentration in the middle of the water column.

Figure 20: Contour map showing how temperature changes with depth and distance offshore. Actual data points plotted on map. Stations are C39, C38, C37, C36 and C35 (E1) in order of distance offshore.

Shows shallowing and strengthening of thermocline with distance offshore, with weak, deep thermocline closer to shore. At approximately 20km offshore the thermocline rapidly shoals. At 30km offshore the thermocline shows a temperature gradient from 16.5C to 12.5C within 10m

Figure 21: Contour map showing how phosphate concentrations change with depth and distance offshore.

Shows depletion in surface waters with an obvious nutricline which shoals with distance offshore. Concentrations are generally higher with depth as remineralisation occurs. There is apparent phosphate depletion at depth at one station, however this could be due to a problem with sample analysis rather than actual depletion at depth.

Figure 22: Contour map showing how nitrate concentrations change with depth and distance offshore.

Nitrate is depleted throughout the water column, especially in surface waters. Less evidence of nutricline of nitrate although there is a gradient throughout the water column. There is evidence of strong remineralisation of nitrate at depth approximately 26km offshore, this could be due to decomposition of a large amount of organic matter at this point.

Each station we visited and sampled offshore varied in both its nutrient and temperature salinity profiles. However, there was a general trend where there was a chlorophyll peak around 20m which corresponded to a nutrient deficit. Chlorophyll indicates the presence of phytoplankton and zooplankton which arise in large concentrations at these depths due to there being high light penetration which is essential for their growth. The nutrient concentrations drop at this point due to the phytoplankton utilising them. The temperature of the water column decreases with increasing depth due to increased distance from the warm air above. The salinity is variable which indicates a well-mixed water column with little stratification.

Figure 1: Station C35 (E1)– 46-35m

This pie chart is indicating the number and proportion of the zooplankton that was found at station C35 at a depth range of 46-35m, it is clearly showing a large proportion of the zooplankton present at this station depth are Cirripedia larvae having 232 per m -3. There is a high diversity of the number of species present at this station depth but with very low numbers found with the dominating species being Cirripedia larvae, Echinoderm larvae, Copepoda, Cladocera and Decapoda larvae.

Figure 2: Station 36 – 24-18m

This pie chart is showing the number and percentage of the zooplankton species present at this station between a depth range of 24-18m. There is a high diversity of species with 6 different species of zooplankton being present in the sample collected. There are four dominating species within the sample however which are: Decapoda larvae, Echinoderm larvae, Copepoda and Copepoda Nauplii.

Figure 3: Station C36 46-24m

This pie chart is showing the number and proportion of the zooplankton species present at this station between a depth range of 46-24m. There is an obvious dominating species of Copepoda with there being 1385 individuals per m3 with there also being a large proportion of Cladocera.

Figure 4: Station C37 – 19-0m

This pie chart is showing the number and proportion of the zooplankton species present at station C37 at a depth range of 19-0m. It is showing that there is high diversity at this location. The dominating species is the Cladocera.

Figure 5: Station C38 – 5-10m

This pie chart is showing the number and proportion of the zooplankton species present at station C38 at a depth range of 5-0m. There is a large diversity of species present at this location however there is a large dominance by one species which is the Copepods, with them having 9982 individuals per m3.

Figure 6: Station C38 – 21-14m

This pie chart is showing the number and proportion of the zooplankton species present at station C38 at a depth range of 21-14m. There is a high diversity of species present with there being 8 species identified within our sample. Cladocera is by far the dominating species with there being 1606 individuals of this species within 1m3 of the water between this depth rage.

Figure 7: Station C39 – 8-0m

This pie chart is showing the number and proportion of the zooplankton species present at station C39 at a depth range of 8-0m. There is a very low diversity at this location with only two species found in the sample, Copepoda and Polychaete larvae. Copepoda are by far the dominating species with 2037 individuals being found per m3 of seawater from this location and depth compared to only 127 polychaete larva found.

Figure 23: Contour map of changing fluorescence with distance offshore and depth.

Fluorescence is used as a proxy for chlorophyll concentrations. Closer to shore fluorescence is low throughout the water column. At around 15km offshore fluorescence and therefore chlorophyll concentration increases between ~20 to 50m. At station C35 (E1) there is a very pronounced deep chlorophyll maximum where fluorescence reaches 0.34 between 20 and 30m, while surface concentrations remain very low. This deep chlorophyll maximum correlates with the depths of the nutricline and the thermocline where the upwelling of nutrients from the nutricline and the light availability at this depth mean that the optimum growth conditions for phytoplankton are at this depth.