Plymouth Field Trip 2019 - Group 1

Offshore

Introduction and Aims

On  2nd July 2019, five stations were sampled across the temperate, deep coastal waters of Plymouth Sound on board the RV Callista (Fig. 13). CTD measurements were taken at each location to study the physical, chemical and biological variability in the overall water column structure. Optimal conditions persisted throughout the whole duration of the survey: the sea was calm, with 2-3 wind force and with cloud cover decreasing from 4/8 to 1/8 by the last station.

The aims were to investigate the physical processes affecting the vertical structure of the water column in the offshore environment across a transect from Plymouth breakwater to station E1; and to determine the characteristic phytoplankton and zooplankton communities at each sampling site in relation to different chemical and physical parameters.

A continuous profile of temperature, salinity and fluorescence (used as a proxy for chlorophyll and thus phytoplankton abundance) was recorded against depth using a CTD rosette. Niskin bottles were fired near the seabed, at the deep chlorophyll maximum (DCM) and at the surface, as well as where any interesting stratification features were observed. The water samples at the selected depths were collected and prepared for laboratory analysis of dissolved oxygen concentration, nutrients (silicon, nitrate and phosphate) and chlorophyll-a using methods as described by Strickland and Parsons (1972). Phytoplankton samples were also taken and preserved using lugols Iodine. Furthermore, nano- and picoplankton samples were preserved with glutaraladehyde and analyzed using flow cytometry, whilst microbial abundance and biomass were determined using epifluorescence microscopy. A zooplankton closing net was then deployed to sample at surface and at the depth of the thermocline, however at station C5 the net was deployed to target the DCM specifically.

Methodology

Figure 13: a map showing the five stations sampled; Plymouth breakwater, L4 buoy, E and W of Eddystone Rocks and E1 buoy.

Results

Zooplankton

A zooplankton closing net, with a diameter of 50cm, was deployed to sample at the surface and at the depth of the thermocline. However, at station C5 the net was deployed to target the DCM specifically. At each station, 1 litre samples were set in formalin, of which 10ml were analyzed in the lab.

Overall, the most abundant zooplankton genus was Copepoda across all stations, varying from 39% to 79%. The abundance was greater at the surface than at the thermocline by >600 cells/m3 at station C2 and by >1500cells/m3 at station C3. Additionally, Copepoda was most abundant at station C4, dominating the surface by >5000cells/m3.

The cell count for stations C2 and C3 both showed that zooplankton diversity was higher at surface than at the thermocline, with five more identified genera at the surface for station C2 and three more at station C3. However, at station C4, the zooplankton diversity was greater at the thermocline than at the surface by one more identified genus.

At station C5, six different genera were identified, of which Copepoda dominated 56% of the zooplankton community. This site also presented the highest abundance of Copepod nauplii (34%), in contrast to the other stations which were dominated by Copepoda followed by Appendicularia.

Phytoplankton

Phytoplankton abundance peaked at different depths across all stations. For instance, at stations C1 and C2 the highest abundance was recorded at 9.5m with 12 cells/ml and at 47.7m with 14 cells/ml respectively. At station C3 a peak of 32 cells/ml was recorded at 10.5m and at station C4 at 25.6m with cell counts reaching 45 cells/ml. The greatest phytoplankton abundance was found at station C5 at 2.1m with a total of 1161 cells/ml. Across stations C3 – C5 a decrease in density following the peak was documented as the depth increased, showing minimum density at depths >40m with cell counts varying between 0-3 cells/ml.  

Overall, 16 different genera were observed, however some diatoms and dinoflagellates remained unidentified. A striking variability in phytoplankton diversity and abundance was observed amongst all stations. At station C1 diversity decreased with depth. This was in contrast to all other stations, which showed an increase in diversity peaking at ­10m, which was then followed by a decrease with depth. The phytoplankton community at station C3 was the most diverse (15 genera) and at station C5 it was the least (6 genera). At both, station C2 and C5 the community was dominated by Guinardia, whereas station C1 was mainly composed of Rhizosolenia, station C3 of Thalassiossira and station C4 of Chaetoceros.

Chemical

Site 2 (L4)

The second CTD was deployed at site C2, which is located just off L4 and is around 9 miles southwest of Plymouth. The temperature profile recorded indicates the thermocline to be situated at 9.0m, dropping from 15.1°C at 8m to 14.3°C at 15m. The fluorescence did not show a chlorophyll maximum at the thermocline but indicates a slight increase of chlorophyll below the thermocline at 23.4m from 0.065 mg/m3 to 0.95 mg/m3. Furthermore, the salinity shows small variation which is as expected due to the coastal offshore location (Smyth et al., 2009) and due to minimal precipitation in the days prior to data collection.

The small increase in chlorophyll below the thermocline along with the weak stratification observed suggests the water column is in the early stages of a deep chlorophyll maximum forming. This theory is further supported by the phytoplankton cell counts observed in figure 22.

Moreover, the depth profiles of oxygen, total nitrate and nitrite and phosphate concentration are indicative of a developing thermocline. Low concentrations of total nitrate and nitrite and phosphate and high concentrations of oxygen are observed above the thermocline which then increase and decrease respectively with depth below the thermocline due to remineralisation and respiration of plankton (Osterropht & Thomas, 2000).

Site 5

Site C5 was located just off station E1 in the open-shelf. The CTD profile shows a double thermocline with a deep chlorophyll maximum located in the lower step at 20-25m. In summer, due to the deep waters at this location the tidal constituents are weaker than the strong solar irradiance acting to thermally stratify the water column, thus allowing the thermocline to prevail, which is shown by the large differences between the near-surface and deep-water temperatures (Le Boyer et al., 2009; Sharples, 2007). As the weather prior to data collection was warm and a calm sea state, this profile is not surprising. C5 temperature profile had the largest temperature change with depth out of the 5 stations from 17°C at the surface to 12.6°C at 35m depth.

Similarly, the phosphate and total nitrate and nitrite concentration show a nutricline at 25m as nutrients are unable to mix across the thermocline boundary (Rippeth, et al., 2005). The low nutrient (nitrate and nitrite and phosphate) concentration in the surface is typical of E1 at this time of year (Smyth, et al., 2010), this can be attributed to the uptake of nutrients by phytoplankton (Probyn & Painting, 1985). Nutrient concentrations increase below the thermocline due to remineralisation and mixing of water below the thermocline (Anderson & Saermiento, 1944).

This explanation is supported by the oxygen trend; oxygen decreases by 62μg/L from the near surface (2.1m) to the near seafloor (69m), as O2 is produced rapidly by high phytoplankton abundances at the deep chlorophyll maximum (DCM) but below the DCM O2 decreases due to low abundances and bacterial respiration during remineralisation (White, et al., 2012).

Physical

C1  

The first CTD was deployed at site C1 which was located just outside of the breakwater that protects Plymouth Sound. The CTD profile (shown above) exhibits a well-mixed water column with the near-surface and deep-water temperatures having less than 0.5°C difference. This is as expected due to the shallow water depth of 19m allowing the tidal mixing forces to overcome the stratifying effects of solar irradiance and thus fully mix the water column (Sharples, 2007).  

C2

The second CTD was deployed at site C2, which is located just off L4 which is around 9 miles southwest of Plymouth. The temperature profile recorded indicates the thermocline to be situated at 9.0m, dropping from 15.1°C at 8m to 14.3°C at 15m. The fluorescence did not show a chlorophyll maximum at the thermocline but indicates a slight increase of chlorophyll below the thermocline at 23.4m from 0.065 mg/m3 to 0.95 mg/m3.

Lastly, the salinity shows small variation which is as expected due to the coastal offshore location (Smyth et al., 2009) and due to minimal precipitation in the days prior to data collection.

C3 and C4  

The third CTD was located at C3 at the western of the Eddystone Rocks.

The fourth CTD drop was located at C4, which was situated on the eastern side of Eddystone Rocks. The temperature profile from C4 indicates the beginning of a development of a weak double thermocline, while the C3 temperature profile shows cold upwelling with temperatures similar to the deep water temperatures right up to surface waters. Furthermore, due to the upwelling and enhanced nutrients from bottom waters, the western side of Eddystone Rocks (C3) has a slight increase in fluorescence and thus phytoplankton abundance compared to the eastern side (C4).

Looking at the underway data recorded on the vessel during the transect from C3 to C4, the temperature significantly decreases due to mixing. These characteristics shown are often phenomena’s observed around outcropping land such as Eddystone Rocks as well as larger islands around the world, collectively known as island mass effects (Doty and Oguri, 1956).

C5

The fifth CTD was deployed at site C5 which was located just off station E1 in the open-shelf. The CTD profile shows a double thermocline with a deep chlorophyll maximum located in the lower step at 20-25m. In summer, due to the deep waters at this location the tidal constituents are weaker than the strong solar irradiance acting to thermally stratify the water column, thus allowing thermocline to prevail which is shown by the large differences between the near-surface and deep-water temperatures (Le Boyer et al., 2009; Sharples, 2007). As the weather prior to data collection were warm and with a calm sea state, this profile is not surprising. C5 temperature profile had the largest temperature change with depth out of the 5 stations from 17°C at the surface to 12.6°C at 35m depth.

Summary

Biological

Figure 22: Phytoplankton abundance (cells/ml) and diversity at each sampled depth across stations C1 to C5.

The upstream site A0 and B0 had the highest nutrient and plankton abundance, suggesting that these are linked. Nutrients such as phosphate and nitrate are required for phytoplankton growth which in turn provides food for zooplankton.

At the seaward side of the estuary, plankton abundance is lower. The increased depth means that tidal influence as well as seabed friction effects reduce mixing causing nutrient depletion in the surface. With lower nutrients, phytoplankton and zooplankton numbers are lower and they cannot grow as efficiently.

For  figure legends please scroll over the figure.

For  figure legends please scroll over the figure.

For  figure legends please scroll over the figure.