Methods

The water between Plymouth Sound and E1 station were sampled by the RV Callista including the area around Eddystone rocks.

11/07/18

Start Time: 07:30 UTC

End Time: 15:45 UTC

Cloud cover varied from 5/8 to 8/8

Wind stayed between 1-3 on the Beaufort scale

Sea-state: Very calm with low variation throughout the day

The calm sea-state allowed for the sampling of station E1 which was far off-shore in comparison to L4 station. A total of 4 stations were sampled during the day, the main aim of this was to investigate the high sea-surface temperature plume observed in the handout given to us in preparation for this activity as well as the depleted sea-surface levels of chlorophyll. At each station a CTD rosette was deployed to get an idea of the water-column using the SeaBird on the downcast and to recover water samples at specific depths of interest on the upcast with the use of remotely fired Niskin bottles. At each station between 3 and 6 bottles were fired. The purpose of taking these water samples was to analyse the nutricline and other chemical constituents within the water column as well as the observed deep chlorophyll maximums we identified as we got further out into the English Channel and away from shore.

At select stations plankton nets were deployed to depths of interest to sample the biology contributing to the deep chlorophyll maximum. Spikes in fluorescence on the downcast highlighted the deep chlorophyll maximums which can be explained by the oligotrophic surface waters as the result of the spring blooms as well as the higher temperature at depth due to the recent warm weather influence.

Physics

As we ventured further from land the sea-surface temperature showed a very slight increase until we hit open water where we noticed a large increase up to 19 degrees Celsius from 17.5 degrees Celcius. This was expected as seen in the handout we received before-hand which showed a large plume within the English channel of warmer surface waters. We attributed the rise in temperature to the recent warm weather as well as gulf stream influence causing a rise in off-shore temperatures. In conjunction with this we saw a drop in sea-surface chlorophyll which also agreed with the handout which can be explained by the recent blooms associated with Spring time causing oligotrophic surface waters as the nutrient uptake exceeds the regeneration.  

The CTD depth profiles we took showed a well defined deep chlorophyll maximum between 23-28m below sea level at station 50 through 52 caused by the oligotrophic overlying waters as well as the temperature penetration reaching this depth. The deep chlorophyll maximum fluctuated along the ADCP transects as seen clearly on ADCP transect 6 which we attributed to internal waves within the water column.

CTD Depth Profiles                                                    ADCP Profiles

Chemistry

Samples were collected from the Niskin bottles attached to the CTD rosette. Dissolved oxygen samples were taken first, as it is a gas, to ensure they were not contaminated, 1 ml of solutions 1 and 2 was added to each sample bottle, forming a precipitate. These were then inverted and stored in water, again to prevent contamination. Samples for phytoplankton were collected in bottles containing lugols solution and stored in the fridge to preserve them.  Zooplankton samples were collected by carrying out vertical trawls with a zooplankton net, these were then stored in bottles containing formaldehyde to preserve any zooplankton in the sample. Nutrient samples were collected and prepared by pre-washing the test tubes with seawater and filtering them using filter paper, trying to obtain as much as possible. The chlorophyll samples were taken from the bottles and filtered using a vacuum pump system to collect the chlorophyll on the filter paper. These pieces of filter paper were then placed in a solution of acetone and inverted before being stored in the fridge.

Graphs including all sampled chemical constituents were then produced using sigma plot and can be seen using the link below or navigating to via the navigation bar.

Chemical Analyses Graphs

Biology

Phytoplankton

The abundance and diversity of phytoplankton across the length of the English Channel sampled was highly variable. Station 51 shows the highest abundance of phytoplankton (~2000cells/10ml)where the toxic dinoflagellate Karenia spp. account for approximately 50% of the phytoplankton community. Station 51 lies between station L4 and E1 in the English Channel where the influence of freshwater from the river Tamar diminishes. Blooms of Karenia are typical along frontal regions during the Summer months (Brand, 2012) and they grow optimally at 30-34 PSU (Magana & Villareal, 2006).

Other dinoflagellate species also have a relatively high abundance in all the sample sites. At stations 48 and 49 chaetoceros spp. accounts for over 50% and around 25% respectively as seen in the stacked bar graph and pie chart. Flagellates, diatoms and ceratium spp. Are also commonly found in the samples.

Phytoplankton was concentrated at a depth of 20-40m at the Deep Chlorophyll Maximum as can be seen in the chl section.

Station 52 shows lower than half the total phytoplankton abundance than station 51 and has the lowest abundance of all the stations (~600cells/10ml).  This may be attributed to the fact that nitrite, being further off-shore, nitrate and phosphate are comparatively low in surface waters here and silicon is reduced at depth compared to other stations.

Zooplankton

The most consistently abundant organisms across all 5 sites were identified as Copepoda, cladocera and gastropoda larvae. Copepoda were originally hypothesized to be the most abundant organism as they make up such a large percentage of the Earth’s biomass.  All other groups of zooplankton were only observed infrequently and in small numbers.

Site 48

The only zooplankton found at site 48 were Copepoda and Cladocera, this site had the lowest abundance and biodiversity of all the sites with only 51 zooplankton per m3. Nutrient levels weren’t particularly low at this site, therefore this difference in abundance could be due to the location of the site.  

Site 49

Site 49 had a much higher diversity of zooplankton species found and had 674 zoopankton per m3. Again the most dominant groups found was Copepoda and Cladocera, but different species of larvae were also observed here. This could be due to the high levels of chlorophyll in the water, increasing the food sources and making it a more habitable environment.

Site 50

At site 50, 7 different groups of zooplankton were observed, with only 331 zooplankton per m3. Copepoda still dominates, with a significantly higher abundance than other zooplankton groups. Cladocera also had high number at this site other than these, site 50 shows no key differences. 331

Site 51

Site 51 showed the highest abundance of total organisms, with a calculated value of 1490 zooplankton per m3 of seawater. Specifically, there was large numbers of Gastropoda larvae, this is different to other sites, and could be due to the conditions at this location making it a preferable habitat for larvae. It also had a high diversity of organisms with 7 different groups of zooplankton being observed. This could be owed to the relatively high concentration of nutrients at the depths where the zooplankton net was towed

Site 52

The highest diversity was observed at site 52, 8 different groups of zooplankton were observed in the sample, however there was only 547 zooplankton per m3. The types of organisms differed in this sample, there were higher number of groups such hydromedusae and Appendicularia compared to other sites.

PHYTOPLANKTON

ZOOPLANKTON