Offshore

Figure 1. Track from RV Callsta’s onboard computer showing sampling stations.

Method

100ml of seawater was added to a brown bottle containing 1ml of Lugols Iodine, which stains and preserves the sample.

This was recorded on a sheet which also notes the Site, depth, temperature, salinity and time the sample was taken.

At the lab, prepping the samples for analysis, a Pastor pipette with an upcurved end, connected to an electricity source acted as a suction pump allowing the sample to be concentrated by a factor of 10.

Using a light microscope, 1ml of the concentrated sample was pipetted onto a 1000 square Sedgewick – rafter cell however only 100 squares are counted so that the phytoplankton counts are per ml.

Any phytoplankton chains or groups, each cell was counted in the chain and the phytoplankton families were noted and inputted into an Excel Spreadsheet.

Findings

At all stations (C42 to C47) Chaetoceradae abundances were higher than any other phytoplankton. Chaetoceradae was found from surface waters to depths of almost 50m with concentrations higher at stations C42 to C44 compared to other stations.  

At C42 There were more phytoplankton in the surface waters than in deeper waters which is a common trend for other stations as well. However, there were different types of phytoplankton at different depths with a toxic diatomic phytoplankton called Pseudo-nitzschia found in surface waters from 8m-24.7m of C42 and C43 but not in shallower or deeper waters. This could be due to a combination of optimum temperature and salinity for growth at both stations or it could be affected by other factors.

Dinoflagellates are found at various depths through the water column as well from 6.9m to 37.3m and are found in all stations apart from C46 which had one type of phytoplankton only, Ceratium sp., at 2ml-1, this is unusually low and could be due to the area being nutrient poor or human error with identifying and counting phytoplankton.

Interestingly, small populations of Stephanophyxis were only found at C45 where there were a coincidental diminished population of Chaetoceradae, a reason for this could be that the two types of phytoplankton compete for resources. Skeletonemaceae populations were also only found at C45.

Ciliates were found in low abundances at C42, C43 and C47.

PHYTOPLANKTON

Aim to identify the changes in the species diversity and distribution of zooplankton in Plymouth, from the river head (Calstock) to the estuary mouth (Plymouth Sound).

Method

On RV Callista, zooplankton samples were collected using a 0.50 m diameter net with a 200mm mesh which decanted into a one litre plastic bottle.  The net, attached to a rope and depth gauge, was cast off the back of the boat and lowered down to a given depth e.g. 35 m or 20m. For certain stations, a weight was sent down the line at certain depths to trigger a mechanism to close the net, preventing any further zooplankton from entering the net. This enabled us to collect samples from different depths e.g. 35 – 15 m. After the net was retrieved from the water, the outside of the net was washed with seawater so that any zooplankton caught on the net were washed into the bottle. Formalin was added to the zooplankton sample to preserve it.

During the Lab analysis, 10ml of the Zooplankton sample were pipetted into an Aliquot and 5ml of this was pipetted into a Bogorov chamber to analyse under a stereo microscope. The number of each individual zooplankton group/order were counted by eye and written down on a sheet.

Zooplankton samples were only taken at C43 and C45.

Findings

There was a greater abundance and diversity of Zooplankton at C45 compared to C43 where only Copepods and Copepod nauplii were found, with the nauplii in a greater abundance than the adults. This could be due to the different sampling depths of 6.9m for C43 compared to 25.27m and 49.87m for C45.

At C45 a plethora of zooplankton were found with a greater abundance of copepods found at 49.87m compared to 25.27m however the inverse relationship was found for diversity with a greater diversity of zooplankton in the shallower sample compared to the deeper sample. Cirriped larvae and Polychaete larvae were found at densities of 33.95m-3 less than that of Cladocera and Gastropod larvae which had denisties of 67.9m-3. Echinoderm larvae had the greatest densities in both the 25.27m and 49.87m samples at 237.7m-3  and 229.2m-3 respectively.

Method

1)Put a set volume of seawater through a filer (e.g 50ml).

2)Then put the filter and let it soak in Acetone

3)Leave it in a fridge until it is analysed in the lab

4)In the lab, pour the acetone into a glass tube, leaving the filter in the plastic test tube.

5)Place the glass tube into the fluorimeter, closing the lid and making sure it is in straight.

6)Press the star button on the fluorimeter and wait for the third beep, before taking the reading on the screen.

7)Write these readings down for each sample.

8)From this value calculate the value for 50ml by doing 7 divided by 50 multiplied by the fluorimeter value.

Findings

The figure shows an increase in concentration of chlorophyll from the surface waters to 25-30m depth at all sites. This indicates that this is the depth with highest densities of phytoplankton. This may be due to this area having a balance between relatively high concentrations of nutrients and light intensities that can still maintain photosynthetic growth. Below this point chlorophyll concentrations decrease due to limiting light and above this, chlorophyll concentrations decrease due to a limitation of nutrients. 30m is the point at which the nutricline pushes up into the photic region.

CHLOROPHYLL

ZOOPLANKTON

Biological Findings…

Above Figure 2. The density of different zooplankton (m-3) at C43 and C45 from different depths.  (10/07/18)

Figure 3. The abundance of Phytoplankton (ml-1) from C42 to C47 at different depths in the water column. (10/07/18)

Figure 4. The chlorophyll concentrations (µg/L) from C42 to C47 at different depths in the water column. (10/07/18)