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Zooplankton trawls were taken at three of the seven stations sampled in the Tamar estuary and Plymouth Sound; stations 29, 30 and 32 at sites D (Lynher River), G (St Johns Lake) and I (Western Channel) respectively. These locations were chosen to show how the zooplankton constitution varies when moving downstream towards the western channel (site I) in Plymouth Sound.
A 200µm mesh phytoplankton net with a 60cm diameter opening attached to a 1L sample bottle at the closed end and a flowmeter attached to the opening was set up. Before being deployed off the back of the vessel at the start of the ADCP transect and towed, the value on the flowmeter was recorded. At the end of the transect the net was retrieved, the flowmeter value was recorded again and the net sprayed with fresh sea water from the ships underway system to wash any zooplankton trapped on the inside of the net down into the sample bottle. The sample taken at station 29 was full of silt on retrieval, so this was emptied into the ocean as it would be impossible to accurately determine zooplankton abundances. 2 pumps of formalin were added to the sample bottle to preserve the samples current state. The sample bottle was placed in a cool box with ice for further preservation.
In the lab, 10ml aliquots of each sample bottle were transferred to two 5ml Bogorov counting chambers. The zooplankton species were identified and counted using a light microscope. Abundances were multiplied by 100 to give the number of zooplankton species per litre. Next, the volume of seawater sampled was calculated using formula:
Equation 1:
= 3.1415926536, r = radius of net opening (m), L = towing distance (m), V =
volume of seawater sampled (m3)
The towing distance was calculated by multiplying the flowmeter revolutions between the start and end of the transect by 0.3. Finally, the number of zooplankton per m3 seawater was calculated using the formula:
Equation 2:
= number of zooplankton in 10ml of a 1L sample, V = volume of seawater sampled (m3), N= number of zooplankton per m3 seawater
Results:
Figures 1 and 2 are pie charts showing that the same species are present at both sites; however, their proportions differ slightly. Both are dominated by Copepoda with proportions 70%. The other zooplankton types’ proportions seem to differ between the two sites. Figure 3 shows a comparison of the two sites using a bar chart. A clear difference in total zooplankton abundance can be observed; lower at site I than site G.
Zooplankton
Analysis:
The difference in total abundance is difficult to explain, as phytoplankton samples weren’t obtained on the day of sampling due to a problem with firing the Niskin bottles attached to the rosette. Also, the average chlorophyll concentration at site I was higher than at site G. So, it can be inferred that the abundance of phytoplankton and hence primary production are greater at site I. Therefore, it’s possible that the zooplankton was in the process of depleting the phytoplankton population by feeding at site G. Whereas at site I, the phytoplankton population was in the process of growing and zooplankton populations hadn’t reached a large enough size to cause significant grazing to deplete the phytoplankton population. Or, this observed difference may be due to biological factors like greater competition of zooplankton with other grazers at site I or greater predation of zooplankton here.
The abundance of
various zooplankton in the
Tamar estuary and Plymouth sound.