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Zooplankton

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Method

Zooplankton trawls were taken every other hour after the CTD was deployed. Depth ranges for the vertical tow were decided after looking at the CTD data, in order to try and sample around the Deep Chlorophyll Maximum (DCM). One deep water tow was done at the first station just to investigate what, if anything, was there.


A 200µm Phytoplankton net with a 50cm diameter opening, 1L sample bottle, and an attached depth meter was deployed to the desired depth, then pulled up through the water column. The net was closed at the upper boundary and retrieved. The net was then washed down with the Callista’s inline hose to wash the plankton down into the bottle. The 1L sample then had 2 pumps of formalin added in order to preserve the sample in its current state.

Samples were analysed in the lab by taking a 10ml aliquot from each bottle, and studying these subsamples in 2x5ml counting chambers under a microscope. Total numbers and individual zooplankton species numbers were counted, then scaled up to give a value for number of plankton per m3.


In total, there were;
3 tows for station 29 (10:00 UTC)
3 tows for station 31 (12:00 UTC)
2 tows for station 33 (14:00 UTC)
2 tows for station 34 (15:00 UTC)

We only had 10 1L zooplankton bottles on the ship, so this limited the number of tows we could do.


A large bloom of Noctiluca scintillans was also observed, but this is a phytoplankton species so not discussed here.

Results

Station 33 15-0m and station 31 15-0m had the most diverse zooplankton samples. Copepods and appendicularia were by far the most common. Relative to the DCM, zooplankton was most abundant above/below. As the vertical tows were done through the DCM it is impossible to pinpoint the zooplankton- the scatter plot was made using the midpoint of each vertical section.

Analysis

Zooplankton was most abundant in and around the DCM, which is expected as they feed on the phytoplankton. Zooplankton was also the most diverse in the upper part of the DCM and just above it, which correlates with the more diverse phytoplankton observed here, which can be seen in the phytoplankton graphs. There is more total zooplankton in the later measurements than the first (10:00 UTC) measurement; this correlates to fewer, less diverse phytoplankton present in the first measurement. Factors affecting these distribution patterns could be diurnal migration, diurnal movement of the thermocline, and abundance/diversity of phytoplankton prey.


An investigation to

determine changes in the

zooplankton community size and composition relative to the Deep Chlorophyll Maximum.