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Method

100ml water samples were taken from each fired niskin bottle on the rosette and transferred to a brown glass bottle containing LUGOLS and labelled with a yellow number.


These samples were then analysed in the lab. A 10ml aliquot was transferred to a small bottle and labelled with the boat reference. Using a pipette, 1ml of the sample was transferred to a Sedgewick Rafter counting chamber. The phytoplankton species present in 100 (20x5) squares of the 1000 square grid were identified and counted using a light microscope at magnification x10. This abundance was scaled up to number of phytoplankton per litre by multiplying by 10000.


Results

Figure 1 displays the abundance of different phytoplankton genera collected in the samples from different depths and times at Station E1. Throughout the day, the total phytoplankton abundance increases. The total number of phytoplankton was lowest at Station 29, equal to 20000 L-1 and highest at station 33, equal to 1635000 L-1. The phytoplankton diversity also increases. Most phytoplankton were sampled between 14-29m depth. Also, a Noctoluca bloom was observed in the zooplankton trawl microscopy but not in these results.





























Analysis

The observed total phytoplankton abundance and number of genera increases with time are likely due to increasing light intensity as the day proceeds. This would act as a que for different phytoplankton species to carry out diel vertical migration. This is supported by an observed shallowing of the deep chlorophyll maximum. Most of the phytoplankton resides at depths between 14-29m because this is where the phytoplankton can acquire the most nutrients upwelled from the nutrient rich depths, and simultaneously absorb enough light for photosynthesis.


Figure 1: Abundance of various phytoplankton genera per litre at different depths in the water column at 6 stations around station E1 in the English Channel south of Plymouth Sound. This data shows a time series of the phytoplankton structure throughout the day on 08/07/2019 between 10:00UTC and 15:00UTC.

Phytoplankton

A time series of the phytoplankton diversity and abundance at the E1 station throughout the water column