Phytoplankton abundance data for the POSEIDON project in the Celtic Seas and English Channel for autumn 2012 to 2016

The POSEIDON project ran an annual autumn pelagic survey in the English Channel, Celtic Sea and Bristol Channel running from 2012-2016 on the RV Cefas Endeavour. This dataset comprises phytoplankton abundance and size data for CEND18 _ 12, CEND20 _ 13, CEND20 _ 14, CEND22 _ 15, and CEND 22 _ 16. Samples were collected from the continuous flow FerryBox water supply (approx. 4m water depth) in a 500 ml container and fixed with 2-3 ml of 25% acidified Lugol iodine solution. Phytoplankton species and abundance (cells per litre) were determined after sedimentation of the samples in 25 ml glass chambers, and using an inverted microscope. Sizes of most common taxa were measured with a micrometer. Abundances of phytoplankton species were then grouped based on functional groups (e.g. centric diatoms, pennate diatoms).