Composition and Patterns of Taxa Assemblages in the Western Channel Assessed by 18S Sequencing, Microscopy and Flow Cytometry

Plankton monitoring by microscopy offers a long-term ecological perspective of plankton communities, but different detection approaches are uniquely biased. Genetic identification of marine plankton has become standard but is still not used in routine monitoring. This study assesses the value that genetic methods bring to microscopic and flow cytometry monitoring methods in the Western (English) Channel. An 18S high throughput sequencing (HTS) diversity survey of plankton taxa was performed on samples collected from an automated Water and Microplankton Sampler (WaMS) deployed on the Continuous Plankton Recorder platform (CPR) from 2011–2012. The 18S�HTS survey of WaMS samples detected contrasting but complementary taxa assemblages to that of microscopic surveys, mostly composed of smaller or naked or thin-walled plankton taxa, with most phytoplankton being under 10 µm but most taxa in the survey being mixotrophic or heterotrophic but picking up rare phytoplankton. In comparison with microscopic phytoplankton counts from the CPR survey and Western Channel Observatory station L4, only 8–12 taxonomic families were common to all three surveys, most of them dinoflagellates, with a bias towards larger diatoms and dinoflagellate taxa in microscopy surveys. Additional quantitative real-time PCR detection of two potentially harmful taxa, the pelagophyte, Aureococcus anophagefferens and four Pseudo-nitzschia from 2011–2013. This confirmed the elevated growth of A. anophagefferens in the Western Channel in the summer of 2011 and the early appearance of Pseudo-nitzschia delicatissima in that year. Individual species’ occurrence or abundance was different from their genera or other same-sized groups. Smaller phytoplankton measured by flow cytometry had distinct seasonality in the mid-Atlantic compared to coastal regions