Picoplankton and nanoplankton composition on and around a seamount, affected by an eddy dipole south of Madagascar

Picoplankton and nanoplankton community abundances were sampled on and around a seamount located on the Madagascar Ridge, in November 2016. Prochlorococcus, Synechococcus, picoeukaryote and nanoplankton community abundances both on the seamount and in the immediate vicinity (approximately 100 km) surrounding it were analysed using flow cytometry. The original aim of the study was to estimate whether or not the seamount had an effect on the picoplankton and nanoplankton community distribution. However, coupled altimetry and in situ ship data indicated that a dipole eddy was also sampled during this time, which had a substantial effect on the results. Results reveal that picoeukaryotes contributed the most to carbon biomass within the anticyclone, showing higher abundances in the stations situated within the southern slopes of the seamount which also fell within the anticyclone. The southern slope of the seamount contained higher Synechococcus, Prochlorococcus and picoeukaryote abundances, whereas nanoplankton dominated in the cyclonic eddy. Species composition and abundance was significantly different on versus off the seamount, driven by nanoplankton biomass. These results indicate significantly high abundance above the slopes of the seamount, driven by picoeukaryotes, which seem to be amplified by the dipole presence. The resulting variability in microbial abundance both on the southern slopes of the seamount as well as within the dipole has far reaching implications with regard to larger organism proliferation and carbon export during such events in this region.