Bio-Optical evidence for increasing Phaeocystis dominance in the Barents Sea

Increasing contributions of prymnesiophytes such as Phaeocystis pouchetii and Emiliania huxleyi to Barents Sea phytoplankton production have been suggested based on in-situ observations of phytoplankton community composition, but the scattered and discontinuous nature of these records confounds simple inference of community change or its relationship to salient environmental variables. However, provided that meaningful assessments of phytoplankton community composition can be inferred based on their optical characteristics, ocean-colour records offer a potential means to develop a synthesis between sporadic insitu observations. Existing remote-sensing algorithms to retrieve phytoplankton functional types based on chlorophyll-a concentration or indices of pigmentpackaging may however fail to distinguish Phaeocystis from other blooms of phytoplankton with high pigment-packaging, such as diatoms. We develop a novel algorithm to distinguish major phytoplankton functional types in the Barents Sea and apply it to the MODIS-Aqua ocean-colour record, to study changes in the composition of Barents Sea phytoplankton blooms in July, between 2002 and 2018, creating time series of the spatial distribution and intensity of coccolithophore, diatom and Phaeocystis blooms. We confirm a north-eastward expansion in coccolithophore bloom distribution, identified in previous studies, and suggest an inferred increase in chlorophyll-a concentrations, reported by previous researchers, may be partly explained by increasing frequencies of Phaeocystis blooms.