Oral Session 2

Coccolithophore blooms can boost particle fluxes in the Nordic Seas

Coccolithophores are calcifying phytoplankton forming an exoskeleton of micron-sized calcite scales called coccoliths. These form an important part of the oceanic pool of particulate inorganic carbon (PIC). Coccolithophore blooms can favor the export of tiny biogenic surface particles towards the abyss which is a major pathway for biological sequestration of atmospheric carbon dioxide. Yet, the factors modulating the timing and magnitude of mesopelagic particle pulses triggered by Coccolithophore blooms is poorly understood and quantified, mainly because the spatial-temporal resolution of conventional carbon flux methods are insufficient. Here, we developed bio-optical and physicochemical metrics combining twelve years of satellite and autonomous robotics observations to track the seasonal development of Coccolithophore blooms and quantify their impact on mesopelagic (100-1000 m) particle fluxes in the Nordic Seas. Twenty-two Coccolithophore blooms lasting 4-5 months were found under strong surface stratification conditions. Twenty-seven percent of these blooms showed peaks of PIC above 1 mmol m-3 and bio-optical fingerprints of low chlorophyll (chl) and high particle backscattering (bbp; bbp:chl = 0.007 m2 mg-1). We show that these blooms increased mesopelagic spikes in backscattering by ~35%, suggesting increased abundance of large particles in the mesopelagic. Furthermore, mesopelagic fluxes of smaller particles also increased by ~107%. We conclude that intense Coccolithophore blooms in strongly stratified waters trigger massive mesopelagic pulses of particles.

Rafael Rasse, Ghent University, Belgium, https://orcid.org/0000-0001-9082-3315

Griet Neukermans, Ghent University and Flanders Marine Institute (VLIZ), Belgium

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