The role of morphology in setting the fate of marine snow in the open ocean

Most of the oceanic primary production is lost from surface marine food webs into the deep ocean as waste sinking particles, known as marine snow. Their morphology influences biogeochemical cycles, yet the fate of various types remains poorly understood. We analyzed four annual time-series of in situ images acquired using UVP6 camera systems mounted on robotic floats at different latitudes between 0 to 50°S. Images were objectively classified to define distinct morphotypes within the continuum of observed morphologies. At high latitudes, surface marine snow was seasonally dominated by elongated morphotypes, possibly fresh phytoplankton-chains, whereas at low latitudes more circular morphotypes predominated possibly as a result of biologically mediated coagulation. Transfer efficiency to the mesopelagic varied substantially between morphotypes. Surface-produced marine snow, predominantly transparent, is replaced by spherical and opaque aggregates below the mixed layer. Given the wide latitudinal range, we propose that marine snow aggregation via coagulation in the mixed layer, followed by zooplankton-mediated control of vertical export at depth, represents a general sequence of particle transformations reflected by shifts in marine snow morphologies.