Assessing Marine Snow Dynamics During the Demise of the North Atlantic Spring Bloom Using In Situ Particle Imagery

David Siegel, Adrian Burd, Margaret Estapa , Erik Fields, Leah Johnson, Elisa Romanelli, Mark Brzezinski, Ken Buesseler, Samantha Clevenger, Ivona Cetinic, Laetitia Drago, Colleen Durkin, Rainer Kiko, Sasha J Kramer, Amy Maas, Melissa Omand, Uta Passow, Deborah Steinberg

The ocean's biological pump, a critical component of the Earth's carbon cycle, transports organic matter from the surface ocean to depth and is dominated by sinking particles, often in the form of marine snow-sized (diameter ≥ 0.5 mm) aggregates. Controls on sinking particle carbon export are thought to be driven largely by ecological processes that create and transform sinking particles. We diagnose the importance of both biotic and abiotic processes in the dynamics of marine snow using image-based determination of their size distribution. These observations were made during the demise of the North Atlantic spring bloom as part of the Export Processes in the Ocean from RemoTe Sensing North Atlantic (EXPORTS-NA) field campaign. We show that a sequence of intense storm events generated high turbulent mixing rates in the upper ocean that impacted the abundance, size distribution, porosity and sinking of marine snow aggregates. Mixed-layer turbulence both created and destroyed marine snow aggregates and the sequence of entrainment and detrainment of the mixed layer induced by repeated storm forcings enhanced the vertical transport of aggregates to depth. Evidence of biological transformations was also observed at mesopelagic depths, both for the consumption of aggregates and in the creation of small particles from larger ones, likely due to interactions with zooplankton. Collectively, these results illustrate the complex interplay of physical and biological processes regulating the dynamics of marine snow and suggest their inclusion in predictive models of the ocean's biological pump.