Award: OCE-1335088

The world’s ocean is the largest habitat in the biosphere, containing the highest amount of reactive carbon among the three reservoirs on Earth (land, atmosphere, ocean). The marine carbon cycle originates in the sunlit euphotic zone where atmospheric CO2 is converted into organic matter by photosynthetic microalgae. Physical and biological processes such as downward mixing of water masses, migrating zooplankton, and passively sinking organic matter (i.e., marine snow) collectively determine the export flux of organic carbon into the ocean’s interior and the seafloor. The interactions between physical and biological processes that are involved in the downward flux of carbon are poorly constrained. Processes involving marine snow formation and sinking, in particular, are little understood due to their complex structures and fragile nature, which complicate marine snow research in situ. Most of our knowledge on marine snow processes within the ocean’s carbon cycle are based on bulk observations of particle sedimentation in sediment traps deployed at certain depths in the water column, and on laboratory experiments conducted under controlled laboratory conditions. The latter are often performed under stagnant and/or non-turbulent conditions. Given that the surface ocean is always in motion, the effects of small-scale turbulence on marine snow processes have rarely been addressed before. Our understanding of how turbulence affects planktonic organisms and marine snow formation is mainly based on fluid dynamic theory and ocean models. This project conducted a series of laboratory experiments to study, in more detail, the effects of varying turbulence on phytoplankton growth and metabolism. One part of this project focused primarily on how turbulence affects the production of algal metabolic byproducts (so called exopolymeric substances -- EPS) that form the underlying matrix for marine snow. We were able to show that small-scale turbulence affected the formation of EPS through enhanced encounter of polymer precursors in the water. In a different set of laboratory experiments, we were able to show that the quantity of EPS within marine snow affects the sinking of aggregates. This has consequences for export efficiencies of sinking organic carbon out of the euphotic zone. A second component of the project focused on the fluid dynamics of how turbulence affects the encounter rates and collisions between particles which coagulate to form marine aggregates. This component involved direct measurements of particle motion and collision using a novel turbulence tank and three-dimensional imaging system. The results have led to a better model for particle encounter rate as a function of particle size and turbulence level, and provided an updated view of a long-standing theory in fluid dynamics. These results will be useful for predicting marine aggregate formation rates and also add new insights to fundamental fluid dynamics in the field of particle-laden turbulent flows. This project led to the training of undergraduate and graduate students at varying levels (PhD and master’s) in experimental work and data presentation and publication of results in at international conferences and peer-reviewed journals, respectively. Last Modified: 07/31/2020 Submitted by: Brian L White