Fragmentation of marine snow and marine oil snow due to small-scale turbulence

Kai Ziervogel, University of New Hampshire, Durham, NH, United States, Julia Sweet, University of California, Santa Barbara, United States, Laura Bretherton, Mount Allison University, NB, Canada, Antonietta Quigg, Texas A&M University at Galveston, Marine Biology, Galveston, United States and Uta Passow, University of California Santa Barbara, Santa Barbara, United States
Abstract:
Fragmentation of marine snow due to small-scale turbulence is important for the fate of sinking organic matter aggregates. In the presence of oil, physical fragmentation of marine oil snow (MOS) leads to redistribution of hydrocarbons in the water column. We conducted laboratory experiments to determine whether the presence of oil affects fragmentation of diatom aggregates formed in roller tanks containing a senescent diatom culture (Skeletonema grethae) and crude oil. Individual aggregates were transferred from roller tanks into a cylindrical acrylic tank (2 L total volume) equipped with a circular grid that oscillates at specific frequencies in the lower part of the tank (between 20 and 70 mm above the bottom), generating small-scale turbulence. When sinking aggregates approached the bottom part of the tank where the grid was oscillating, the grid was turned off to avoid physical contact with the aggregates. Once the aggregate entered the area of turbulent mixing, it stayed either intact and sank to the bottom of the tank or disaggregated into daughter particles. Prior to and after the transit through the turbulence zone, aggregates or their daughter particles, were sized microscopically and then collected on pre-combusted GF/F filters to determine their organic carbon content. Additionally, we determined the concentration of microscopic particles (<100 µm) in the turbulence tank using a Coulter counter. The fragmentation potentials of aggregates (with or without oil) were measured at 3 turbulence levels (dissipation rates), reflecting conditions from open ocean surface waters to high turbulent coastal environments. MOS fragmentation was observed only at the highest turbulence level (ɛ = 1.2 cm2 s-3) whereas diatom aggregates without oil already fragmented at intermediate turbulence (ɛ = 0.5 cm2 s-3). Oil droplets may lead to a tighter packaging of cells within MOS, possibly decreasing aggregate porosity which could explain a greater physical strength of MOS compared to non-oil containing aggregates.
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