Award: OCE-2326735

This project advanced our understanding of marine aggregate formation and fragmentation and its implication for biogeochemical cycling in the ocean. The settling of marine aggregates, or particle clusters, are a key component of how carbon-rich phytoplankton are transported from where they grow at the surface ocean to depth. This process is a key part of the ocean biological carbon pump and is related to the rate at which the oceans can absorb carbon dioxide from the overlying atmosphere. The breakup of these particles due to fluid shear from turbulence in the ocean reduces their physical size, slowing their sedimentation and the overall transport of this material. These fragmentation processes are poorly understood and they are not currently incorporated in our ability to predict carbon transport in the ocean. As part of this project new experimental and field methods were developed to measure the strength of marine phytoplankton aggregates to determine their susceptibility to fragmentation in the ocean. Detailed laboratory investigations showed how even the very weak fluid shear experienced by particles in the ocean can cause elongation deformations that result in fragmentation. The strength of these aggregates varied with their size and the interparticle bond forces of their component particles. Aggregate strength varied not only with phytoplankton species, but also on the overall health of the population, with older or stressed microalgae aggregating more quickly and forming aggregates that were harder to fragment. Importantly, all aggregates behaved as viscoelastic materials with significant, non-linear deformation behaviors prior to fragmentation. To extend these findings to particles in the real world, new methods were developed to fragment particles in-situ in the ocean to estimate their strength. These methods were deployed on a cruise on the Northeast US Continental Shelf in April 2022 and in more recent field investigations. Extensive datasets were collected on particle characteristics, the phytoplankton community, biopolymers secreted by these organisms, and the turbulence in the water column. These datasets are openly available and can be used to develop marine particle disaggregation models to improve our predictions of particle transport in the oceans. Furthermore, this project contributed to the traineeship of three undergraduate researchers, one Masters student, two PhD students, and one postdoctoral scholar. It also resulted in the creation of outreach material to encourage students to pursue oceanography by applying engineering skillsets to the challenging and interdisciplinary challenges inherent to marine science. Last Modified: 09/29/2024 Submitted by: MatthewRau