Dissertation Defense- Huanqing Huang

Particle concentration and particle sinking velocities govern export fluxes and transfer efficiencies of the ocean biological pump and are thus critical components in ocean biogeochemical models. As a component of particulate organic carbon pool, the evaluation of the ocean gel organic carbon pool has remained especially elusive due to their fragile nature and low detectability via optical techniques. Here, the application of a focused shadowgraph imaging system (FoSI) to marine gel research provides novel insights into the ecological significance, distribution, and transport of gel-like particles across a broad environmental gradient extending from the Azores to the upwelling region off northwest Africa. 

To lay the foundation, a novel image processing protocol was developed tailored towards the recognition of small and gel-like particles in the ocean using FoSI, including illumination correction, image subtraction, edge detection and artifact removal, effectively recognizing the edges of small particles for segmentation. Gel-like particles exhibited significantly distinct geometrical characteristics among depth layers and geographical distribution, and were one or two orders of magnitude more abundant than opaque particles. The same camera system was deployed at 600 meters depth in the Sargasso Sea off Bermuda to determine the relationship between sinking velocity, particle size and particle characteristics with a focus on the small particle size range from 24 to 631 micrometers.  Particle size explained only 25% of the total variance in sinking velocity, while the application of machine learning tools to include particle characteristics approximately doubled the predictive power of the model. Ocean mixing is a large contributor to the flux of carbon via the transport of small particles. É«¾Ûing the 1D Mellor-Yamada turbulent model demonstrated the physical mechanism of the mixed layer pump with strong particle detrainment processes in winter and spring which injects slow-sinking particles from the surface to the mesopelagic zone, and the relatively weaker particle detrainment in the summer, providing an additional organic carbon source to the deep sea.

Therefore, small and gel-like particles may contribute to the ocean biological pump more substantially than previously recognized, suggesting that their role in marine biogeochemical cycling has likely been underestimated. Continued efforts to quantify this elusive particle pool are essential, as a better understanding of its abundance, distribution and vertical transport may necessitate a reassessment of current estimates of particulate organic carbon inventories and their parameterization in ocean biogeochemical models.

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