Spring 2020

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


Carbon is transported to the deep ocean through physical mixing and gravitational sinking of organic particles. Carbon that sinks to the seafloor is consumed by benthic organisms who rely on the detrital particles as their source of food. Station M is a long-term deep-sea study site in the Northeast Pacific where large episodic pulses of particulate organic carbon (POC) sinking to the sea floor at a depth of 4000 meters have been recorded for the past 30 years. The episodic pulses of POC have increased in frequency and magnitude over the past decade, driving a long-term increase in carbon export observed in sediment traps deployed at this location. The goal of this study is to investigate changes in phytoplankton communities within the sinking particles to provide an indication of the ecological mechanisms associated with the high-flux events and why they might be increasing over time. Samples collected by sediment traps were analyzed by microscopy to determine phytoplankton community composition. Phytoplankton community compositions observed in high-flux samples were significantly different from those in low-flux samples. Particles sinking during low-flux periods were relatively more enriched in phytoplankton cells (higher cell:POC fluxes) compared to particles sinking during high- flux events. When separating intact and fragmented diatom cells only the fragmented diatoms had significantly different community compositions in the high- and low-flux events. Since fragmentation of diatoms was significantly different it suggests that grazing and repackaging of the cells was occurring in the water column as the particles sank. Biogenic silica (BSi) and particulate inorganic carbon (PIC) were measured in the particles to test whether mineral ballasting may be driving the large pulses of POC sinking to depth. BSi:POC was constant with increasing POC flux, while the PIC:POC decreased with increasing POC. This indicates that ecological processes leading to high-flux events involve silicifying phytoplankton more than calcifying phytoplankton at Station M. While the biomineral data does not support the ballast hypothesis, it suggests instead that high POC flux is due to production by diatoms in the surface ocean followed by transformation and repackaging through the mesopelagic food web.