EXPORTS: Microbial attenuation of sinking particle flux in the mesopelagic
Collaborator: Philip Boyd, U. Tasmania
Sinking particulate organic matter is a major route of carbon export from the sunlit surface ocean to the ocean's interior. Both single-celled microbes and zooplankton play a role in setting the fate of particles in subsurface mesopelagic waters. Microbes exploit the rain of carbon-rich material as a substrate for growth, often attaching to sinking particles, partially dissolving them and respiring the carbon as CO2. Although we know that particle flux is markedly attenuated with depth, predicting the microbial contribution to this loss term using surface measurements is currently not possible. The goal of this project is to develop a mechanistic link between microbial attenuation of sinking particle flux and surface ocean properties, with the following objectives:
• Measure, in situ, the magnitude of microbial respiration as a sink for carbon throughout the upper mesopelagic during the two EXPORTS field campaigns
• Refine the existing conceptual model of the relationship between surface ecosystems, subsurface biogeochemical characteristics, microbial respiration, and transfer efficiency of carbon through the mesopelagic
To accomplish these objectives, we are deploying replicated sets of particle capture devices equipped with oxygen optode-based respiration chambers throughout the mesopelagic during the EXPORTS field campaigns. These systems, known as RESPIRE traps, allow for the in situ capture of sinking particles and subsequent tracking of oxygen consumption. We will compare respiration rates to subsurface ecosystem and biogeochemical characteristics including particle sinking rates, geochemical characterization of particles, and microbial community structure. We will further compare our results to remotely-sensed properties such as net primary production, phytoplankton community composition, and particle size spectra to determine the mechanistic basis for the relationship between surface ocean properties and subsurface activity. This research will determine the importance of microbial processes relative to other potential sinks, such as zooplankton particle consumption.