The Impact of Surface Ocean Mixing on Phytoplankton Communities and Associated Carbon Pools

• Main Author: VerWey, Brian J.
• Other Authors: Graff, Jason R., Letelier, Ricardo, Westberry, Toby, Botany and Plant Pathology
• Format: Master Thesis
• Language: English; unknown
• Published: Oregon State University
• Subjects: North Atlantic
• Online Access: https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gt54kw54t

Deep mixing events in the ocean’s surface layer act as physical drivers of carbon export by detraining dissolved and particulate organic matter, including surface phytoplankton communities, to depth. Once removed from the sunlit surface ocean environment, phytoplankton accumulation rates are dependent on the relative contributions of loss processes, such as grazing and viral lysis. While heterotrophic grazers play a large role in determining the accumulation rate of phytoplankton in the surface ocean, they have not always been found to be the dominant driver of phytoplankton loss at depth. Viruses are by far the most abundant biological entity in the ocean, with some estimates suggesting more than 1030 viruses living in the ocean. Although very small, marine viruses play an outsized role in the mortality of marine organisms, the structuring of marine communities, and ultimately the fate of carbon and nutrient cycling in the ocean. To better understand the fate of phytoplankton communities and associated carbon pools following a deep mixing event, incubation experiments designed to mimic mixing events were conducted during the EXPORTS North Atlantic field campaign. Surface phytoplankton communities were diluted with mesopelagic water and incubated in the dark for up to seven days while monitoring plankton and viral community abundances and rates and dissolved and particulate organic carbon concentrations. Additionally, a series of four storms occurred during the field campaign, resulting in the mixing of surface phytoplankton communities below the euphotic zone and allowing for further investigation into the in-situ responses of phytoplankton, viruses, and bacteria to detrainment below the euphotic zone. Key findings from this study suggest that viral lysis plays an important role in phytoplankton mortality at depth, rapidly moving phytoplankton-associated carbon into a dissolved pool of relatively fresh carbon for use by heterotrophic microbes.