Delineating Planktonic Habitats in Dynamic Marine Environments: From Oceanographic Expeditions to Autonomous Platform
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Pham, Anh H.
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Abstract
Phytoplankton are pivotal to marine ecosystems and biogeochemical cycles, serving as the foundation for oceanic food webs and playing a critical role in carbon sequestration. Recognizing their significance, oceanographers and marine biologists have prioritized understanding phytoplankton biogeography through examination of environmental niches and species distribution patterns. Traditional efforts to characterize planktonic habitats have relied heavily on global-scale analyses, amalgamating extensive data compilations and literature reviews. However, it's become increasingly evident that mesoscale and sub-mesoscale processes can significantly influence phytoplankton growth and community structure. To resolve this multi-scale complexity, a novel method has been developed to dynamically define distinct planktonic habitats utilizing standard oceanographic measurements.
My work combined cutting-edge observational technologies, computational techniques, and advanced statistical analyses to provide a computational framework to enhance delineation of phytoplankton habitats. This framework facilitated a flexible and effective integration of diverse oceanographic data, leading to improved understanding of phytoplankton distributions and their environmental drivers.
I used this approach to investigate the spatial and temporal distribution of planktonic habitats over a span of ten years in waters of the Western Tropical North Atlantic influenced by the Amazon River Plume. This study unveiled consistent, distinct phytoplankton habitats associated with the Amazon River's two dominant seasonal flow regimes. Phytoplankton diagnostic pigment analysis and in vivo fluorescence analysis showed that diatoms dominated the riverine input and plume core regions near the coast of French Guiana and Suriname; blue water cyanobacteria were found abundance in the oceanic waters eastward of the river discharge while cryptophytes were found in abundance to the north of the plume regions. Direct cell counts also reveal different diazotroph communities among the intermediate habitat types between the riverine input and oceanic waters. These findings underscored the significant influence of freshwater inputs on marine biogeographical patterns and carbon sequestration in the Western Tropical North Atlantic.
Building on this foundation, I extended the habitat delineation framework to incorporate data from the BGC-Argo fleet of robotic profilers deployed in the Tropical North Atlantic. This extension yielded a higher-resolution and more dynamic overview of phytoplankton biogeography relative to the widely used and static biogeochemical provinces of Longhurst. My habitat delineation captured the elevated abundance of microphytoplankton and nanophytoplankton following the seasonal displacement of the Intertropical Convergence Zone (ITCZ), and the Amazon and Orinoco River discharges into the Tropical Atlantic and Eastern Caribbean Sea. At midlatitudes, the delineation approach was able to capture both the hydrographic and spatial variations of the oligotrophic region of the North Atlantic Subtropical Gyre. These findings shed light on the seasonality of phytoplankton distributions driven by meteorological and oceanic forcings and demonstrated the potential of the BGC-Argo fleet in studying and monitoring marine ecosystems.
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2024-04-27
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Dissertation