Ecological and Physiological Characteristics of Island Mass Effect

The Island Mass Effect (IME) describes the localized enhancement of phytoplankton biomass around oceanic islands and atolls, disrupting the nutrient-poor conditions typical of the subtropical oligotrophic Pacific Ocean. Despite its recognized importance, the spatial extent, physiological dynamics, and ecological consequences of IME remain poorly studied. This dissertation aims to (1) improve the delineation of the spatial extent of IMEs, (2) characterize the physiological dynamics of phytoplankton populations within IMEs, and (3) evaluate the ecological consequences of IMEs on community composition and diversity, using a multidisciplinary approach that integrates satellite remote sensing, in situ bio-optical measurements, and high-resolution taxonomic analyses across the Pacific Ocean. We developed a novel satellite-based method that merges multi-sensor observations and surface current models to detect and track IME features over time to better delineate the spatial extent of the Island Mass Effect (IME). This approach reveals that IME patches can persist and be advected over distances exceeding 1000 km, substantially extending the known spatial influence of islands on their surrounding oligotrophic ocean. We characterized the physiological dynamics of IME phytoplankton communities using satellite-derived stress indices and in situ data from the Tara Pacific expedition. Both iron and macronutrient enrichments characterize IMEs across the South Pacific Subtropical Gyre; however, unlike iron, which remains high during offshore advection of coastal IMEs, macronutrients are rapidly consumed. To evaluate the ecological consequences of IMEs, we analyzed phytoplankton community succession using a combination of metabarcoding, automated imaging, and in situ bio-optical proxies. Phytoplankton community composition differs significantly between coastal IMEs, advected IMEs, and background oligotrophic waters. Distinct shifts in size structure and taxonomic composition emerge between these regions, with evidence suggesting that large, persistent offshore IMEs occur when specific phytoplankton taxa can escape grazing pressure during the advection of coastal water masses into the open ocean. Together, these studies enhance our understanding of the physiological and ecological consequences of IMEs and the complex interactions between bottom-up and top-down processes involved in IMEs.