The Seasonal Impact of the Island Mass Effect on Coastal Biogeochemistry and Phytoplankton Assemblage: The Case of the Tropical and Volcanic Island of Guadeloupe (French West Indies)

Abstract The Island Mass Effect (IME) in tropical regions remains understudied, despite its potential to fertilize oligotrophic oceanic gyres and impact on higher trophic levels. Guadeloupe provides an ideal case study of IME due to its diverse nutrient sources, dual exposure to the Caribbean Sea on the western coast and Atlantic Ocean on the eastern coast, and seasonality. Two field campaigns (June 2023, early wet season; January 2024, dry season) assessed macro‐ and micro‐nutrient sources including dissolved iron (DFe) and nitrate (NO 3 − ) and their effects on phytoplankton biomass and composition. Key Fe sources included river outflows influenced by hydrothermalism, weathering, sediment resuspension on the Atlantic shelf, and Saharan dust deposition, with contributions varying seasonally and spatially. Nitrate inputs were mainly linked to anthropogenic river influence, with additional Sargassum contributions in June. On the Atlantic coast, seasonal shifts in DFe and NO 3 − fluxes, associated with river inputs in the dry season and dust and Sargassum in the wet season, did not lead to major changes in phytoplankton assemblages. Diatoms dominated the continental shelf, where NO 3 − and DFe were higher. Marked seasonal changes in phytoplankton occurred only on the Caribbean coast, associated with increased DFe and localized diatom blooms near river outflows in June, although cyanobacteria generally remained the dominant group on this coast. Overall, two distinct IMEs were identified, shaped by contrasting nutrient regimes on each coast. However, offshore nutrient export was minimal and cyanobacteria predominated in offshore oligotrophic waters. These findings enhanced our understanding of IME variability and island‐driven nutrient cycling in tropical systems.