The comparative island mass effect in volcanic and ultramafic tropical islands

Tropical islands play a significant role in oligotrophic gyres by supplying macro- and micronutrients that support phytoplankton production in otherwise nutrient-poor waters. This so-called island mass effect remains poorly documented and is likely to be highly system-specific. Here, we compare the island mass effects of two contrasting tropical environments: a volcanic island in the Caribbean Sea (Guadeloupe, French West Indies) and an ultramafic island enriched in nickel, iron, and other trace metals within the lagoon system of New Caledonia (Coral Sea).This study combines oceanic and terrestrial field campaigns, including river surveys, to identify and quantify nutrient sources surrounding each island, together with analyses of phytoplankton community distributions. Complementary laboratory experiments were conducted to assess the role of coastal sediments as a source of trace metals and their effects on phytoplankton.Our results highlight marked differences in the island mass effect both within and between the two systems. These differences likely result from a combination of environmental and physical drivers, including soil composition, hydrological regime, wind exposure, land use, hydrothermal activity, island-scale circulation, and the presence of a continental shelf. The impacts of these contrasting nutrient sources on natural phytoplankton assemblages are discussed.