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In oligotrophic gyres of the tropical ocean, islands can enhance phytoplankton biomass and create hotspots of productivity and biodiversity. This "island mass effect" (IME) is typically identified by increased chlorophyll concentrations next to an island. Here we use a simple plankton model in a Lagrangian framework to represent an unexplained open ocean bloom, demonstrating how islands could have triggered it remotely. This new type of IME, termed "delayed IME," occurs when nitrate is limiting, N:P ratios are low, and excess phosphate and iron remain in water masses after an initial bloom associated with a "classical" IME. Nitrogen fixers then slowly utilize leftover phosphate and iron while water masses get advected away, resulting in a bloom decoupled in time (several weeks) and space (hundreds of kilometers) from island‐driven nutrient supply. This study suggests that the fertilizing effect of islands on phytoplankton may have been largely underestimated. Plain Language Summary: In the poor and nutrient‐depleted waters of the tropical Pacific, islands act as sources of nutrients fertilizing nearby waters. These nutrients are consumed by microscopic photosynthesizing algae, the phytoplankton. The resulting phytoplankton enrichments (blooms) in turn support productive ecosystems. This phenomenon, termed the "island mass effect," has been known for 60 years and is classically defined by increased chlorophyll (representing phytoplankton biomass) next to an island. Blooms also occur in the open ocean and are usually attributed to vertical processes such as mixing or uplifting that locally supply nutrients from subsurface reservoirs. In this paper, we demonstrate that a different type of island mass effect exists, where the phytoplankton response is delayed because they grow very slowly. These blooms are supported by the nitrogen fixer Trichodesmium. Since phytoplankton get carried away from islands by oceanic currents while they grow, this can lead to a bloom located hundreds of kilometers away with no apparent connection to the islands. Nutrient inputs by islands followed by advection can thus trigger remote blooms in the open ocean. Our study suggests that the fertilizing effect of islands may currently be largely underestimated, particularly in the warm waters of the tropical Pacific where Trichodesmimum is common. Key Points: Previously undescribed delayed island mass effects can generate intense blooms decoupled from island fertilization both in time and in spaceThese occur when diazotrophs slowly utilize excess phosphate and iron after a classical island effect while being advected away from islandsThe fertilizing impact of islands on phytoplankton may thus currently be largely underestimated in the oligotrophic ocean [ABSTRACT FROM AUTHOR]

The Marquesas islands are a place of strong phytoplanktonic enhancement, whose original mechanisms have not been explained yet. Several mechanisms such as current−bathymetry interactions or island run-off can fertilize waters in the immediate vicinity or downstream of the islands, allowing phytoplankton enhancement. Here, we took the opportunity of an oceanographic cruise carried out at the end of 2018, to combine in situ and satellite observations to investigate two phytoplanktonic blooms occurring north and south of the archipelago. First, Lagrangian diagnostics show that both chlorophyll-a concentrations (Chl) plumes are advected from the islands. Second, the use of Finite-size Lyaponov Exponent and frontogenesis diagnostics reveal how the Chl plumes are shaped by the passage of a mesoscale cyclonic eddy in the south and by a converging front and finer-scale dynamic activity in the north. Our results based on these observations provide clues to the hypothesis of a fertilization from the islands themselves allowing phytoplankton to thrive. They also highlight the role of advection to disperse and shape the Chl plumes in two regions with contrasting dynamical regimes. [ABSTRACT FROM AUTHOR]

Most of the tropical Pacific is characterized by low phytoplankton biomass, and is often considered to be relatively homogeneous and static. In this mostly unproductive region, chlorophyll concentration however often increases in the vicinity of oceanic islands. Coined the Island Mass Effect (IME), these island-driven biological enhancements are very common across the tropical Pacific. But while the IME has been known for 60 years and the corresponding nutrient supply processes are well understood, the vast majority of islands remain unstudied and impacts at the basin scale are unknown. Additionally, little is known regarding changes in phytoplankton community structure despite implications for higher trophic levels. Here we revisit the impact of islands on phytoplankton by conducting the first systematic study of the IME in the tropical Pacific from a suite of physical and biological satellite data. An algorithm automatically identifying and characterizing the IME was developed and applied to hundreds of tropical Pacific islands. The algorithm identifies the zone of influence of each island from satellite chlorophyll, and contrasts biological and physical variables within and outside of this zone of influence. Physical variables were used to identify enrichment processes and include sea surface temperature, sea level, surface currents, winds, significant wave height, and precipitation. Information on phytoplankton community was obtained from the satellite PHYSAT method that clusters spectral anomalies into classes representing given phytoplankton characteristics. The ultimate goals are to 1) quantify the impact of islands on phytoplankton and physics in the tropical Pacific and 2) classify the islands as a function of enrichment processes and phytoplankton response. [ABSTRACT FROM AUTHOR]