The Delayed Island Mass Effect: How Islands can Remotely Trigger Blooms in the Oligotrophic Ocean.

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]