Work during SAPPHIRE included three main studies: (1) a local analysis of a bloom driven by the Tonga islands, notably using data from the 2015 oceanographic cruise OUTPACE, (2) a regional analysis of chlorophyll variability and links with island effects in the southwest tropical Pacific, conducted by a M2 student during a 5.5-month internship, and (3) a basin-scale, systematic analysis of the island mass effect in the tropical Pacific. The work and main results achieved in these studies are described below.
(1) An unexpected bloom was observed east of the Tonga islands during OUTPACE. This bloom was hypothesized to be triggered by an island mass effect, even though it was located hundreds of km away from the islands. We revisited this bloom, demonstrating how the Tonga islands indeed triggered it. Doing so, we demonstrated the existence of a new type of island effect, which we termed “delayed island mass effect”, where the phytoplankton responds so slowly to island fertilization that the bloom gets disconnected from the islands as water masses get advected away. Our work suggests that the fertilizing impact of islands may have been largely underestimated, since the island mass effect is classically defined as increased chlorophyll near an island.
(2) During his master’s internship, M2 student Ibrahima Afoula Coly analyzed 17 years of satellite chlorophyll data. He found relationships between high precipitation events and chlorophyll enrichments during austral winter near Pacific islands, suggesting that island runoff significantly contributes to phytoplankton variability in the region.
(3) An algorithm was developed, that automatically identifies the island-enriched areas from satellite chlorophyll data. We built a database of tropical Pacific islands and systematically applied the algorithm to all islands, at the mean, seasonal and monthly time scales for the 2002-present time period. We contrasted several physical and biological variables within and outside of the islands’ zone of influence, building a database of island impacts. Our results identified a strong influence of islands on phytoplankton in the Pacific. We found that islands significantly impact phytoplankton biomass, but also species and biodiversity. On average, areas fertilized by islands represent about 2% of the tropical Pacific, and island-driven chlorophyll increases are around 10%.
Scientific papers are in preparation for studies (1) and (3), and study (2) was written as a master’s thesis. Several communication actions are planned after the papers are published, including a dedicated page on the project website, articles on the MIO website, and letters within the University and French scientific community. In addition, the algorithm software and the database of island impacts will be made publicly available on the SAPPHIRE website, at publication date or within one year, whichever comes first.
