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Systematic analysis of Pacific phytoplankton and islands regional effects

Periodic Reporting for period 1 - SAPPHIRE (Systematic analysis of Pacific phytoplankton and islands regional effects)

Reporting period: 2018-04-02 to 2019-07-01

The tropical Pacific ocean is a largely unproductive region, characterized by low concentrations of nutrients and chlorophyll (a proxy for phytoplankton). However, local sources of nutrients exist due to the presence of islands. They create hotspots of productivity and biodiversity supporting rich ecosystems. This process, termed “island mass effect” and first identified sixty years ago, represents an important contributor to fisheries potential and to the welfare of islands human populations. However, the vast majority of islands remain unstudied and impacts at the basin scale are not well characterized. Additionally, little is known regarding changes in phytoplankton community structure despite implications for higher trophic levels.
In this context, the objective of the SAPPHIRE project (Systematic Analysis of Pacific PHytoplankton and Island Regional Effects) was to elucidate the impact of islands on phytoplankton by conducting a systematic study of the island mass effect from a suite of physical and biological satellite data. The goal was to quantify the impact of islands on phytoplankton biomass and biodiversity, and classify the islands as a function of enrichment process and phytoplankton response.
The SAPPHIRE project concluded that islands exert a strong impact on phytoplankton biomass and biodiversity in the tropical Pacific ocean. We also discovered an undescribed type of island effect beyond the classical definition, suggesting that the impact of islands on phytoplankton may currently be underestimated.
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.
The SAPPHIRE project made significant progress beyond the state of the art, providing novel insights into the impact of islands on phytoplankton community. In particular, we identified a new type of island mass effect that is not being captured by the “state of the art” definition of increased biomass near an island. This discovery, which was not foreseen at the time of proposal writing, has strong implications because it means that the impact of islands is currently underestimated.
Significant advances were also made in our understanding of the global importance of this phenomenon. For the first time, the impact of island was systematically identified, mapped and analyzed for all tropical Pacific islands. We discovered widespread and significant impacts on phytoplankton biodiversity, which had not been described previously. This has implication for higher trophic levels, and for ecosystem health.
We anticipate this work to be useful to other researchers, and potentially to local island populations. The public release of the island impact database, planned within a year, will allow for more in-depth analysis of specific islands, and for an identification of regions that may be at risk (e.g. decreasing trend in the island fertilizing effect). This will be the focus of future studies, either by SAPPHIRE researchers or others.