Revisiting the island mass effect

Phytoplankton, the tiny algae that fix carbon dioxide via photosynthesis, represent the base of the oceanic food chain. As such, they support essentially all life in the oceans. In the tropical Pacific, where phytoplankton biomass is low, islands act as sources of nutrients. This natural fertilization process supports productive ecosystems characterized by higher biomass and biodiversity than surrounding waters. Termed “island mass effect”, the impact of islands on phytoplankton has been known for sixty years. 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.

Photo by Thomas Nilsson (license CC BY-SA 2.0).

The SAPPHIRE project

The objective of the SAPPHIRE project (Systematic Analysis of Pacific PHytoplankton and Island Regional Effects) was to elucidate the impact of islands on phytoplankton. To do so, we conducted a systematic study of the island effect in the tropical Pacific from a suite of physical and biological satellite data. The project leveraged the OUTPACE oceanographic cruise that took place in the western south tropical Pacific near several islands in 2015. SAPPHIRE also used data from the satellite-based PHYSAT method that detects phytoplankton functional groups from space.

Using in situ and satellite data in the OUTPACE region, we first developed an algorithm that automatically identifies and characterizes the island effect. Then we applied the algorithm to the entire tropical Pacific using satellite data. The ultimate goals were to:
1) quantify the impact of islands on phytoplankton biomass and biodiversity in the tropical Pacific, and
2) classify the islands as a function of enrichment process and phytoplankton response using machine learning techniques.

Oceanographic campaign investigating nitrogen fixation and biological production in the Southwest Pacific.

Using ocean color signals to determine the dominant phytoplankton groups in surface waters.

Progress and main results

SAPPHIRE started on April 1st, 2018 and ended in June 2019. Our work revealed a strong influence of islands on phytoplankton in the Pacific. We confirmed that islands significantly increases phytoplankton biomass, but also found strong impacts on phytoplankton species and biodiversity. On average, areas fertilized by islands represent about 2% of the tropical Pacific, and island-driven chlorophyll increases are around 10%. We also identified a new type of island effect, where the phytoplankton response is “delayed” in time. Since phytoplankton get carried away from islands by oceanic currents while they grow, this leads to a bloom without any apparent connection with the islands that triggered it. Finally, an analysis of chlorophyll in the southwest tropical Pacific (Ibrahima Afoula Coly’s master 2 internship) revealed a strong influence of islands during the rainy season.

Two papers presenting these results are in preparation; check back for more information after they are published!

Basin-scale analysis of the island mass effect in the tropical Pacific ocean (to be updated)

Description of a new type of island mass effect (to be updated)

Analysis of chlorophyll variability for the 2002-2018 time period

Presentations and publications

Ocean Optics XXIV conference, October 7-12, 2018: Delayed island effects can trigger nitrogen fixer blooms. (abstract, poster).
EGU General Assembly, April 8-12, 2019: Revisiting the island mass effect: a systematic study in the tropical Pacific. (abstract, poster).

Data and software

SAPPHIRE products will be made available for download here when ready.

Team and collaborators

SAPPHIRE is a 15-month project funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement SAPPHIRE No. 746530 (MSCA individual fellowship to Monique Messié).
Team and collaborators include Anne PetrenkoSéverine AlvainAndrea DoglioliThierry MoutinSophie BonnetElodie Martinez and Ibrahima Afoula Coly.