High-frequency observations from the HOPE buoy, deployed in New Caledonia, provide new insights for WP2 of the MAHeWa project on the biological impacts of marine heatwaves (MHWs).
The data reveal a very rapid increase in surface plankton biomass, which can quadruple within a few hours during MHW events, promoting the accelerated formation of carbon-rich marine snow and its export to depth. These processes are notably associated with blooms of diazotrophs (Trichodesmium), which are typically seasonal in tropical oligotrophic regions.
In July 2025, a persistent MHW detected using MAHeWa forecasting tools (in partnership with Mercator Ocean) was linked to unusual Trichodesmium blooms during the austral winter. These events create temporary biological oases that enhance carbon fixation in the open ocean, while also raising questions about the resilience of coastal ecosystems, particularly already vulnerable coral reefs.
Experiments conducted in situ on the HOPE buoy and in the laboratory allow the quantification of marine snow sedimentation and the estimation of long-term CO₂ sequestration, suggesting that some MHWs may, counter-intuitively, temporarily enhance the oceanic carbon sink.
Overall, these results strengthen MAHeWa’s assessment of the ecological vulnerability of coastal and now semi-offshore ecosystems, improving the anticipation of biological impacts associated with marine heatwaves.
