The increasing concentration of greenhouse gases has intensified ocean warming, with the ocean absorbing over 90% of the excess heat due to its high heat capacity. This warming is spatially heterogeneous, leading to regions frequently affected by marine heatwaves (MHWs), prolonged periods of anomalously high sea surface temperature lasting from days to months. MHWs have major ecological, biogeochemical, and socio-economic consequences, with global losses in ecosystem services already amounting to billions of dollars and projected to increase with the rising frequency, intensity, and duration of these events.

Phytoplankton, as primary producers and key regulators of carbon fluxes, are highly sensitive to such thermal extremes. Their responses to MHWs are region-dependent: biomass may decrease in some areas and increase in others, generating strong spatial and temporal variability in surface chlorophyll-a (Chl-a) concentrations and potentially affecting particulate and dissolved matter dynamics and related carbon fluxes. Understanding these processes is crucial to predict ecosystem resilience, biogeochemical feedbacks, and the impacts of climate change on marine ecosystems. Despite the global relevance, phytoplankton, and more largely the biogeochemical, responses to MHWs remain poorly characterized in coastal shelf waters. Understanding the impacts of MHWs on the coastal ecosystems is especially timely given important evidence of their increasing frequency, intensity, and duration over recent decades. Each passing year has brought record-breaking MHW events.

This PhD project aims to improve understanding of phytoplankton and biogeochemical responses to MHWs using contrasted marine environments, including the western tropical Atlantic and the French coastal waters. The specific objectives are to:

1. Characterize the regional and temporal response of phytoplankton Chl-a to MHWs.

2. Investigate the biogeochemical consequences of MHWs by integrating satellite-derived products such as dissolved and particulate organic matter, organic carbon stocks, and surface pCO₂.

To achieve these goals, a multi-scale (spatial and temporal) approach will be adopted, considering the synergy between multi-platform satellite (ocean color, SST, altimetry) and in situ observations. The study will explore multiple spatial and temporal scales, from a few kilometers to basin level and from days to interannual variability, linking physical forcing to biogeochemical responses to provide an integrated view of MHW impacts on coastal/shelf marine ecosystems.

Satellite observations: MHWs will be detected using satellite data from the NOAA Optimum Interpolation SST (OISST) and ocean reanalyses that assimilate satellite SST providing access to subsurface MHWs. Medium-resolution ocean color data (OC-CCI, GlobColour, MODIS 1 km, TOSCA-MOSAICO (under evaluation) merged coastal data) will give a basin-scale context to identify MHWs affected regions, while higher-resolution data (Sentinel-3, 300 m; Sentinel-2, 60 m) will allow finer analyses of coastal variability and sub-basin processes. This PhD will benefit from recent methodological advances for coastal waters (Chl-a, SPM, POC, DOC, and pCO₂) developed in the ANR COCOBRAZ (PI V. Vantrepotte), CO₂COAST (PI H. Loisel), and TOSCA OSYNICO projects. Moreover, the contribution of fine-scale dynamics to MHW development and phytoplankton blooms will be examined using high-resolution SWOT data, enabling unprecedented insights into small-scale dynamics that are often unresolved by conventional satellite data.

In situ observations: Complementary field data will be collected under the CNES-funded SWOT-SWATI project. A quasi-permanent observatory will be established off Recife (2026–2028) with moorings under SWOT track 379 and biannual cruises. Additional datasets from SOMLIT and REPHY will support the French waters study. These observations will provide key validation for satellite products, detailed physical drivers of MHWs, and insight into phytoplankton bloom and coastal biogeochemical dynamics.

This PhD will deliver a multi-scale assessment of phytoplankton and biogeochemical responses to MHWs. By linking satellite observations to in situ validation, it will generate new insights into MHWs biogeochemical impacts. While focused on two regions, the developed tools will be transferable to larger-scale studies. The work directly aligns with CNES objectives, demonstrating the critical role of space-based Earth observations for monitoring and understanding ocean–climate interactions. It will contribute to SWOT-SWATI, PPR Riomar, PEPR FuturObs, and the ODATIS CES “Couleur de l’Océan”, while reinforcing French–Brazilian cooperation through the TAPIOCA IRD LMI, fostering international collaboration and expertise in satellite oceanography.

This work will be performed in collaboration between LOG, LOCEAN (C. Artana) and LEGOS (J. Llido).

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - vincent.vantrepotte@univ-littoral.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online  before March 13th, 2026 Midnight Paris time!