06 Jan 2023
06 Jan 2023
Status: this preprint is open for discussion.

Satellite data reveal earlier and stronger phytoplankton blooms over fronts in the Gulf Stream region

Clément Haëck1, Marina Lévy1, Inès Mangolte1, and Laurent Bopp2 Clément Haëck et al.
  • 1LOCEAN-IPSL, Sorbonne Université, CNRS, IRD, MNHN, Paris, France
  • 2LMD-IPSL, École Normale Supérieure / Université PSL, CNRS, École Polytechnique, Paris, France

Abstract. Fronts affect phytoplankton growth and phenology by locally reducing stratification and increasing vertical nutrient supply. Biomass peaks at fronts have been observed in-situ and linked to local nutrient upwelling, and reduced stratification over fronts has been shown to induce earlier blooms in numerical models. However observation of these biophysical interactions through satellite imagery have been scarce, despite the opportunity to quantify them at synoptic scales. Here we used twenty years of Sea Surface Temperature (SST) and Chlorophyll-a satellite data in a large region surrounding the Gulf Stream to quantify the impact of fronts on phytoplankton in contrasting regimes, from oligotrophy to bloom, and throughout the year. We computed an Heterogeneity Index (HI) from SST, and used it to sort fronts into weak and strong fronts based on HI thresholds. We observed that the localization of strong fronts corresponded to western boundary current fronts, and weak fronts to more ephemeral submesoscale fronts. We compared Chlorophyll-a distributions over strong fronts, weak fronts and outside of fronts. We assessed three metrics, the local enhancement of Chlorophyll-a over fronts, the global enhancement of Chlorophyll-a due to fronts at the scale of the region, and the lag in spring bloom onset due to fronts. We found that weak fronts lead to a local enhancement of Chlorophyll-a weaker than strong fronts, but because they are also more frequent they contribute equally to the regional Chlorophyll-a budget. We also find the the local enhancement of Chlorophyll-a was two to three times larger for the spring bloom than in the oligotrophic subtropical gyre. We also provide observational evidence that blooms start earlier over fronts, by one to two weeks. Nevertheless our results suggest that the spectacular impact of fronts at the local scale may be misleading, considering their impact on a regional scale budget remains limited.

Clément Haëck et al.

Status: open (until 17 Feb 2023)

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Clément Haëck et al.

Clément Haëck et al.


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Short summary
Phytoplankton vary in abundance in the ocean over of large regions, and with the seasons, but also because of small-scale heterogeneities in surface temperature, called fronts, where phytoplankton growth can be favoured. Here, using satellite imagery, we found that fronts enhance phytoplankton much more where it is already growing well, but despite large local increases the enhancement for the region is modest (5%). We also found that blooms start by one to two weeks earlier over fronts.