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

Abstract

Fronts affect phytoplankton growth and phenology by locally reducing stratification and increasing nutrient supplies. Biomass peaks at fronts have been observed in situ and linked to local nutrient upwelling and/or lateral transport, while reduced stratification over fronts has been shown to induce earlier blooms in numerical models. Satellite imagery offers the opportunity to quantify these induced changes in phytoplankton over a large number of fronts and at synoptic scales. Here we used 20 years of sea surface temperature (SST) and chlorophyll a (Chl a) satellite data in a large region surrounding the Gulf Stream to quantify the impact of fronts on surface Chl a (used as a proxy for phytoplankton) in three contrasting bioregions, from oligotrophic to blooming ones, and throughout the year. We computed an heterogeneity index (HI) from SST to detect fronts and used it to sort fronts into weak and strong ones based on HI thresholds. We observed that the location of strong fronts corresponded to the persistent western boundary current fronts and weak fronts to more ephemeral submesoscale fronts. We compared Chl a distributions over strong fronts, over weak fronts, and outside of fronts in the three bioregions. We assessed three metrics: the Chl a excess over fronts at the local scale of fronts, the surplus in Chl a induced at the bioregional scale, and the lag in spring bloom onset over fronts. We found that weak fronts are associated with a local Chl a excess weaker than strong fronts, but because they are also more frequent, they contribute equally to the regional Chl a surplus. We also found that the local excess of Chl a was 2 to 3 times larger in the bioregion with a spring bloom than in the oligotrophic bioregion, which can be partly explained by the transport of nutrients by the Gulf Stream. We found strong seasonal variations in the amplitude of the Chl a excess over fronts, and we show periods of Chl a deficit over fronts north of 45∘ N that we attribute to subduction. Finally we provide observational evidence that blooms start earlier over fronts by 1 to 2 weeks. Our results suggest that the spectacular impact of fronts at the local scale of fronts (up to +60 %) is more limited when considered at the regional scale of bioregions (less than +5 %) but may nevertheless have implications for the region's overall ecosystem.