On the role of light and vertical mixing in shaping Southwestern Atlantic shelf blooms

Abstract. The influence of light availability and mixed layer depth (MLD) on phytoplankton bloom dynamics was examined across the Argentine Continental Shelf in the Southwest Atlantic Ocean (SWAO). Using satellite-derived chlorophyll-a concentration (Chl-a), photosynthetically available radiation (PAR), and euphotic depth (Zeu, defined as the depth at which the irradiance is 1 % of its PAR value at surface) data, together with reanalysis products for MLD and wind fields, we analyzed the spatial and temporal variability of key phenological parameters computed from the Chl-a time series, including bloom initiation, peak timing, and bloom intensity, over the 1998–2019 period. Distinct mean spatial distribution patterns in bloom dynamics were observed. In the Central Shelf (CS), blooms typically initiate (May–August) and peak (September–November) relatively early which correlated with shallow MLDs and increasing light, while coastal areas showed even earlier initiation (April) due to highly variable environmental conditions. In turn, the Patagonian Shelf (PS) experienced delayed initiation (September onwards) and peaks (December–January) probably due to deeper MLDs as result of the colder Subantarctic waters. Bloom intensity also exhibited spatial variability, with the highest values observed in the southern PS and regions influenced by frontal systems, where nutrient-rich upwelling and favorable light conditions enhanced phytoplankton growth. Statistical modeling revealed that light penetration (Zeu) and its interplay with vertical mixing (Zeu:MLD ratio) were the strongest predictors of bloom anomalies at most sites. However, the predictive power of these relationships varied in regions influenced by local processes, like tidal mixing or frontal zones. Predictive models need to be integrated with regional oceanographic features to improve assessments of bloom phenology and primary production in such highly variable shelf ecosystems.