Biophysical coupling of seasonal chlorophyll-a bloom variations and phytoplankton assemblages across the Peninsula Front in the Bransfield Strait

Abstract. This study investigates the spatio-temporal variations of the chl-a blooms in the Bransfield Strait (BS) at a climatological scale (1998–2018). We propose that a suitable monitoring of these blooms can be achieved through remotely-sensed observations only if the Bransfield Strait is divided following the Peninsula Front, which ultimately influence the phytoplankton assemblage. Our analysis is based on characterizing climatological fields of Sea Surface Temperature (SST), air temperature, Sea-Ice Coverage (SIC), chl-a concentrations and wind stress, guided by synoptic novel and historical in situ observations which reveal two niches for phytoplankton assemblage: the Transitional Bellingshausen Water (TBW) and Transitional Weddell Water (TWW) pools. The TBW pool features stratified, less saline, warmer waters with shallow mixed layers, while the TWW pool features well-mixed, colder, and saltier waters. We identify that the 0.6 ºC isotherm corresponds to the climatological Peninsula Front location, effectively dividing the Bransfield Strait in two different scenarios. Furthermore, the 0.5 mg m⁻³ chl-a isoline aligns well with the 0.6 ºC isotherm, serving as a threshold for chl-a blooms of highest concentrations around the South Shetland Islands. These thresholds enable for the first time the monthly climatological description of the two blooms developing in BS at both sides of the Peninsula Front. We think this approach underscores the potential of combining SST and chl-a data to monitor the year-to-year interplay of the chl-a blooms occurring in the TBW and TWW pools contoured by the Peninsula Front.