Spatio-temporal variability of the Peninsula Front and the surface chlorophyll-a bloom in the Bransfield Strait

Abstract

The Bransfield Strait (BS) is a relatively narrow region located between the South Shetland Islands (SSI) and the Antarctic Peninsula (AP), where the dominant cyclonic circulation is composed by two major inflows, which appear to influence the development of the seasonal chlorophyll bloom. On the one hand, the Bransfield Current transports Transitional Zonal Water with Bellingshausen influence (TBW) northeastwards along the SSI slope. TBW is characterised by well-stratified, relatively warm (Ɵ > -0.4ºC) and fresh (<34.45) waters. On the other hand, the Antarctic Coastal Current (CC) transports Transitional Zonal Water with Weddell influence (TWW) southwestwards along the AP coastline, being distinguished by homogeneous, colder (Ɵ < -0.4ºC) and saltier (>34.45) waters (Sangrà et al., 2017). These two water masses confront each other forming the Peninsula Front (PF; García et al., 1994; López et al., 1999). Interestingly, the chlorophyll-a (chl-a) spatial distribution in the BS has already been linked in the past to the spatial distribution of both water masses and their water column vertical stability, among other factors (Lipski and Rakusa-Suszczewski, 1990; Basterretxea and Arístegui, 1999). Thus, higher chl-a concentrations have been reported around the SSI and Gerlache Strait where TBW flows, while lower concentrations have been traditionally found north off the SSI and closer to the AP coastline, where more homogeneous surface waters prevail (AASW and TWW, respectively) (Corzo et al., 2005).In this work we aim to provide a further understanding on the bio-physical coupling occurring in the Bransfield Strait, focused on the physical drivers controlling the surface distribution of the chl-a bloom and the location of the PF at seasonal and interannual scales. To do this we use various remotely-sensed observations over the period 1998-2018: Sea Surface Temperature (SST), Sea-Ice Coverage (SIC), chlorophyll-a, wind stress and Photosynthetically Active Radiation (PAR). Preliminary results confirm that the spatial distribution of the surface chl-a bloom in the Bransfield Strait is strongly influenced by the location of the PF, both seasonally and interannually. Also, a shift in the strength of the chl-a bloom has been identified, where significantly stronger events are found from 2005 onwards; when mean chl-a bloom values are slightly greater, and about 0.61 mg m-3, than in previous years, when they averaged about 0.49 mg m-3. We hypothesize this shift might be linked to observed changes in the seasonal evolution of the SIC and SST over the same period. Ongoing analyses attempt to elucidate the major mechanisms accounting for this apparent variability of the bio-physical coupling controlling the chl-a blooms in the Bransfield Strait.