Oceanographic features of the Sub-Antarctic and Polar Fronts in the South-Western Atlantic Ocean during summer 2001.

PAPARAZZO F.E.; SCHLOSS I.R.; SOLÍS M.; RUIZ PINO D.; POISSON A.; ESTEVES J.L.

CADIC-CONICET - Ushuaia - Argentina

Simposio; IBMANT - ANDEEP; 2003

CADIC-CONICET

The present work analyzes the distribution of some chemical (inorganic nutrient), physical (temperature and salinity) and biological (chlorophyll-a) parameters in the Sub-Antarctic Front (SAF) and the Polar Front (PF) in the South-Western Atlantic Ocean. This oceanographic information was collected during summer 2001 in the frame of a joint Argentinean-French Project (Argau I) to relate the above parameters to CO2 dynamics, onboard the icebreaker "Almirante Irizar". Both fronts can usually be identified surrounding the Antarctic continent, approximately between 48º and 60º S, depending on the longitude. The SAF separates the Sub-Antarctic Zone from the Polar Frontal Zone, while the PF separates this last zone with the Antarctic Zone. The area was visited on four occasions during Argau I, between January and May 2001. Data corresponding to an area 2º North and 2º South of the fronts are presented. Temperature and salinity were measured with a Seabird probe connected to a continuous sampling system, which pumped sea surface water (10 m) from the bow of the ship. Nutrient analysis was done on GF/F filtered sea water; the filters were frozen for chlorophyll-a analysis. Inorganic nutrients were analyzed by means both of an automatic analyzer (NO3-, NO2-, and SiO32-) and in manual form (PO43-). Chlorophyll-a was measured spectrophotometrically on extracts. The four transects crossed the fronts at four different geographical longitudes. Three different situations were found: in the easternmost transect, the SAF and the PF were clearly distinguished, while in the westernmost transects the SAF was either not met at all or the fronts crossed displayed mixed characteristics of both fronts (here named SAF+PF), with characteristics that could be an average of both fronts. The SAF was identified around 49ºS 50ºW, with salinities changing from values >34.4 psu to <34.1 psu and 3 oC difference in surface temperature, while in the PF it was distinguished by the temperature that changed from values >1.7ºC to below 0.1ºC and salinity, changing from 34 psu to values below 33 psu. The SAF+PF structure was met around 57ºS, with N-S changes in average values of all the parameters between both fronts. All nutrient salts analysed showed different concentrations across the fronts, but only silicate was significantly different when the northern and southern parts of the different fronts were compared, always showing higher values in the southern part of the fronts. Chlorophyll-a mainly changed in the SAF and SAF+PF, with concentrations that changed from values <0.08μg L-1 to >0.35 μg L-1. Our results indicate that surface physical and chemical data may only on occasions allow to clearly identify the position of the SAF and PF in the Drake Passage area, which can merge together as it happens in other areas surrounding the Antarctic Continent. On the other hand, close to 49ºW both fronts are well separated and physical and chemical parameter could both act as indicator of their presence.