Temporal and spatial distribution of chlorophyll-a in surface waters of the Scotia Sea as determined by both shipboard measurements and satellite data

Abstract

Abstract Chlorophyll- a (Chl- a ) concentrations in surface waters were measured at 137 hydrographic stations occupied by four research vessels participating in the CCAMLR 2000 Survey and the values were compared to estimates from data acquired by the SeaWiFS satellite. The Chl- a concentrations measured on board ship ranged from 0.06 to 14.6 mg m −3 , a range that includes most surface Chl- a concentrations during mid-summer in the Southern Ocean. Owing to persistent cloud cover over much of the Southern Ocean, it was necessary to acquire multi-day composites of satellite data in order to obtain reliable estimates of Chl- a at each of the hydrographic stations. The correlation between the median value for the eight-day composites and the Chl- a concentrations measured on board ship had an R 2 value of 0.82, with the satellite data under-estimating the values obtained on board ship at high Chl- a concentrations and slightly over-estimating the shipboard data at Chl- a concentrations of −3 . For Chl- a concentrations of −3 , the ratio of the satellite estimates divided by the shipboard values was 0.89 ± 0.45 ( n =50). As the mean Chl- a concentration in most pelagic Antarctic waters is close to 0.5 mg m −3 , satellite estimates for Chl- a concentrations in surface waters are thus close to shipboard measurements, and offer the advantage of providing synoptic maps of Chl- a distribution over extensive areas of the Southern Ocean. Satellite Chl- a images for the months preceding (December 1999) and following (February 2000) the CCAMLR 2000 Survey cruises showed that the general pattern of Chl- a concentration in the Scotia Sea and adjoining waters was similar in all three months, but that the phytoplankton biomass was generally lowest in December, reached maximal values in January, and started to decline in February. In contrast, Chl- a concentrations in Drake Passage declined progressively from early December through February.