SeaWiFS in the southern ocean: spatial and temporal variability in phytoplankton biomass around South Georgia

Abstract

Abstract SeaWiFS was used to examine the spatial and temporal distribution of chlorophyll-a (chl-a) concentrations around the island of South Georgia during four summers between October 1997 and March 2001. A comparison was made between SeaWiFS and ship-collected (in situ) chl-a gathered during three of these seasons. A significant correlation was found between daily SeaWiFS and in situ match-ups (r2=0.62). At low chl-a concentrations ( 5 mg m−3). Analysis of SeaWiFS data from two survey boxes (the Western and Eastern Core Boxes) sampled during cruises indicated significant differences between seasons and regions; the 1999 and 2000 seasons were nearly twice as productive as the 1998 and 2001 seasons. In addition, phytoplankton biomass was significantly higher in the Western (∼1.4 mg m−3) than in the Eastern Core Box (∼0.6 mg m−3). These seasonal and regional differences also were reflected in the distribution of copepods, one of the major zooplankton groups around South Georgia. SeaWiFS revealed patterns of growth outside of the survey boxes and cruise periods (January only). Blooms regularly occurred to the southwest and directly north of the island, and occasionally occupied the entire area between South Georgia and the Polar Front. In addition, blooms took place any time between October and March but not continuously. Primary production at South Georgia is mainly a local feature with little growth initiated upstream. However, chl-a may remain elevated downstream of the island (∼40°W) as part of a continuous feature along the Polar Front to the 0° meridian. The passage of the Antarctic Circumpolar Current over the ridge and shelf waters in the vicinity of South Georgia may contribute micronutrients such as iron, and could partially explain the elevated chl-a levels associated with the island. We also examine the role of temperature, light and grazing on controlling phytoplankton productivity in this region of the Southern Ocean.