Distribution and recurrence of phytoplankton blooms around South Georgia, Southern Ocean

[摘要] South Georgia phytoplankton blooms are amongst the largest of the SouthernOcean and are associated with a rich ecosystem and strong atmospheric carbondrawdown. Both aspects depend on the intensity of blooms, but also on theirregularity. Here we use data from 12 yr of SeaWiFS (Sea-viewing WideField-of-view Sensor) ocean colour imagery and calculate the frequency ofbloom occurrence (FBO) to re-examine spatial and temporal bloomdistributions. We find that upstream of the island and outside the borders ofthe Georgia Basin, blooms occurred in less than 4 out of the 12 yr(FBO < 4). In contrast, FBO was mostly greater than 8 downstream of theisland, i.e., to the north and northwest, and in places equal to 12,indicating that blooms occurred every year. The typical bloom area, definedas the region where blooms occurred in at least 8 out of the 12 yr, coversthe entire Georgia Basin and the northern shelf of the island. The timeseries of surface chlorophyll a (Chl a) concentrations averaged over thetypical bloom area shows that phytoplankton blooms occurred in every yearbetween September 1997 and September 2010, and that Chl a values followed aclear seasonal cycle, with concentration peaks around December followed inmany years by a second peak during late austral summer or early autumn,suggesting a bi-modal bloom pattern. The bloom regularity we describe here isin contrast with results of Park et al. (2010) who used a significantlydifferent study area including regions that almost never exhibit bloomconditions.

[学科分类] 地球化学与岩石