Annual changes in sea-ice, chlorophyll a, and primary production in the Ross Sea, Antarctica

A satellite-based study was conducted to quantify the seasonal-to-interannual changes in phytoplankton production in the southwestern Ross Sea between 1997 and 2001. The size of the Ross Sea polynya from 1992 to 2001 was related to the climate state as expressed by the Multivariate ENSO Index (MEI). These data show that 76% of the variation in sea-ice extent between 1992 and 2000 can be explained by the MEI, with the El Niño year of 1997–1998 exhibiting the greatest sea-ice cover and the La Niña year of 1999–2000 the least. Whether the relationship between ice area and the MEI is caused by interannual changes in wind patterns or changes in the movement of circumpolar deep water onto the Ross Sea continental shelf has yet to be determined. The year 2000–2001 was excluded from the climate analysis, due to the effects of the enormous B-15 iceberg, which inhibited the normal advection pattern of sea-ice out of the Ross Sea, resulting in higher than normal sea-ice conditions. Phytoplankton blooms were markedly less extensive in the heavy sea-ice years of 1997–1998 and 2000–2001 and were significantly delayed in their development, with concentrations of Chl a peaking 2 months later (February) than in reduced sea-ice years (December). Annual production during the 1997–1998 El Niño (27.4 Tg C) was similar to that of the iceberg impacted year 2000–2001 (28.3 Tg C). These annual rates were approximately 40% below values estimated between November 1998 and March 2000 (47.5 Tg C) when open-water area during spring and summer was much greater. This decreased production during heavy sea-ice years is more a function of the delay in the start of the bloom than in the reduction of open water area.