Monsoonal differences in phytoplankton biomass and production in the Indonesian Seas: tracing vertical mixing using temperature

Measurements made in the Indonesian Seas during the 1994 northwest monsoon (January–February) and the 1993 southeast monsoon (August–September) show up to an order of magnitude increase in average surface chlorophyll a from 0.25 mg mṫ during the northwest monsoon to over 2.5 mg m−3 during the southeast with a strong east-west gradient. Carbon assimilation was similar for both seasons in the eastern seas (Banda, Flores and Seram), 1.2 g C m−2 day−1 but dropped by half in the western seas (Sulawesi and Makassar Strait) in February, from 1.2 to 0.5 g C m−2 day−1. Areal chi a decreased in the western seas from 20.2 to 5.5 mg m−2 from the northwest to southeast monsoons. The eastern seas showed an opposite trend, with areal chi a increasing from 20.8 to 55.8 mg m−2 between seasons. The increase in surface and integral euphotic zone phytoplankton biomass in the eastern seas is correlated with cooler sea surface temperatures, suggesting that phytoplankton biomass is proportional to the strength of upwelling and vertical mixing.