Seasonal dynamics of primary production in the pelagic zone of southern Lake Baikal

We measured primary production by phytoplankton in the south basin of Lake Baikal, Russia, by in situ C-13-bicarbonate incubations within the period March-October in two consecutive years (1999 and 2000). Primary production was highest in the subsurface layer, possibly due to near-surface photoinhibition of photosynthesis, even under 0.8m of ice cover in March. Areal primary production varied from 79 Mg Cm-2 day(-1) (March) to 424 mg Cm-2 day(-1) (August), and annual primary production was roughly estimated as 75g Cm-2 year(-1), both of which are within the lower range of previous estimates. Size fractionation measurements revealed that phytoplankton in the <20 mum fraction accounted for 72%, 96%, and 85% of total primary production in March, August, and October, respectively. The contribution of picophytoplankton (<2mum) to total primary production ranged from 41% to 62%. A large fraction (82%-98%) of particulate organic carbon was associated with particles in the <20mum fraction. These results suggest that nano- and picophytoplankton play an important role as primary producers in the pelagic ecosystem of Lake Baikal.