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

Abstract

 We measured primary production by phytoplankton in the south basin of Lake Baikal, Russia, by in situ ¹³C-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 0.8 m of ice cover in March. Areal primary production varied from 79 mg C m⁻² day⁻¹ (March) to 424 mg C m⁻² day⁻¹ (August), and annual primary production was roughly estimated as 75 g C m⁻² year⁻¹, both of which are within the lower range of previous estimates. Size fractionation measurements revealed that phytoplankton in the <20 μm fraction accounted for 72%, 96%, and 85% of total primary production in March, August, and October, respectively. The contribution of picophytoplankton (<2 μm) to total primary production ranged from 41% to 62%. A large fraction (82%–98%) of particulate organic carbon was associated with particles in the <20 μm fraction. These results suggest that nano- and picophytoplankton play an important role as primary producers in the pelagic ecosystem of Lake Baikal.