Phytoplankton Photosynthesis in Southern Lake Baikal: Size-Fractionated Chlorophyll a and Photosynthetic Parameters

Abstract

Phytoplankton photosynthesis was measured on water samples from southern Lake Baikal (Siberia, USSR) during autumn 1989. Stations were selected to characterize differences between offshore and nearshore communities, and to address the influence of the Selenga River. The contribution of small size classes to total phytoplankton biomass and photosynthesis was estimated by sieving samples prior to analyses. Photosynthesis was estimated by the uptake of 14C along a gradient of light intensities in a ship-board incubator. The resulting P vs. I parameters were used, along with measurements of chlorophyll a and light penetration, to estimate areal photosynthesis. The results of this study showed that the < 10 μm phytoplankton accounted for 52–88% of the total chlorophyll a and 66–100% of the total 14C uptake during this time of the year. Also observed was a negative correlation between this dominance and the total chlorophyll a in the sample. The P vs. I parameters also allow insight into physiological differences between the whole water and < 10 μm communities. The < 10 μm fractions were characterized by higher assimilation numbers (0.36 – 5.03 mgC mgChl −1 hr −1) than the whole water samples (0.16 – 2.76 mgC mgChl −1 hr −1). The <10 μm fractions also showed higher assimilation efficiencies (1.13 – 15.01 mgC mgChl −1 mgChl −1 E −1 m 2) and lower saturating irradiances (55 – 102 μE m −2 s −1) than the whole water samples (0.53 – 5.28 mgC mgChl −1 E −1 m 2 and 62 – 152 μE m −2 s −1). Whole water assimilation numbers were highest in the region of the Selenga River Delta, and decreased away from the influence of the river.