We measured primary production by phytoplankton in the south basin of Lake Baikal, Russia, by in situ 13 Cbicarbonate 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 Ϫ2 day Ϫ1 (March) to 424 mg C m Ϫ2 day Ϫ1 (August), and annual primary production was roughly estimated as 75 g C m Ϫ2 year Ϫ1 , both of which are within the lower range of previous estimates. Size fractionation measurements revealed that phytoplankton in the Ͻ20 µm fraction accounted for 72%, 96%, and 85% of total primary production in March, August, and October, respectively. The contribution of picophytoplankton (Ͻ2 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 Ͻ20 µm fraction. These results suggest that nano-and picophytoplankton play an important role as primary producers in the pelagic ecosystem of Lake Baikal.

No abstract

Fluctuation of algal alkaline phosphatase activity, and the possible mechanisms of hydrolysis of dissolved organic phosphorus by alkaline phosphatases that could be isolated from algae, were investigated seasonally at Lake Barato. The fluctuation of specific enzymatic activity was affected by algal species rather than by the concentration of external phosphorus and amount of phytoplankton. Most (more than 74%) of the organic phosphorus, extracted from phytoplankton, was hydrolyzed by the three variants of alkaline phosphatase purified from Melosira spp., Anabaena sp., and Escherichia coli. By contrast, the organic phosphorus in lake water and that extracted from sediment muds were only partially hydrolyzed (up to 32% and 20%, respectively) by the same enzymes. These results indicate that phytoplankton cannot utilize most of the dissolved organic phosphorus in the lake water and in the sediment muds through enzymatic hydrolysis by alkaline phosphatase, in spite of the high levels of activity of this enzyme in the organisms, In&duction

Lake Katanuma is a strongly acidic volcanic lake (average pH 2.2) located in NE Japan in which only 2 algal species are found: Pinnularia acidojaponica, a benthic diatom, and Chlamydomonas acidophila, a phytoplankton species. Although the δ 13 C of phytoplankton generally varies seasonally in lake ecosystems, in Lake Katanuma the mean δ 13 C of particulate organic matter (POM, mainly phytoplankton) is constrained to a narrow range from -26.4 to -23.7 ‰. A major reason for this is the continuous supply of dissolved CO 2 gas available to C. acidophila from fumaroles at the lake bottom. The δ 13 C of P. acidojaponica in Lake Katanuma varied seasonally and was positively correlated with P. acidojaponica abundance at 1 and 4 m depths. This suggests that the higher P. acidojaponica biomass at 1 and 4 m produced the 13 C-enrichment in the high-density algal mats because of the limited dissolved CO 2 gas. The mean δ 13 C of benthic diatoms was higher than that of phytoplankton in Lake Katanuma, although the diatoms seemed to assimilate the same carbon source (CO 2 gas) in the lake water. KEY WORDS: Carbon stable isotope · POM · Phytoplankton · Benthic diatoms · Inorganic carbon · Seasonal variation · Acidic conditions Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 33: [87][88][89][90][91][92][93][94] 2003 phytoplankton species Chlamydomonas acidophila and the benthic diatom species Pinnularia acidojaponica are found in the lake.Usually, the phytoplankton and benthic algal communities in lake ecosystems are composed of many species. The complexity of the species composition may make it difficult to analyze the relationships between the carbon isotope ratios of algae and their physiological and environmental conditions in situ, since the isotope ratios of microalgae are mainly measured for an entire algal community (Yoshioka et al. 1989, Takahashi et al. 1990b). The seasonal variation in the δ 13 C of planktonic and benthic microalgal communities is inevitably influenced by changes in species composition (Fry 1996). Only few studies have reported seasonal variation in the carbon isotope ratio at the species level (Zohary et al. 1994).This study examines the seasonal changes in the δ 13 C of phytoplankton and benthic diatoms in a strongly acidic lake and compares the difference in the δ 13 C between the 2 algal species that dominate it. In Lake Katanuma, because of its limited phytoplankton and benthic algal species, we could analyze the relationships between the carbon isotope ratios of the phytoplankton and benthic diatoms and physiological and environmental factors more accurately than in an environment with a greater variety of species. Moreover, we could determine the difference in the δ 13 C of benthic diatoms and phytoplankton in the same natural environment. MATERIALS AND METHODSStudy area. Lake Katanuma is a volcanic lake located in NE Japan (38°44' N, 140°43' E) and is situated 306 m above sea level. Lake Katanuma is strongly acidic, with an average pH...

No abstract