Nutrient limitation of the primary production of phytoplankton at some stations in southern and central Lake Baikal was studied by nutrient enrichment experiments in August 2002. Chlorophyll (Chl.) a concentrations ranged from 0.7 to 5.8 µg l −1 . Inorganic nutrient concentrations were low: soluble reactive phosphorus ranged from 0.05 to 0.20 µmol l −1 , ammonia from 0.21 to 0.41 µmol l −1 , and nitrite plus nitrate from 0.33 to 0.37 µmol l −1 . In the five enrichment experiments, phosphate spikes and phosphate plus nitrate spikes always stimulated primary production. Nitrate spikes also stimulated primary production in four of the experiments. Significant differences were detected between the controls and phosphate spikes and between the controls and phosphate plus nitrate spikes. Thus, the first limiting nutrient is thought to be phosphorus, but once phosphorus is supplied to the surface water, the limiting nutrient will quickly shift from phosphorus to nitrogen.
In Lake Baikal, picocyanobacteria are the most important primary producers during the summer. Freshwater picocyanobacteria are discriminated into either the phycoerythrin (PE)-rich or the phycocyanin (PC)-rich types according to their pigment composition. The distributions of these two types of picocyanobacteria were investigated in Barguzin Bay. The PC-rich type accounted for [98% of the total picocyanobacteria at the station near the shore of the bay where river water flows directly in. In the offshore area of the lake, all of the picocyanobacteria cells were of the PE-rich type. In addition, the occurrence of the PC-rich type was restricted to the station, where the attenuation coefficient exceeded 0.25 m -1 . Near the shore, where the turbidity was high ([1 NTU), the cell densities of both the PE-and PC-rich types increased away from the river mouth. This indicates that the PC-rich type cells grow near the shore of the bay where turbidity is high. Since the PCrich type could not grow well when cells were incubated in offshore lake water, restricted distribution of the PC-rich type could also be explained by their growth capability. The present study clearly demonstrated the shift in the pigment type composition of picocyanobacteria from the coastal to the pelagic zone of Lake Baikal. The co-existence of the two pigment types probably enables the abundance of the picocyanobacterial community to be stable over a broader range of environmental conditions than would be possible for a single pigment type.
The abundance and composition of phytoplankton were investigated at six stations along a transect from the Barguzin River inflow to the central basin of Lake Baikal in August 2002 to clarify the effect of the river inflow on the phytoplankton community in the lake. The water temperature in the epilimnion was high near the shore at Station 1 (17.3°C), probably due to the higher temperature of the river water, and gradually decreased offshore at Station 6 (14.5°C). Thermal stratification developed at Stations 2-6, and a thermocline was observed at a 17-22-m depth at Stations 2-4 and an 8-12-m depth at Stations 5 and 6. The concentrations of nitrogen and phosphorus nutrients in the epilimnion at all stations were \1.0 lmol N l -1 and \0.16 lmol P l -1 , respectively. Relatively high concentrations of nutrients (0.56-7.38 lmol N l -1 and 0.03-0.28 lmol P l -1 ) were detected in the deeper parts of the euphotic zone. Silicate was not exhausted at all stations ([20 lmol Si l -1 ). The chlorophyll a (chl. a) concentration was high ([10 lg l -1 ) near the shore at Station 1 and low (\3 lg l -1 ) at five other stations. The\2 lm fraction of chl. a in Stations 2-6 ranged between 0.80 and 1.85 lg l -1 , and its contribution to total chl. a was high ([60%). In this fraction, picocyanobacteria were abundant at all stations and ranged between 5 9 10 4 and 5 9 10 5 cells ml -1 . In contrast, chl. a in the[2 lm fraction varied significantly (0.14-11.17 lg l -1 ), and the highest value was observed at Station 1. In this fraction, the dominant phytoplankton was Aulacoseira and centric diatoms at Station 1 and Cryptomonas, Ankistrodesmus, Asterionella, and Nitzschia at Stations 2-6. The present study demonstrated the dominance of picophytoplankton in the pelagic zone, while higher abundance of phytoplankton dominated by diatoms was observed in the shallower littoral zone. These
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