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.

Dissolved silica (DSi) and its associated biological and physicochemical factors were measured in Lake Biwa, Japan and its watershed from 2002 to 2003 in order to clarify seasonal variations in the magnitude of the sink of silica and the factors that influence it within the limnetic system. Consequently, it is concluded that Lake Biwa is a noticeable body of water where a massive sink of silica is caused. Calculated silica sedimentation in Lake Biwa was 2.0 · 10 7 kg Si year À1 (7.1 · 10 8 mol Si year À1 ) which is equivalent to about 80% of the annual inflow discharge of DSi to Lake Biwa. The magnitude of the sink varies seasonally by increasing in the winter holomictic stirring period, since it is greatly affected by the species composition of phytoplankton, the load of phosphorus and the condition of stratification. It seems reasonable to suppose that the DSi in Lake Biwa is removed mainly by biological processes, i.e., the assimilation of DSi by large centric diatoms and its accumulation in their frustules. Such silica sinks occur naturally in deeper stagnant waters, providing extended water residence time and supplying a certain amount of nutrients. These findings indicate that an increase in nutrient loads and abundance of stagnant water due to the construction of large dams lead to an expansion in the magnitude of the silica sink in a limnetic system.

-In order to clarify the mechanisms underlying high efficiency of the silica sink in monomictic Lake Biwa in Japan, vertical flux of biogenic silica (BSi) was measured using sediment traps over a period of 15 months. The sediment traps were deployed at depths of 30 and 70 m. On a global scale, BSi fluxes in Lake Biwa were very high, ranging from 20 to 1087 mg Si.m In the winter period, when nutrients are supplied from the hypolimnion to the epilimnion, the distribution of photosynthetically active diatoms was almost homogeneous in all layers, including the aphotic layer. At this time, the diatoms assimilated dissolved silica (DSi) in a wider layer containing a part of aphotic layer in order to produce rigid frustules, which accumulated rapidly in bottom sediments as DSi concentration in the water column decreased. Thus, size of the silica sink in Lake Biwa is enhanced during the winter holomictic mixing period through interaction between physical (thermocline disruption: transfer of diatoms to deep layers by vertical convection), chemical (nutrient supply from deep layers) and biological (dominance of active diatoms in all layers) processes.

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