Limnological characteristics and vertical distribution of phytoplankton in oligotrophic lake Akan-Panke [Japan]

The limnological characteristics and vertical distribution of phytoplankton in Lake Akan-Panke were surveyed to clarify the metabolism in an oligotrophic lake. Maximum water temperature was 19.6°C in August 1992, and the concentration of dissolved oxygen was always over 4.8 mg 1-1 in the hypolimnion. Transparency ranged from 7.1 to 17.6 m, and was relatively high in July and August. Standing stocks of nitrogen, phosphorus, and chlorophyll-a (chl-a) were 8.20, 0.33, and 0.114 g m-2, respectively. In particular, that of phosphorus was at a low level in July and August. Therefore, phosphorus may be the limiting factor in this lake. These data suggest the theory that Lake Akan-Panke is in an oligotrophic state. However, this lake had a relatively high concentration of chl-a at a deeper layer (30-45 m) during the early thawing season in May 1992 and July 1993, and large size phytoplankton (>20μm) dominated at those times. When thermal stratification was developing in the lake in July and August, the phytoplankton population and chl-a concentration decreased in the surface layer and formed a sub-surface maximum in the metalimnion, mainly dominated by large size phytoplankton (> 20 μm) . In contrast, relatively small size phytoplankton (< 20 μm) dominated the hypolimnion in August and October. Since most of the chl-a was trapped in the 45 m layer rather than in the 20 m layer by the sediment trap method in May 1992 and August 1993, the high chl-a concentration observed in the deeper layer might be due to the growth of phytoplankton in the layer and/or to sinking of the phytoplankton from the upper layer which increased in May (spring bloom). This suggests that the high biomass of phytoplankton in the deeper layer derived from sinking.