Long-term fluctuation in sedimentary remains of Aegagropila linnaei in Lake Takkobu in Kushiro Mire, Northern Japan

The purpose of this study was to examine the long-term fluctuation in remains of Aegagropila linnaei associated with phytoplankton dynamics and sediment accumulation in Lake Takkobu, northern Japan. A. linnaei is a green alga requiring good light conditions (low phytoplankton production and a low density of fine sediment particles) for growth. Six lake sediment core samples were obtained, and lake sediment-core samples contained two tephra layers as Ko-c2 (1694) and Ta-a (1739). Fresh tephra deposition is the source of phosphorus in the lake, and induces an increase in phytoplankton. For the period 1694-1739, low sediment accumulations and low phytoplankton productions were observed, and A. linnaei remains were low, likely due to nutrient deficiency. Ko-c2 tephra deposition had no detectable influence on phytoplankton and A. linnaei dynamics, as insufficient tephra was deposited to induce a nutrient deficiency. However, phytoplankton productivity was attributed to Ta-a tephra deposition, which was at a higher influx than that of Ko-c2, in addition to human activities, such as deforestation and agricultural development. A. linnaei remains in 1739-1898 were increased despite the enhanced phytoplankton productivity caused by Ta-a tephra deposition. This evidence indicates that a phytoplankton impact on the transparency from 1739-1898 was probably insignificant, but the released nutrient from Ta-a tephra acted as a stimulus to make the A. linnaei grow. Therefore, A. linnaei existence depends on interaction between light conditions and sediment accumulation as well as nutrient conditions. Human activities after 1898 have directly influenced increased phytoplankton productivity and sediment accumulation, and result in A. linnaei degraded. Moreover, A. linnaei remains mainly were concentrated in sediment from northern zone close to lake outflow because their spherical form and free-floating filaments tend to be moved by water flow. In recent years, the distribution of A. linnaei in the lake was reduced due to the shading effect caused by the increase in phytoplankton production and the attenuation of sunlight by high sediment accumulation.