Seasonal Fluctuations of Phytoplankton in Lake Onuma and Konuma, Nanae, Hokkaido

Onuma and Konuma are belonging to the Onuma Quasi-National Park and are located in southern Hokkaido. The fisheries and tourism are important industries in this lake area. Eutrophication has progressed in these lakes since the 1980s, and nuisance blooms of cyanobacteria have occurred every summer to autumn. The outbreaks of cyanobacterial blooms substantially destroy the ecosystem due to the production of cyanobacterial toxins, and effective countermeasures are urgently needed. However, in the lakes of Oshima Onuma, there is a paucity of knowledge about the appearance trends of phytoplankton including cyanobacteria. Therefore, seasonal monitorings were carried out on the phytoplankton community in the water column and the relatioships were discussed between dynamics of phytoplankton communities and changes in environmental factors in the lakes of Oshima Onuma. The survey was conducted once a month as a rule at Stns. 1-5 (Stn. 1 is the northeastern end of Onuma, only Stn. 5 is in Konuma) and at Stn. OP and Stn. OC along the shore of the Lake Onuma during the period of May-November 2015 and April- October 2016. The parameters of hydraulic environments were measured about water temperature, pH, transparency, dissolved oxygen, nutrient concentrations (NO3-N, NO2-N, NH4-N, PO4-P, SiO2-Si), chlorophyll a concentration, and pheophytin. The chlorophyll a concentration of the surface water showed a single-peak type fluctuation with the maximum value (28.7 μg L-1 at Stn. 2) in August at all stations in 2015. In 2016, the largest single-peak type fluctuation was observed in September- October with the exception of Stn. 3. Concerning the seasonal variation of the phytoplankton species, the proportion of Uroglena volvox (Chrysophyceae) was high at Stn. 5 in May, but the pennate diatoms Fragilaria crotonensis and Asterionella formosa, and the centric diatoms Aulacoseira spp. at other stations other than Stn. 5. As for cyanobacteria in August 2015, Dolichospermum planctonicum, possessing an ability of nitrogen-fixation, dominated (maximum 4.4×104 cells mL-1) at all the stations under the severe nitrogen-deficient conditions (N/P < 16). In the following year 2016, the cell densities of Dolichospermum spp. were low, and Microcystis spp. dominated at all the stations (up to 5.6×104 cells mL-1) with the enough nitrogen conditions. It is hence thought that the N / P ratio determined the dominant species of cyanobacteria in the summer season. In September and thereafter, the number of phytoplankton cells decreased on the whole in both years, and the centric diatoms Aulacoseira spp., Cyclotella spp. and the cryptophyte Cryptmonas spp. tended to increase. Considering the occurrence mechanisms of cyanobacterial blooms based on the fluctuation trends of cyanobacteria in the water columns, it is found that the supply of Microcystis aeruginosa from the lake bottom sediment to the water column (water temperature of 10-15ºC is required) is progressing at all stations in April-June. Since Onuma and Konuma are shallow with an average depth of 4.7 m, wind-inducing resuspension of bottom sediments probably contribute to the supply of cyanobacteria to water columns. In addition, since cyanobacterial cells tend to float and accumulate in surface water, it is needed to take physical factors such as wind and flow into consideration regarding the distribution of the blooms of cyanobacteria