Abstract
The nutritional state of phytoplankton communities in the South Basin of Lake Biwa was examined every other day from 24 August to 12 September, 1993. During this period, a windy typhoon (Typhoon 9313) and a rainy typhoon (Typhoon 9314) moved along Lake Biwa, on 4 and 9 September, respectively. The concentration of PO4-P remained at a low level (<0.7 μgP⋅1-1) from 26 August to 3 September, but it increased to 2.5 μgP⋅1-1 on 4 September just after the arrival of the windy typhoon. NH4-N varied in a similar way as PO4-P, and the highest value, 64 μgN⋅1-1, was observed on 4 September. NO2 + NO3 - N, which was 151 μgN⋅1-1 on 24 August, decreased gradually until 10 September, reaching a concentration of 6 μgN⋅1-1. It increased again to 76 μgN⋅1-1 on 12 September after the arrival of the rainy typhoon, whereas concentrations of PO4-P did not increase after that typhoon. Chl-a concentration also increased after the windy typhoon, and reached the highest concentration of the summer, 26.5 μg⋅1-1 on 10 September.
The results of an in situ nutrient-addition experiment indicated that phosphorus was a limiting factor for phytoplankton growth in the South Basin of Lake Biwa before the arrival of the windy typhoon. The sestonic C: N: P ratios also indicated moderate phosphorus deficient phytoplankton from 24 August to 12 September, except on 4 and 5 September just after the windy typhoon. The phytoplankton were not nitrogen deficient at any time during our study. Thus, we suggest that the growth of phytoplankton communities from 5 to 10 September in Lake Biwa was mainly induced by an increase in available phosphorus, supplied from the disturbance of the bottom sediments by the windy typhoon.
