River Primary Productivity of benthic algae and its characteristics in the Yahagi River, Central Japan, by laboratory and bag methods.

Abstract

We conducted a study to characterize the primary production by benthic algae from the upper to middle reaches in the Yahagi River, Central Japan, using two methods, the light and dark bottle method in the laboratory and the in-situ bag method in Feb. (winter) and in Aug. (summer) 2018. The amount of the benthic algal community in the Yahagi River was large in winter when diatoms were predominant; however, the maximum photosynthetic rates on photosynthesis‒irradiance curves obtained from the light and dark bottle method and primary productions from the bag method were high in summer, when the cyanobacteria Homoeothrix jantina was dominant. It was assumed that in summer, the accumulation of benthic algae is suppressed by the predation pressure of algae‒eating fish, such as ayu Plecoglossus altivelis, and the riverbed disturbance caused by flooding. The average daily gross production (P_g‒d) by the bag method was 0.9 g C m^-2 d^-1 in summer and 0.4 g C m^-2 d^-1 in winter which were less than values of domestic and foreign rivers. To estimate the River Primary Productivity in the riffles, we determined the daily net production (P_n‒d) from the upper to middle reaches: 1,620 kg C d^-1 in summer and 180 kg C d^-1 in winter. In the Yahagi River, in summer, the primary production was just or slightly short for the estimated feeding amount of predators. But it was insufficient in winter. Therefore we believe that this River Primary Productivity is one of the essential indexes when considering the conservation of river ecosystems and the environmental impact assessment.