Long-term variations in water quality of lakes in Rokkasho, Aomori, Japan, from 2004 to 2015

Abstract

Long-term variations in water quality were assessed in two freshwater lakes, Tamogi and Ichiyanagi, and two brackish lakes, Takahoko and Obuchi, in the village of Rokkasho, Aomori Prefecture, Japan, via monthly sampling from April 2004 to March 2015. Lake Takahoko has been separated into eastern (seaside) and western (inland) basins since 1993 by a levee with gates that are partially open to accommodate coastal tides. Mean concentrations of total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand in the surface waters were higher in the two freshwater lakes and Lake Takahoko than in Lake Obuchi. Blooms of cyanobacteria in the freshwater lakes were observed every year between August and October, an indication of the eutrophic condition of the water. The TN and TP concentrations of the western basin were higher than in the eastern basin of Lake Takahoko but comparable to earlier survey data from before the levee was built. In contrast, all surveyed parameters in Lake Obuchi indicated that water quality was good throughout our observations, an indication of relatively high exchange rates between lake water and tidal seawater. The water quality of each of the four lakes was fairly constant during the survey period and was slightly improved over that from 40-year-old data in the literature.