Original paper

Long-term changes in lake sediments and their infl uences on lake water quality in Japanese shallow lakes

Fukushima, Takehiko; Kamiya, Koichi; Onda, Yuichi; Imai, Akio; Matsushige, Kazuo


In order to elucidate the nutrient dynamics in lakes with special reference to sediments, the changes in vertical distributions of sediment nutrients over 20 years were analyzed with the aid of sediment dating at the centers of two shallow, eutrophic lakes in Japan, i.e.Lakes Kasumigaura and Suwa. Peaks of137 Cs and apparent density gave estimated sedimentation rates of around 1 and 2 kg m-2 y-1 in Lakes Kasumigaura and Suwa, respectively. Slight decreases of TP and TN contents (0.07 and 0.63 mg g-1 for TP and TN, respectively, in Lake Kasumigaura and 0.90 mg g-1 for TN in Lake Suwa) were observed by comparison between the cores obtained around 1980 and those of recent years except for a considerable decrease in TP in Lake Suwa (1.0 mg g-1). This decrease probably came from the higher percentage of easily releasable phosphorus in the past sediments of Lake Suwa. Using the TP and TN increases in lake waters during summer, the averaged nutrient release rates were estimated to be within the ranges of those observed by laboratory core experiments and by measuring the vertical profi les of interstitial concentrations except for a smaller estimation of TN in Lake Kasumigaura. On the other hand, the nutrient release rates were also determined using the long-term changes in sediment nutrient contents. The estimated rates were a little lower than the rates determined by the method using the increase in lake nutrient concentrations other than the higher ones for TP in Lake Suwa, which were originally based on the higher percentage of easily releasable phosphorus as mentioned above. Net sedimentation rates for 24 years in Lake Kasumigaura and those for 26 years in Lake Suwa calculated as the sum of new sedimentation and changes in old sediments during these periods were compared with the net input loads based on the per capita/exporting method. In the case of TP, we observed a fairly good agreement because of its conservation, whereas the residuals were larger for TN due to denitrifi cation in the overlying waters of sediments. The rates estimated from the imbalances in TN were in the ranges of the measured denitrifi cation rates in laboratory tests with the sediments. Thus, the information on the long-term changes in sediment nutrients could provide the average rates of these fundamental processes in lake environments.


sedimentsphosphorusnitrogenshallow lakesvertical profi ledatingbudgetary analysis