Lakes as Sources and Sinks: Social and Ecological Dynamics Affecting Downstream/Pollution-Accumulating Lakes

fukushima Takehiko Graduate School of Life Environmental Sciences, University of Tsukuba

tsujimura Maki Graduate School of Life Environmental Sciences, University of Tsukuba yamanaka Tsutomu Graduate School of Life Environmental Sciences (Terrestrial Environment Research Center), University of Tsukuba kuroda Hisao College of Agriculture, Ibaraki University maruoka Teruyuki Graduate School of Life Environmental Sciences, University of Tsukuba onda Yuichi Graduate School of Life Environmental Sciences, University of Tsukuba imai Akio National Institute for Environmental Studies kohzu Ayato National Institute for Environmental Studies ozaki Noriatsu Graduate School of Engineering, Hiroshima University matsushita Bunkei Graduate School of Life Environmental Sciences, University of Tsukuba nishihiro Jun Graduate School of Agriculture and Life Sciences, University of Tokyo shirakawa Naoki Graduate School of Systems and Information Engineering, University of Tsukuba endo Takahiro Graduate School of Life Environmental Sciences, University of Tsukuba iijima Yoshihiko Ibaraki Prefecture hanazato Takayuki Institute of Mountain Science, Shinshu University torigoe Hiroyuki Faculty of Human Sciences, Waseda University

A downstream and pollution-accumulating lake (DPA lake) is an inland body of water that is both an active source of water for humans, agriculture and industry and a sink for upstream waste. This project investigates present and likely future (50-100 years) problems in DPA lake environments in the context of global environmental sustainability and regional development. It will propose a set of indices related to DPA lakes from which solutions or mitigations can be based, and which will guide the design of ideal governance of human-watershed systems.

Project overview

Photo 1  Lake Kasumigaura at the time of an outbreak of Koi Herpes Virus (KHV) disease

Water quality of downstream and pollution-accumulating (DPA) lakes is particularly vulnerable to degradation (Fig. 1). The number of DPA lakes is expected to increase in the world, particularly in and near the metropolitan cities that have poor water resources. The purposes of this study are to predict and discuss the problems specific to such lakes in the next 100 years, to propose appropriate governance techniques, and to design a human society that has a rich sense of futurability. Lake Kasumigaura northeast of Tokyo is a representative DPA lake (Photo 1).

Figure 1  Hydrological (black) and nutrient (brown) cycles in a DPA lake

We will test two working hypotheses. First, we hypothesize that increases in water use will accelerate accumulation of pollutants in the lake, which in turn will reduce availability of water. In order to test this hypothesis, we will construct a numerical model capable of simulating hydrological and biogeochemical cycles in the lake and watershed. This model will show both subsystems with short turnover times, such as precipitation and specific river and lake dynamics, and those such as groundwater and sedimentary processes with long turnover times and which are also affected by various stakeholders around the water environment (Fig. 2). The model will be used to indicate nutrient cycles in the watershed and to predict changes in water quantity. We will apply sustainability indices to DPA lakes in order to evaluate the watersheds in light of environmental factors and human development.

Secondly, we hypothesize that decreases in both the diversity of water use and human population in the watershed area will reduce the resilience of the lake, and thus the viability of the regional human-nature system. We will investigate water and lake uses and their change through time in order to test this hypothesis. Several lakes in Asian countries such as China and Indonesia will be considered as DPA lakes. We will conduct inter-lake comparisons based on indices that describe economic and environmental constraints such as global carbon limitation and population decline and aging.

Figure 2  Subsystems of short and long turnover times