Evaluation of biological water purification functions of inland lakes using an aquatic ecosystem model

Abstract

An effective measure to cope with eutrophication of lakes is to remove nutrients that can cause algal blooming by taking advantage of natural water purification processes. Here the term “purification” is defined, in a wide sense, as the potential role of a water body to contribute to the reduction of pollutants and thus controlling eutrophication. Also regarded as a kind of ecological regulating services, biological purification involves various processes concerning seasonal nutrient fixation, such as uptake by aquatic macrophytes, biofouling onto foliage substrates, feeding by organisms in higher trophic level, and eternal loss or removal of substance from the water. In order to evaluate the water purification ability, a numerical lake ecosystem model highlighting the role of macrophyte colonies in the shore zone was developed and applied to Lakes Suwa, Kasumi and Biwa, as well as five small lakes attached to Lake Biwa.

The model includes the biological interactions between pelagic compartments (phyto- and zooplankton, detritus, dissolved organic matter, pelagic fish and nutrients) and benthic compartments (macrophytes, epiphytic algae, phytal animals, macro- and megalobenthos, and demersal fish). Under dynamic forcing of meteorological and hydrological parameters, the model was run over a year to evaluate the annual nutrient cycles and purification functions. The reproducibility of the model was validated for each water body by comparison with the field data from the water quality monitoring campaign. Numerical results revealed that the water purification ability, from viewpoints of both stock and flux, generally tends to be higher in the shore region than in the offshore region owing to a wide variety of aquatic biota. Moreover, it increases almost in proportion to the density of vegetation in the shore region. Nutrient fluxes representing the purification processes turned out to be closely connected with the surface area or equivalently with the retention time of the water bodies, suggesting that biological purification does not become dominant against physical turnover until the surface area reaches around 10³ ha or the retention time exceeds the order of 10² days.

Introduction

In Japan, with the aim of environmentally protecting inland water bodies, water quality standards have been established for many lakes, and continuing efforts have been made to reduce the loading inputs of pollutants or to remove contaminants from the water. Most of the water bodies, however, have been involved in highly advanced eutrophication and organic water pollution has caused a long duration of deteriorated water quality. To watch the annual trend of chemical oxygen demand (COD), used in Japanese environmental administration as an index of organic water pollution, the rate of achieving the environmental standards for lakes has remained at a low level of about 50% for more than three decades (Ministry of the Environment, 2006).

In eutrophic water bodies, a large amount of nutrients supplied through terrestrial input, bacterial decomposition in the water column as well as in the sediment, enhance the production of organic substances mainly through phytoplankton growth. This not only leads to water pollution, but causes serious environmental problems such as water bloom, hypoxia, or ultimate change in an aquatic ecosystem. Eutrophication of lakes is nowadays a matter of social concern, and emergency steps are therefore strongly advised for the environmental protection and management of fishery resources. Besides physical operations like enhancing water circulation or cleaning up the sediment, measures to cope with eutrophication include the utilization of biological approaches such as the natural water purification functions that belong intrinsically to shore zones with macrophyte colonies, attached small lakes and ponds or surrounding wetlands. Inshore zones, for example, contribute to water purification by assimilating and binding nutrient substances with the aid of various organisms assembling in the aquatic plant community. In addition to the biological functions, small-scale water bodies also play an important role in filtering out suspensions behaving as depositing reservoir. Accordingly, such purification functions attract a keen interest of many limnological and ecological researchers around the world (e.g., Howard, 1981, Reddy, 1984, Suzuki et al., 1988, National Research Council, 1992, Dennison et al., 1993, Okino and Watanabe, 1996, Yamamuro, 1996, Bratl et al., 1999, Xu et al., 1999, Ramachandra et al., 2001). As a typical example of environmental policy in Japan, Shiga Prefecture started a campaign to reevaluate and restore the water purification functions of the attached small lakes, with aim of implementing a comprehensive management plan for the environmental protection of Lake Biwa.

In order to elucidate the nature of the water purification functions useful to eutrophication control, as part of the environmental study project conducted under a grant from the Fisheries Agency of Japan, a numerical lake ecosystem model was developed and applied to several typical eutrophic lakes and small water bodies to provide a quantitative assessment of their purification ability: those are the three major inland freshwater lakes, Lake Suwa, Lake Kasumi and Lake Biwa, and the five small lakes of different sizes attached to Lake Biwa.

In this study, we focus on the interactions between pelagic compartments (phyto- and zooplankton, detritus, dissolved organic matter, pelagic fish and nutrients) and benthic compartments (macrophytes, epiphytic algae, phytal animals, macro- and megalobenthos, and demersal fish). Based on the model results, we study the water purification ability of these water bodies in relation to their hydrological characteristics, namely, surface area and retention time, and present the notable ecological features.