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

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.