Model Estimation of the Water Purification Capacity of Lost Tidal Flats

The water environment of Ariake Bay has been degraded for decades, with some observers suggesting that this problem might be due to the decrease in tidal flat areas. However, for lost tidal flats, the water purification capacity cannot be estimated without sufficient historical data. A new procedure for estimating the water purification capacity of the Isahaya Tidal Flat (i.e., a lost tidal flat) was developed based on an ecosystem model. Given their similarities, an ecosystem model was first developed for the tidal flats of the Shiota River, and then recalibrated using the limited historical data available for the Isahaya Tidal Flat. The water purification capacity of the Isahaya Tidal Flat was estimated to be 0.39 t-N d1 in 1988, when the tidal flat covered a total area of 35.64 km2, which is equivalent to just 36% of total nitrogen loading from the basin (1.07 t-N d1). The denitrification capacity of the Isahaya Tidal Flat was shown to be 7.3 × 103 t-N km2 d1. Both the water purification capacity and denitrification capacity estimated for the muddy Isahaya Tidal Flat were much less than those of sandy tidal flats (e.g., tidal flats at Sambanse). Discipline: Agricultural engineering Additional key words: Ariake Bay, bottom sediment, ecosystem model, denitrification, water quality *Corresponding author: e-mail toshiya_nishio980@maff.go. jp Received 5 November 2015; accepted 7 January 2016. Introduction Ariake Bay is located in northwestern Kyushu, Japan. The tidal range, which reaches a maximum of 6 m, has resulted in one of the largest tidal flat systems in Japan. Land reclamation for agriculture and industry has been done since the 1960s and 1970s, in order to support rapid economic growth; however, concerns remain over the loss of tidal flat areas. In recent years, Ariake Bay has experienced serious degradation of its water environment, including anoxic water and red tides, with some observers suggesting that such problems might be due to reduced water purification capacity of the tidal flat areas. The water purification capacity of tidal flats can be estimated based on field studies; however, for lost tidal flats, this process is only possible with sufficient historical data. As a result, mathematical models represent the only way to estimate the previous water purification capacity of a reduced (or lost) tidal flat. Suzuki (2006) estimated the water purification capacity of tidal flats in the Mikawa and Ise bays as 360 t-N yr1 based on field data collected using an enclosed chamber, and then input that capacity into a mathematical model of the ecosystem. In contrast, the Chiba prefectural government estimated the capacity of nitrogen removal from the tidal flats at Sambanse to be 574 t-N yr1, a rate that could remove 45% of total nitrogen loading into the flats (Kimura et al. 1992). Nishio et al. (2013) showed that the water purification capacity of tidal flats differs and is highly dependent on the characteristics and environmental conditions of a given tidal flat. An ecosystem model based on chemical reactions, biological reactions, and nutrient transfer can be used to estimate purification capacity under limited but varying conditions, assuming that sufficient data are available to calibrate and validate the model (e.g., field studies). However, for lost tidal flats, such data cannot be collected, and if not already available, cannot be created. In this study, we developed a new method of estimating water purification