Hydrological and chemical budgets of a mire pool formed on alluvial lowland of Hokkaido, northern Japan

Mire pools – permanently water-filled depressions on peatlands – provide important habitats for myriad organisms. Recently, water balance change and eutrophication resulting from agricultural development are increasingly evident in mire pools of alluvial lowlands. Conservation of mire pool hydrochemistry is necessary. We investigated the hydrological and chemical budgets of a pristine mire pool, Akanuma Pool (95,280 m2 area; 1.8 m mean depth), located in Kushiro Mire in Hokkaido, northern Japan, during its ice-free period (April–November) in 2007–2008. Thereby we elucidated the hydrochemical characteristics of mire pools formed on alluvial lowlands. Surface water inflow and surface water outflow dominated the hydrological budget, respectively representing 18.3 and 20.2 mm day−1. Groundwater seepage through the pool bottom and surface water inflow mainly supplied the lake water with total nitrogen and Ca2+. Total phosphorus was supplied mostly by groundwater seepage through the bottom. These chemical constituents were run off from the pool mostly by surface water outflow. The input and output fluxes of water were 16–20 times greater than those of North American mire pools because of Hokkaido’s higher values of precipitation minus evapotranspiration (P–ET). Moreover, the Ca2+ input into the Akanuma Pool was several times greater than those reported from North American studies. Alluvial mineral soil under the peat layer supplied large amounts of nutrients and mineral ions including Ca2+. These results demonstrate that Hokkaido mire pools’ hydrochemical characteristics differ greatly from those of pools in North America. Furthermore, each hydrological budget component maintained a constant fraction throughout the two year study period, although the absolute flow rate varied concomitantly with the precipitation level. Maintaining this budget stability is important for the conservation of mire pool hydrochemistry.