Growth of Margaritifera laevis in the upper reaches of a river with diurnal increases in Na＋ and Cl- concentrations

Abstract

Margaritifera laevis, an endangered species of freshwater bivalves, inhabit the upper reaches of rivers. The survey site for this study was a small river, with about 1-4 m width in a river basin dotted with springs in inland Iwate prefecture. Four survey points were established, spanning about 6.5 km. For this study, diurnal increases in Cl- and Na＋concentrations were observed during winter in the survey area upstream of the target river. The maximum value during the study period was 453 mg Cl- L-1. Concentrations exceeding 283 mg Cl- L-1, equivalent to EC 100 mS m-1, were recorded 12 times in 2018 and 7 times in 2019, which suggests that sodium chloride, an anti-freezing agent, adhered to car bodies and subsequently flowed into the river from a car wash near a highway interchange upstream of the study area. Based on results of transplant surveys conducted over a two-year period at three locations(upstream, midstream, and downstream of the study river), almost no growth of the M. laevis was observed in the upstream area near the car wash. However, annual growth in the middle and downstream study areas was around 1.5 mm, which was less than earlier findings of 3-5 mm. Further investigations must elucidate factors affecting this species habitat.