Effect of hypoxic water mass in summer on the distribution of ark shells, Anadara kagoshimensis, in Lake Nakaumi, Japan

Abstract

This study was conducted to clarify the effect of summer hypoxic water on the spatial distribution of ark shells (Anadara Kagoshimensis) in Lake Nakaumi, a brackish lake in the Japan Sea coast in Western Japan. From July to September in 2011 and 2012, dissolved oxygen (DO) concentration was monitored monthly at 29 sites in the lake, along with weekly/hourly DO measurements at a site in the center of the lake. Sixteen sites were sampled for ark shells using towed dredge in March and November 2012. In the July–September both in 2011 and 2012, hypoxic and/or anoxic water extensively covered the bottom surface of the lake excluding the northern part near Nakaura Channel, which is a major supplier of sea water from Japan Sea to the lake. Among the sampled sites, ark shells were collected from 8 and 5 sites in March and November, respectively. The collection sites were located near the southern end of Nakaura Channel. No ark shells were collected in the rest of the sites that were away from the channel. In March, collected ark shells were larger (shell length >40 mm) and had higher density (>11 ind./230 m²) in the sites near the channel than those in the other sites (<32 mm, <5 ind./230 m²). The relation between density of ark shells that survived >1 yr after settlement and collection site distance to the inlet of Nakaura Channel strongly suggested that high-DO water that flowed through the channel into the lake may have contributed toward the adult ark shells colonization in Lake Nakaumi.