Semiaquatic spiders Alopecosa cinnameopilosa rely on prey derived from macrophyte-based food web: evidence from Lake Izunuma, Japan

Although lacustrine food webs have been extensively studied, food webs formed on floating-leaved macrophytes are poorly understood. In recent years, lotus Nelumbo nucifera, which was often seen in shallow waters, has rapidly expanded and covered a significant part of the surface of the eutrophic shallow Lake Izunuma in Japan. We investigated the diet of the semiaquatic spider Alopecosa cinnameopilosa collected from lotus vegetation and the shore, using carbon and nitrogen stable isotopes. We categorized the spiders' potential diet as aquatic prey (aquatic insects and small fish), terrestrial insects, or semiaquatic leaf beetles (Donacia provostii and Galerucella nipponensis) that forage mainly on floating-leaved macrophytes. In lotus vegetation, A. cinnameopilosa relied mostly on leaf beetles (mean dietary contribution: 57.3% in July and 46.0% in September). On the lake shore, A. cinnameopilosa relied on both leaf beetles (41.0%) and aquatic prey (36.4%) in July, whereas they relied mostly on aquatic prey (83.1%) in September. The difference in diets between sites could reflect differences between sites in the abundance of leaf beetles. Therefore, macrophyte-based food webs that start from lotus or water chestnuts could significantly support semiaquatic spiders on macrophyte vegetation in Lake Izunuma.