Stable-isotope determination of mayfly (Insecta: Ephemeroptera) food sources in three tropical Asian streams

Abstract

Carbon and nitrogen stable isotopes were used to determine the foods assimilated by mayflies in three Hong Kong streams: Tai Po Kau Forest Stream (Forested 1), Shing Mun River (Forested 2), and an open (Open 1) and a shaded section (Shaded 1) of the Lam Tsuen River. Analyses were carried out during both wet and dry seasons. A total of seven mayfly species (Heptageniidae, Leptophlebiidae and Baetidae; four or five species per site) were included. During the dry season, mayflies in the two forest streams (I and 2) fed mainly on autochthonous foods (33-100 %) especially cyanobacteria and Cladophora spp. in Forested 1. During the wet season, the use of allochthonous food increased (13-100 %) in Forested 1 and 2, but the importance of autochthonous foods in shaded streams with ample stocks of detritus was nevertheless surprising. Mayflies in both Open 1 and Shaded 1 derived most of their biomass from a variety of autochthonous foods during both seasons, but epilithon was the dominant wet season food. Moreover, allochthonous foods did not appear to contribute significantly to the diet of mayflies during any season at Shaded 1. When compared with published literature, these results suggest that use of allochthonous foods in tropical streams may be greater than in temperate latitudes where allochthonous energy sources appear to dominate community energy flows in small streams.