A COMPARISON OF THE FOOD SOURCES OF STONEFLIES (PLECOPTERA) FROM AN OPEN-AND A CLOSED-CANOPY HEADWATER STREAM IN SOUTH AFRICA USING STABLE-ISOTOPE TECHNIQUES

Summary

The effects of different local food resources on the diets of some filipalpian stonefly nymphs (Aphanicerca spp.) were investigated. Animals from two headwater reaches, one primarily allochthonously driven and the other an open-canopied autochthonous-based system were compared. However, gut and stable carbon isotope analyses revealed no real differences in plecopteran diets between the systems, with leaf detritus being a major source of carbon, and algae contributing virtually nothing to their diets. δ¹⁵N analysis suggested that the animals obtained their nitrogen from sources other than leaf material itself, and it was hypothesized that this source was the microbial slime layer formed on decomposing leaves. It was concluded that the feeding behaviour of Plecoptera is consistent with that of shredders of leaf detritus, and as such they uphold River Continuum Concept predictions of large shredder populations being indicative of a predominance of coarse particles in headwater reaches. This study also highlighted the danger of making broad generalisations as to an organism's diet, based on the overall nature of the system, rather than on the micro- level of what may actually be available to small populations within a species.

Keywords:

• Plecoptera
• River Continuum Concept
• stable isotopes in freshwater systems
• dietary analysis