FOOD WEBS IN FOREST AND PASTURE STREAMS IN THE WAIKATO REGION, NEW-ZEALAND - A STUDY BASED ON ANALYSES OF STABLE ISOTOPES OF CARBON AND NITROGEN, AND FISH GUT CONTENTS

Stable isotopes of carbon (C) and nitrogen (N) were studied in 11 stre am communities in the Waikato region of New Zealand. From comparisons of mean delta(13)C and delta(15)N values, food webs in the shaded, for est streams were clearly based on allochthonous material (conditioned leaf litter and terrestrial invertebrates). Autotrophs in forest strea ms were not a significant C source for the food webs. However, the C s ource of food webs in the unshaded pasture streams appeared to be a mi xture of allochthonous and autochthonous material. Conditioned leaf li tter appeared to contribute to the pasture stream food webs, and the d elta(13)C and delta(15)N of some samples of epilithic diatoms indicate d their consumption by invertebrates in pasture streams. Fish ate a wi de range of aquatic invertebrates; longfinned eels (Anguilla dieffenba chii) and banded kokopu (Galaxias fasciatus) also had a large proporti on of terrestrial invertebrates in their diet. Filamentous green algae were found only at pasture sites, where they were sometimes abundant. The wide range of delta(13)C values of filamentous green algae (-18.8 to -29.7 parts per thousand) complicated understanding of their role in the stream food webs. The delta(13)C values of Cladophora were rela ted to water velocity, with more C-13-enriched values in pools than in runs (-23.2 parts per thousand in pools, mean velocity 0.12 m s(-1); -28.1 parts per thousand in runs, mean velocity 0.24 m s(-1)). Crayfis h and the gastropod mollusc Potamopyrgus appeared to be the only inver tebrates to eat filamentous green algae.