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

Abstract

Stable isotopes of carbon (C) and nitrogen (N) were studied in 11 stream communities in the Waikato region of New Zealand. From comparisons of mean δ13C and δ15N values, food webs in the shaded, forest streams were clearly based on allochthonous material (conditioned leaf litter and terrestrial invertebrates). Autotrophs in forest streams were not a significant C source for the food webs. However, the C source of food webs in the unshaded pasture streams appeared to be a mixture of allochthonous and autochthonous material. Conditioned leaf litter appeared to contribute to the pasture stream food webs, and the δ13C and δ15N of some samples of epilithic diatoms indicated their consumption by invertebrates in pasture streams. Fish ate a wide range of aquatic invertebrates; longfinned eels (Anguilla dieffenbachii) and banded kokopu (Galaxias fasciatus) also had a large proportion of terrestrial invertebrates in their diet. Filamentous green algae were found only at pasture sites, where they were sometimes abundant. The wide range of δ13C values of filamentous green algae (‐18.8 to ‐29.7‰) complicated understanding of their role in the stream food webs. The δ13C values of Cladophora were related to water velocity, with more 13C‐enriched values in pools than in runs (‐23.2‰ in pools, mean velocity 0.12 m s−1; ‐28.1‰ in runs, mean velocity 0.24 m s−1). Crayfish and the gastropod mollusc Potamopyrgus appeared to be the only invertebrates to eat filamentous green algae.