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
Bj. Hicks, 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, New Zealand Journal of Marine and Freshwater Research, 31(5), 1997, pp. 651-664
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.