New Zealand Journal of Marine and Freshwater Research abstracts

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Trophic pathways and diet of blue duck (Hymenolaimus malacorhynchos) on Manganuiateao River: a stable carbon isotope study

KEVIN J. COLLIER

Science and Research Division
Department of Conservation
P. O. Box 10-420, Wellington, New Zealand

GRAEME L.LYON

Institute of Nuclear Sciences
DSIR Physical Sciences
Department of Scientific and Industrial Research
P. O. Box 31-312, Lower Hutt, New Zealand

Abstract Samples of algae, epilithon, tree leaves, aquatic invertebrates, and feathers of blue duck (Hymenolaimus malacorhynchos) were collected for stable carbon isotope analysis from Manganuiateao River, central North Island, New Zealand, to elucidate the diet of blue duck and the carbon pathways involved. 8¹³C values for leaves were typical of terrestrial vegetation elsewhere (-27 to -31%o), but, in contrast to other values found in New Zealand, algae were much richer in ¹³C (> -17.0%c) than terrestrial vegetation. Larvae of the caddisflies Olinga feredayi and Aoteapsyche colonica, and the dipteran Aphrophila neozelandica were usually enriched with ¹³C (mostly > -20.0%e) relative to other taxa, indicating that they had been assimilating higher proportions of algal-derived carbon. In contrast, 8¹³C values < -20.0%c were recorded for the mayflies Deleatidium spp. and Coloburiscus humeralis, and the caddisfly Beraeopterc roria, indicating that their diets were composed of a higher proportion of organic matter of terrestrial origin. Feathers from blue ducks living in the lower part of the study section were enriched with ¹³C (-21.9 to -15.6%o) compared with those from the upper part of the study section (-25.0 to -23.2%c) where steep walls shade the stream bed. This indicates that trophic pathways in this upper section of Manganuiateao River were dominated more by terrestrially derived carbon sources. Birds living in the same territory generally had similar 8¹³C values and this is thought to reflect a high degree of behavioural synchrony typically displayed by pairs of birds when feeding.

Keywords blue duck; diet; stable carbon isotope analysis; Hymenolaimus malacorhynchos; aquatic invertebrates; Manganuiateao River, New Zealand

New Zealand Journal of Marine and Freshwater Research, 1991, Vol. 25: 181-186 0028-8330/2502-0181 $2.50/0 © Crown copyright 1991
Received 7 February 1991; accepted 8 April 1991

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