TROPHIC PATHWAYS AND DIET OF BLUE DUCK (HYMENOLAIMUS-MALACORHYNCHOS) ON MANGANUIATEAO RIVER - A STABLE CARBON ISOTOPE STUDY

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. delta-C-13 values for leaves were typical of terrestrial vegetation elsewhere (-27 to -31 parts per thousand), but in contrast to other values found in New Zealand, algae were much richer in C-13 (> -17.0 parts per thousand) than terrestrial vegetation. Larvae of the caddisflies Olinga feredayi and Aoteapsyche colonica, and the dipteran Aphrophila neozelandica were usually enriched with C-13 (mostly > -20.0 parts per thousand) relative to other taxa, indicating that they had been assimilating higher proportions of algal-derived carbon. In contrast, delta-C-13 values < -20.0 parts per thousand were recorded for the mayflies Deleatidium spp. and Coloburiscus humeralis, and the caddisfly Beraeopteraroria, 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 C-13(-21.9 to -15.6 parts per thousand) compared with those from the upper part of the study section (-25.0 to -23.2 parts per thousand) 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 delta-C-13 values and this is thought to reflect a high degree of behavioural synchrony typically displayed by pairs of birds when feeding.