Changes in substrate and diet of blue duck on Tongariro River after the 1995 Mt Ruapehu eruption (Part 1 of 2)

Sampling of three sites on the upper Tongariro River was carried out in summer (January) 1996 to evaluate impacts of the Mt Ruapehu eruption and subsequent flushings of accumulated sediment from behind Rangipo Barrage on substrate, periphyton, invertebrates and blue duck (Hymenolaimus malacorhynchos) diet. Levels of sand and gravels increased markedly in summer 1996 in blue duck feeding habitat at the head of a large island, while sand levels were low at the upstream reference site and immediately below Rangipo Barrage. Amounts of suspendable interstitial fine sediment in the middle part of Rangipo Reach were comparable to those at the upstream reference site, while directly below Rangipo Barrage there were extensive accumulations of fine volcanic silt in slow flowing areas. The accumulations of volcanic silt below the barrage apparently smothered algae growing on stones in slow flowing areas, and may have contributed to higher (but non-significant) periphyton inorganic levels at this site. Periphyton biomass and inorganic content in the middle part of the reach were similar in summer 1996 to those at the upstream reference site. In contrast, there were marked reductions in the diversity, biomass and density of invertebrates at this site associated with the high levels of sand and gravels. Summer 1996 values for invertebrate richness, biomass and density were the lowest recorded at this site over a one year period, whereas at the upstream reference site summer 1996 values were not significantly different to those recorded on other dates. There had been an increase in the relative abundance of Chironomidae on cobbles and gravels in the middle part of Rangipo Reach since the eruption, and there was some suggestion of a corresponding increase in the proportion of Chironomidae in the diet of blue duck. The inorganic content of blue duck faeces at this site also increased after the eruption, although it was within the range previously recorded elsewhere in the reach. Continued inputs of volcanic sediment to Rangipo Reach are likely to have adverse impacts on feeding habitat, food supplies and diet of blue duck, at least for birds feeding amongst boulders at the heads of islands. Two options for intercepting volcanic sediment delivered to Tongariro River are discussed: 1. Construction of sediment retention devices on tributary streams carrying high suspended sediment loads. 2. Continued use of Rangipo Barrage to trap sediment delivered to the river.