EFFECTS OF HEAVY-METALS ON BENTHIC MACROINVERTEBRATE COMMUNITIES IN NEW-ZEALAND STREAMS

We performed chemical analyses of heavy metals in water and periphyton , toxicity tests with Daphnia magna and an indigenous mayfly (Deleatid ium sp.), and field surveys of benthic macroinvertebrates to estimate the degree of metal pollution in three catchments in the Coromandel Pe ninsula of New Zealand. Good agreement was found between toxicity test s and measures of benthic community structure, particularly at station s with the highest metal levels. Responses of benthic communities at s tations with low or moderate levels of metal contamination were variab le and were probably confounded by factors other than heavy metals. Ef fects of heavy metals on benthic communities in New Zealand streams we re similar to those reported for metal-polluted streams in North Ameri ca and Europe, suggesting that responses to metal contamination are pr edictable. Abundance and species richness of mayflies, number of taxa in the orders Ephemeroptera, Plecoptera, and Trichoptera, and total ta xonomic richness were the best indicators of heavy metals in New Zeala nd streams. In contrast, the quantitative macroinvertebrate community index (QMCI), a biotic index proposed for assessing effects of organic enrichment in New Zealand streams, could not distinguish between refe rence and metal-polluted streams. The poor performance of the QMCI was primarily due to incorrect tolerance scores for some taxa to heavy me tals. Because of concerns regarding the subjective assignment of toler ance values to species, we recommend that tolerance values for dominan t species in New Zealand streams should be verified experimentally in stream microcosms.