Stable carbon isotope analysis as a means of determining the fate of the Kinleith pulp and paper mill effluent in the Waikato River

The trout fishery downstream of Lake Maraetai is considerably more vigorous than that upstream. It has been speculated that carbon from pulp mill effluent discharging into the lake may be contributing to the food chain and being incorporated into biological tissues. Since the Waikato River is derived from the oligotrophic Lake Taupo which has a residence time of about 14 years, its dissolved inorganic carbon upon entering the chain of hydro electric dams is in isotopic equilibrium with the atmosphere. The carbon isotope composition of plants growing within the river, and that of the animals feeding on them were expected to be considerable enriched compared to that of the Pinus radiata derived pulp mill effluent. Stable carbon isotope analysis were used to trace the Kinleith pulp mill effluent carbon through the food web of Lake Maraetai and the Waikato River. Water, fish, weed, plankton and aquatic invertebrate samples were collected and analysed over a period of 18 months. The 13C composition of the mill effluent was found to be distinctly different from the 13C of the Waikato River. However there did not appear to be significant direct uptake of isotopically light carbon from the pulp mill effluent by any of the fish species analysed. It was also found to be difficult to distinguish isotopically between the pulp mill effluent and drainage from other small tributary streams. (authors)