Heavy metal concentrations in the tissues of kākahi, Echyridella menziesii (Gray, 1843), and water from wadable depths in Lake Tarawera, Aotearoa–New Zealand

Mahinga kai (food gathering) activities are culturally important and still practised in the Te Arawa Rotorua Lakes, Aotearoa–New Zealand. Owing to active geothermal inputs in this region, heavy metal concentrations can be high and knowledge of their distributions is critical for undertaking mahinga kai practices. Whether it is safe to harvest kai (food) from Lake Tarawera, and whether risk varies spatially within the lake, were explored, guided by and centred on the priorities of the iwi (tribe) Tūhourangi. The concentrations of As, Cd, Cu, Fe, Pb, Mn, Hg and Zn in kākahi, Echyridella menziesii (Gray, 1843), and water, were analysed from five wadable locations around the lake. Heavy metal concentrations in kākahi tissues and the lake water varied among locations, but rank orders for each did not vary similarly. Kākahi near the lake outlet had the highest concentrations of all heavy metals except Pb, which was highest near a residential area. In the water samples, the highest concentrations of each heavy metal varied among sites. Water movement near the outlet likely increased mussel clearance rates and the transfer of heavy metals and organic matter towards this site, leading to higher concentrations in kākahi there. A potential significant risk may arise when consuming kākahi or lake water from Lake Tarawera. However, spatial variation in heavy metals means that representing the whole lake with a single risk value for consumption may not accurately reflect the variability of risk. Tūhourangi can use this study to inform mahinga kai practices within Lake Tarawera.