Land-derived metals impact the survival and settlement of larval mussels.

Global declines in wild mussel populations and production have been linked to the impacts of climate change and pollution. Summer die-offs of mussels (Perna canaliculus), spat retention issues, and a severe decline in mussel spat settlement have been reported in the Marlborough Sounds, an important area for mussel farming in New Zealand. Preliminary evidence suggests that naturally occurring contaminants and changing land use in the surrounding areas, could contribute to the decline of this species. The aim of this study was to determine whether trace metals affect the larval survival and settlement of mussels. Concentrations of trace elements in surface waters, mussel gill tissues and sediments were characterized across the Marlborough Sounds. Over a two-year period, diffusive gradients in thin film (DGT) passive samplers were deployed at three locations, to estimate trace metal levels in the water column. Mussel gills and sediment were also collected to complement metal monitoring. Lead and zinc were the most abundant bioavailable metals detected during in-situ monitoring, with strong temporal variability. Ecotoxicity thresholds affecting P. canaliculus embryo-larval development and settlement were determined for these metals and overlapped with levels of lead and zinc found in the field during the reproductive season of mussels. Furthermore, high concentrations of zinc and lead in waters were strongly correlated with rain events, suggesting land runoff in the surrounding catchments as their main source. We hypothesize that vulnerable early life stages of mussels are exposed to these metals, especially following summer rainfalls, delivering pulses of these metals into the environment while triggering mussel spawning events. Climate-associated stressors (intense storms, marine heatwaves, freshwater and sedimentation inputs, acidification) are likely to exacerbate any toxic effects from land-derived inputs; a better understanding of these complex interactions is essential to prevent further P. canaliculus recruitment declines occurring in NZ, and in other mussel species globally.