Spatial delineation of the depositional footprint of the Motueka River outwelling plume in Tasman Bay, New Zealand

Abstract

Progress towards development of a river plume ecosystem basis for management of coastal resources requires a thorough understanding of the nature and spatial extent of terrestrial influences on receiving water and seabed characteristics. As a case study, we estimate suspended sediment (SS) loading rates from the Motueka River catchment into Tasman Bay, New Zealand and describe benthic characteristics along a series of transects encompassing the coastal river plume. A mineral-rich geological formation in the headwaters of the river was identified as a storm-generated source of highly elevated concentrations of nickel and chromium in river margin sediments, and coastal sediments extending >5 km offshore. A major storm in 2005, focused in the upper catchment, resulted in an estimated SS discharge of 161,000 tonnes into the Bay. Spatial gradients of a suite of sediment trace metal signatures, organic content and infauna community structure were used to define a river plume depositional footprint of c. 180 km². We suggest that the boundaries of the Motueka catchment be redefined to include this area in order to facilitate engagement of marine stakeholders in management decisions that may affect coastal biological resources.

Keywords:

• integrated catchment management
• ICM
• river plume ecosystem
• suspended sediments
• depositional footprint
• trace metals
• macrofauna

Acknowledgements

This project was funded by the Foundation for Research Science and Technology, Contract C09X0305. Some of the data presented here was collected as part of an unpublished MSc thesis, Trace metals in sediments and biota of the Motueka River Plume, Tasman Bay, completed in 2007 by Reid Forrest through the University of Otago, Dunedin, New Zealand. The authors gratefully acknowledge the Community Trust of Otago for their generous contribution enabling the establishment of the Community Trust of Otago Centre for Trace Element Analysis. NIWA staff Rob Merrilees, Michelle Wild and Jo Hoyle provided suspended sediments samples, field data and analyses.