Marlborough District Council recently commissioned NIWA to study sediment sources in Te Hoiere/Pelorus Sound using the latest scientific techniques.
The Council’s Coastal Scientist, Oliver Wade says the new study improves understanding of the sources of sediment that impact the marine environment and how these have changed over time.
“The report identifies some of the sources of sediment deposited in Te Hoiere/Pelorus River catchment and Mahau Sound,” he said.
“The report identifies a complex dynamic of material deposited in the sound. Around 70% is termed ‘legacy sediment’ that, although originating from the land, has been in the sound for a long time and continues to move around.”
“30% of the sediment deposited comes from contemporary sources. Subsoils and streambank erosion make up the largest proportion, with smaller proportions attributed to erosion from land associated with primary industry and native forest. These proportions vary according to location,” Mr Wade said.
“Legacy sediment from deforestation, mining and burning during the mid to late 1800s has accumulated as flood plain deposits and throughout Te Hoiere/Pelorus Sound. This historical catchment disturbance and land use activity brought a ten-fold increase in sediment accumulation rates, relative to previous centuries.
“The effects of increased soil erosion, sedimentation and harvesting have had a profound effect on the ecology of the subtidal environment in Mahau Sound. Shellfish diversity is now at its lowest point at any time in history.”
The ‘legacy sediment’ presents a challenge, he says, since the marine environment continues to suffer the impacts of past human activities like goldmining, native forest clearance and pastoral farming.
“This suggests that despite our best efforts to improve land management, the marine environment will take longer to recover as this sediment continues to have an impact well into the future.”
“Integrated catchment and marine management will be needed in the future to halt any further degradation and help realise measurable improvements in the system’s environmental state.”
“The Te Hoiere/Pelorus Restoration Project, which is already well underway, provides a platform for the community to come together and begin that process,” Mr Wade said.
For further information go to www.marlborough.govt.nz/environment/coastal/sedimentation
The NIWA report is: ‘Sources of fine sediment and contribution to sedimentation in the inner Pelorus Sound Te Hoiere’ and is accompanied by a short summary document called ‘Tracing the sediment in Pelorus Sound’.