New study finds pine forest link to fine sediment in Waimea, Moutere estuaries

Monday, 2 April 2018

A plume of sediment from Waimea Inlet mars the blue of Tasman Bay in January 2009.

Almost 90 per cent of the environmentally-damaging fine sediment at the mouth of the Moutere River came from pine forest, a new study has found.

Tasman District Council and NIWA have been investigating the effects of sediment on the district's river systems. The resulting report, which is not yet available in full, also found that recently harvested pine forests along with bank erosion were responsible for a high proportion of sediment in the Waimea Inlet.

A model of land use soil contributions to the Moutere River system.

Council resource scientist Trevor James said the study represented a 'snapshot in time' but he hoped to organise a meeting with the forestry companies as well as sediment experts from NIWA and Landcare Research to discuss its findings.

'I know the forestry companies will be concerned,' he said.

Tasman District Council resource scientist Trevor James says he hopes to organise a meeting with forestry companies as well as sediment experts from NIWA and Landcare Research to discuss the findings.

A model of land use soil contributions to the Waimea River system.

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Excessive fine sediment in fresh and sea bed environments has been linked to many adverse effects.

In early March, James said fine sediment could fill spaces between stones in rivers and creeks where invertebrates tended to be most active, the effects of which then ricocheted up the food chain.

Sediment could also cause death by clogging the gills of fish and other aquatic animals. Many scientists 'strongly suspect' it caused the demise of the scallop fishery, James said.

A council report outlines the key findings of the study, which comes after dramatic increases in sediment were noted over recent years in the Moutere and Waimea estuaries.

Monitoring of the Moutere Inlet revealed the coverage of soft and very soft mud increased from 99ha in 2006 to 274ha in 2012, covering 38 per cent of the estuary. In the Waimea Inlet, the coverage of very soft mud soared from 10ha in 1999 to 551ha in 2013 with soft and very soft mud covering 40 per cent of the estuary.

Those worrying increases 'led us to try and find the source', James said.

'We waited in a queue,' he said. 'NIWA has had a lot of demand for those services.'

The long-awaited investigation used a 'chemical fingerprinting' technique to analyse samples collected from 30 sites, mostly in the Waimea and Moutere catchments.

James said the technique had been used for about 10 years and had been tested in the Environment Court.

It worked by identifying the 'fatty acid biomarkers' that plants exuded into the soil and determining how much of that soil was present in fine sediment samples collected from waterways. Reference soil samples were collected from within the dominant land uses in the catchments.

The council report lists the key findings of this study as:

* Native forest and mature pine forest plantations were found to produce little fine sediment.

* A 'substantial proportion' of fine sediment was found to originate from forest harvesting.

* At some river sampling sites, fine sediment from land covered with gorse and broom was found to be a significant source. Often, areas of harvested production forest can become dominated by these weedy plants, which are less effective at protecting the soil from erosion than a closed canopy forest.

* Bank erosion is a major source of sediment.

* The Waimea estuary is receiving a 'high proportion' of legacy sediment from bank erosion but also from harvested pine forest at locations down the river, particularly the Wairoa, Lee, Roding and Pidgeon Valley catchments where forest harvesting recently occurred. The lower Hacket Stream had a disproportionately higher amount of sediment than the Roding upstream of the confluence. A truck crossing in the lower Hacket was suspected to be the cause of higher-than-expected amounts of fine sediment found in the river sampled downstream. The Eighty Eight Valley Stream had a disproportionately higher amount of sediment than expected. The source is suspected to be from land cultivation, development and roading.

* The Moutere estuary is receiving a high proportion of sediment directly attributable to pine forest harvesting. The large-scale conversion of pine forest to pasture and rural residential in the catchment in 2007-08 may have been the origin of the sediment in Gardner Valley and Upper Moutere. Downstream of these catchments, the main sources of fine sediment were from recently harvested pine forest.

James said it would be good to repeat the test, adding there were plans to collect core samples from the Waimea and Moutere inlets to determine the dominant sediment sources historically.

The study findings come as many Tasman district residents call for tougher controls on forestry operations in the wake of devastation wrought by ex-Tropical Cyclone Gita and just weeks before the Government's new National Environmental Standards for Plantation Forestry are set to take effect on May 1.