Excessive fine sediment in stream beds and coastal sea bed environments is well known for causing significant adverse effects. No level of fine sediment (defined as particles less than 2mm in diameter) is beneficial to the ecological health of waterways, particularly aquatic invertebrates. Even natural slips that discharge fine sediment into waterways will adversely affect waterways. When combined with other contaminants such as nutrients and disease-causing organisms, the ecological effect can be greater than the additive effects of each of these stressors on their own. Economic values are often adversely affected. Examples include, the cost of drinking water treatment increasing significantly with increasing concentrations of fine sediment, costly maintenance from increased wear on irrigation pumps, reduction or collapse of shellfish harvesting (cockles and scallops stop feeding and die when suspended sediment concentrations are high), trout fishing guides lose business as they can’t see their fish.
Public concern about fine sediment has long been high in Tasman, particularly in the Waimea, Moutere, Motueka and Buller catchments.
Council has been monitoring fine sediment trapped within the gravel/cobble stream beds for years and found some streams (particularly spring and lowland-fed) to have very high levels of fine sediment in the bed. There is a clear relationship between macro-invertebrate community health and the level of sediment.
With these adverse effects in mind, Council commissioned NIWA to undertake a study to determine the relative contribution of fine sediment from various land uses.
A ‘chemical fingerprinting’ technique using compound-specific stable isotopes (CSSI) was employed to analyse samples collected in December 2016 from 30 sites mostly in the Waimea and Moutere catchments. This technique has been proven over many years at over 20 catchments throughout the country. It works by identifying the fatty acid biomarkers of exudates from particular plants into the soil and determining how much of that soil is present in fine sediment samples collected from waterways. Reference soil samples were collected from within the dominant land uses in the catchments.
Native forest and mature pine forest plantations were found to produce very little fine sediment.
A substantial proportion of fine sediment was found to originate from forest harvesting, although loads could not be calculated without mass transport data (which is very expensive to collect).
At some river sampling sites fine sediment from land covered with gorse and broom was found to be a significant source. Very often areas of harvested production forest can become dominated by these weedy plants that are less effective at protecting the soil from erosion than a closed canopy forest.
Bank erosion is a major source.
The Waimea Estuary is receiving a high proportion of legacy sediment from bank erosion, but also from harvested pine forest at various locations down the river, particularly the Wairoa, Lee, Roding and Pigeon Valley catchments where forest harvesting had recently occurred. The proportion of sediment arising from the various tributaries relative to their catchment size as estimated from the CSSI technique closely match that of the Hicks 2011 sediment yield model (annual suspended sediment loads) with the exception of two sites: Roding/ Hacket confluence and 88 Valley Stream/Wai-iti confluence. 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. 88 Valley Stream had a disproportionately higher amount of sediment than expected. The source of this 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 this catchment in 2007-08 to beat climate change regulations 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, with only a small pasture contribution. Almost 90% of the sediment at the Moutere River mouth was identified as being from pine forest origin.
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