Too much mud in Tasman Bay

02:37, Nov 15 2012

Sediment runoff from intensive pastoral farming, residential development and forestry is filling Tasman and Golden bays with mud.

Habitat mapping of 611km of Tasman’s coastline from Waimea Inlet to Kahurangi Point by Barry Robertson and Leigh Stevens of Wriggle Coastal Management has found half of the district’s estuaries are excessively muddy.

The mud is degrading shellfish habitat and covering rocky shores.

Their contracted state of the environment report on Tasman’s coast, which included habitat mapping, ecological risk assessment and monitoring recommendations, was presented to Tasman District Council in September.

At the time councillors voted to remove the $120,000-report’s management goals deciding that in accepting the report, the council was, by default, accepting the management recommendations.

The report, which includes significant new fieldwork information from 2010 and detail from all known previous studies, is now publicly available.


Workshops on the report are to be held in Motueka and Golden Bay later this month.

Other key issues in the report include:

The elevated disease risk from bathing and shellfish collecting along the district’s beaches and estuaries, which was primarily pastoral run-off and found after heavy rain in the catchments.

The risk posed human health issues and potential economic losses through shellfish bed closures.

Nutrient enrichment from the run-off of intensive pastoral fertilisers was affected the upper Motupipi Estuary, a number of small tidal river estuaries and the Waimea Estuary in particular.

The decline in Tasman’s smaller eelgrass beds which were prime habitat for many species through changes in sediment loading and eutrophication.

The over-stabilisation of 30 percent of the district’s duneland through the planting of exotic forest and sand-binding plants and the development of pasture or housing has halted their ability to migrate in response to physical changes, help protect the coastal margins from erosion and sea level rise and provide providing specialised habitat.

The loss of about one-thirds of the district’s saltmarsh since 1900 through increased sedimentation rates.
The Moutere (50 per cent ) and the Ruataniwha (40 per cent) have lost the most of the productive marsh which serves as a nursery ground and wildlife habitat, provides flood and erosion control, water quality improvements and for atmospheric gas regulation (as estuaries tend to be ‘carbon sinks and absorb carbondioxide in the photosynthesis carried out by the plant growth).

The construction of seawalls, stopbanks and reclamation has seen habitat lost across 65km (28 per cent) of the Tasman Bay, and 21km (12 per cent) of the Golden Bay, and 4km (2 per cent) of the West Tasman coastal and estuarine shoreline.

And the modification of around 65 per cent of natural vegetated margin which acts as a buffer from development protects against introduced weeds and grasses, naturally filters sediment and nutrients, and provides ecological habitat.

The natural shoreline 200m margin had been replaced by intensive pastoral grazing, residential properties and forestry.