Federated Farmers: Eye opening estuaries study

Waikareao Estuary

There is growing concern about the condition of some of New Zealand's estuaries.

Estuaries are the receptacles for all contaminants in our waterways. Both those flushed from urban stormwater and sewerage outlets and those leached from rural land have a potential impact on estuary health.

Arguably the biggest risk to estuaries is excess sediment and nutrients that aren't flushed away during tidal processes. These contribute to the deterioration of estuary ecosystems, which shows up through algal blooms, depleted oxygen in water and sediment, and reduced macro-invertebrate communities.

Under the New Zealand Coastal Policy Statement, regional councils are required to maintain coastal water quality.

For many councils, this adds to the already complex and resource-intensive water quality and quantity limit-setting process required of them under the National Policy Statement for Freshwater.

In Southland, the regional council is responding by ramping up its research programme to get a better understanding of water quality issues from 'mountains to sea'.

Part of this programme is a two-stage pilot research project which uses innovative forensic testing methods to help determine the sources of sediment entering the Jacobs River Estuary and the New River Estuary.

The first stage of the project (Jacobs River Estuary) was undertaken by NIWA and funded by Environment Southland and DairyNZ, using sediment samples collected by Environment Southland.

The research used a new forensic compound specific stable isotope technique that allowed NIWA to 'fingerprint' sediment samples to determine where in the catchment the sediment came from, as well as which land use it derived from. The results have provided an interesting snap-shot of estuarine sediment processes.

In brief, NIWA's work indicated that the majority (approximately 90 per cent) of the sediment at the four Jacobs River estuarine monitoring sites was coastal sediment transported back into the estuary.

The remainder of the studied sediment (10 per cent) was derived from within the catchment, and much of this is seemingly reworked material.

The majority was sourced from bank erosion. Sheep pasture contributed approximately 3 per cent of the remaining load, and dairy and deer pasture contributed approximately 1 per cent. The slightly higher sediment from sheep pasture is to be expected considering Southland's historical land use.

While trying to source where sediment comes from is a great first step, more work is under way to understand how ecosystems in Southland's estuaries function, and what measures can be taken to improve the health of the estuaries.

What is really going to be relevant to farmers in the region will be Environment Southland's ongoing investigation into what level of sediment from the land they consider acceptable, and what types of contaminants pose the biggest risks to estuary ecosystems.

In any case, NIWA's sediment pilot study reinforces the fact that improving the condition of Southland estuaries will be complex, long-term and multi-faceted. Any fixes for the issue have to go beyond the all too common refrain of "make more rules for farmers", and be supported by robust science.