I launched our fourth “Dirty Water Tour” this week at Invercargill’s New River Estuary, with Southland Green Party member Dave Kennedy, and Maurice Rodway of Fish and Game. Invercargill could potentially overtake Oamaru as the heritage capital of the south – distinctive and attractive heritage buildings abound. Natural heritage at the nearby New River Estuary, however, has fared less well.
Estuaries are a natural sink for sediments but forest clearance, land development for agriculture and settlement and wetland drainage means that sedimentation is happening at rates more than ten times those of before human settlement. Sedimentation means sand flats become mud flats, there is less oxygen and more toxic sulphides in the sediments, and plant and invertebrate communities are degraded, which reduces food sources for fish and birds.
With the long views to the extinct volcanic cone of Motupohue/Bluff Hill, New River Estuary is great for paddling, but it’s no place to swim (thanks to Dave’s very stable double sit on kayak there was no risk of that) and it’s off-limits to shellfish gathering.
Environment Southland began regular monitoring of the ecological health of Southland’s estuaries in 2001. Two recent reports rang loud alarm bells about Invercargill’s New River and Riverton’s Jacob’s River estuaries. There are more nutrients and sediment entering each estuary than they can cope with. In sheltered arms of the estuary such as the Waihopai Arm where we paddled, “there are significant areas of gross eutrophic conditions”. This means thick, gloopy mud which has very low levels oxygen (if any at all), and elevated levels of sulphide dioxide. These conditions can kill invertebrates, and cause nuisance growths of macroalgae.
At one site in the upper Waihopai Arm, sediment is being deposited at the rate of 17.5 mm annually compared to less than 1mm which would occur naturally. So much sediment is being deposited that marker pegs for the monitoring sites which were set 190mm above the surface in 2011 were completely buried in January 2012.
Also of concern, is the 68% loss in the extent of sea grass beds in the New River Estuary since 2000. Seagrass (or eel grass) is a vital nursery area for fish. The smothering of these aquatic plants by nutrient rich mud has potential implications for fish stocks in the estuary and off the coast.
So what can be done?
Environment Southland has yet to formally identify nutrient and sediment sources, but the intensive farming in the Oreti and Waihopai River catchments are likely to be contributing much of the estuary’s nutrient and sediment burden. Invercargill’s treated sewage, which is discharged on an outgoing tide near the estuary mouth, is also likely to add to the nutrient load.
Environment Southland is still in catch-up mode in relation to intensive farming. The regional council’s Plan Change 13 requires a Farm Management Plan for new dairy farming (but imposes few controls on existing dairy operations to control nutrient losses which submitters such as Forest and Bird and EDS have called for).
Through its Water and Land 2020 planning exercise the regional council aims for a 10% improvement in water quality (nutrients, sediment and bacteria) by 2020 across a range of catchments. It plans to set interim limits by 2015 and catchment load limits for contaminants such as phosphorus and nitrogen by 2023.
That’s likely to be far too late for the New River Estuary. Federated Farmers is likely to continue to challenge at every step as it is doing with Horizon Regional Council’s One Plan. Strong and effective national environmental standards are needed now to buttress the work of the regional council, and avoid this sad situation in other parts of the country.