Date: 05 February 2009
Scientists visiting the Whanganui River estuary last week were surprised to find it is in pretty good health. Barry Robertson and Leigh Stevens spent two days walking the length of the estuary, from the tip of the sand spit at the South Mole, to the mouth of the Matarawa Stream in Kōwhai Park, then back again out to Castlecliff Beach. Like many people, they thought the estuary would be clogged with soft mud, but instead found many areas of clean sand, and only some places with deeper mud. Too much soft mud clogs the estuary bottom, and reduces the amount of oxygen available for creatures living in the sediment. A good amount of sand, on the other hand, means lots of oxygen and therefore plenty of worms and insects for birds to feed on.
Dr Robertson said, “Whanganui has an estuary to be proud of, and it is likely to be ranked among the best tidal river mouth estuaries in the country when our report comes out later this year. Its large size, clean sediments, high human use and ecological diversity all contribute to its good health”. He believes the estuary probably receives a lot of sand from the ocean on incoming tides which contributes to its good health.
The study was commissioned by the Department of Conservation and is part of a nationwide initiative to map the current state of estuaries. In Whanganui, Dr Robertson and Mr Stevens walked the estuary, marking aerial photos with careful notes of vegetation in the area, what the estuary bottom is like, and how deep the oxygen reaches into the sediment.
These notes and photos are then used to make detail maps of the estuary which are used for managing the area. After mapping the area, some spots will be recommended for further study, which may involve tests on heavy metal contamination and a detailed look at what is living in the sediment. The maps will also pinpoint weeds which threaten the estuary and need to be removed.
Many people don’t think about the Whanganui River mouth being an estuary, but Dr Robertson pointed out that an estuary is any area where the saltwater penetrates inland, mixing with the freshwater. That includes the mouths of even very small farm streams that flow directly to the sea, and in Whanganui it reaches to somewhere near the Cobham Bridge. However, the Whanganui Estuary is classified as a ‘freshwater dominated’ estuary, which means that saltwater creatures are rarer in the Whanganui estuary than in other, more open, estuaries. The engineered North and South moles serve to further reduce the entry of seawater into the estuary.
For part of the journey, Dr Robertson and Mr Stevens got a reprieve from negotiating the dunes and beaches when locals Toiora Hawira, Manahi Cribb and Hemi Gray took them on a six man waka to complete their survey. The scientists commented that it was a much better way to travel, and got a chance to pick up some valued local knowledge. The locals, on the other hand, got to learn about the different types of worms in the sediment, and how these are affected by oxygen penetration.
Apart from a healthy estuary, Dr Robertson and Mr Stevens also pointed out the extensive spinifex dunes at both Castlecliff and South Beach are among the best they have seen. They commented that both the estuary and the dunes are a real asset to Whanganui, and should be protected.