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From: WADE DOAK
_: 5
Date: 15-08-10
Time: 11:02
Aerial view of Ngunguru by Adrian Gilbert
CHOCOLATE RIVERS, COFFEE SEAS: N.Z. GOING DOWN THE GURGLER Wade Doak Today beneath the warm, high tide river I wondered how the school of tiny silvery trevally would survive if heavy rain hits, as last year, filling their whole world with a choking cloud of mud.
Inputs of sediments to the New Zealand coastal zone are now especially high by world standards, approaching almost 1% of total world sediment yields. Recent NIWA studies show in detail how sediment is having a huge effect on the survival of many of our commercial fish species at the juvenile stage.
Heavy downpours send plumes of dark brown water far out to sea from Ngunguru River in Northland. It curves north, off Dolphin Bay and beyond. It almost gets to the precious Poor Knights Islands. This ocean pollution happens several times a year. Even a 12mm rainfall has produced a mud plume. A slug of sediment is wallowing down the tidal reaches of the river. So much mud, even in the inshore waters, is a serious matter for marine life, local fishing, diving and the tourism potentials of which Tutukaka Coast is so proud. It comes from multiple disturbances in the catchment that should be controlled. Some day, perhaps, those who love the sea or profit from it, may engage actively in campaigning against the bad practices happening on land....
Landowners and developers can bulldoze and send clay into river catchment without control provided the area is no greater a certain limit. Then there is pine forestry activity and its constant cycles of land clearance. The Northland Regional Council informs me that most of it comes from farming: pugged paddocks; over-grazing; slumping stream edges. Having visited four large pine plantations in the upper catchment I find it hard to believe that the vast amount of bare clay and extensive areas of erosion are not a major contributing factor. There are scientific methods for establishing the origin of sediments. But whatever, it would be hard to deny that clear felling of ground covering forest in very steep country does have an effect on the rapidity of run off after heavy rains. The times from peak rainfall to flood peaks are greatly reduced when there is no retaining mattress of forest to retain run off. During recent draught I have noted that streams in native forest are still flowing nicely after months of minimal input. Without cover the steep, bare hills shed their load rapidly and soon dry out.
I recently heard a national radio discussion on the future development of forestry for ethanol fuel. Forestry people are excited about this. We could grow forty percent of our current fuel use, they say. I would think unstable hill country would be best planted in trees that can be harvested without replanting. Then the root systems stay intact and represent capital in the bank. Only the interest is harvested.
On our own land beside the Ngunguru River I have a demonstration area with big, straight poles of blackwood that are all spontaneous coppicings above a landslip from the 1975* one hundred year deluge. Such coppicings occur wherever there is any land disturbance. Blackwoods are nitrogen fixers too. Their root systems join up into super tree. *On Friday 30 May 1975 a storm left a narrow ban of destruction from Onerahi, through Tutukaka, Matapouri and ran out at Sandy Bay. At the centre of the storm near Taiharuru Harbour, a total of 360mm of rain fell. Floods that ensued pushed the Ngunguru River across the neck of the sandspit. N.R.C. records.
THE CARBON SEQUESTRATION DEBIT OF SEDIMENT IN SEA Several scientists I have discussed it with have supported my suggestion that sedimentation of the ocean from land misuse involves a carbon sequestration debit: reducing light penetration inhibits carbon uptake by plant plankton and other marine flora. Our government has already shown concern about goats grazing native forest in relation to our Kyoto obligations.
In Northland pine forestry is often on the cheapest land they could find; land unsuited to farming because it is too steep. I contend that, because of our Kyoto obligations rotational pine forestry is no longer an appropriate land use in sensitive watersheds on steep land with the highest of erosion factors, in these times of climate change, when major weather events are increasingly chaotic and frequent. EFFECTS OF SEDIMENT ON BIODIVERSITY Estuarine studies in the Whitford catchment, near Auckland, show that the number of animals and species change with different amounts of mud. Some species are more sensitive than others but if a 2cm mud layer persists more than 5 days, all life will perish from lack of oxygen. Lesser thicknesses reduce species numbers and densities and so alter the community structure. Frequent deposition of thin layers, less than half a centimeter, still have long-term impacts that may change the community structure. Research suggests that cockles in poor quality habitats don't grow very big, nor very abundant.
In Whangateau estuary, just north of Auckland, massive numbers of cockles have recently died. All harvesting has been suspended. Once cockles disappeared at Cheltenham Beach in Auckland, they have not returned. Larval recruitment of cockles may be local. Cockles have another important function: because they filter huge volumes of water as they feed, they enhance water clarity and purity. As these shellfish are of primary importance to the whole food web in our estuaries and, as kaimoana, to local communities, they represent a challenge: water quality in our estuaries is vital to the quality of life in New Zealand.
Last changed: 23-Jul-2011