Weekend Herald editorial: Hawkes Bay's poisoned lake may not be alone

Editorial

Lakes have a beauty all of their own. They lie lovely, still and quiet, surrounded by the hills or mountains that trapped the water. They do not have the sound of surf or the movement of rivers. A gentle ripple might be seen and heard on a windy day but otherwise nothing disturbs their lonely ancient silence.

Perhaps that is why they have not been in the forefront of discussion of the state of New Zealand's freshwater resources. When we refer to "rivers, lakes and aquifers" we usually have rivers and underground water in mind. But our environment reporter Jamie Morton today draws attention to the particular problems of lake pollution that defy easy or even known solutions.

Rivers and aquifers are quickly flushed and their water quality would soon recover if contaminants from pasture or urban activities are contained. But lakes hold the same water for much longer. Lake Tutira in Hawkes Bay, fed and drained by streams at the same end, takes a decade to replace its water. And the sediments in its bed containing high loads of nitrogen and phosphorus could continue to release those chemicals into the lake water for a century.

The lake is still a beautiful sight for travellers when it appears — suddenly as lakes do — on the route north of Napier. Its shore may still be a popular picnic spot, unless an epidemic of dead fish washes up, as has happened more than once in Morton's report. But signs warn against swimming in the lake and anglers no longer find many of the trout that used to be in abundance there.

Lake Tutira's problems do not have a recent cause. They were apparent by the 1970s, which means they predate the intensified dairy farming that bears most of the blame for the deterioration of rural water courses.

The lakes of northern Hawkes Bay were created by massive landslips and protected from further erosion by the forests that grew around it. Over the past 500 years most of the forest cover has been removed, replaced by scrub and bracken and later by pastoral farming. The erosion in the lake's catchment lies on its deep bed as sediment bearing chemicals from fertiliser, especially the quantities dropped from planes in post-war aerial top dressing.

Lake Tutira is probably not alone among those lying in agricultural catchments. Morton quotes a freshwater scientist, Professor David Hamilton, who says the lessons of the lake need to be heeded everywhere in New Zealand. "A price is being paid for past legacies of sediment and nutrients that have entered the lake as a result of forest clearing and agriculture."

And the Hawkes Bay Regional Council's water-quality scientist, Andy Hicks, told him Tutira is a microcosm of New Zealand's freshwater problems. "You've got different landscapes, some intensively farmed, some of it hill country. Nationally there has been a lot of attention on those more intensified landscapes but that hill country, which generally you are not making money off, that's quite a big challenge too."

It sounds like it. Lakes that take water from large areas and hold it a long time, are going to need more attention. Complete replanting of high country forest is not feasible, no matter how marginal its value as pasture may be. But the lakes cannot be abandoned, a solution must be found.