Lake Taupo – keeping it clear for future generations
The time to take action to protect the water quality of New Zealand’s largest lake is now – before the lake shows further signs of water quality decline.
In every story of lake decline, and there are depressingly many such stories from around the world, there are always 'if onlys'. In this context, 'if only' is usually a lament that if protection measures had been taken before serious declines in water quality occurred, then not only would the lakes still be in a good shape, but also highly expensive remedial actions would not be necessary. For Lake Taupo, it seems that this pattern might just be broken.
In recent years we have seen the first indications that the water quality of Lake Taupo is beginning to decline. Monitoring has shown three indicators of decline: a steadily increasing phytoplankton biomass, increased internal nutrient loading, and increased external nutrient loading. Taken in isolation, none of these effects provides an overwhelming picture of declining water quality, but when coupled with our understanding of lake functioning, and the time scales over which changes to the factors driving this decline will respond, they provide a clear signal that action now is a wise investment in the future health of this great lake.
Since 1998, monitoring by Environment Waikato has shown a gradual increase in the amount of chlorophyll-a (a proxy for phytoplankton biomass) in Lake Taupo.
This steady increase is exactly what would be expected if the supply of nutrients were slowly increasing, and monitoring of inflows shows that the inflow of nitrogen in readily available forms has been increasing in recent years. Not only that, but the internal load of nitrate (the form of nitrogen that accumulates in summer in the lowest layer, where algae cannot grow) has also been steadily increasing.
To date, the effects of these gradual changes have been slight. The lake is still in excellent condition, with clear water, but the prognosis is for continued gradual decline if nothing is done to reduce the loading of nutrients. The decline would be expected to accelerate if the nutrient loading increased. One particular problem for Lake Taupo is the long lag time between nutrients entering the pumice soils and emerging as groundwater in the lake or its tributaries.
The decision to protect the catchment of Lake Taupo while the lake is still in good condition is a far-sighted one, supported by lake scientists. We could wait until the signs of decline are incontrovertible. However, action now will ensure that future generations are more likely to have a clean lake to enjoy, rather than a restoration task to pay for.
Ian Hawes [ [email protected] ] Clive Howard-Williams [[email protected] ]