Lakes slowly warming

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Long viewed as clean and green, it would appear New Zealand is not immune to the ongoing effects of climate change.

That's the warning from the University of Waikato's Professor David Hamilton, who says the gradual warming of surface air temperatures is threatening the health of New Zealand - and the world's - lakes.

Until recently scientists had little knowledge of how atmospheric heating and increasing weather extremes were affecting lakes throughout the world - until they began sharing their data with each other.

Professor Hamilton is part of a group of more than 60 scientists across six continents who took part in a lake warming trends research project that was recently published in the Geophysical Research Letters journal. The database for the project was published earlier this year in Nature - Scientific Data.

"In collating and comparing our data, we've found that lakes are warming an average of 0.34 degrees Celsius, every decade," says Professor Hamilton. "That's greater than the warming rate of either the oceans or the atmosphere, and it can have profound effects on lakes."

Temperature is one of the most fundamental and critical physical properties of water, controlling a wide range of properties that include intricate living processes that have evolved within strict boundaries over time.

Professor Hamilton says that with sudden changes in temperature, life forms in lakes can change dramatically and in some cases, disappear completely.

"The consequences of climate warming on lakes are numerous and diverse. Effects include increases in harmful algal blooms, which can rob the water of oxygen as they decay or may be toxic to fish and animals.

"This is particularly harmful in a New Zealand context, where we have a lot of end-users who rely on our lakes - particularly iwi, local and regional councils and recreational users."

Professor Hamilton has researched New Zealand's lakes for more than three decades, and is leading a new project funded by the Ministry of Business, Innovation and Employment tasked with identifying and remediating threats to lake ecosystems in New Zealand. It draws together New Zealand's leading lake researchers from the University of Waikato, the University of Otago, NIWA and the Cawthron Institute.

"The results the global study has found indicate that big changes in our lakes are not only unavoidable, they're probably already happening."

New Zealand lakes are unique in that they are also affected strongly by the El Nino weather pattern which produces strong westerly winds and drought in eastern areas.

"Extreme wind increases the amount of water column mixing, and reduced rainfall can turn low-lying freshwater lakes salty as there may be insufficient freshwater to stop the ingress of seawater."

The lake warming trends paper stems from the Global Lake Temperature Collaboration (GLTC) which earlier this year tackled the challenging task of pulling together data from 235 lakes around the world between 1985 and 2009.

The database assesses the global and regional patterns of water temperature change over 25 years for 235 lakes. While that's a fraction of the world's lakes, they contain more than half the world's freshwater supply.

Professor Hamilton and Dr Mat Allan from the University of Waikato and Piet Verburg from NIWA contributed data from New Zealand's lakes, including individual lakes' climate and geographical features .

"Many lake temperatures are rising faster than the average air temperatures.

"There is an urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes, not just in New Zealand, but around the world."

- Hamilton News

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