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Ecosystem mystery

By examining the Lake Ellesmere ecosystem, Otago researchers aim to determine to what extent lowland lakes and wetlands provide a little-known but valuable ecosystem service by converting nitrate to harmless nitrogen gas or, conversely, contribute to greenhouse gases.

Lake Ellesmere/Te Waihora is Canterbury's largest lake and of national importance as a waterfowl habitat and fishery. Unfortunately, as a natural sink for the run-off from farms in the Canterbury Plains catchment, it is also one of New Zealand's most polluted wetlands. This makes it a model ecosystem for studying the conflict between freshwater ecology and agriculture – and also something of a mystery.

Dr Sergio Morales (Microbiology and Immunology) explains that agricultural waste increases nutrients in water, potentially triggering persistent algal blooms. However, Otago researchers found that the prevalence of nitrogen and phosphorous (the two main agricultural contaminants) is greater in Lake Ellesmere's tributaries than in the lake itself.

“This suggests that something in the lake is playing a strong role in purifying the water.”

To find out exactly what is happening, Morales and his colleagues are focusing on the role of micro-organisms.

“Nitrogen [nitrate or urea] flowing into the lake can encourage the growth of algae, or denitrifying bacteria can actually purify waterways by transforming nitrogen into harmless nitrogen gas. Are these two processes competing for nitrogen in the lake and, if so, which process wins?

“However, there could be a sting in the tail of this. Under certain conditions, some of the nitrogen is converted into nitrous oxide which has a greenhouse gas potential roughly 300-times greater than carbon dioxide.”

Photo: Graham Warman
Dr Sergio Morales (above centre) with Dr Federico Baltar and Dr Candida Savage.
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