Freshwater Ecosystems Under Threat In A Warming Aotearoa

Nature is taking with one hand and giving with the other when it comes to Aotearoa New Zealand’s freshwater ecosystems.

While the increasing number of severe droughts is playing havoc with our waterways, more floods could prove to be their friend.

Professor Angus McIntosh, a specialist in freshwater ecology at University of Canterbury, is heading research into how climate warming is affecting our freshwater ecosystems and how to best deal with those influences in the future.

“A major climate-drive threat to freshwater life in New Zealand is increased frequency and magnitude of drought. We know that river communities collapse when rivers dry completely, but we’re trying to find the early warning signs indicating when communities start that process so we can offer better advice in water allocation decisions.”

A big challenge, especially in some recent Environment Court decisions, has been how to manage river flows when interactions between trout and native fish (galaxiids) are involved, Professor McIntosh says.

“It’s often thought that native fish will benefit when rivers are lower because there will be fewer of their predators – trout. What actually happens with no trout present is that the number of non-migratory native fish decline as the river flow slows. Just how to manage native fish populations and how their interactions are affected by longer periods of low flow is going to be tricky as climate change bites.”

Professor McIntosh says the flip side of climate warming is that large magnitude floods could also be very influential.

“We were worried that Canterbury’s autumn floods last year would have wiped out native fish in the headwater and alpine areas. In some of our study streams around Porters Pass, for example, the stream beds were scoured down many metres.

“But, to our surprise, all the populations have held on. They’ve been greatly reduced, but they’re still there. We’re now interested in how quickly they recover, whether the trout were more or less affected and what the long term effects are.”

Professor McIntosh says the ability of those native fish to deal with ongoing extreme events such as flooding will depend on the resilience of their populations and his Freshwater Ecology Research Group’s research is now focusing on how to enhance that ability.

The research group has also recently finished work that reveals the relationship between river size (ie flow) and their ability to absorb sudden environmental disturbances such as floods.

Mimicking the impact of climate change, they used a waterblaster to simulate the influence of scour over small reaches of stream bed and observed the recovery. In general, the bigger a river, the more able it was to absorb the effects and maintain structure. The smaller the river, especially if water is taken out, the more vulnerable they become.

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