Estuaries Are Literally In Hot Water, Says Researcher

Research on the threat of rapidly escalating heat waves in New Zealand estuaries has just been launched by University of Auckland Professor Karin Bryan.

The Marsden Fund has granted $1 million to Bryan and her team of researchers to find out what drives sudden, short-lived heat waves in estuaries and to develop tools to predict them.

As the climate warms, more frequent and more extreme heat waves occur in estuaries, says Bryan, from the University’s Centre for Climate, Biodiversity and Society – Ngā Ara Whetū.

Research into the issue is vital, because New Zealand has about 350 estuaries, which provide habitat for a wide range of marine life and shore birds, she says. Estuaries are also treasured sources of kai moana.

Heat waves in estuaries can cause catastrophic die-offs of shellfish and toxic algal blooms.

“When a whole bed of shellfish dies off, it can be obvious and everyone has a theory about the causes,” says Bryan.

Mudflats bake in summer sun when the tide is out.

“Super-heated sediments can cook the water, so within a few short hours the temperature is intolerable for some species,” she says.

High temperatures can also cause cascades of different stressors, acidifying the estuary environment, depleting oxygen and increasing the toxicity of contaminants.

“The health of estuaries is important because they sequester carbon from the atmosphere and mangroves protect the surrounding land against sea-level rise and coastal storms,” Bryan says.

Over the next three years, her research team will examine to what extent temperature extremes in estuaries are driven by tidal currents bringing warm water from the ocean, by atmospheric conditions, or by other factors.

Satellite data from the past 20 years and council records from coastal monitoring, along with new measurements from the research project, will be used to determine how much water temperatures are rising within estuaries every year and how often heat waves occur.

The researchers aim to develop tools that could be used globally to predict temperature increases in estuaries.

This information could be used to assess the potential impacts of rising heat levels on food chains and to inform plans that might help species, such as sea and shore birds, survive soaring temperatures.

“The temperature in estuaries is important, because it adds to the many stressors on biodiversity.

“We don’t know exactly why many of our coastal taonga species, like the hoiho or yellow-eyed penguin, are declining. But most species are extremely sensitive to water temperature, so heating is adding to issues such as degraded habitat, disease and poor food supply.”

The researchers will spend three weeks studying two New Zealand estuaries – probably Whangateau Harbour on Auckland’s east coast, where a massive cockle die off occurred in 2009, and a west coast estuary with dark sediment.

Sensors and a drone will be used to map heat patterns within the estuaries.

Detailed studies at the two sites will help gauge whether computer models can accurately predict temperature changes in estuaries.

Seagrass beds around New Zealand coastlines are increasing, so the researchers will look at whether that might be linked with waters becoming warmer, or with other factors such as lack of predation or increasing nutrient levels.

Bryan says vegetation provides shade that cools estuaries, so local planting projects on the fringes of estuaries could help buffer the impacts of climate change.

“Planting around an estuary won’t solve this complex problem, but if being involved in restoration projects helps convince people of the value of reducing their greenhouse gas emissions, that is a positive step.”

Having grown up in an inland area of Canada, Bryan says it is special to live near the sea - but she notices problems, such as declining water quality, as she walks around our coasts.

“We have a huge coastline, so it should be at the top of the list of things we look after.

“In New Zealand, our estuaries are our food-baskets, and they’re deeply engrained in our collective cultures and national psyche,” she says.

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