Estuaries are vital to Aotearoa New Zealand’s cultural and economic wellbeing; they also play a crucial role in the carbon cycle and can act as a carbon sinks. However, little is known about carbon cycling in intertidal sand and mudflats or how this will be affected by climate extremes such as heatwaves.

Study lead and marine ecologist Dr Emily Douglas says that these ecosystems are particularly vulnerable environments because of their dynamic nature.

“Estuary intertidal zones naturally experience large temperature changes because of incoming and outgoing tides, meaning they are exposed to both marine and atmospheric heatwaves. Estuaries are also threatened by sediments and nutrients from land, which results in muddy degraded habitats,” said Dr Douglas.

Climate change is increasing the frequency, intensity and duration of heatwaves. Scientists therefore wanted to know how temperature extremes may be impacting these ecosystems and their carbon absorption.

Dr Douglas and her team simulated a multiday atmospheric heatwave at two estuary intertidal flats at low tide – one was sandy, i.e. healthy, and the other muddy, i.e. degraded. They tested how the heatwave impacted biodiversity and shellfish, as well as the uptake and emissions of methane and carbon dioxide.

“We found that muddy sites emit more methane (a potent greenhouse gas) than sandy, healthier habitats. We also found that the sandy site was absorbing more carbon dioxide, and this increased with heatwave duration. These results tell us that climate change may alter the greenhouse gas source/sink status of our estuaries and that effects will be worse in muddy degraded estuaries,” said Dr Douglas.

The researchers also found that heatwaves changed the seafloor community. Again, these effects were greater at the muddy site compared with the sandy site.

“Overall, our findings indicate that healthy biodiverse intertidal habitats will be more resilient to global warming, and this emphasises the importance of preventing further degradation of our estuaries,” said Dr Douglas.

The team are planning another heatwave experiment this summer this time focussing on the impact of heatwaves on shellfish beds.

The research was funded by Earth Sciences New Zealand's Strategic Science Investment Fund.

It was published in Nature Communications.