The impact of megafires on estuaries

Australian researchers have explored the impact of bushfires on estuaries in New South Wales, Australia’s largest state. They found that megafires can increase the load of fire-derived pollutants with potentially profound environmental effects.

Estuaries (where the river meets the sea) are some of the most valuable habitats on earth. They are biologically diverse and productive places. They are also places where ports are located, and a significant portion of the world’s population is concentrated.

Published in Environmental Pollution, the researchers say their findings prompt a call for riverside vegetation to be prioritised for protection in fire management plans. Bushfires also need to be considered in catchment management plans. These plans are developed to protect the environment. It is also an opportunity to regulate resources, such as fish and water, from the whole catchment area.

Unlike regular bushfires, megafires such as those during Australia’s 2019-2020 ‘Black Summer’ can surround estuaries. Following the Black Summer fires, the research team measured a rapid increase in the concentration of nutrients, metals and pyrogenic carbon (carbon formed by fires) in nearshore areas. They thought that could affect the behaviour, survival and reproduction of estuarine species.

“In some catchments, more than 90 per cent of the vegetation was burnt, and the fire went right down to the water’s edge. It led to a massive amount of pollution in the form of sediment, metals and nutrients entering our waterways,” said senior author Professor Emma Johnston, a marine ecosystems expert and the Deputy Vice-Chancellor (Research) at the University of Sydney.

The researchers found that the material from the Black Summer fires contained traces of metals and nutrients that sunk onto estuarine sediments.

“If there is consistent flushing of the estuary, these trace elements will prime them for productivity. However, if water flow is low and flushing is minimal, they can cause microalgae to breed exponentially. They can lead to plankton blooms that deplete oxygen supplies and kill fish. It would have a domino effect on the entire ecosystem,” said Johnston.

Fire carbon from megafires

The study also revealed how much pyrogenic carbon was deposited in estuaries—the bushfires’ calling card.

“We’ve not been concerned about pyrogenic carbon in these waters before. Now climate change is increasing the extent and severity of wildfires worldwide. We suddenly need to know what concentrations of pyrogenic carbon cause harm. That should be the subject of further study,” Professor Johnston said.

Impact of megafires and actions

With current climate projections suggesting megafires like Black Summer will occur more frequently in Australia and around the world, the authors say governments must factor our vital estuary habitats into their fire prevention and management plans.

“Considering that excessive nutrient input is one of the main stressors on estuaries, the changes found in our study might have serious implications due to its potential to alter ecological and physicochemical processes,” said lead author Thayanne Barros, a PhD candidate from the University of New South Wales.

Among the actions the authors propose are maintaining natural vegetation buffers to prevent fires from reaching the edge of estuaries. They also encouraged ecotoxicological assessments of wildfire impacts on estuarine areas.

The author’s concern about the impact of fires on waterways is shared by an international research consortium, of which Johnston is a member. The consortium’s new paper names wildfires as one of 15 emerging threats to ocean biodiversity.

About the study

The researchers studied six New South Wales coast estuaries at the mouths of the rivers, Hastings. They were the Karuah, Georges; Shoalhaven; Clyde; and Moruya immediately before the fires.

Barros said: “We had just finished collecting samples for a different project when the fires started. We saw a unique opportunity to conduct a Before-After-Control-Impact study in these areas. The initial samples became our ‘before the fires’ dataset. We collected the samples for our ‘after the fires dataset’ when the fires ceased.”

To investigate the potential impacts of bushfires on estuaries, the researchers decided to focus on the soft sandy bottom. This habitat is an important source and sink of elements. It plays a major role in global biogeochemical cycles, simultaneously supporting high biodiversity and productivity.

They analysed the sediments to directly check for changes in nutrient concentrations, sediment silt content, metals, and different forms of carbon. It included pyrogenic carbon (formed by fires), to link carbon concentration changes to bushfires.

Estuaries were categorised according to the percentage of burnt catchment vegetation and the burnt zone’s proximity to the waterway.

The study shows that in those estuaries with a large proportion of the catchment burnt and little gap between the fire and the waterway, the concentration of different forms of carbon, nutrients, metals and silt content significantly increased after the fires. At the same time, no significant changes were detected in the unburnt estuaries or those that retained a buffer zone.

The study was a collaboration between researchers at the University of Sydney, CSIRO, Macquarie University, and the University of New South Wales.