Rising seas threaten to drown important mangrove forests, unless we intervene


Neil Saintilan; Catherine Lovelock, The University of Queensland, and Kerrylee Rogers, University of Wollongong

Mangroves are some of the world’s most important trees. They provide food and resources for people and animals, protect coasts, and store huge amounts of carbon. The world’s largest mangrove forest – the Sundarbans in the Bay of Bengal – supports millions of livelihoods. In terms of the services they provide, they are worth nearly US$200,000 per hectare per year.

But these coastal forests are threatened by rising seas and human development. In a study published today in Nature, we show that some of these forests will drown unless we help them.

Catherine Lovelock explains her new mangrove study

Getting to the root of it all

Mangroves grow along tropical coasts. Unique amongst the world’s plants, they can survive in salt water and can filter seawater. The rain of leaf-fall from tropical mangrove forests provides food for crabs and other herbivores, the foundation of a food web that extends to fish (and therefore people) right across the tropics.

One of the distinguishing characteristics of mangroves are their roots, used to anchor the plant on unstable ground and buttress against wind, waves and currents. The form of root architecture varies greatly between families of mangrove, including the dense prop-roots (Rhizophora), cathedral-like buttresses (Bruguiera), and numerous pneumatophores – literally narrow breathing–tubes – of the common grey mangrove of southeast Australia (Avicennia).

Prop roots on a mangrove
Ruth Reef

A high proportion of the living mass of mangroves exists below-ground. This means mangroves are the most efficient ecosystem globally in the capture and sequestration of atmospheric carbon dioxide. The uniquely oxygen-poor, salty characteristics of mangrove soil provides the perfect setting for long-term preservation of carbon below ground. The typical mangrove forest sequesters several times more carbon dioxide than a tropical rainforest of comparable size.

Mangrove roots trap sediment as currents carrying suspended particles are intercepted and slowed. Between the carbon sequestered below-ground, and the sediment trapped within the tangle of roots, mangroves are effectively able to raise the height of the land over time.

Keeping up with rising seas

Analysis of these sediments shows mangroves can deal with low to moderate sea-level rise by building up land. But how will mangroves respond to future rising seas when people are in the way?

We and other colleagues measured how fast mangrove forests in the Indo-Pacific region increase the height of the land. We used a tool called Surface Elevation Table-Marker Horizon, as you see in the video below.

Mangroves also build up land height by accumulating roots below ground. Previous studies have focused on this. Our study, using up to 16 years of data across a range of coastal settings, shows that sediment build up is also important.

We also compared the rate of land height increase in mangroves to local tidal gauges, to assess whether mangroves were keeping pace with the local rate of sea-level rise.

In most cases (90 out of 153 monitoring stations) mangroves were lagging behind. This is not an immediate problem if mangroves are already high enough to delay the effect of expected sea-level rise. However, mangroves at the low end of their elevation are highly vulnerable.

We used this insight to model how long mangroves might survive rising seas across the Indo-Pacific. We used a range of sea-level rise projections from the Intergovernmental Panel on Climate Change, including a low-range scenario (48 cm by 2010), high-range (63 cm by 2100) and extreme (1.4 m by 2100).

Mangrove forests with a high tidal range and/or high sediment supply such as Northern Australia, eastern Borneo, east Africa and the Bay of Bengal proved to be relatively resilient. Most of these forests will likely survive well into the second half of the century under low and moderate rates of sea-level rise.

The prospect of mangrove survival to 2070 under the 63 cm and 1.4 m scenarios was poor for the Gulf of Thailand, the southeast coast of Sumatra, the north coasts of Java and Papua New Guinea and the Solomon Islands.

Dams holding mangroves back

Our results imply that factors that prevent sediment building up may prevent mangroves responding to sea-level rise. This might include dams holding sediment within water catchments.

This impact is already being felt. An 80% reduction in sediment delivery to the Chao Phraya River delta has, for example, contributed to kilometres of mangrove shoreline retreat.

Similar developments are planned for the Mekong River. These threats compound those already being felt, including the widespread conversion of mangrove to aquaculture.

Appreciation of the financial contribution of mangroves has been slowing the trend of decline. However, long-term survival will require planning that includes both the continued provision of sediment supply, and in many cases the provision of retreat pathways, to allow mangroves to respond to sea level in ways they always have.

The Conversation

Neil Saintilan, Head, Department of Environmental Science; Catherine Lovelock, Professor of Biology, The University of Queensland, and Kerrylee Rogers, ARC Future Fellow, University of Wollongong

This article was originally published on The Conversation. Read the original article.

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Rising seas could drown turtle eggs: new research


James Whitmore, The Conversation

Immersion in seawater kills sea turtle eggs, suggesting that sea turtles are increasingly at risk from rising seas, according to research published today in Royal Society Open Science.

In a laboratory experiment, researchers immersed green turtle eggs in seawater for varying lengths of time. The researchers tested eggs of various ages, and then counted the number of eggs that hatched. They found that immersion for six hours reduced survival by a third.

The study partly explains reduced numbers turtle of hatchlings recorded at Raine Island, home to the largest population of green sea turtles in the world.

David Pike, lecturer in tropical biology at James Cook University and lead author of the study, said turtle nests low down on beaches could be underwater for six hours during abnormally high “king” tides or storm surges.

Michele Thums, ecologist at the Australian Institute of Marine Science, said that given climate projections for increased severe weather events, this could mean fewer hatchlings survive in the future.

But every beach will see different impacts from rising seas, said Tim Dempster, senior lecturer in marine biology at University of Melbourne.

“You can’t just take [a…] scenario of a certain degree of warming, say that will lead to a certain amount of sea level rise, project how much land will be inundated and then project what proportion of nesting habitat will be affected,” he said.

Turtle embryos need oxygen to develop into baby turtles, and immersion in water prevents oxygen from the soil entering the eggs. The embryos effectively suffocate, a process known as “hypoxia”.

Thums said that while most turtles nest above the high tide line and are rarely immersed for six hours, “there are always inexperienced turtles that will lay further down the beach and also there is competition at high density nesting sites like Raine Island”.

Compared to the rest of the world, green sea turtles on Raine Island have a much lower level of breeding success, which could lead to a large decline in the number of breeding adults in the future.

Pike said the low level of success could be partly explained by inundation, but there were likely other factors at work.

“One possibility is that the sand is full of bacteria from all of the rotting eggs that are beneath the sand, and that any fresh eggs laid there may be exposed to bacteria that overgrow the egg and kill the embryo,” he said.

“Another possibility is that contaminants (heavy metals, pesticides) are being passed from the mother turtle to the eggs, and that may cause the embryos to die.”

The Queensland Department of Environmental Heritage and Protection is currently trying to raise low lying spots on Raine Island by moving sand. The island could lose between 7 and 27% of its area thanks to rising seas.

With Janelle Braithwaite, editor at The Conversation.

The Conversation

James Whitmore is Editor, Environment & Energy at The Conversation.

This article was originally published on The Conversation.
Read the original article.