Mangrove Tree Offspring Travel Through Water Currents. How will Changing Ocean Densities Alter this Process?

The pods were everywhere. Thousands of mangrove propagules, or germinated seeds, had washed up along shorelines in Corpus Christi, Texas. Biologists were puzzled. They knew these bean-shaped pods couldn’t be from local black mangrove trees—nearly all the Texas trees were killed during the February freeze event that chilled the state in 2021. Could these mangrove propagules have drifted from forests in Louisiana, Florida or Mexico? “We don’t actually know,” said marine biologist Anna Armitage. 

For Armitage, a professor at Texas A&M University at Galveston, it was an example of the importance of mangrove seed dispersal to replenish and grow the species over long distances. And what the consequences might be if this water-dependent process is interrupted.

Climate change may influence how far the trees can spread. That’s according to a recent study published in the journal Nature, which found that the density of seawater could change as temperatures rise, precipitation increases and the salinity of ocean water drops. An international group of geographers and biologists found that mangrove propagules will sink more quickly in less-dense oceans. That could potentially limit their dispersal over long distances. These findings suggest seawater density may deserve some additional research attention as an important effect of climate change, particularly for species like mangroves that depend on an exact ocean chemistry to survive.

Mangroves are resilient ecosystems that help build coastlines. Their roots are home for fish nurseries while birds nest in their canopies. But they are also threatened by a lot of different aspects of climate change ranging from sea level rise, storms, warmer temperatures and drought, said Michael Osland, an ecologist at the United States Geological Survey’s Wetland and Aquatic Research Center in Lafayette, Louisiana. “The study shows that there’s these really important interactions that are less frequently considered and can really affect the adaptive capacity of ecosystems,” said Osland, who was not involved in the research.

Temperature and precipitation are factors that are often considered in mangrove dispersal and survival. In this new study, researchers calculated seawater density using publicly available datasets on sea surface temperature and salinity, along with information on global mangrove ranges and habitats. The density of water varies with its temperature and salinity, said Tom Van der Stocken, lead author on the study and a researcher at Vrije Universiteit Brussel in Belgium. Van der Stocken and colleagues then calculated future sea surface conditions under four different greenhouse gas concentration scenarios. 

In all of the warming scenarios there were decreases in sea surface density across the globe. However, certain regions experienced greater changes. In Indonesian and West Pacific mangrove regions, the ocean salinity decreased. Fresh water is less dense than salty water so mangrove propagules that float just under the water’s surface might sink faster in this scenario. This “freshening” of the water could be from longer rainy and monsoon seasons that dump rain into the ocean, said Van der Stocken. Regions with large estuaries like the Sundarbans mangrove forest on the delta of the Ganges, Brahmaputra and Meghna rivers in India are predicted to receive an increasing flood of freshwater from melting Himalayan glaciers, he added.

This puts long distance propagule dispersal at risk, said Kyle Cavanaugh, a professor of geography at University of California, Los Angeles who was co-author on the study. This region also has a lot of islands, he said, so dispersal between islands may be particularly threatened.

Most mangrove propagules don’t travel far. They drop straight under their parent tree or get trapped in the tangled roots of the surrounding mangrove forest and take root a few feet or miles away. But long distance dispersal across oceans and seas is still important, said Van der Stocken. If mangroves need to expand their range because of unsuitable environmental conditions from climate change or weather events, as in Texas, being able to spread their genetic material long distances on ocean currents becomes important, he said.

Across the globe in the Atlantic and Eastern Pacific mangrove populations, including those along the Gulf of Mexico, reduced water density was less apparent in the study models. However, the model found these regions might develop saltier water than the Western Pacific. Even if propagules travel the same distance, increased salinity might affect their survival, which will have implications for the dispersal dynamics of mangroves in those regions.

One study in Japan recorded mangrove propagules sinking faster along a local river’s freshwater gradient. Recording this density effect at a global scale is the next challenge and will require additional observations and modeling.

“I hope that this research will kind of trigger the dispersal science for mangroves even more…just to better understand this process,” said Van der Stocken. In future studies, he wants to combine ocean water density with an existing mangrove dispersal model he developed with colleagues in 2018. 

For Samantha Chapman, the results of this new study are already leading to questions in her own research. “I haven’t seen something really linking dispersal directly to warming, so I think it’s really exciting,” she said. Chapman is a professor of biology at Villanova University whose work looks at how warming ambient air temperatures and soil chemistry influence mangrove growth, survival and dispersal.

It’s important for mangroves to survive because they build coastlines and protect from storm damage, both characteristics that will be helpful with sea level rise and intense storms from climate change. Mangroves have long roots that collect sand behind them. The red mangrove species are sometimes called “walking trees” because of how the roots hang above the water and prop them up. 

“This is why mangroves are so fun to study with climate change because they, like birds, can move pretty far,” said Chapman. Mangroves can even create new land where there used to be water. In a time when coastlines need better storm protection and higher elevation from sea level rise, changes in density may limit how far mangroves can travel to provide this service.

Yet mangrove migration is not always seen as positive. Over the last three decades, mangroves have migrated northward along the east coast of Florida, and up into Louisiana and Texas. The trees have encroached on areas that were historically saltmarsh habitats, which support different species and vegetation. This is a result of fewer winter freezes, like the Texas event, which normally prevent mangroves from extending too far north. As global temperatures have warmed, these freeze events have become less frequent and have allowed mangroves to grow further north in areas that had never supported mangroves before. 

The invasion into saltmarsh habitat brings tradeoffs, said Osland, the USGS ecologist. Marshes provide many similar benefits to mangroves, including coastline erosion protection, water filtration and habitats for nursery fish, birds and shellfish. When the walking trees move into these marshy areas, the flora, fauna and benefits of salt marshes are replaced by those of mangroves.

“It’s also kind of scary,” he said.

Studying density and other mangrove ecosystem features affected by climate change will help scientists understand the impacts of this transformation.