Seagrass Meadows Decline in Every Climate Scenario in Stanford Study

Seagrasses are ancient plants that evolved in the ocean before moving to land and then back to sea about 140 million years ago, according to a Stanford University press release.

Lying just below the surface of the coastal tides, seagrass meadows remove carbon dioxide from the air and produce the oxygen essential to life on Earth.

Manatees, sea turtles and other herbivores graze on seagrasses, and they provide shelter for fish, shrimp and marine invertebrates. About 20 percent of the biggest commercial fisheries in the world use them as nurseries.

In a new study from Stanford, researchers modeled seagrass species’ distribution worldwide at two separate timepoints in the future and found that these underwater savannahs may be in peril.

Marine species are predicted to be greatly affected by climate change, partially due to the fact that oceans absorb about 80 percent of the excess heat created by greenhouse gas emissions. Scientists don’t know just how well seagrasses will do in the future, or if existing marine protected areas will be enough to save them.

“The simple question we ask in this paper is, ‘How will seagrasses – which are a foundational group in the marine food chain – respond to climate change?’” said Barnabas Daru, assistant professor of biology in the Stanford School of Humanities and Sciences, in the press release.

The study, “Reorganization of seagrass communities in a changing climate,” was published in the journal Nature Plants.

Though many marine species are directly dependent on seagrasses to survive, many others benefit indirectly from the versatile plants.

“For example, sharks feed on marine animals that, in turn, may feed directly or indirectly on plants,” said Daru, who is the lead author of the study conducted with Brianna M. Rock, a researcher at Clearwater Marine Aquarium Research Institute in Florida, in the press release.

“If anything affects these foundational species at the beginning of the food chain, it will have cascading effects on other organisms that depend on them high up in the food chain, including humans,” Daru said.

Seagrasses expand outward from about 116,000 square miles of land along the coastlines of 191 countries on every continent but Antarctica, so creating models of how they might be affected by climate change globally was a big undertaking.

Rock and Daru started out by mapping how much of each species of seagrass there was and where they were located using a century or so of samples collected from ecosystems along the various coasts. This information was combined with data from the field, as well as statistics on seagrass prevalence from public databases such as Seagrass-Watch and the Global Biodiversity Information Facility.

In modeling-predicted seagrass habitats in areas that had been under-sampled, like the Indo-Pacific and Southeast Asia, the researchers used data from areas like Europe and North America that had been well-sampled.

The scientists used environmental and geophysical data taken from the Bio-ORACLE website to create global “snapshots” of today’s ocean climate, as well as snapshots for the years 2040 to 2050 and 2090 to 2100.

For the future time periods, as well as the present, Daru came up with models for four distinct climate scenarios. The first was a “best case” that had low concentrations of greenhouse gases; the second and third were a pair of scenarios where the levels of greenhouse gases had plateaued; and the last was a “worst-case” scenario plagued with high concentrations of greenhouse gases.

All of the possible scenarios included data on variables that are known to have a significant influence on the growth, photosynthesis and distribution of seagrasses, like salinity, sea temperature and velocity of sea currents.

In order to predict how populations and distributions of seagrasses could change from now until the future timepoints, Daru used a computer model of each climate scenario’s observed species occurrences.

The study found that widespread species composition and diversity reductions will occur in a significant number of seagrasses that exist in hotspots outside of the present system of marine protected areas.

Perhaps most importantly, the findings showed that within every scenario — even the “best case” — seagrasses were found to decline in composition and abundance.

“It probably means that ‘the best’ is still not enough,” Daru said in the press release. “We have to be more intentional in how conservation efforts are prioritized and this sort of analysis points to the places where these efforts should be done.”

The study also indicated that the network of existing marine protected areas isn’t enough.

“One of the signatures of this modern era of profound human impact on the environment is not even the loss of species, but the reorganization of biotic communities. The homogenization of communities is likely to lead to a more profound impact on biodiversity than even the loss of species,” Daru said.

Reductions in diversity due to homogenization makes ecosystems more prone to extreme weather and disease. These impacts could affect the marine life that relies on seagrasses, as well as the ecosystem services they provide. It could also force marine species specializing in particular types of seagrass to move to another location or adapt to a different, less favored species of seagrass, which could lower their rates of fitness and survival.

However, Daru still has hope for the future.

“We highlighted hotspots of change in species diversity and phylogenetic diversity that represent priority regions to target for conservation efforts,” Daru said in the press release. “Our goal, our hope is that by pointing policymakers and conservationists to focus on these hotspots, marine protection will be increased in these areas and the future of seagrasses will – to some extent – be safeguarded.”