In a study published at the beginning of 2024, an American and Dutch research team highlighted the vulnerability of turtle grass (Thalassia testudinum), an emblematic feature of tropical and subtropical ecosystems in the western North Atlantic, to predicted overgrazing by herbivorous species migrating northwards as a result of warmer waters.
By Maud Lénée-Corrèze
Under the clear, turquoise waters of the Caribbean Sea, their long, flat green leaves clinging to the sandy substrate slowly undulate. There are many species of seagrass in these tropical and subtropical seas, but the most emblematic of them all, whose range extends from the Gulf of Mexico to Bermuda, is the turtle grass, or Thalassia testudinum.
However, the turtle’s living space could well be reduced by voracious grazers migrating northwards as a result of warmer waters. The presence of greater numbers of herbivores in areas where turtle grass is not accustomed to excessive grazing would be ‘likely to have profound effects on the ecology of seagrass ecosystems in the northern Gulf of Mexico’, explains Tom Frazer, Professor and Dean of the School of Marine Sciences at the University of South Florida.
What then would be its resilience? Luminosity, which varies according to latitude? This is the question explored by biologists and ecologists from several universities in the United States and the Netherlands in a paper published in January 2024.
The researchers cut the turtle grass at different intensities to simulate grazing
Several studies dating from the 2010s, cited by the researchers, have highlighted this migration, particularly that of emerald parrots, whose numbers have increased 22-fold since 2010. They are said to consume grass at a rate almost five times greater than that of native grazers.
The scientists also point out in their publication that certain successful animal conservation initiatives, such as the green turtle (Chelonia mydas), which is very fond of Thalassia testudinum, have led to ‘overgrazing along the Bermuda shelf’.
A green turtle grazing on turtle grass, or Thalassia testudinum, in Akumal Bay, on Mexico’s Yucatan Peninsula. © P. Lindgren/WikimediaCommons
To verify that light is the crucial factor for marine plants, in 2018 scientists set up a one-year experiment on a sector stretching from the 9th to the 32nd parallel north to test the response of seagrass beds to various grazing pressures. Other factors such as temperature variations and the availability of nutrients (nitrogen and phosphorus) were also monitored.
They marked out 650 plots of turtle grass in 13 large meadows, some of which were completely isolated from grazers by means of cages. Twice a month, divers used scissors to cut back the shoots of Thalassia testudinum, inflicting two levels of cut. A ‘severe’ one to simulate turtle grazing, and a ‘moderate’ one to mimic fish grazing. Other cages were left alone.
The researchers delimited 650 plots, some of which were completely closed to grazers and others left accessible. © Maggie Johnson
In addition, for the duration of the experiment, part of the plots was fertilised with nitrogen and phosphorus, while temperature and light were recorded every six minutes by instruments installed above the meadows. Plant growth was measured twice during the year, at the end of summer 2018 and then at the end of the following winter, in April-May 2019.
Good exposure to light encourages photosynthesis and therefore the production of reserves
The initial results confirm the researchers’ hypothesis: seagrass productivity decreases with increasing latitude, and this decrease is linked to the level of light exposure. In Bermuda, seagrass beds are therefore more vulnerable, and the effects of grazing are all the more negative the more intense the cutting. The most southerly grasses are the most resistant.
This is because the plants are exposed to more sunlight and can therefore photosynthesise more than their northern cousins. This allows them to store more carbohydrates, reserves that they use when they are overgrazed. Carbohydrate reserves declined at all the sites studied,’ add the authors, ’but high-latitude grasslands were more affected, given that their reserves were already at fairly low levels, and declined to near-zero levels at some sites (Bermuda), when grasses were subjected to sustained grazing.’
Limiting pollutants and turbidity to help the resilience of marine plants
One of the study sites, near the coast of Panama, where the water is very turbid due to dredging activity, confirms the role of light: although located at low latitudes, the Thalassia testudinum meadows showed little resilience to the simulated grazing. Turbidity prevents light from penetrating well and the plants from photosynthesising properly.
As for the environmental fertilisation experiment, it showed that in an environment rich in nitrogen and phosphorus nutrients, the meadows were less resilient to intensive grazing, because they would then produce fewer carbohydrate reserves.
To study plant growth, the divers recovered samples of shoots from the plots defined. © Maggie Johnson
‘If we want to give these grasslands the best chance of resisting the anticipated increase in grazing, we need to guarantee them access to light’, concludes Justin Campbell, lead author and marine biologist at Florida International University. In particular, by limiting the human activities that cause high levels of water turbidity and inputs of nitrogen and phosphorus. This would not only guarantee the survival of the turtle grass, but also the entire ecosystem that depends on it.
Source : Campbell, J.E., Kennedy Rhoades, O., Munson, C.J. et al. Herbivore effects increase with latitude across the extent of a foundational seagrass. Nat Ecol Evol 8, 663–675 (2024). https://doi.org/10.1038/