Model reveals role of pungent gas in seagrass ring patterns

Seagrasses play a vital role in the coastal ecosystem, acting as carbon sinks. In certain conditions, they form interesting patterns that have not been fully explained. For example, circular bands of seagrass vegetation have been observed in the Mediterranean Sea and in other locations, where they move a few centimetres each year. Hydrogen sulphide has been proposed as a possible cause of this phenomenon: it derives from the bacterial decomposition of dead plants, and can also be introduced by human pollution. 

A research team, including marine ecologist, Carlos Duarte of King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, derived a mathematical model to simulate the growth patterns of the seagrass Posidonia oceanica in the presence of hydrogen sulphide. The model takes into consideration seagrass branching rate, its mortality, which increases with sulphide concentration, and the plant sensitivity to sulphides. The model showed that a dense-enough round patch of seagrass can experience decay in its centre, where sulphides accumulate, and growth at its edges. As a result, the round patch becomes a ring that moves outward and can break down into spirals. Bare soil with a high concentration of sulphide is left in the ring’s centre. 

The simulated patterns are consistent with aerial photos and samples of seagrass and sediment taken on the shores of Mallorca, Spain over the last 50 years. The model also simulated the merging of two expanding seagrass rings, which was observed in the most recent photos from 2021. 

“These findings are now ready to be applied to diagnose the state of endangered meadows or to design strategies to maximize the success of restoration projects. Such a mechanism is likely to be present also in many other ecological contexts and species, so the results of this work can be extended to other ecosystems,” says Damià Gomila from the Universitat de les Illes Balears (Spain), who led this study. 

“This work goes a step further than most ongoing work in the area because it empirically tests some of the pattern-forming feedbacks assumed by the model. It also includes the patterns’ temporal dimension, which is often neglected in many studies due to the impossibility of observing ecosystems over a long timescale,” says Ricardo  Martínez-García from the Center for Advanced Systems Understanding (CASUS) in Germany, who was not involved in this study. 

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