Researchers investigated what happens to the material generated by Posidonia seagrass meadows. Conducted in the Mediterranean at STARESO, the study found that the fallen leaves of Neptune grass gather in shallow waters, where they decompose similarly to compost, aiding in the remineralization of organic matter. This process has previously been overlooked regarding its impact on carbon exchanges in Mediterranean coastal zones. Interestingly, while CO2 emissions were noted, oxygen production was also observed. This oxygen output is associated with photosynthetic organisms found within this marine compost, setting it apart from land-based compost.
Researchers at the University of Liège (BE) explored the fate of the material from Posidonia seagrass meadows. This investigation, conducted in the Mediterranean Sea at STARESO, reveals that dead leaves, commonly referred to as Neptune grass, accumulate in shallow regions and decompose like compost, which helps to remineralize organic matter. This has significant implications for carbon dynamics in Mediterranean coastal habitats that were previously underestimated. Unexpectedly, alongside CO2 emissions, they also detected oxygen production. This is attributable to photosynthetic organisms residing within the marine compost, distinguishing it from terrestrial compost.
Posidonia, a flowering plant that symbolizes the Mediterranean Sea, also known as Neptune grass, creates expansive meadows (underwater prairies) in shallow waters (less than 40 m deep). “It is a terrestrial plant that re-adapted to marine conditions millions of years ago, a curious twist of evolution,” explains Alberto Borges, an oceanographer at ULiège. “Like other terrestrial plants, Posidonia loses its oldest leaves each autumn. These fallen leaves form litter (similar to what collects at tree bases) in substantial clusters near the seagrass meadows.” Researchers were intrigued by how these dead leaves accumulate, decompose, and transform, prompting their trip to STARESO, an underwater and oceanographic research center in Calvi, Corsica, to study the primary production and degradation of organic matter in Posidonia litter.
“In the litter, organic matter decomposes and releases nutrients and CO2, resembling the compost used in gardens,” states Gilles Lepoint. “The litter builds up in open, sunlit areas. Every gardener understands that plants require nutrients and light to flourish. This understanding directed our study, which led to a surprising initial discovery: in the litter formed from what one might initially regard as dead and lifeless materials, we recorded oxygen production, resulting from the photosynthetic activity of macroalgae washed from rocks, living Posidonia shoots that detached from the meadow, and diatoms (tiny algae) found in the litter.”
In summary, in this nutrient-rich setting, all living plants associated with the litter flourish and photosynthesize. While the oxygen production is notable, it does not counterbalance the oxygen consumed during the breakdown of dead leaves. Thus, these accumulations are net consumers of oxygen, and therefore net emitters of CO2, similar to compost and litter in terrestrial systems.
The second finding of this study surprised the researchers. “We initially believed that Posidonia litter broke down relatively swiftly, but we found the opposite, based on litter mass loss measurements—it degrades more slowly,” shares Alberto Borges. “We measured respiration through short-term (1-day) incubations utilizing very precise oxygen assessments.” These measurements yielded a more realistic estimate with lower values than those typically gathered via monitoring mass loss over extended periods (several months). This outcome could alter the current carbon balance estimations for these ecosystems, which have relied on conventional mass loss measurements.
During this study, the researchers also looked at the primary production and degradation of organic matter from the macroalgae growing on nearby rocks adjacent to the Posidonia meadows. “We suspected there could be interactions between the two systems, which might initially seem separate and isolated. Once again, we obtained an unexpected finding,” remarks Willy Champenois with enthusiasm. “These macroalgae, despite undergoing photosynthesis, were net consumers of oxygen instead of net producers! This suggests that the communities of bacteria and invertebrates residing within the algae consume more organic matter than the algae produce. This indicates that the excess organic matter must originate from an external source.” By calculating a mass balance, the researchers concluded that this surplus organic matter likely comes from Posidonia as dissolved organic molecules flowing from the seagrass meadow and litter to the rocks.
In conclusion, there exists a reciprocal exchange between the macroalgae on the rocks and the Posidonia meadows. The macroalgae drifting from the rocks may accumulate in the Posidonia litter and contribute to primary production there. Conversely, the seagrass is capable of supplying organic molecules that diffuse to the rocks and are assimilated by the bacterial communities associated with the macroalgae. A truly mutually beneficial relationship!
This study offers new perspectives on quantifying and understanding the organic carbon balance of Posidonia seagrass meadows in the Bay of Calvi, an area that has been a focus of study for oceanographers and marine biologists at the University of Liège since the 1980s, particularly through the STARESO marine research station.
