Posidonia, an emblematic flowering plant of the Mediterranean Sea, commonly called Neptune’s grass, forms vast seagrass beds (underwater meadows) in shallow waters (less than 40 m). “This is a plant of terrestrial origin that recolonized the marine environment several million years ago—a little whim of Evolution,” explains Alberto Borges, an oceanographer at ULiège. “Like the majority of terrestrial plants in our regions, in the fall, the Posidonia loses its oldest leaves. These dead leaves accumulate in the form of litter (like at the foot of trees) in large patches near the seagrass beds.”
It is these piles of dead leaves and their degradation and transformation that interested Willy Champenois, Gilles Lepoint, and Alberto Borges—all who are researchers at the FOCUS Research Unit (ISOTOPY Platform)—who went to STARESO, the underwater and oceanographic research station of ULiège located in Corsica in Calvi, to conduct a study on the primary production and degradation of organic matter in the Posidonia litter.
“In the litter, organic matter is degraded and releases nutrients and CO2, like compost in gardens,” explains Gilles Lepoint. “Litter accumulations form in open and, therefore, bright areas. Every gardener knows that to grow plants, you need nutrients and light. This is the premise on which we based our study, which led to a surprising first result: in the litter resulting from the accumulation of material that we imagine a priori to be dead and inert, we measured oxygen production, a consequence of the photosynthetic activity of macroalgae torn from the rocks, bundles of living Posidonia detached from the neighboring seagrass and diatoms (microscopic algae) present in the litter.”
To summarize, in this environment rich in nutrients, all the living plants associated with the litter are doing well and are photosynthesizing. This production of oxygen is significant but does not compensate for the consumption of oxygen by the decomposition of dead leaves. These accumulations, therefore, remain net consumers of oxygen and, consequently, net emitters of CO2, such as compost and litter, from terrestrial environments.
The second result obtained in this study somewhat surprised the researchers. “While we thought that the Posidonia litter degraded relatively quickly, this study showed us the opposite, starting from measurements based on the mass loss of the litter, it degrades more slowly,” continues Alberto Borges. “We measured respiration by short-term incubations (1 day) based on very precisely measured oxygen monitoring.”
These measurements gave a more realistic and precise estimate, with lower values than the measurements traditionally made by monitoring mass loss over very long periods (several months). A result that could modify the calculations of the carbon balances currently established for these ecosystems, which are based on measurements by mass loss.
Still, within the framework of this study, the researchers were also interested in the primary production and degradation of organic matter of macro-algae that grow on the rocks surrounding the Posidonia meadows. “We considered the hypothesis that there could be exchanges between the two systems, which one could a priori imagine to be separate and compartmentalized. We obtained again an unexpected result,” rejoices Willy Champenois. “These macro-algae, which nevertheless carry out photosynthesis, were at the same time net consumers of oxygen rather than net producers! This means that the communities of bacteria and invertebrates that live associated with the algae consume more organic matter than the algae produce. Which then necessarily implies that this excess organic matter must be brought from outside.” By carrying out a mass balance, the researchers came to the conclusion that this excess organic matter was probably provided by the Posidonia in the form of dissolved organic molecules that diffused from the seagrass and litter to the rocks.
In summary, the exchanges between the macro-algae on the rocks and the Posidonia meadow are two-way. The macro-algae torn from the rocks can accumulate in the Posidonia litter and contribute to primary production. The meadow can, in turn, provide organic molecules that will diffuse to the rocks and will be assimilated by the communities of bacteria associated with the macro-algae on the rocks. Good neighborly relations, all in all!
This study provides new elements for the quantification and understanding of the organic carbon balance of Posidonia meadows in the Bay of Calvi, which has been the subject of research by oceanographers at the University of Liège since the 1980s, thanks in particular to the STARESO marine research station.
