Two research teams from the University of Liège have made some surprising discoveries on the ultimate fate of Posidonia seagrass, or Neptune grass, in the Mediterranean Sea. It is due to research at STARESO, the marine research station ULiège opened in Calvi, Corsica. Dead Posidonia leaves sink in shallow waters where they increasingly degrade: such changes impact carbon fluxes in coastal environments. Traditionally considered to break down rapidly, the leaves also discharge nutrients and CO2, much like a compost on land. However, the research proved that Posidonia litter breaks down much slower than previously determined, forcing the current views on carbon balance in these ecosystems into reevaluation.
Surprisingly, the study also showed that the photosynthetic organisms in the form of macroalgae, living Posidonia shoots, and the diatoms occupying the litter itself perform their photosynthesis within the litter. Although this oxygen production is quite important, still, it does not compensate for the oxygen that is consumed by the decomposition of dead leaves. This means that the litter remains an oxygen consumer and a carbon dioxide emitter.
The research team of Willy Champenois, Gilles Lepoint and Alberto Borges of the FOCUS Research Unit guided this study of these accumulations using the insight of precise oxygen measurements through short-term incubations giving a better representation of the slow degradation of litter. This new perspective might alter carbon balance calculations for Posidonia ecosystems and thus is critical to understanding carbon cycles in the Mediterranean.
An interesting feature of the study also was the relationship between macroalgae on adjacent rocks and the Posidonia meadows. Interestingly, although the macroalgae contained chlorophyll and looked like macrophytes that carry out photosynthesis, they were proven to be net consumers of oxygen. It has been observed that consumption of organic matter by microbial communities living within algae is the cause for this oxygen consumption. The scientists concluded that this additional organic material used by these communities must have been supplied from Posidonia meadows in dissolved organic molecules. Such an observation would imply that there is already complex interaction between the seagrass and macroalgae systems, both sharing mutual benefits.
This new research therefore significantly contributes new knowledge to the long-standing studies of Posidonia seagrass meadows at STARESO and underlines intricate interactions which contribute to the Mediterranean coastal carbon cycle.