A publication dans Estuarine, Coastal and Shelf Science

Dynamics of Posidonia litter accumulations

Posidonia meadows in the Bay of Calvi have been the focus of research by ULiège scientists since the 1980s. Willy Champenois, Gilles Lepoint, and Alberto Borges (UR FOCUS | ISOTOPY Plateform) present the work, initiated as part of the FNRS research project "Dynamique des accumulations de litière de posidonies" (PDR FNRS), which has led to a new publication in Estuarine, Coastal and Shelf Science.

n addition to algae, the seabed is also covered with meadows (herbariums) of flowering plants (angiosperms) descended from superior terrestrial plants that recolonized the marine environment several tens of millions of years ago. In the Mediterranean, we find Posidonia oceanica (Posidonia or Neptune's grass), established at depths of between 0 and 40 meters along the coast. These meadows flower in spring, produce fruit and lose their leaves in autumn. These dead leaves accumulate in “heaps” known as “litter”, which are coveted by heterotrophic organisms such as fungi, bacteria and invertebrates (worms and crustaceans) that degrade the organic matter over time. Through this degradation, some of the organic carbon in the leaves is released in the form of CO₂, along with nutrient salts. But there are also photosynthetic organisms in this litter in the form of micro-algae (diatoms), macro-algae that have been plucked from nearby rocks, and living posidonia clusters also plucked from the nearby seagrass bed. The macro-algae and posidonia bundles drifted with the currents, washed up and accumulated in the litter. These photosynthetic organisms produce organic matter, while heterotrophic organisms degrade it. It was this duality of production and degradation of organic matter in litter that interested Willy Champenois, Gilles Lepoint, and Alberto Borges (UR FOCUS | Plateforme ISOTOPY), whereas all studies up to now had considered that litter only degraded, and moreover this had been quantified with very approximate methods giving values that we thought to be erroneous and probably not transposable to real conditions. This study, just published in Estuarine Coastal and Shelf Science, proved us right.

The research project (PDR) “Dynamics of Posidonia litter accumulations” began in October 2009 and ended in October 2013. The PhD theses of François Remy and Thibault Mascart (co-diploma with UGent, Prof Marleen De Troch) were carried out within the framework of this PDR and two FRIA doctoral fellowship funding (2011-2015). They focused on the small fauna (between 38 µm and 3 cm) that develops in Posidonia litter. This fauna is less diverse than in living seagrass beds (a few dozen species vs. several hundred species) but much more abundant (10 to 100 times). It is dominated by different groups of crustaceans (copepods, amphipods, isopods and decapods), polychaete annelids and gastropod molluscs. As in other types of litter, amphipods are particularly dominant. This fauna presents diversified trophic ecologies based on the detrital material making up the litter, but also decomposer microorganisms (bacteria, fungi) and living material plucked from adjacent seagrass beds and rocks. We have shown that these diets vary seasonally according to litter inputs.

Any attentive and patient diver will notice that these litters are much frequented by fish, such as goatfish and many labrids. So it's hardly surprising to find the various organisms that inhabit the bedding in the stomachs of juvenile and then adult fish, which actively frequent and forage on this bedding. This creates an essential trophic link between this abundant detrital biomass and the coastal fish community. This is all the more essential as very few animals directly consume living Posidonia leaves in the meadow. Live Posidonia leaves are in fact too “tough”, not easily digestible and not nutritious enough to be consumed directly. This transfer of matter therefore takes place via the detrital food web (i.e. the brown web) rather than via direct consumption of living posidonia (i.e. the green web). Posidonia is like cheese: it has to age to taste good.

As part of Willy Champenois' thesis on carbon fluxes in the Posidonia meadow, a chapter was devoted to primary production and respiration in the litter and macro-algae growing on nearby rocks. This chapter was in draft form when the thesis was submitted in 2021, and sat in the bottom of a (digital) drawer until spring of this year, when we decided to dust it off, finalize it, and publish it.

Willy Champenois' research continued beyond the end of the PDR in 2013, and we collected primary production and respiration data on the Posidonia meadow until 2018. The idea was to describe and quantify year-on-year variations in primary production on the Posidonia meadow, and above all to try to understand the causes. For example, by looking at the response to a heatwave, we can get an idea of the response to future global warming. Over the period from 2006 to 2018, the most marked variation was the exceptionally low primary production in 2007 compared with other years. In autumn, posidonia leaves fall like tree leaves in our forests, accumulating in the litter. In “normal” years, storms in autumn and winter evacuate the litter, which is transported out of the meadow, either to deep water or to the beaches forming the banks. The meadow is thus cleaned. The winter of 2006 was extremely mild in the Mediterranean, with fewer storms, and the litter was not evacuated. The garbage cans weren't collected, so to speak. All this material, which usually doesn't accumulate in the meadow from one year to the next, led to a drop in primary production the following year (2007). Growth was not as good. So, in fact, Willy Champenois's study of the seagrass bed went back to the litter to explain year-on-year variations in seagrass primary production.

Primary production is the production of organic matter by plants, based on photosynthesis, which is the transformation of carbon dioxide (CO₂) into simple sugar molecules using light energy. Other plant molecules (lipids, proteins, DNA) are then made from these sugars supplied by photosynthesis. These sugars also serve as a source of energy to power the cellular machinery. The first stage of photosynthesis involves a whole range of molecular machinery (including chlorophyll) that transforms light energy (photons) into chemical energy. It's during this stage that oxygen (O₂) is produced, in fact, as a waste product. Once chemical energy is available, it is used to transform CO₂ into sugar, corresponding to the second stage of photosynthesis, which takes place elsewhere in the cell.

In fact, all the oxygen present on the planet (20% of all gases in the atmosphere, after all) has been produced as “waste” by photosynthesis, accumulating over billions of years. The first photosynthetic micro-organisms appeared 2.5 billion years ago, in the form of bacteria equipped with photosynthetic pigments, and still present today in the form of blue-green algae, cyanobacteria, incredible “living fossils”. However, life appeared on Earth 3.8 billion years ago. So the first micro-organisms lived without oxygen for over a billion years. That's a very long apnea, when you consider that exceptional athletes can only hold their breath for 10 minutes, and that the vast majority of human beings can only hold their breath for 2 minutes!

Oxygen is also the most powerful oxidizing agent on Earth, destroying organic matter and extracting energy from it. Respiration is what the vast majority of organisms on Earth do, with the exception of a few ultra-specialized micro-organisms that can destroy organic matter in the absence of oxygen using other oxidants such as nitrate and sulfate.

Oxygen is therefore a waste product of primary production and is the oxidant for the degradation of organic matter (respiration). It is on this duality that we can quantify primary production and respiration by looking at variations in oxygen in the water, since oxygen release is proportional to primary production, and oxygen consumption is proportional to respiration. The advantage of oxygen is that it can be measured very precisely but relatively simply. We could also have measured changes in carbon dioxide (CO₂), since primary production assimilates CO₂, by definition, and respiration releases it. But CO₂ measurements are much more complicated to perform with the required precision, and CO2 in seawater is also affected by calcification (biological production of calcareous structures such as skeletons), so this requires the additional measurement of other chemical components to be able to correct CO₂ measurements for the effect of calcification. In short, why make it complicated when you can make it simple? You might as well measure oxygen in a simple way rather than CO₂ in a much more complicated way.

The other side of the coin is that oxygen concentrations have to be measured at very precise times during the 24-hour dive. At dawn, at sunset and again at dawn the following day. Then all the samples have to be measured the following day. That's a lot of work over two days. We were helped by sensors that measured oxygen autonomously, but as we weren't sure of their reliability, we still took manual samples and then measured them in the laboratory. We did, however, prove their reliability, which should simplify the future work of other researchers.

The study was carried out in the Posidonia meadow in the Bay of Calvi, which is considered a reference site, and is in continuity with the Revellata marine nature reserve. We worked from the Station de Recherches Sous-Marines et Océanographiques (STARESO), which was founded in 1972 by the University of Liège. ULiège researchers such as Daniel Bay, Albert Distèche, Jean-Marie Bouquegneau, Patrick Dauby and Michel Frankignoulle carried out pioneering research into the ecology, ecotoxicology and CO₂ fluxes in seagrass beds. Gilles Lepoint and Alberto Borges belong to the third generation of Liège researchers to have worked at STARESO, their thesis having been supervised by Jean-Marie Bouquegneau, Patrick Dauby and Michel Frankignoulle. The research presented here was carried out as part of the doctoral theses of the fourth generation of researchers (François Remy, Thibault Mascard, and Willy Champenois).