The same is true, explains the researcher, of seasonal variations: it is hardly surprising that litter is more abundant in the Autumn and in the Winter, i.e. when P. oceanica sheds its leaves, than in the Spring and in the Summer. However, while these seasonal variations do have an influence on the number of invertebrates living in the litter, have virtually no effect on their diversity. There is another type of phenomenon observed in the P. oceanica litter that is much more random, intense, and limited in time: ‘resource pulses’. Resource pulses occur during storms and gusts of wind, and can have a significant structuring role on the invertebrates living in the litter. When strong currents stir up the litter deposits and remove dead leaves (up to 99% of the litter), some organisms follow these dead leaves or migrate to a different environment and might eventually return at a later point. Others, however, are strongly attached to the litter and choose to remain in the few remaining accumulations. ‘In other words, they would rather remain in a harsher environment with a higher density of organisms and, therefore, more competition. This is probably because they would be unable to survive optimally in other environments.’ Such is the case of amphipod Gammarella fucicola: this small crustacean is present, as mentioned previously, in layers of the litter that have both high and low oxygen concentrations, and it is among the organisms that, when many dead leaves are removed, prefer to stay in what is left of the litter. ‘A number of hypotheses can be suggested to explain this: this species may have lower chances of survival in other environments and would not entirely belong, for instance, in the meadow itself, only a few metres from the accumulated dead leaves – it is not found in the meadow itself, but only in its litter –, or it may be more advantageous for Gammarella fucicola to temporarily live with increased competition with other gammarellidae or with otheramphipods, as dead leaves always eventually return, rather than spend considerable energy migrating to another environment. In any case, Gammarella fucicola seems to be remarkably well adapted to living in the litter.’

Looking at stomachs under the microscope

An inventory of the community of invertebrates living in the litter should, according to François Remy, immediately be followed by an analysis of their diets. Could a study of these diets corroborate the hypothesis that litter-consuming invertebrates are responsible for transferring the energy stored during P. oceanica’s leaf production to all coastal food chains in the Mediterranean? After all, P. oceanica litter is not limited to dead leaves: it also includes over potential food sources such as algae, but also epiphytes (bryozoans, hydrozoans, bacteria, fungi, microalgae, etc.) that grow on the surface of P. oceanica leaves. So François Remy spent months analysing the stomach content of some 560 microorganisms and, by comparing the results with analyses of stable isotopes of carbon, nitrogen and sulphur, attempted to determine the general diet of 19 species out of the 115 that were identified initially. This allowed him to observe, in addition to unexpected content such as textile fibre (read Microplastics in fish stomachs), the existence of trophic levels, that is to say several levels within a single trophic network (a series of food chains): primary consumers such as Gammarella fucicola, omnivores such as Nebalia strausi, carnivores such as Liocarcinus navigator, a small crab, or Palaemon xiphias, a shrimp, and even possible top predators who prey on carnivores, such as young fish in the Gobius genus. However, each of these trophic levels seems to have its own dietary preferences: some primary consumers such as G. fucicola are mixed consumers – they ingest a combination of algae, epiphytes, and dead leaves – while others have more of a detritivore diet, eating essentially dead leaves (e.g. Gammarus aequicauda, 60% of whose diet is dead leaves). ‘Since predators consume these invertebrates, and since top predators then eat those predators, it is easy to understand how the organic matter from dead leaves is transferred from the litter to top predators through these “litter consumers”. Litter therefore appears to be a non-negligible source of food for all coastal ecosystems in the Mediterranean, even though P. oceanica meadows contain other sources of food that can be assimilated more easily.’

(1) Characterization, dynamics and trophic ecology of macrofauna associated to seagrass macrophytodetritus accumulations (Calvi Bay, Mediterranean Sea), Thèse de Doctorat 2016

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