P. oceanica (Posidonia oceanica) is a flowering plant that lives in abundant meadows covering the seabed along the Mediterranean coast. Its considerable influence on Mediterranean coastal ecosystems is well known, and is being studied by several researchers of the University of Liège from it’s oceanographic research station (STARESO) in Calvi, Corsica. During the four years that François Remy spent working on his PhD thesis(1) at ULg’s oceanology laboratory, he spent nearly one year on STARESO where he explored – both by diving and conducting laboratory analyses – an aspect of these meadows that has not yet been studied much: the role of their litter as a habitat and a food reserve for the communities of invertebrates that inhabit it. This study is unprecedented in scale.

‘P. oceanica meadows are characterised by a very high annual primary production, comparable to that of certain tropical forests. As is the case in forests, P. oceanica leaves – whose average length is around 75 cm – wither and fall off in the Autumn, then grow back again in the Spring,’ explains François Remy. ‘Another particularity of this plant is that its leaves, which are both dense and in high numbers, fall off when they are still mostly untouched by the local herbivore organisms. These dead leaves, or macrophytodetritus, then become stuck in the meadow and decompose there, creating what is known as a litter. The litter sometimes remains inside the meadow, and is sometimes massively pushed out by waves towards unvegetated areas – often sand patches – where it accumulates. These are called exported litters.’ It is no coincidence that P. oceanica leaves are hardly ever consumed live: they contain toxic and repellent compounds that prevent them from being eaten by herbivores and make them edible only to some species of fish and urchin. ‘However, when the leaves fall off, the toxic compounds are released from the dead cells and dissolve in the water. As a result, they are not present in dead P. oceanica leaves.’

While the dead leaves can then be assimilated, they have lost many of the nutrients they once contained, either because they also dissolved in the water or because the plant reabsorbed them just before shedding each leaf (in a process known as remobilisation). ‘This is similar to how trees retrieve nutrients from their leaves before the Autumn, in order not to waste what they have spent so much time producing.’ So these accumulations of dead P. oceanica leaves are not toxic, yet largely also non-nutritious; still, they are sought after by many detritivore micro-organisms: fungi, bacteria, and microalgae immediately start degrading the leaves and take advantage of what few nutrients are still present. These microorganisms are then consumed by a series of invertebrates who, while they did not have to digest dead P. oceanica leaves themselves, directly benefit from them as they consume the microorganisms that digested the leaves. P. oceanica litter is therefore a much more significant ‘detrital compartment’ than it seems, and not just a habitat. ‘They are not just dead leaves.’François Remy even speculates that this organic matter and the detritivores that consume it allow foliar organic matter produced by P. oceanica to be transferred to all food chains on the Mediterranean coast.

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