The necromass of the Posidonia oceanica seagrass meadow: fate, role, ecosystem services and vulnerability

Posidonia oceanica is a seagrass endemic to the Mediterranean Sea. Most of the primary production of the P. oceanica meadow is not directly consumed by herbivores and plays a role as dead rhizomes and roots, dead leaves and drift epibionts (hereafter necromass). The fate of this necromass is (i) sequestration within the matte, (ii) consumption by detritus-feeders within the meadow, (iii) export towards other marine ecosystems, where it constitutes a source for food webs, (iv) export towards beaches, where it constitutes banquettes, reduces the impact of waves and contributes to the beach ecosystem, and (v) export towards the terrestrial dune ecosystem. These five stocks can exchange necromass. The ecosystem services of the P. oceanica necromass are pivotal. For example, the role of P. oceanica banquettes is fundamental in protecting beaches from erosion, and the carbon sequestration within the matte contributes to the mitigation of emissions of CO2. Human impact on each of these stocks can affect the other stocks and their ecosystem services. The removal of banquettes from beaches can have a dramatic negative impact on P. oceanica ecosystem services, including the sustaining of beaches. The erosion of matte due to trawling and anchoring can remobilize the sequestrated carbon stock.