Ecology of Posidonia oceanica necromass (seagrass berms) in Es Trenc, a semi-exposed beach in Mallorca, Balearic Islands

Abstract

[eng] Posidonia oceanica leaves and rhizomes can reach the shore and accumulate on many Mediterranean beaches forming seagrass berms (also known as banquettes) of up to 2,5 m height, with a demonstrated role in ecosystem services such as shoreline protection and offering suitable conditions (food and shelter) for a large number of species to thrive in. However, little is known about the structural and depositional features of seagrass berms, the spatio-temporal diversity patterns of associated fauna and possible role in nutrient cycles. This study assesses the variation in fauna composition, seagrass berm structures and nitrogen fixation rates in a semi-exposed Mallorcan beach (Balearic Islands, Western Mediterranean) over a year, with fieldwork carried out in December, February and June. Samples of seagrass berm and fauna were collected along perpendicular transects to the shore, (front-shore, mid-shore and back-shore depending on berm size) and at 2 or 3 different depth levels, and associated N2-fixing activities were assessed in leaves following the same procedure and also in sediment samples. Temporal differences in seagrass berm bulk density as well as in berm morphology demonstrate two depositional dynamics for the seagrass berms, either accumulating after storm events during the winter or due to coastal alongshore drift during the summer. This difference in depositional dynamics coupled with the swash morphodynamics could also explain the sediment size variations across seasons. In terms of species composition, no temporal variation was observed, however, presence of a seagrass berm led to a higher species diversity and abundance of fauna. Amphipods (beach hoppers or sand fleas) were the most abundant taxonomic group followed by dipterans (flies and mosquitoes). In particular, the species with the highest abundance were an amphipod, Talitrus saltator (22%) and an isopod, Ligia italica (19%). Additionally, nitrogen fixing activity has been reported for the first time in the dead matter of P. oceanica, with rates comparable to those of seagrass matte living leaves, highlighting the ecological importance of this resource. Thus, all the results obtained in this study confirm the ecological role of these seagrass berms, thus serving as a guide for future experiments and contributing to their future management.