Seasonal dynamics of Posidonia oceanica beach-cast wracks in a Sicilian beach: an overlooked system for carbon dynamics

Seagrass detritus accumulates along the coasts forming beach-cast wracks. Being subject to environmental gradients and hydrodynamism, the size and composition of beach-cast wracks may vary in space and time. Also the composition may vary depending on both proximities to and productivity of the adjacent seagrass meadows. Despite their dynamic nature, seagrass beach-casts provide important ecosystem services, by contributing to the nutrient and carbon flow between marine and terrestrial systems, supporting beach biodiversity and trophic webs and protecting the coastline from erosion. However, the knowledge about beach-cast dynamics, ecological role and functions is still fragmented and deserves a greater effort. Here, we aimed to assess the seasonal dynamics of beach-casts in terms of mass, nutrient and carbon content, and composition. The study was conducted in November 2018 and February 2019 in a Sicilian beach (Capo Feto, TP), adjacent to large nearshore Posidonia oceanica meadows. Surficial and deep (0.5-2 m) seagrass beach-cast samples were collected through a handsaw along the seaward side of the wrack, at three equidistant sites (100 m). Beach-cast mass (kg DW m coastline-1) exhibited wide temporal fluctuations, significantly decreasing from November to February, as an effect of the winter sea storms. A higher mass was also evident in the deep layer than in the surficial one only in November, suggesting a higher compactness of deep layers in the highest beach-cast wracks. Accordingly, nitrogen and organic carbon content (kg DW m coastline-1) showed the same pattern revealing a key role of the beach-cast as a nutrient source between adjacent systems. Despite the high seasonal variability, beach cast composition was rather constant over time, showing an overall dominance of seagrass detritus, and an increase in sediment in the deep layers. This indicates a relevant input from the adjacent P. oceanica meadow and confirms the role of banquette as sediment trap