Sea warming caused by climate change poses severe threats to Mediterranean marine ecosystems. Here, meadows of the seagrass Posidonia oceanica are among most threatened habitats. While the effects of sea warming on P. oceanica leaves chemistry are increasingly made clear, the warming effects on the below-ground sedimentary compartment of seagrass meadows remain poorly assessed, also due to poorly known patterns of seasonal variations.

To provide insights on the seasonal fluctuations in quantity, biochemical composition and degradation of organic matter (OM) in the below-ground compartment of two P. oceanica meadows in the surroundings of the Ischia Island, we collected sediment samples in summer and autumn at Lacco Ameno (LA) and Castello (CA). Samples were analyzed to quantify protein, carbohydrate, lipid, and phytopigment contents, and to evaluate OM ageing (protein-to-carbohydrate ratio), nutritional quality (algal fraction of biopolymeric C), extracellular enzymatic activities, C degradation rates and turnover times.

At both stations, below-ground OM contents, but carbohydrates, were lower in autumn than in summer. Phytopigment contents did not show any change, while the algal fraction of biopolymeric C and the protein-to-carbohydrate ratio resulted higher in LA in autumn, suggesting a fresher and more labile nature of organic loads. OM biochemical composition differed significantly between the two seasons in both stations, with seasonal differences mostly explained by lower lipid contents in autumn. Extracellular enzymatic activities and C degradation rates were significantly higher in LA in both seasons, and varied seasonally only in CA, where values were significantly higher in autumn.

Though limited to two seasons only, our results suggest that temperature seasonal variations in the seagrass below-ground, as well as local variations, can  largely influence OM quantity, nutritional quality, biochemical composition and C degradation rates.

This study is part of the project BORIS (P2022R739J; CUP F53D23008230001) funded by the PRIN PNRR 2022 program.