Abstract. The ecosystem associated to the Mediterranean seagrass Posidonia oceanica shows a clear distinction in two subcompartments regarding turnover time: aboveground and belowground. Aboveground parts (leaves) are highly dynamic, and most of the leaf material is decomposed or exported in less than one year, representing a net loss of nutrients. In contrast, belowground biomass (roots and rhizomes) has a turnover time of the order of centuries, with a consequent accumulation of organic matter in the sediment. The accumulation rates for the single elements rank in the order C > N > P. This ecosystem may be considered as a sink for biogenic elements.

Abstract. Biotic and abiotic factors influence the accumulation of organic carbon (Corg) in seagrass ecosystems. We surveyed Posidonia sinuosa meadows growing in different water depths to assess the variability in the sources, stocks and accumulation rates of Corg. We show that over the last 500 years, P. sinuosa meadows closer to the upper limit of distribution (at 2–4 m depth) accumulated 3 to 4-fold higher Corg stocks (averaging 6.3 kg Corg m−2) at 3 to 4-fold higher rates (12.8 g Corg m−2 yr−1) compared to meadows closer to the deep limits of distribution (at 6–8 m depth; 1.8 kg Corg m−2 and 3.6 g Corg m−2 yr−1). In shallower meadows, Corg stores were mostly derived from seagrass detritus (88 % in average) compared to meadows closer to the deep limit of distribution (45 % on average). Also, sediment accumulation rates and fine-grained sediment content (< 0.125 mm) in shallower meadows (2.0 mm yr−1 and 9 %, respectively) were approximately 2-fold higher than in deeper meadows (1.2 mm yr−1 and 5 %, respectively). The Corg stocks and accumulation rates accumulated over the last 500 years in bare sediments (0.6 kg Corg m−2 and 1.2 g Corg m−2 yr−1) were 3 to 11-fold lower than in P. sinuosa meadows, while fine-grained sediment content (1 %) and seagrass detritus contribution to the Corg pool (20 %) were 8 and 3-fold lower than in Posidonia meadows, respectively. The patterns found support the hypotheses that Corg storage in seagrass soils is influenced by interactions of biological (e.g. meadow productivity, cover and density), chemical (e.g. recalcitrance of Corg stocks) and physical (e.g. hydrodynamic energy and sediment accumulation rates) factors within the meadow. We conclude that there is a need to improve global estimates of seagrass carbon storage accounting for biogeochemical factors driving variability within habitats.

Abstract. Biotic and abiotic factors influence the accumulation of organic carbon (C org ) in seagrass ecosystems. We surveyed Posidonia sinuosa meadows growing in different water depths to assess the variability in the sources, stocks and accumulation rates of C org . We show that over the last 500 years, P. sinuosa meadows closer to the upper limit of distribution (at 2-4 m depth) accumulated 3-to 4-fold higher C org stocks (averaging 6.3 kg C org m −2 ) at 3-to 4-fold higher rates (12.8 g C org m −2 yr −1 ) compared to meadows closer to the deep limits of distribution (at 6-8 m depth; 1.8 kg C org m −2 and 3.6 g C org m −2 yr −1 ). In shallower meadows, C org stocks were mostly derived from seagrass detritus (88 % in average) compared to meadows closer to the deep limit of distribution (45 % on average). In addition, soil accumulation rates and fine-grained sediment content (< 0.125 mm) in shallower meadows (2.0 mm yr −1 and 9 %, respectively) were approximately 2-fold higher than in deeper meadows (1.2 mm yr −1 and 5 %, respectively). The C org stocks and accumulation rates accumulated over the last 500 years in bare sediments (0.6 kg C org m −2 and 1.2 g C org m −2 yr −1 ) were 3-to 11-fold lower than in P. sinuosa meadows, while fine-grained sediment content (1 %) and seagrass detritus contribution to the C org pool (20 %) were 8-and 3-fold lower than in Posidonia meadows, respectively. The patterns found support the hypothesis that C org storage in seagrass soils is influenced by interactions of biological (e.g., meadow productivity, cover and density), chemical (e.g., recalcitrance of C org stocks) and physical (e.g., hydrodynamic energy and soil accumulation rates) factors within the meadow. We conclude that there is a need to improve global estimates of seagrass carbon storage accounting for biogeochemical factors driving variability within habitats.

Molecular composition of plant parts and sediment organic matter in a Mediterranean seagrass (Posidonia oceanica) mat.Aquatic Botany http://dx.doi.org/10. 1016/j.aquabot.2016.05.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Graphical abstract Highlights-Analytical pyrolysis applied to Posidonia seagrass organs and marine mat deposit -Posidonia phenolic composition dominated by p-hydroxybenzoic acids -Peat-like mat deposits composed predominantly of root, rhizome and sheath materials -We suggest a link between Posidonia chemistry and C accumulation in mats AbstractPosidonia oceanica forms extensive peat-like deposits (mats) in Mediterranean coastal waters, which have a potential as carbon sinks and archives of environmental change.Nonetheless, the organic chemistry of both P. oceanica plant materials, as well as the environmental and diagenetic effects on the composition of its detritus, is poorly understood.We analyzed plant organs of P. oceanica and the coarse organic matter from a mat core spanning 750 yrs using pyrolysis techniques (PY-GC-MS and THM-GC-MS) to improve our understanding of their molecular properties and their preservation upon mat development. It