Carbon and Nitrogen Stocks and Burial Rates in Intertidal Vegetated Habitats of a Mesotidal Coastal Lagoon Ma ´rcio Martins, 1 Carmen B. de los Santos, 1 Pere Masque ´, 2,3,4,5 A. Rita Carrasco, 6 Cristina Veiga-Pires, 6 and Rui Santos 1 * 1 CCMAR - Centre of Marine Sciences, University of Algarve, Faro, Portugal; 2 International Atomic Energy, 4a Quai Antoine 1er, 98000 Principality of Monaco, Monaco; 3 School of Science and Centre for Marine Ecosystems Research, Edith Cowan University, Joondalup, WA 6027, Australia; 4 Institut de Cie `ncia I Tecnologia Ambientals, Universitat Auto ` noma de Barcelona, 08193 Bellaterra, Barcelona, Spain; 5 Departament de Fı ´sica, Universitat Auto ` noma de Barcelona, 08193 Bellaterra, Barcelona, Spain; 6 CIMA – Centre for Marine and Environmental Research, FCT, Universidade Do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal ABSTRACT Coastal vegetated ecosystems such as saltmarshes and seagrasses are important sinks of organic car- bon (OC) and total nitrogen (TN), with large global and local variability, driven by the confluence of many physical and ecological factors. Here we show that sedimentary OC and TN stocks of inter- tidal saltmarsh (Sporobolus maritimus) and seagrass (Zostera noltei) habitats increased between two- and fourfold along a decreasing flow velocity gradient in Ria Formosa lagoon (south Portugal). A similar twofold increase was also observed for OC and TN burial rates of S. maritimus and of almost one order of magnitude for Z. noltei. Stable isotope mixing models identify allochthonous particulate organic matter as the main source to the sedimentary pools in both habitats (39–68%). This is the second esti- mate of OC stocks and the first of OC burial rates in Z. noltei, a small, fast-growing species that is widely distributed in Europe (41,000 ha) and which area is presently expanding (8600 ha in 2000s). Its wide range of OC stocks (29–99 Mg ha -1 ) and burial rates (15–122 g m 2 y -1 ) observed in Ria Formosa highlight the importance of investigating the dri- vers of such variability to develop global blue car- bon models. The TN stocks (7–11 Mg ha -1 ) and burial rates (2–4 g m -2 y -1 ) of Z. noltei were gen- erally higher than seagrasses elsewhere. The OC and TN stocks (29–101 and 3–11 Mg ha -1 , respec- tively) and burial rates (19–39 and 3–5 g m -2 y -1 ) in S. maritimus saltmarshes are generally lower than those located in estuaries subjected to larger accu- mulation of terrestrial organic matter. Key words: Blue carbon; Nitrogen; Seagrass; Saltmarsh; Sediment stocks; Burial rates; Flow current velocity. Received 14 October 2020; accepted 20 May 2021 Supplementary Information: The online version contains supple- mentary material available at https://doi.org/10.1007/s10021-021-0066 0-6. Author contributions: MM: Methodology, Formal analysis, Investigation, Data curation, Visualization, Writing - original draft. CBdlS: Methodol- ogy, Supervision, Investigation, Writing—review and editing. RS: Con- ceptualization, Supervision, Methodology, Funding acquisition, Resources, Writing—review and editing. CV-P: Conceptualization, Supervision, Methodology, Resources, Writing—review and editing. PM: Methodology, Formal analysis, Resources, Writing—review and editing. ARC: Methodology, Funding acquisition, Resources, Writing—review and editing. *Corresponding author; e-mail: rosantos@ualg.pt Ecosystems https://doi.org/10.1007/s10021-021-00660-6 Ó 2021 The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature