Carbon and Nitrogen Stocks and Burial Rates in Intertidal Vegetated Habitats of a Mesotidal Coastal Lagoon

Abstract

Coastal vegetated ecosystems such as saltmarshes and seagrasses are important sinks of organic carbon (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 intertidal 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 estimate 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 m2 y−1) observed in Ria Formosa highlight the importance of investigating the drivers of such variability to develop global blue carbon models. The TN stocks (7–11 Mg ha−1) and burial rates (2–4 g m−2 y−1) of Z. noltei were generally higher than seagrasses elsewhere. The OC and TN stocks (29–101 and 3–11 Mg ha−1, respectively) 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 accumulation of terrestrial organic matter.