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 m 2 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.

Vertical intertidal variation of organic matter stocks and patterns of sediment deposition in a mesotidal coastal wetland

“…Specifically, we aimed at providing an empirical function model to estimate the OM superficial sedimentary stocks, as a proxy for blue carbon stocks, in Ria Formosa lagoon (South Portugal) based on those drivers. We hypothesise that organic matter stocks will vary spatially at two dimensions: horizontally along a flow velocity gradient (as previously shown by Santos et al, 2019, Martins et al, 2021, with larger stocks at low flow velocity sites, and vertically, with bed elevation, hydroperiod, and vegetation composition, expecting the lower the bed elevation, the larger the stocks within the same vegetation type. The OM stocks were quantified along the vertical distribution of the two co-occurring intertidal vegetated habitats, the lower intertidal seagrass Zostera noltei and upper saltmarsh Sporobolus maritimus, in sites along a horizontal current flow velocity gradient.…”

“…Four sampling sites (S1 to S4, Fig. 1B) were selected along a wellstudied gradient of flow current velocity, a key factor determining sedimentary organic matter and organic carbon stocks in the Ria Formosa (Santos et al, 2019;Martins et al, 2021). The gradient was characterised by an increase in the relative frequencies of low velocities (0-0.2 m s − 1 ) from S1 to S4, and a decrease in the frequency of high velocities (0.2-0.6 m s − 1 ) from S1 to S4 (Santos et al, 2019), based on a validated process-based model of depth-average current velocities for the Ria Formosa lagoon (Carrasco et al, 2018).…”

“…The model was validated against field measurements of water depth at two points along the S3 transect (points 0 and 30 m), measured with a water level logger (Solinst ® Levelogger and In-Situ level troll) at 4 Hz, during two tidal cycles, from 28 th to 29 th March. Full model explanation, script, data and validation are available at a software repository (de los Santos and Martins, 2021).…”

“…The spatial variation in the sedimentary organic carbon stocks in seagrass and salt marshes can be controlled by a variety of biophysical drivers at the regional level (Ouyang and Lee, 2014;Mazarrasa et al, 2018;Ewers Lewis et al, 2020;Ricart et al, 2020;Novak et al, 2020). When these ecosystems are present in tide-controlled systems, common drivers are vegetation habitat composition, hydrodynamics, tidal range, and bed elevation (Ouyang and Lee, 2014;Kelleway et al, 2017;Zhang et al, 2017;Santos et al, 2019;Jiménez-Arias et al, 2020;Lima et al, 2020;Martins et al, 2021). These environmental drivers can be significant when comparing different wetlands (e.g., wetlands subjected to different tidal regimes), but also within the same system if they vary substantially in space.…”

“…Sedimentation patterns in tidal marshes can be controlled by a series of biophysical factors with a high spatial variability, both horizontally and vertically (e.g., Cahoon and Reed, 1995;French et al, 1995;Leonard, 1997;Butzeck et al, 2015;Kelleway et al, 2017). The intensity of the horizontal flow velocity, the turbulent energy of the flow, and the particle settling velocity determines if the sediment is transported or if it settles down (Christiansen et al, 2000 and references therein), influencing the organic carbon stock and sequestration rate in intertidal areas (Santos et al, 2019;Martins et al, 2021). The tidal marsh vegetation can also influence the settling of the suspended particles and prevent further sediment resuspension of the particles trapped (Leonard and Luther, 1995;Leonard et al, 1997;Nepf et al, 1997;Christiansen et al, 2000;Kelleway et al, 2017).…”