Abstract
Coastal vegetated ecosystems are major organic carbon (OC) and total nitrogen (TN) sinks, but the mechanisms that regulate their spatial variability need to be better understood. Here we assessed how superficial sedimentary OC and TN within intertidal vegetated assemblages (saltmarsh and seagrass) vary along a flow gradient, which is a major driver of sediment grain size, and thus of organic matter (OM) content. A significant relationship between flow current velocity and OC and TN stocks in the seagrass was found, but not in the saltmarsh. OC and TN stocks of the saltmarsh were larger than the seagrass, even though that habitat experiences shorter hydroperiods. Mixing models revealed that OM sources also varied along the flow gradient within the seagrass, but not in the saltmarsh, showing increasing contributions of microphytobenthos (17–32%) and decreasing contributions of POM (45–35%). As well, OM sources varied vertically as microphytobenthos contribution was highest at the higher intertidal saltmarsh (48%), but not POM (39%). Macroalgae, seagrass and saltmarsh showed low contributions. Local trade-offs between flow current velocities, hydroperiod and structural complexity of vegetation must be considered, at both horizontal and vertical (elevation) spatial dimensions, for better estimates of blue carbon and nitrogen in coastal ecosystems.
Highlights
Blue carbon designates the carbon stored and sequestered in marine ecosystems, vegetated coastal systems including seagrasses and saltmarshes[1]
The relative frequency of lower flow current velocities below or equal to 0.2 m s−1 increased from S1 to S4 sampling stations, whereas the relative frequency of higher flow current velocities from 0.2 to 0.6 s−1 decreased from S1 to S4
This indicates that conditions for the settlement of fine grain sizes increase from S1 to S4 as opposed to resuspension conditions
Summary
Blue carbon designates the carbon stored and sequestered in marine ecosystems, vegetated coastal systems including seagrasses and saltmarshes[1]. Despite the good reasons to consider flow current velocity as a key driver of sedimentary carbon storage by coastal vegetated ecosystems, as highlighted in a recent conceptual model to explain carbon storage in seagrasses[15], this relationship has been poorly addressed. Because relevant gradients of flow current velocity occur within the intricate channel system of the lagoon, we investigated how the sediment grain size, OC and TN, and associated sediment properties, vary along a flow gradient in a tidal channel where extensive meadows of the high intertidal saltmarsh Spartina maritima and intertidal seagrass Zostera noltei co-occur. The flow gradient reported refers to the depth-average current velocities along sample sites, predicted by the application of a numerical model recently developed for Ria Formosa lagoon[40]
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