Blue carbon inventories in the warm temperate NE Atlantic seagrass meadows

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

Publication . de los Santos, Carmen B.; Lahuna, François; Silva, André; Freitas, Cátia; Martins, Márcio; Carrasco, A. Rita; Santos, Rui

Tidal coastal wetlands, common home to seagrass and salt marshes, are relevant carbon sinks due to their high capacity to accumulate and store organic carbon in their sediments. Recent studies demonstrated that the spatial variability of this organic carbon within the same wetland system can be significant. Some of the environmental drivers of this spatial variability remain understudied and the selection of the most relevant ones can be context dependent. Here we investigated the role of bed elevation, hydrodynamics, and habitat type (salt marsh and seagrass) on the organic matter (OM) net deposition-resuspension rate and superficial sedimentary stocks (top 5 cm) at the tidal wetlands of the Ria Formosa, a mesotidal coastal lagoon in South Portugal. Results showed that two vectors of spatial variation need to be considered to describe the intertidal sedimentary OM stocks: the bed elevation that imposes a decrease of the hydroperiod and thus the change of habitat from the lower seagrass Z. noltei to the upper saltmarsh S. maritimus, and the horizontal spatial variation along the secondary channels of the lagoon that imposes a decrease in the current flow velocity magnitude. The multiple linear regression analyses, using data from 40 sampling points, explained 59% of the variation of the superficial sedimentary stocks of OM in salt marshes and seagrasses of the Ria Formosa lagoon and revealed that stocks generally decrease with elevation, yet with variation among sites and habitats. It was also found that the decrease of the OM net deposition-resuspension rate with bed elevation was exponential. Our study emphasizes the importance of considering multiple environmental drivers and spatial variation for regional estimations of organic matter (and organic carbon) sedimentary stocks in coastal wetlands.

2022-08-05Journal article

Open access

Sedimentary organic carbon and nitrogen sequestration across a vertical gradient on a temperate wetland seascape Including salt marshes, seagrass meadows and rhizophytic macroalgae beds

Publication . Barrena de los Santos, Carmen; Egea, Luis G.; Martins, Márcio; Santos, Rui; Masqué, Pere; Peralta, Gloria; Brun, Fernando G.; Jiménez-Ramos, Rocío

Coastal wetlands are key in regulating coastal carbon and nitrogen dynamics and contribute significantly to climate change mitigation and anthropogenic nutrient reduction. We investigated organic carbon (OC) and total nitrogen (TN) stocks and burial rates at four adjacent vegetated coastal habitats across the seascape elevation gradient of Cádiz Bay (South Spain), including one species of salt marsh, two of seagrasses, and a macroalgae. OC and TN stocks in the upper 1 m sediment layer were higher at the subtidal seagrass Cymodocea nodosa (72.3 Mg OC ha−1, 8.6 Mg TN ha−1) followed by the upper intertidal salt marsh Sporobolus maritimus (66.5 Mg OC ha−1, 5.9 Mg TN ha−1), the subtidal rhizophytic macroalgae Caulerpa prolifera (62.2 Mg OC ha−1, 7.2 Mg TN ha−1), and the lower intertidal seagrass Zostera noltei (52.8 Mg OC ha−1, 5.2 Mg TN ha−1). The sedimentation rates increased from lower to higher elevation, from the intertidal salt marsh (0.24 g cm−2 y−1) to the subtidal macroalgae (0.12 g cm−2 y−1). The organic carbon burial rate was highest at the intertidal salt marsh (91 ± 31 g OC m−2 y−1), followed by the intertidal seagrass, (44 ± 15 g OC m−2 y−1), the subtidal seagrass (39 ± 6 g OC m−2 y−1), and the subtidal macroalgae (28 ± 4 g OC m−2 y−1). Total nitrogen burial rates were similar among the three lower vegetation types, ranging from 5 ± 2 to 3 ± 1 g TN m−2 y−1, and peaked at S. maritimus salt marsh with 7 ± 1 g TN m−2 y−1. The contribution of allochthonous sources to the sedimentary organic matter decreased with elevation, from 72% in C. prolifera to 33% at S. maritimus. Our results highlight the need of using habitat-specific OC and TN stocks and burial rates to improve our ability to predict OC and TN sequestration capacity of vegetated coastal habitats at the seascape level. We also demonstrated that the stocks and burial rates in C. prolifera habitats were within the range of well-accepted blue carbon ecosystems such as seagrass meadows and salt marshes.

2022Journal article

Restricted access

Seagrasses in coastal wetlands of the Algarve region (southern Portugal): past and present distribution and extent

Publication . Ito, Paula; Martins, Márcio; von Sachsen-Coburg und Gotha, Simon; Santos, Rui; Barrena de los Santos, Carmen

The global biodiversity crisis urges to update the distribution maps of natural habitats, especially those that are in rapid decline and play a key role in ecosystem functioning, such as seagrasses. In Portugal, seagrass meadows have declined substantially since the 1980s and are considered one of the most endangered marine habitats. Here we aimed to 1) compile records on the distribution and area extent of seagrass meadows in the four wetlands of the Algarve region (southern Portugal) with historical occurrence of seagrasses, based on a systematic review, and 2) update the distribution maps and area extent of seagrass meadows in this region using a combination of manual and machine learning mapping techniques. The systematic review showed a general lack of data on the distribution of seagrass meadows in the Algarve, with patchy information at different times and using incomparable mapping techniques. Based on the updated mapping from 2019 to 2023, well-developed seagrass meadows were only found in the Ria Formosa lagoon and the Guadiana estuary. In the Ria de Alvor, some sparce scattered patches of seagrass were observed in the intertidal zone, while no seagrass was found in the Arade estuary. The total extent of seagrass in the Algarve region was estimated at 1231 ha, with nearly all (99 %) occurring in the Ria Formosa, where 85 % was intertidal. Although assessing temporal trends remains challenging due to methodological differences and data limitations, the estimated 20 % decline and ongoing pressures on seagrass meadows in Algarve wetlands underscore the urgent need for continuous, standardized monitoring of their distribution, extent, and condition.

2025-06Journal article

Open access