Bare Sediment Research Articles

Erratum - Macrobenthos of lagoon ecosystems: a comparison in vegetated and bare sediments

This corrects the article published on Advances in Oceanography and Limnology 2023;14:11124 doi: https://doi.org/10.4081/aiol.2023.11124

Stable isotopes of nitrate (δ15N and δ18O) as functional indicators of nitrogen processing in constructed wetlands

The effectiveness of nitrogen removal in wetlands relies heavily on the biological processes that control its removal. Here, we used δ15N and δ18O of nitrate (NO3−) to assess the presence and the dominance of transformation processes of nitrogen in two urban water treatment wetlands in Victoria, Australia over two rainfall events. Laboratory incubation experiments were undertaken in both light and dark to measure the isotopic fractionation factor of nitrogen assimilation (by periphyton and algae) and benthic denitrification (using bare sediment). Highest isotopic fractionations were observed for nitrogen assimilation by algae and periphyton in the light, 15ε = −14.6 to −25 ‰ while the 15ε = −1.5 ‰ in bare sediment, consistent with that of benthic denitrification. Transect water samplings of the wetlands showed different rainfall patterns (discrete versus continuous) affect the removal capability of the wetlands. During the discrete event sampling, the observed 15ε of NO3− (an average of 3.0 to 4.3 ‰) within the wetland falls between the experimental 15ε of benthic denitrification and assimilation; coinciding with the decrease in NO3− concentrations, suggesting that both denitrification and assimilation were important removal pathways. Depletion of δ15N-NO3− throughout the whole wetland system also suggested the influence of water column nitrification during this time. In contrast, during continuous rain events, no fractionation effect was observed within the wetland and was consistent with limited NO3− removal. The difference in fractionation factors within the wetland during different sampling conditions suggested that nitrate removal was highly likely limited by changes in overall nutrient inputs, residence time and water temperature which impeded biological uptake or removal. These highlight that consideration of sampling condition is crucial when assessing the efficacy of a wetland in removing nitrogen.

Population structure and habitat assessment for two commercial clam species exploited in small-scale fisheries

Small-scale fisheries play a crucial role in providing food and jobs in local communities worldwide. Nonetheless, their environmental impact remains poorly understood. To assess the effect of different levels of harvesting pressure on clam population dynamics, we selected three areas for study within each of three intertidal shellfish beds (in NW Spain) on the basis of historical harvesting pressure. The abundance (up to 149 ind m−2) of the introduced clam Ruditapes philippinarum of marketable size was much greater than that of the native R. decussatus (up to 20 ind m−2) in all three beds, which is consistent with the low level of recruitment of the native species. Our results suggest that the harvesting pressure did not significantly affect reproduction, which was asynchronous across beds, and that the reproductive period was longer in R. philippinarum than in R. decussatus. Nonetheless, the intertidal system was strongly affected by harvesting, as bare sediment was typical in frequently harvested areas, while sparse or dense patches of the seagrass Zostera noltei occurred in areas where harvesting pressure was scarce or null. The abundance and diversity of non-commercial infaunal species were greatest in unharvested areas. However, commercial clams were not abundant in these areas, possibly due to natural habitat preferences or artificial seeding and movement of clams along the beds. Management plans based on local knowledge of ecosystems are needed to maintain sustainable stocks of R. decussatus and also to limit the effects of harvesting on the systems.

The Ecological Role of Ruppia cirrhosa (Petagna) Grande in a Choked Lagoon

We studied the ecological and functional aspects (such as role in sediment characteristics and plant–animal interactions) of a Ruppia cirrhosa (Petagna) Grande meadow during its vegetative season in the choked Valle Campo lagoon, a sub-basin of the wider Valli di Comacchio, on the Northern Adriatic coast. Sampling campaigns were carried out with a roughly fortnightly frequency in 2017 at two sites, one with the Ruppia meadow and one with bare sediment. Sediment parameters analyzed were microphytobenthic chlorophyll-a, protein, carbohydrate, and lipid content, and total organic matter. The macrobenthos was identified at the lowest possible taxonomic level. Chlorophyll a, as a surrogate of microphytobenthos, showed differences between the two sites, probably mainly related to light intensity; thus, it is expected that the absence of seagrass canopy results in the higher production of microphytobenthos. At both sites, proteins were the dominant class of labile compounds, suggesting that detrital organic matter present at both study sites is of high nutritional quality. The high protein/carbohydrate ratio also suggests the presence of non-aged organic matter. We recorded a total of 18 macroinvertebrate taxa. The Ruppia meadow showed a positive influence on macrofauna abundance, diversity, species richness, and composition of trophic groups. Only the infaunal taxa Capitella capitata and Chironomus salinarius exhibited higher abundance at the bare site. The ecological quality status measured by the M-AMBI index was unsatisfactory everywhere. However, the presence of the Ruppia meadow resulted in index values being consistently higher. The role of this minor seagrass has been proved to be of great importance, improving the nutritional quality of the organic matter in the sediments and, above all, providing new habitats and new niches for a number of benthic macrofauna species.

Macrobenthos of lagoon ecosystems: a comparison in vegetated and bare sediments

The classic paradigm, not always unequivocal though, that seagrass macrobenthic abundance and diversity are greater than those in adjacent unvegetated areas, was tested in a Mediterranean lagoon for which evidence is lacking. We compared the community structure and species composition of macrobenthic assemblages in three distinct areas of the Mistras Lagoon (Sardinia, Italy) dominated by i) the seagrass Cymodocea nodosa (Cym), ii) mixed-macrophyte/detritus (Mix), and iii) unvegetated sediments (Unv). Samplings were conducted in each area twice in spring (April and May) and twice in autumn (October and November) 2010. Multivariate analyses showed significant differences among the three areas and a marked separation between the two vegetated (Cym and Mix) and the unvegetated (Unv) areas. The top discriminating species, indicated by SIMPER analysis, were characterized by direct development lacking free living larvae. They included marine species, e.g. Cerithium lividulum, Microdeutopus gryllotalpa, Loripes orbiculatus and Gammarus aequicauda, at Cym vs Mix, whereas the pair discrimination of the areas Cym vs Unv and Mix vs Unv mostly depended on the marine Abra tenuis, the opportunistic Chironomidae and Capitella capitata, and the brackish Hydrobia acuta. Both vegetated areas showed a higher species number than the unvegetated area. Differently, abundance was higher in Unv than in Cym and Mix in all sampling dates, most notably in spring likely because of reproductive events, due to both opportunistic (C. capitata, chironomids) and halolimnobic (H. acuta, Cerastoderma glaucum) species. Overall, the present study demonstrated the importance of biological factors in structuring the macrobenthic assemblages of the Mistras Lagoon. Habitatforming phanerogams hosted species-rich assemblages, species-specific reproductive cycles and adaptive strategies contributed to drive species colonization and abundance distribution. Both vegetated and unvegetated patches were found to contribute greatly to the local-scale heterogeneity of the habitat, highlighting the importance of coastal lagoons as hotspots for benthic biodiversity. For these reasons, lagoons must be considered major targets for conservation measures.

Where to fish in the forest? Tree characteristics and contiguous seagrass features predict mangrove forest quality for fishes and crustaceans

Abstract Mangroves often support rich fish and crustacean communities, although faunal abundance and diversity show strong spatiotemporal variability. Consistent patterns in mangrove animal communities might be dictated by forest characteristics, by seascape context or by some combination of these factors. Predicting drivers of spatial heterogeneity in mangrove faunal communities can better support the zoning of forests for management purposes, for example by identifying sites important for fisheries nursery provision. We sampled 14 sites within a large (4000 ha) mangrove forest in Kenya, quarterly over a period of 2 years. There were clear and consistent differences in the quality of sites for fish and crustacean abundance and diversity. Forest characteristics (as summarised by the complexity index, CI) and seascape metrics (the presence, area and configuration of contiguous seagrass) were strong predictors of site differences. However, they showed opposite influences on dominant members of the fish and crustacean faunas, with CI correlated negatively with fishes and positively with crustaceans, and seagrass area correlated positively with fishes and negatively with crustaceans. Synthesis and applications. Sites within the same mangrove forest exhibit consistent differences in fish and crustacean abundance. However, the fish and crustacean communities (and particularly dominant species within them) act differently in response to forest and seascape characteristics. Old growth, mature forest, set in a seascape of seagrass patches with bare sediment, was associated with highest crustacean abundance. In contrast, denser smaller trees and seascapes with larger, continuous areas of seagrass correlated better with fish abundance. Zoning for management, as mandated in new Kenyan policy, will need to consider these differences in seascape use between fish and crustaceans.

The Near‐Bed Flow Structure and Bed Shear Stresses Within Emergent Vegetation

AbstractThe structure of the bottom boundary layer (BBL) in aquatic flows influences a range of biophysical processes, including sediment transport, hyporheic exchange, and biofilm formation. While the structure of BBL above bare sediment beds has been well studied, little is known about the complex near‐bed flow structure within aquatic vegetation. In this study, we used high‐resolution laboratory measurements and numerical Large Eddy Simulations to investigate the near‐bed mean and turbulent flow properties within staggered‐ordered emergent vegetation under a wide range of flow conditions and densities. There is strong spatial variability of key near‐bed flow characteristics on the scale of the vegetation elements. Measurement locations that provide single‐point flow characteristics closest to the spatially averaged values were identified. The spatially averaged BBL thickness is influenced strongly by vegetation density. This impact of vegetation density is engendered through its direct control of near‐bed turbulent kinetic energy (TKE), which in turn is negatively correlated with BBL thickness, both locally in a given flow and across the range of flow conditions studied here. A model based on the near‐bed TKE is developed to predict the BBL thickness and, ultimately, the bed shear stress. Model predictions were in close agreement with the experimental and numerical results. These findings provide new insights into the physical links between near‐bed flow variables and therefore contribute to the understanding of some of the complex biophysical processes present in vegetated flows.

Seasonality of methane and carbon dioxide emissions in tropical seagrass and unvegetated ecosystems

Seagrass ecosystems are important carbon dioxide sinks that can sequester carbon for centuries as organic matter in sediment. They are also a major source of methane, a potent greenhouse gas, which limits their carbon sink capacity. However, data are lacking on their methane emission dynamics. Here, we conduct a one-year survey of carbon dioxide and methane concentrations and air-sea fluxes in Red Sea seagrass, mudflat, and coral backreef ecosystems. All ecosystems were sources of methane and carbon dioxide. Methane concentrations were lowest in the reef lagoon. We suggest that lagoons may be a globally important source of greenhouse gases. Methane concentrations were lower in seagrass than mudflat ecosystems at temperatures below 29.2 °C. Seagrass had the highest annual methane air-sea fluxes but the lowest global warming potential in carbon dioxide equivalent due to a decrease in its flux. Hence, seagrasses can help climate change mitigation compared to bare sediments.

Beach-cast and drifting seaweed wrack is an important resource for marine and terrestrial macroinvertebrates in high latitudes

Seaweeds are foundation species across near-subtidal and intertidal zones, including when detached and free-floating and then cast ashore as wrack. Wrack is sometimes removed by humans for aesthetics or to be used as fertilizer. The study of wrack as an important habitat and resource for macroinvertebrates in high latitudes has been limited. To determine which taxa might be impacted when wrack is removed, the composition and relative abundance of macroinvertebrates were quantified monthly and compared in areas with and without wrack in Kachemak Bay, Alaska. Relationships were assessed between macroinvertebrates and wrack line (tidal height, moisture content, seaweed biomass) and beach characteristics (wave exposure, beach slope, substrate types). Approximately 47,000 animals were counted and a total of 87 taxa were identified from beach-cast wrack, drifting wrack, and bare sediment habitats. Macroinvertebrate communities within beach-cast wrack and bare sediment habitats were significantly different. Beach-cast wrack generally had more terrestrially-derived animals, especially Coleoptera and Diptera. Bare beach sediment was predominantly occupied by Enchytraeida (annelids). Macroinvertebrate communities were most strongly influenced by seaweed biomass and tidal height of the wrack line. Beach-cast wrack and bare sediments were also compared to drifting wrack in shallow, nearshore waters. Drifting wrack was different and generally occupied by more marine-derived animals, especially Amphipoda, Gastropoda, Mytilida, and Polychaeta. Ecological succession in decaying beach-cast wrack was documented, with decomposers (e.g., Amphipoda and Diptera) being early colonizers, and predators (e.g., Coleoptera and Hymenoptera) arriving later. Understanding the importance of this unique and ecologically important habitat to macroinvertebrates is essential, as removals and reductions in wrack availability could influence macroinvertebrate community structure, higher trophic level consumers, and key ecological processes on beaches. This study is the first direct investigation into seaweed wrack-associated macroinvertebrate communities in a sub-Arctic system.

River delta eco-morphodynamics under changing scenarios: The case of Lake Turkana, Kenya

In this work we explore the impact that changes in local climate and river hydrology have on the morphodynamics of a river delta, particularly focusing on the evolution of the delta generating at the lake inlet. We investigated the case of the delta in the lower reach of the Omo River in Ethiopia, which flows into lake Turkana, Kenya. The lake is the fourth largest lake and the largest desert lake in the world. This case study is of particular interest because within the last decades three dams have been built within the Omo basin. Among these, the Gibe III dam had a huge impact on the river hydrology and the sediment supply. To quantify changes in land use and river morphology and relate these to climate change and river hydrology we collated a historical dataset by combining information from different satellite sources. We observed that the amount of bare sediments progressively diminished and the biomass became denser compared to the existence of sparse biomass in the past. We argue that this is due to the changes in river hydrology and sediment load imposed by the dam, which increased sediment erosion by deepening the channel. The presence of the dam also increased the low flow level and reduced the peak flow during flood season, reducing the natural oscillations of the water table. In consequence, the possibility to plant for the local population was removed during floods. Our findings also indicate that the hydrology and reduced sediment discharge have changed the downstream (e.g. lake level), and upstream (e.g. mouth bar deposition) boundary conditions controlling the evolution of the delta structure, respectively. As a result, the delta morphology became less dynamic and less complex.

Sedimentary blue carbon dynamics based on chronosequential observations in a tropical restored mangrove forest

Abstract. Among the many ecosystem services provided by mangroves, the sequestration of large amounts of organic carbon (OC) in marine ecosystems (also known as “blue carbon”) has given these unique ecological environments enormous global attention. While there are many studies on the blue carbon potential of intact mangroves (i.e., naturally growing), there have been very few studies on restored mangroves (i.e., planted). This study aims to address this knowledge gap by examining the sediment development process during the early colonization (rehabilitation) of mangroves in an OC-poor estuary in Panay Island, Philippines. Based on source apportionment of multiple end-members in the sedimentary organic matter, the contribution of mangrove plant material was higher at the older sites compared to the younger sites or bare sediments where there is a higher contribution of riverine input. A clear increasing gradient according to mangrove development was observed for bulk OC (0.06–3.4 µmol g−1), porewater OC (292–2150 µmol L−1), sedimentary OC stocks (3.13–77.4 Mg C ha−1), and OC loading per surface area (7–223 µmol m−2). The estimated carbon accumulation rates (6–33 mol m−2 yr−1) based on chronosequence are within the global ranges and show an increasing pattern with the age of mangroves. Hence, the sediments of relatively young mangrove forests appear to be a significant potential C sink, and short-term chronosequence-based observations can efficiently define the importance of mangrove restoration programs as a potential carbon sequestration pathway.

Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass.

Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass.

Microplastic accumulation in Halophila ovalis beds in the Swan-Canning Estuary, Western Australia

Small ephemeral seagrass (Halophila ovalis) beds in the Swan-Canning Estuary, Western Australia, were sampled to determine if microplastics attach to seagrass blades and accumulate in higher concentrations in seagrass sediment compared to bare sediment. Three microplastics were observed attached to sampled seagrass blades (n = 108). Microplastics had a mean concentration in seagrass sediments of 1000 ± 100.37se particles kg−1 and in bare sediment of 972 ± 92.19se particles kg−1. ATR FTIR further verified 64.2 % of subsample particles as plastic (n = 28). This is the first known study to identify microplastics within the leaf canopy of H. ovalis however we could not support our hypothesis that this seagrass species acts as a sink for microplastic particles in sediments, as seen in studies on other seagrass species. The ability for seagrass habitats to trap and accumulate microplastics in sediments is likely influenced by species morphology, seagrass canopy density and life history.

In situ differences in nitrogen cycling related to presence of submerged aquatic vegetation in a Gulf of Mexico estuary.

Estuaries provide a suite of ecosystem services to people but are also under heavy stress from human development including excess nutrient loading and alterations in benthic habitat that affect nutrient cycling. Here we examine the interaction of two important and common ecosystem management priorities in estuaries: limiting eutrophication and restoration of submerged aquatic vegetation (SAV). Rates of benthic nitrogen processing can vary by habitat type and there is need for more complete data on the contribution of SAV to overall nitrogen cycling in estuaries, as well as a need to examine nitrogen cycling in situ to better characterize the role of SAV areal coverage in mediating estuarine eutrophication. We compare nitrogen cycling between two common and adjacent habitat types (SAV and adjacent bare sediment [BS]) in an index coastal estuary using an in situ chamber-based approach to better capture realized habitat differences. We also examined genomic community structure of sediment bacteria and archaea to identify biological indicators of nitrogen exchange. Both mean sediment-water exchange of dissolved N2 and microbial functional community structure differed between SAV and BS. Habitat differences were more consistent with lower variability at locations with low salinity and when sediment organic content was highest, which aligns with findings in other studies. Habitat types differed significantly in microbial composition, including functional groups and genes, like nifH, that may contribute to observed differences in nitrogen cycling. Overall, habitat type appeared most important to nitrogen cycling near the river mouth where sediment nitrogen was higher, and this information has implications for integrated management of habitat restoration/conservation and nutrient loading.

The role of seagrass meadows in the accumulation of microplastics: Insights from a South African estuary

Microplastics are widespread in marine ecosystems, where they threaten biota and human wellbeing. Seagrasses may act as natural filters of microplastics due to their particle trapping abilities, yet little is known about the extent of microplastics in the sediment of seagrass beds. The aim of this study was to compare microplastic accumulation in the sediments of Zostera capensis meadows with adjacent bare sediments at a small spatial scale (~5 km) in the Knysna estuary in South Africa. No significant difference in total microplastic counts were found between seagrass and unvegetated sediments. However, fibre microplastic counts differed significantly between the two locations at which samples were collected. This may be due to pollution levels and proximity to larger human population densities. Importantly, our study reveals variability in microplastic abundance and type even at small spatial scales, an important consideration for the design of future studies aiming to monitor for microplastics.

Habitat-builders complexity boosts associated fauna functional trait richness (Zostera marina meadows, Baltic Sea)

Seagrass has been widely recognized as ecosystem engineers and habitat-builders increasing biodiversity and abundance of associated fauna. However, seagrass habitats are being degraded globally due to modification of coastline, terrestrial inputs, and climate change. While the effects of seagrass meadows vegetation density on associated macrofauna standing stocks and taxonomic richness have been widely investigated, the relationships with functional diversity aspects remain poorly understood. Here, we investigate if macrofauna standing stocks and richness indicators (both taxonomic and functional) are related to the habitat-builders presence and structural characteristics (density, biomass, diversity) in temperate Zostera marina Linnaeus, 1753 meadows (Baltic Sea). Relationships between faunal (density, biomass, species and functional trait composition and diversity), vegetation and sediment indicators were tested based on materials collected by scuba divers. Macrofauna density (ranging from 587.6 ± 188.2 to 4514.1 ± 977.8 ind. 0.1 m−2), biomass (ranging from 43.1 ± 28.3 to 13.6 ± 30.8 g 0.1 m−2) and species richness (ranging from 7 ± 1.4 to 20 ± 1.4) responded positively to seagrass presence regardless of meadows characteristics. Functional trait richness ranged from 203.7 ± 64.3 to 44.7 ± 41.1 and was enhanced only in presence of the greatest macrophyte diversity. Therefore, it is the diversity of meadows that boosts the functional richness of associated fauna. This implies that 1) monospecies seagrass meadows do not necessarily provide a higher number of functions than neighboring bare sediments in systems with low diversity such as Baltic Sea, 2) both taxonomic and functional indicators should be used to describe macrofauna communities. This result is an important message for conservation planning for systems with low diversity, as restoration of at least a few macrophytes species not only the dominant seagrass species could be considered to secure greater taxonomic and functional richness.

Blinded by the bright: How species‐poor habitats contribute to regional biodiversity across a tropical seascape

AbstractAimConservation efforts have traditionally addressed the threat of biodiversity loss by prioritizing regions and habitats with high endemic species richness. However, species‐poor habitats often harbour distinct, valuable and/or functionally unique species that contribute to regional diversity. In the tropical marine realm, the ‘mangrove‐seagrass‐coral reef continuum’ has dominated both the scientific literature and marine spatial planning. Bare sediment habitats are also part of this continuum and often comprise the majority of bottom area, as they do in most regions worldwide, but are understudied and rarely considered in conservation. To address this information gap, we partition the relative contributions of five habitats in the coastal mosaic (forereefs, patch reefs, seagrass, mangroves and bare sand) to fish diversity across a tropical seascape.LocationCarrie Bow Cay, Belize Barrier Reef.MethodsUsing diver visual censuses, we assessed fish community composition at three replicate sites from each of the five habitats annually for 5 years. We then partitioned diversity into within (α) and between (β) habitat contributions, and further partitioned β‐diversity into components attributed to species replacement and richness difference. Finally, we determined the local contributions to β‐diversity (LCBD) by each habitat and identified the key individual species behind these contributions.Main ResultsTropical sand flats contributed uniquely and disproportionately to regional fish species diversity, despite having the fewest species overall. Sand flats also had higher‐than‐expected local contributions to β‐diversity (LCBD) and had the highest percentage of species not found in other habitats.Main ConclusionsOur results demonstrate that previously understudied sand habitats harbour a relatively unique assemblage of species compared with other nearby habitats. Marine spatial planning aimed at maximizing the preservation of total biodiversity (including unique species) in the tropics should consider sand habitats.

Meta-analysis of ecosystem services associated with oyster restoration.

Restoration of foundation species promises to reverse environmental degradation and return lost ecosystem services, but a lack of standardized evaluation across projects limits understanding of recovery, especially in marine systems. Oyster reefs are restored to reverse massive global declines and reclaim valuable ecosystem services, but the success of these projects has not been systematically and comprehensively quantified. We synthesized data on ecosystem services associated with oyster restoration from 245 pairs of restored and degraded reefs and 136 pairs of restored and reference reefs across 3500km of U.S. Gulf of Mexico and Atlantic coastlines. On average, restoration was associated with a 21-fold increase in oyster production (mean log response ratio=3.08 [95% confidence interval: 2.58-3.58]), 34-97% enhancement of habitat provisioning (mean community abundance=0.51 [0.41-0.61], mean richness=0.29 [0.19-0.39], and mean biomass=0.69 [0.39-0.99]), 54% more nitrogen removal (mean=0.43 [0.13-0.73]), and 89-95% greater sediment nutrients (mean=0.67 [0.27-1.07]) and organic matter (mean=0.64 [0.44-0.84]) relative to degraded habitats. Moreover, restored reefs matched reference reefs for these ecosystem services. Our results support the continued and expanded use of oyster restoration to enhance ecosystem services of degraded coastal systems and match many functions provided by reference reefs.

Effectiveness of Moorings Constructed from Rope in Reducing Impacts to Seagrass

Seagrass meadows commonly reside in shallow sheltered coastal environments which are typically safe havens for mooring boats. There is evidence from around the globe that the use of common swinging chain moorings leads to halos of bare sediment in otherwise productive seagrass. These halos reduce animal abundance and diversity and lead to a loss of the carbon stored within sediments. To protect and enhance seagrass ecosystem services, low-cost simple solutions are required that can solve the problems of boating-based disturbance. In the present novel study, we provide evidence that the simple replacement of mooring chains with rope can significantly reduce damage to sensitive benthic habitats such as seagrass. At three locations across a range of environmental conditions, we provide evidence that well-established moorings constructed from rope do not damage seagrass. Overall, there was a significant effect (F1,756 = 299.46, p < 0.001) of the mooring type and distance from the mooring base. This equates to a 44% increase in seagrass cover within areas around a rope mooring relative to a chain one. Most small boat mooring activity happens within the summer months, therefore large heavy-duty winter mooring systems are not required in many situations, opening opportunities for adapted systems that have a reduced environmental impact. The present study suggests that there is a ready-made, low-technology, low-cost solution already in existence for halting the widespread loss of seagrass from small boat mooring damage and allowing recovery and opportunity for restoration.

Sedimentary organic carbon and nitrogen stocks of intertidal seagrass meadows in a dynamic and impacted wetland: Effects of coastal infrastructure constructions and meadow establishment time

Seagrass meadows, through their large capacity to sequester and store organic carbon in their sediments, contribute to mitigate climatic change. However, these ecosystems have experienced large losses and degradation worldwide due to anthropogenic and natural impacts and they are among the most threatened ecosystems on Earth. When a meadow is impacted, the vegetation is partial- or completely lost, and the sediment is exposed to the atmosphere or water column, resulting in the erosion and remineralisation of the carbon stored. This paper addresses the effects of the construction of coastal infrastructures on sediment properties, organic carbon, and total nitrogen stocks of intertidal seagrass meadows, as well as the size of such stocks in relation to meadow establishing time (recently and old established meadows). Three intertidal seagrass meadows impacted by coastal constructions (with 0% seagrass cover at present) and three adjacent non-impacted old-established meadows (with 100% seagrass cover at present) were studied along with an area of bare sediment and two recent-established seagrass meadows. We observed that the non-impacted areas presented 3-fold higher percentage of mud and 1.5 times higher sedimentary organic carbon stock than impacted areas. Although the impacted area was relatively small (0.05–0.07 ha), coastal infrastructures caused a significant reduction of the sedimentary carbon stock, between 1.1 and 2.2 Mg OC, and a total loss of the carbon sequestration capacity of the impacted meadow. We also found that the organic carbon stock and total nitrogen stock of the recent-established meadow were 30% lower than those of the old-established ones, indicating that OC and TN accumulation within the meadows is a continuous process, which has important consequences for conservation and restoration actions. These results contribute to understanding the spatial variability of blue carbon and nitrogen stocks in coastal systems highly impacted by urban development.