Intertidal Marsh Research Articles

Effects of variations in hydrological connectivity on the macrobenthic community structure in reclaimed wetlands

The Nanhui Dongtan Wetland is the most extensively reclaimed part of the Yangtze River Estuary wetland. In recent decades, urbanization has led to the extensive reclamation of the intertidal wetlands of Nanhui Dongtan. Macrobenthos are crucial as secondary production groups in the food web. However, there is a lack of in-depth research on the response mechanisms of macrobenthic communities to environmental disturbances in reclaimed wetlands. This study investigated the impact of hydrological connectivity changes caused by land reclamation on the macrobenthic community based on the macrobenthic community composition in preserved tidal flats and closed and open reclamation areas in Nanhui Dongtan. The results showed that the macrobenthos species richness in the closed reclamation area was significantly lower than that in the other areas. After dividing the functional groups of macrobenthos, structural equation modeling revealed a negative correlation between the salinity and the functional group composition. Analysis of the food sources revealed significant positive correlations between predatory and sediment-feeding populations and sediment organic matter content, between detritivorous group and environmental chlorophyll-a content, and between herbivorous group and suspended organic matter content in water. Therefore, variations in hydrological connectivity in different reclamation areas caused differences in food source distribution, which led to different compositions of macrobenthic functional groups. The results of this study provide a theoretical reference for the study of intertidal wetland habitat restoration.

Small wetland‐fringed estuaries deliver disproportionately large carbon loads to the ocean

AbstractPrevious estimates of dissolved carbon export from estuaries focused on larger systems in the Northern Hemisphere, with little data for smaller tropical estuaries often fringed by intertidal wetlands. We investigated lateral export (outwelling) and transformation rates of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and total alkalinity (TA) as well as CO2 emissions from 18 diverse Australian estuaries. Most estuaries acted as net sources for DOC (72%), DIC (83%), and TA (50%). On average, estuaries exported 120 ± 55 and 344 ± 150 mmol m−2 catchment yr−1 DOC and DIC, respectively. Estuarine CO2 emissions (33 ± 20 mmol m−2 estuary d−1) equalled 13% ± 16% of the dissolved lateral carbon export. Carbon export positively correlated with runoff, rain, and intertidal wetland cover, and negatively correlated with estuary and catchment area. Mangroves and saltmarshes cover < 1% of all catchments but can contribute 46% ± 11% of the DOC and 67% ± 13% of the DIC exported to the ocean. Upscaling our observations, Australian estuaries export 2.8 ± 2.2 TgC yr−1 DOC, 8.1 ± 6.2 TgC yr−1 DIC, and 0.7 ± 0.6 Tmol yr−1 TA. Small catchments (< 10 ha) making up 70% of all estuaries and accounting for 18% of the total freshwater flow provided 27% to the total dissolved carbon export. Overall, small tropical estuaries fringed by highly productive intertidal wetlands are hotspots of carbon exports and should be considered in marine carbon budgets.

Aerobic granular sludge inoculant for enhancing high salinity wastewater treatment: Performance and value-added biopolymer recovery

In this study, two lab-scale aerobic granular sludge (AGS) systems, seeded with intertidal wetland sediment (IWS) and activated sludge (AS), were constructed to compare their treatment performances and alginate-like exopolymers (ALE) recovery potential in high salinity wastewater treatment. Both reactors (RIWS and RAS) exhibited excellent total organic carbon (TOC) reduction rates > 95.0 %, while RIWS exhibited an improved total nitrogen (TN) removal than RAS (84.4 % vs 75.7 %). The presence of several inorganic particulates (acted as nucleus) in IWS accelerated the formation of granules in RIWS. The ALE yield of RIWS reached 466.6 ± 52.2 mg/g VSS, which was 1.6 times higher than that of RAS (286.2 ± 5.2 mg/g VSS). Moreover, to illustrate the different ALE recovery potentials, the proposed ALE biosynthesis pathway was also assessed. It was observed that the abundance of function genes encoding ALE biosynthesis of RIWS was significantly higher than that of RAS, revealing its superior ALE recovery potential. This study proposed a novel and effective treatment method for high-salinity wastewater with high potential resource recovery.

Sediment water content drives movement of intertidal crab Helice tientsinensis more strongly than salinity variations

Intertidal wetlands undergo dynamic water and salinity variations, creating both promising and challenging habitats for diverse organisms. Crabs respond strongly to these variations by means such as altering their movements, thereby restructuring their spatial distribution and influencing coastal ecosystem resilience. However, the movements of crabs under varying environmental conditions require further elucidation. We conducted a systematic mesocosm experiment using the ubiquitous intertidal crab species Helice tientsinensis with four amount levels and six salinity levels of sprayed water applied through a custom apparatus, with a primary focus on crab movement. Crab movement from the experimental side of the apparatus (with altered conditions) to the control side (resembling field conditions of the intertidal wetlands of China's Yellow River Delta) and vice versa was recorded. The results revealed significant differences in moving out of the experimental side and moving in among the different water and salinity conditions, both separately for the two factors and simultaneously. Decreases in water content had a more pronounced effect on crab movement, leading to an increased number of crabs moving out of the experimental side of the apparatus. Conversely, as the experimental side became wetter, crabs tended to move towards it, and this movement was intensified by increases or decreases in water salinity. A structural equation model revealed that the moving-out and moving-in played fundamental roles in determining the number of resident crabs at the end of each experiment. While crabs preferred moist sediment with lower salinity, changes in salinity alone had minimal direct effect compared to sediment water contents. Our results clarify crab movements under varying water and salinity conditions, offering valuable insights to support adaptive interventions for crab populations and inform adaptive conservation and management strategies in intertidal wetlands.

Seasonal Temperature Distributions and Variations in Salt Marshes: Field Investigation and Numerical Simulation

AbstractSoil temperature regulates biogeochemical processes and is a key environmental factor affecting salt marsh ecosystems. Previous studies on soil temperature and heat transport in intertidal marshes predominantly focused on short‐term changes, leaving seasonal variations unclear. This study conducted a yearlong field and modeling investigation to examine temporal and spatial temperature variations in a creek‐marsh section under estuarine and meteorological influences. The results showed that inundating tidal water propagated the seasonal water temperature signal to marsh soils, especially at low elevations, thereby modulating air temperature‐induced soil temperature variations and distributions. The response of soil temperature to air and tidal water temperatures exhibited a damped and delayed pattern. In contrast, tide‐induced porewater circulations near the creek facilitated the temperature responses. A regression model incorporating a Gamma distribution function was developed to quantify the delayed and cumulative thermal effects of tidal water and air on shallow soil temperatures. The model coefficients varied along the creek‐marsh section, capturing the seasonal regulation of periodic tidal inundation on soil temperature in the low‐elevation marsh and near the creek. Sensitivity analyses indicated that relative sea level rise lowered yearly‐averaged temperatures for the marsh platform by enhancing latent heat export. Surface water warming increased the marsh temperatures at lower elevations. This study demonstrates that the creek‐marsh topography, warming, and relative sea level rise jointly affect soil temperature dynamics, advancing our understanding of temperature‐dependent biogeochemical processes in marsh ecosystems.

Molecular mechanism of vivipary as revealed by the genomes of viviparous mangroves and non-viviparous relatives

Vivipary is a prominent feature of mangroves, allowing seeds to complete germination while attached to the mother plant, and equips propagules to endure and flourish in challenging coastal intertidal wetlands. However, vivipary-associated genetic mechanisms remain largely elusive. Genomes of two viviparous mangrove species and a non-viviparous inland relative were sequenced and assembled at the chromosome level. Comparative genomic analyses between viviparous and non-viviparous genomes revealed that DELAY OF GERMINATION 1 (DOG1) family genes (DFGs), the proteins from which are crucial for seed dormancy, germination, and reserve accumulation, are either lost or dysfunctional in the entire lineage of true viviparous mangroves but are present and functional in their inland, non-viviparous relatives. Transcriptome dynamics at key stages of vivipary further highlighted the roles of phytohormonal homeostasis, proteins stored in mature seeds, and proanthocyanidins in vivipary under conditions lacking DFGs. Population genomic analyses elucidate dynamics of syntenic regions surrounding the missing DFGs. Our findings demonstrated the genetic foundation of constitutive vivipary in Rhizophoraceae mangroves.

A Fresh Take: Seasonal Changes in Terrestrial Freshwater Inputs Impact Salt Marsh Hydrology and Vegetation Dynamics

Salt marshes exist at the terrestrial-marine interface, providing important ecosystem services such as nutrient cycling and carbon sequestration. Tidal inputs play a dominant role in salt marsh porewater mixing, and terrestrially derived freshwater inputs are increasingly recognized as important sources of water and solutes to intertidal wetlands. However, there remains a critical gap in understanding the role of freshwater inputs on salt marsh hydrology, and how this may impact marsh subsurface salinity and plant productivity. Here, we address this knowledge gap by examining the hydrologic behavior, porewater salinity, and pickleweed (Sarcocornia pacifica also known as Salicornia pacifica) plant productivity along a salt marsh transect in an estuary along the central coast of California. Through the installation of a suite of hydrometric sensors and routine porewater sampling and vegetation surveys, we sought to understand how seasonal changes in terrestrial freshwater inputs impact salt marsh ecohydrologic processes. We found that salt marsh porewater salinity, shallow subsurface saturation, and pickleweed productivity are closely coupled with elevated upland water level during the winter and spring, and more influenced by tidal inputs during the summer and fall. This seasonal response indicates a switch in salt marsh hydrologic connectivity with the terrestrial upland that impacts ecosystem functioning. Through elucidating the interannual impacts of drought on salt marsh hydrology, we found that the severity of drought and historical precipitation can impact contemporary hydrologic behavior and the duration and timing of the upland-marsh hydrologic connectivity. This implies that the sensitivity of salt marshes to climate change involves a complex interaction between sea level rise and freshwater inputs that vary at seasonal to interannual timescales.

Spatial-temporal impacts of invasive Spartina anglica on the rates and pathways of organic carbon mineralization and resulting C-Fe-S cycles in the intertidal wetland of the Han River Estuary, Yellow Sea

To elucidate the spatial-temporal impact of invasive saltmarsh plant Spartina anglica on the biogeochemical processes in coastal wetlands, we investigated the rates and partitioning of organic carbon (Corg) mineralization in three representative benthic habitats: (1) vegetated sediments inhabited by invasive S. anglica (SA); vegetated sediments by indigenous Suaeda japonica; and (3) unvegetated mud flats. Microbial metabolic rates were greatly stimulated at the SA site during the active growing seasons of Spartina, indicating that a substantial amount of organic substrates was supplied from the high below-ground biomass of Spartina. At the SA site, sulfate reduction dominated the Corg mineralization pathways during the plant growing season, whereas iron reduction dominated during the non-growing season. Overall, due to its greater biomass and longer growing season than native Suaeda, the expansion of invasive Spartina is likely to greatly alter the Corg-Fe-S cycles and carbon storage capacity in the coastal wetlands.

Extracting waterline and inverting tidal flats topography based on Sentinel-2 remote sensing images: A case study of the northern part of the North Jiangsu radial sand ridges

Muddy tidal flats with landforms vary widely over space and time. The evolution of tidal flats topography has had a significant impact on the region. Therefore, there is a need to develop methods for rapidly and accurately characterizing tidal flats topography. This study applied thresholding segmentation (TS), machine learning, and two image classification methods to Sentinel-2 remote sensing images to extract the instantaneous waterline of the northern part of the North Jiangsu Radial sand Ridges (NJRSR). Tidal level data at specific time intervals were obtained using the Delft3D and TPXO9 models, after which tidal flats topography inversion was conducted. As a result, varied among the different classification methods, with the K-Nearest Neighbors (KNN) algorithm producing the optimal waterline extraction. Furthermore, the accuracy of tidal level simulation was improved by employing Delft3D software and constructing a scaled-down model of the study area. Comparisons with Delft3D model simulations of tidal levels with observations at three tide gauge stations yielded accuracies of 17.5 cm, 18.5 cm and 17.9 cm, this result showed that the establishment of the tidal level model can enhance the accuracy of tidal flats Digital Elevation Model (DEM) inversion. In fact, the combined application of the KNN algorithm and Delft3D model within retrieval of tidal level provided the most accurate topographic inversion, with an average error of 0.297 m, which was reduced to 0.292 m after median filtering, and the results indicated a trend of southward erosion in the tidal flats in the study area, with a turning point roughly located in the southern part of Doulong Port. The results of this study can help improve the future monitoring of dynamic changes in intertidal wetland topography and the conservation and development of tidal flats areas.

Warming sea surface temperatures are linked to lower shorebird migratory fuel loads.

Warming sea surface temperatures (SSTs) are altering the biological structure of intertidal wetlands at a global scale, with potentially serious physiological and demographic consequences for migratory shorebird populations that depend on intertidal sites. The effects of mediating factors, such as age-related foraging skill, in shaping the consequences of warming SSTs on shorebird populations, however, remain largely unknown. Using morphological measurements of Dunlin fuelling for a >3000 km transoceanic migration, we assessed the influence of climatic conditions and age on individuals' migratory fuel loads and performance. We found that juveniles were often at risk of exhausting their fuel loads en route to primary wintering grounds, especially following high June SSTs in the previous year; the lagged nature of which suggests SSTs acted on juvenile loads by altering the availability of critical prey. Up to 45% fewer juveniles may have reached wintering grounds via a non-stop flight under recent high SSTs compared to the long-term trend. Adults, by contrast, were highly capable of reaching wintering grounds in non-stop flight across years. Our findings suggest that juveniles were disproportionately impacted by apparent SST-related declines in critical prey, and illustrate a general mechanism by which climate change may shape migratory shorebird populations worldwide.

Temporal turnover of viral biodiversity and functional potential in intertidal wetlands

As the central members of the microbiome networks, viruses regulate the composition of microbial communities and drive the nutrient cycles of ecosystems by lysing host cells. Therefore, uncovering the dynamic patterns and the underlying ecological mechanisms mediating the tiniest viral communities across space and through time in natural ecosystems is of crucial importance for better understanding the complex microbial world. Here, the temporal dynamics of intertidal viral communities were investigated via a time-series sampling effort. A total of 1911 viral operational taxonomic units were recovered from 36 bimonthly collected shotgun metagenomes. Functionally important auxiliary metabolic genes involved in carbohydrate, sulfur, and phosphorus metabolism were detected, some of which (e.g., cysH gene) were stably present within viral genomes over time. Over the sampling period, strong and comparable temporal turnovers were observed for intertidal viromes and their host microbes. Winter was determined as the pivotal point for the shifts in viral diversity patterns. Notably, the viral micro-diversity covaried with the macro-diversity, following similar temporal patterns. The relative abundances of viral taxa also covaried with their host prokaryotes. Meanwhile, the virus–host relationships at the whole community level were relatively stable. Further statistical analyses demonstrated that the dynamic patterns of viral communities were highly deterministic, for which temperature was the major driver. This study provided valuable mechanistic insights into the temporal turnover of viral communities in complex ecosystems such as intertidal wetlands.

Spatiotemporal variation in size and abundance of juvenile Chinese mitten crab (Eriocheir sinensis) in Yangtze Estuary

IntroductionIntertidal habitat preferences and spatiotemporal variation in the abundance of juvenile Chinese mitten crab Eriocheir sinensis in Yangtze Estuary are reported.MethodsThe size and abundance of this crab are reported for mud flat, gravel, root belt, and marsh habitats in this estuary’s lower, middle, and upper reaches from June 2021 (spring) to February 2022 (winter) using quadrat method.ResultsJuvenile E. sinensis of carapace length (CL) 5.5 ± 2.1 mm (mean ± standard deviation) were collected; no juveniles were found in February 2022. Crab abundance in root belt and gravel habitats usually exceeded that of marsh habitat; no juveniles were found in mud flat habitat. The greatest abundances and smallest individuals were found when megalopa recruited in early spring (June); juvenile abundance decreased sharply afterwards, and crabs were absent from the intertidal during winter. Size and relative growth rate of juvenile crabs were greater in root belt and gravel habitat than in marsh habitat from June to August. Recruitment primarily drove changes in crab abundance and size during June and July, and temperature best correlated with changes in the winter. Many stage I juveniles (CL < 3.1 mm) occurred in the lower estuarine reaches, while stage III and IV juveniles (CL 3.9–6.5 mm) primarily occurred in the middle and upper estuarine reaches.DiscussionAlthough intertidal wetland habitat in Yangtze Estuary is severely degraded and reduced in area, it remains important for recruitment and maintenance of mitten crab populations. An understanding of the habitat requirements of this species will benefit management of this crab resource and the prioritized restoration of intertidal habitat.

How Artificial Salt Marsh Vegetation Reduces The Threshold For Sediment Resuspension In Wave-Current Flows

Suspended sediment transport and retention within salt marshes is a key factor in their resilience against erosion associated with threats such as sea level rise and coastal squeeze. Salt marshes are vegetated intertidal wetlands found in temperate climate zones. Their presence contributes to flood protection, coastal flora and fauna, and carbon sequestration (Temmerman et al., 2013). Vegetation-induced resuspension of fine sediment helps to transport fine particles deeper into salt marshes. The interaction between waves, currents, and vegetation may resuspend sediment sooner than it would without vegetation. It has been shown in conditions with only currents (Liu et al., 2021; Tinoco & Coco, 2014) or only waves (Tinoco & Coco, 2018) that the threshold for resuspension is lower within vegetated meadows than without vegetation. However, the threshold has not yet been studied for combined wave-current conditions, which often exist in the marsh environment. Identifying this threshold will enable us to predict when in a tidal cycle sediment resuspension occurs and can be used to improve sediment transport model. We use flume experiments to methodically identify the threshold of sediment resuspension in artificial salt marsh meadows of three different area densities under combined wave-current flows.

Seasonal and spatial characteristics of microbial community in intertidal wetlands: Aspect of microbial coupling

The simultaneous reductions of pollution and carbon are widely concerned in sewage treatment. Various microorganisms with synergies are existed in the intertidal wetlands, which have the potential to be the microbial inoculation used in sewage treatment. In this study, the microorganisms in the reed wetlands of the Yellow River Delta in four seasons and at four layers (0–5, 5–10, 10–15, and 15–20 cm) were investigated by high-throughput sequencing technology. The results showed that the majority of functional bacteria were denitrifying bacteria in the spring and summer. Spring was the primary season for hydrogen autotrophic denitrification, whereas summer was the primary season for sulfur autotrophic denitrification. A range of bacteria connected to one another by cable bacteria cooperated to finish the cycle of several elements in the 10–15 cm layer in autumn. A distinctive characteristic of functional bacteria was the anaerobic methane oxidation mediated by NC10 facultative bacteria in the 0–5 cm layer in winter. The results of this study provide an inspiration for utilizing suitable microbial inoculation to realize the simultaneous reductions of pollution and carbon in the future sewage treatment.

Effects of Seasonal Variations in Seagrass Density and Storms on Sediment Retention and Connectivity Between Subtidal Flats and Intertidal Marsh

AbstractBoth submerged aquatic vegetation and salt marsh are important coastal ecosystems known for their effectiveness in sediment trapping and carbon burial. However, their proximity can lead to competition for limited sediment resources, potentially compromising their capacity to facilitate sufficient sediment deposition to withstand sea‐level rise. Here we applied the Delft3D flow and sediment transport model in a shallow coastal bay to assess how seasonal variations in seagrass density and storms modulate sediment retention and connectivity between subtidal flats and intertidal marsh. Results show that submerged seagrass meadows acted as temporary storage for fine sediment and altered the timing of sediment transport to an adjacent marsh through seasonal growth cycles. When seagrass occupied subtidal flats, sediment flux to the marsh was controlled by seasonal seagrass density variations, with peak fluxes during winter senescence. In contrast, wave‐induced sediment resuspension on the flats was the major contributor for marsh sediment input in the simulation with no seagrass, with peak storm‐driven fluxes in summer/autumn. Overall, seagrass meadows significantly increased annual sediment accumulation on the vegetated subtidal flats by tenfold, while reducing annual marsh sediment flux and edge erosion by 20%. Seagrass meadows and marsh were both able to maintain a sediment deposition rate comparable to the rapid sea‐level rise at the study site despite the absence of terrestrial sediment sources. Our findings highlight the strong seasonal control subtidal aquatic vegetation has in the vertical and horizontal dynamics of tidal flat‐marsh systems and provide insights that can inform wetland restoration and management strategies in similar systems.

Spatiotemporal evolution of coastal wetland resources in northern Rizhao under the influence of human activities

Coastal wetlands are a vital resource for the sustainable development of coastal areas. This paper presents a comprehensive monitoring of coastal wetland resources and their evolution in the northern region of Rizhao using multi-period remote sensing data and field investigations. The results show that as of the 2020s, wetland resources in northern Rizhao include subtidal wetlands, artificial wetlands, and intertidal wetlands, accounting for 38.5%, 34.2%, and 26.7% of the total wetland resources, respectively. Over the past 35 years, the evolution of wetlands has been divided into two stages according to the impact of human activities. Before 2000, the wetland area expanded rapidly due to beach aquaculture development. Subsequently, after 2000, the beach wetland area experienced a decline, whereas the landscape wetland area increased. The reason for the change in the wetland area in the later stage was the artificial change in wetland type and beach erosion. By 2020, coastal erosion gradually became the main factor driving the change in wetlands. Therefore, strategies and measures aimed at promoting natural beach growth should be implemented by regulating coastal engineering activities.

Effects of tidal creeks on Spartina Alterniflora expansion: A perspective from multi-scale remote sensing

Rapid expansion of Spartina Alterniflora (S. alterniflora) has led to severe ecological consequences in coastal wetlands in China. Better understanding of the mechanism of S. alterniflora expansion is of great importance for its control and management. Although previous research has reported that the expansion of S. alterniflora is closely related to tidal creeks via field investigations, few studies have quantitatively investigated their relationships using remote sensing at different scales. In this study, we utilized time series satellite remote sensing imagery with different spatial resolutions to investigate the effects of tidal creeks on S. alterniflora expansion at both landscape scale and patch scale. We took the Yellow River Delta (YRD), China as our study area, where S. alterniflora has spread across the intertidal wetlands during the past decade. At landscape scale, we first derived the yearly maps of S. alterniflora and tidal creeks during 2009–2020 based on Landsat-series satellite imagery, then analyzed the spatio-temporal relationship between S. alterniflora and tidal creeks. By using grid analysis method, we evaluated the change of S. alterniflora area and tidal creek length and compared their change time. The results showed that 30.46 % of the grids experienced no change in tidal creeks while expansion of S. alterniflora, and 54.02 % of the grids experienced S. alterniflora area increase after the lengthening of tidal creeks. The results indicated that relationship between tidal creeks and S. alterniflora in YRD was dominated by “tidal creeks promoting S. alterniflora expansion”. We then simulated the S. alterniflora expansion at patch scale using an improved spatially explicit individual-based model based on S. alterniflora maps derived from Landsat imagery (30 m) and GF-1 high spatial resolution imagery (2 m), respectively. The results showed that consideration of long-distance dispersal of seeds along the direction of tidal creeks considerably enhanced the simulation accuracy, regardless of the spatial resolution of the S. alterniflora maps. High-resolution S. alterniflora maps yielded higher simulation accuracies. Our study suggests that the long-distance dispersal of seeds led by tidal creeks is an important manner for S. alterniflora expansion. Satellite remote sensing imagery have great potential to discover the mechanism of S. alterniflora expansion, facilitating control of S. alterniflora at the early invasion stage.

Role of n-DAMO in Mitigating Methane Emissions from Intertidal Wetlands Is Regulated by Saltmarsh Vegetations.

Coastal wetlands are hotspots for methane (CH4) production, reducing their potential for global warming mitigation. Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) plays a crucial role in bridging carbon and nitrogen cycles, contributing significantly to CH4 consumption. However, the role of n-DAMO in reducing CH4 emissions in coastal wetlands is poorly understood. Here, the ecological functions of the n-DAMO process in different saltmarsh vegetation habitats as well as bare mudflats were quantified, and the underlying microbial mechanisms were explored. Results showed that n-DAMO rates were significantly higher in vegetated habitats (Scirpus mariqueter and Spartina alterniflora) than those in bare mudflats (P < 0.05), leading to an enhanced contribution to CH4 consumption. Compared with other habitats, the contribution of n-DAMO to the total anaerobic CH4 oxidation was significantly lower in the Phragmites australis wetland (15.0%), where the anaerobic CH4 oxidation was primarily driven by ferric iron (Fe3+). Genetic and statistical analyses suggested that the different roles of n-DAMO in various saltmarsh wetlands may be related to divergent n-DAMO microbial communities as well as environmental parameters such as sediment pH and total organic carbon. This study provides an important scientific basis for a more accurate estimation of the role of coastal wetlands in mitigating climate change.

Benthic contribution to seasonal silica budgets in two macrotidal estuaries in North-Western France

The paper aims to build seasonal silica budgets in two macrotidal estuaries, the Elorn and Aulne estuaries of the Bay of Brest (North-Western France), based on modeling and measurements, in order to increase our understanding of the silica (Si) cycle at land-sea interfaces. A diagenetic model was developed to quantify benthic Si fluxes, e.g. aSiO2 deposition fluxes that are difficult to assess through direct measurements. Sediment cores were also seasonally sampled at six stations to provide data essential to parametrize and validate the model. Vertical profiles of porosity, burrowing depth, biodiffusive coefficients, concentrations of amorphous silica (aSiO2) and silicic acid (Si(OH)4 and the proportion of reactive aSiO2 were measured. The results show that sites sampled along the Elorn and Aulne estuaries constitute significant net Si deposition areas (1-4.5 mmol Si m-2 d-1), particularly in the upstream during winter and in midstream and downstream during summer. Year round, reprecipitation is negligible (&amp;lt; 3%) while burial accounts for the retention of ~ 30-80% of deposited aSiO2. In winter, burial dominates the benthic Si budget. As surface-integrated benthic Si fluxes are low compared to riverine aSiO2 fluxes, the Si export to coastal waters is high (93%) during winter. In contrast, in summer, burial accounts for 38% of river Si fluxes, and Si(OH)4 flux from the sediment is high as a result of enhanced benthic recycling and bioirrigation. Internal estuarine processes, e.g., benthic and pelagic primary production, dissolution and benthic Si fluxes, surpass river fluxes in magnitude during summer. Overall, we conclude that the Elorn and Aulne macrotidal estuaries are efficient filters of Si, retaining about 4-38% of river Si fluxes, and even 6-67% when accounting for retention in intertidal marshes, but with massive exports occurring during winter floods.

Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes

Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol m−2 d−1 in mangroves and 57 ± 104 mmol m−2 d−1 in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism.