Sediment Bulk Density Research Articles

Precision and bias of carbon storage estimations in wetland and mangrove sediments.

Peaty sediments in coastal wetlands play an important role in the sequestration of atmospheric carbon dioxide and its belowground storage. Sediment cores are used to quantify organic matter (OM) density, estimated by multiplying the bulk density of a core segment by its OM fraction. This method can be imprecise, as repeated samples often differ widely. Recent studies have shown that sediment bulk density and OM fraction are not independent but tightly related by a function called the ideal-mixing model. Thus, the bulk density of the sediment can be directly estimated from its OM fraction. Statistical theory and simulations demonstrate that the high variance in the product estimation of OM density is the result of error propagation in the product of two functionally related variables with independent errors. Estimating OM density in wetland sediments using the ideal-mixing model is more precise than the traditionally used product estimate, especially in highly organic sediments.

Seagrasses in the Eastern Tropical Pacific: species, distribution ecology, blue carbon, and threats

Eastern Tropical Pacific (ETP) seagrasses are composed of three genera and four species: Halophila baillonii, Halodule beaudettei, Halodule wrightii, and Ruppia maritima. These are colonizing seagrass species and meadows in the ETP can be ephemeral. Current seagrass distribution in this region remains unknown, with verified extant presence at a limited number of locations and mapping heavily reliant on historical reports. Suitable environmental conditions for seagrasses in the ETP consist of sheltered bays <10 m depth with fine sediment, 19-35 salinity, 26-32°C temperature, and water transparency of up to 10 m Secchi depth. In this region, seagrass organic carbon (OC) biomass pools (<0.2 Mg ha-1) have been reported from three locations, while sediment bulk density (<1.4 g mL-1) and OC (<24 Mg ha-1) have been reported from eight locations, all found on the Pacific coast of Costa Rica. Recent blue carbon reports from the ETP have not been included in global assessments to date. OC sequestration and sediment accumulation rates are currently unknown. Seagrasses provide key ecosystem services yet they are also threatened by anthropogenic and natural stressors. Seagrasses have already disappeared from two locations within the ETP, with restoration efforts currently underway on the northern Pacific coast of Costa Rica. This overview of our current understanding of seagrasses in the ETP and their services highlights the need for further research in this understudied region.

Spatial distribution characteristics of denitrification functional genes and the environmental drivers in Liaohe estuary wetland.

Genes nirS, nirK, and nosZ are specific for the denitrification process, which is associated with greenhouse gas N2O emission. The abundances and diversities of community containing these three genes are usually used as a common index to reflect the denitrification process, and they would be affected by differences in environmental factors caused by changes from warm to cold conditions. The quantification of denitrification in natural wetlands is complex, and straightforward identification of spatial distribution and drivers affecting the process is still developing. In this study, the bacterial communities, gene diversities, and relative abundances involved in denitrification were investigated in Liaohe Estuary Wetland. We analyzed the relative abundances, diversities, and communities of bacteria containing the three genes at warm and cold conditions using Illumina MiSeq sequencing and detected the potential environmental factors influencing their distribution by using a random forest algorithm. There are great differences in the community composition of the bacteria containing genes nirS, nirK, and nosZ. All the abundant taxa of nirS and nirK communities belonged to phylum Proteobacteria. Compared with the community composition of bacteria containing nirS and nirK, the community of bacteria containing nosZ is more diverse, and the subdivision taxa of phylum Euryarchaeota was also abundant in the community containing nosZ. The distribution characteristics of the relative abundance of nirS and nirK showed obvious differences both at warm and cold climate conditions. The oxidation-reduction potential, nitrite nitrogen, and salinity were detected as potential variables that might explain the diversity of nirS. The total nitrogen and nitrite nitrogen were the important variables for predicting the relative abundance of nirS at warm climate condition, while oxidation-reduction potential and pH contributed to the diversity of nirS at cold condition. The bulk density of sediment was detected as a potential variable affecting the relative abundance of nirK at warm and cold conditions, and diversity of nirK at warm condition, while nitrite nitrogen was detected as an important environmental factor for predicting the diversity of nirK at cold condition. Overall, our results show that the key environmental factors, which affect the relative abundance, diversity, and community of bacteria containing the functional denitrification genes, are not exactly the same, and the diversities of nirS, nirK, and nosZ have a higher environmental sensitivity than their relative abundances.

The Role of Boreal Seagrass Meadows in the Coastal Filter

AbstractBy removing nutrients from the water, coastal ecosystems serve as a filter between land and the open sea. Seagrasses contribute to the coastal filter by trapping and absorbing nutrients. Understanding the processes and environmental conditions underpinning the variability in nutrient retention among and within seagrass meadows is important to evaluate their role in the coastal filter across geographic regions, especially in less studied regions. This study evaluates the role of eelgrass (Zostera marina) meadows in the coastal filter in boreal Newfoundland, Canada, and identifies environmental traits driving variability in nutrient fluxes. We measured carbon (Corg) and nitrogen (N) proportions and stable isotopic composition in the surface sediment (top 5 cm) of three eelgrass meadows. Sediment cores were collected from different locations (i.e., inside, edge, outside) relative to each meadow. Sediment %N (0.22%), %Corg (2.82%), Corg stock (11.1 Mg Corg ha−1), and N stock (0.91 Mg N ha−1) were elevated in our study sites; however, nutrient content was not consistently higher inside the meadow than at the edge or outside. Variability in nutrient retention was best explained by a negative relationship with sediment bulk density. Additionally, differences in carbon isotopic (δ13Corg) enrichment between eelgrass tissue (−11.6‰) and sediment (−22.1‰) within sites indicated that sediment nutrients were predominantly derived from allochthonous marine sources, where variability was best explained by salinity. This study improves the understanding of the role of eelgrass to nutrient cycles in boreal coastal systems and the potential of eelgrass as a blue carbon ecosystem.

Assessing the Erosional Properties of Muddy Sediments: Asan Bay Case Study

The study's outcomes reveal that both the critical shear stress needed to initiate erosion and the sediment erosion rate are influenced by various parameters, including the bulk density of sediment and bed shear stress. This comprehensive analysis underscores the vital role that these parameters play in determining sediment erosion characteristics. The presented research contributes to the understanding of erosional behaviors of muddy sediments and their implications. The findings emphasize the importance of comprehending such properties, especially in designing effective erosion control structures and managing water bodies prone to erosion.

Below ground carbon stock and carbon sequestration potentials of mangrove sediments in Eastern Niger Delta, Nigeria: Implication for climate change

The carbon sequestration capacity and climate change mitigation potentials of mangrove sediments in eastern Niger Delta were investigated. The aim was to determine the carbon stock in the sediments as a measure of carbon sequestration. A total of 312 sediment samples were obtained from 24 sampling 100 cm deep borings at seven locations using an open-cylindrical gouge auger of 100-cm length and 5-cm diameter. The procedures for field sampling, laboratory analysis and result presentation followed protocols for forest carbon measurement, monitoring and reporting. The lithology at the site, which is a major control on the distribution of organic carbon, included peat, peaty clay, and clayey peat with pockets of sands. Sediment bulk density (SBD), organic carbon concentration (%C), were used to estimate the organic carbon stock, carbon dioxide equivalent (CO2e), and to calculate carbon sequestration rate (CSR) and sequestration potential (CSP). Mean values of SBD were 1.30 g/cm3) and 12.48 %) respectively. Similarly, carbon averaged 39.80 Mg ha–1. The total carbon stock of the study area was estimated at 199,617.36 ± 55.64 Mg C, equivalent to 732,595.71 ± 55.64 Mg CO2. CSR was estimated at 4.54 g C cm–2 yr–1 while CSP of mangrove sediments was estimated at 21.56 Mg C yr–1. The result showed that 732,595.71 ± 55.64 Mg of carbon dioxide could either be removed or released to the atmosphere by the mangrove forest of Niger Delta through forest management practices and/or degradation. The study reinforces the need for the protection of mangroves against all anthropogenic-related degradation to improve carbon sequestration potential and achieve target in line with Sustainable Development Goal 13.

Stok Karbon Organik Sedimen di Kawasan Ekosistem Mangrove Pesisir Kabupaten Kolaka Sulawesi Tenggara

Mangrove ecosystem can absorb and store carbon stock in their biomass and sediment. This ecosystem plays an important role in the carbon cycle. This study aimed to analyze the C-organic content and organic carbon stock of sediment, also CO2 absorption. The sediment sampling was conducted in the Kolaka Coastal line mangroves using a Peat Auger for 1 m of sediment depth. Then the sediment was separated into four layers. Then the sediment samples were oven dried at 50°C for 1 week for sediment bulk density analysis (SBD). The analysis of the C-organic content of sediment was done by the LOI method. The estimation of sediment carbon stock using sediment C-organic data and SBD data. The results showed that the highest average of C-organic content was in station 3 (4.12±0.15%). In the othher hand, the highest average of SBD was in station 1 (0.67±0.02 g cm-3). Overall, the total of sediment C-organic stock in station 1 was higher than other station (233.67±11.90 Mg C ha-1). The total capacity of CO2 absorption by sediment in the station 1 was also higher than other station (856.77±37.66 Mg CO2 ha-1). The sediment C-organic stock was influenced by C-organic content and SBD value.

Changes in the spatial distribution of total phosphorus in sediment and water column of a shallow subtropical lake

Julian PJ II, Schafer T, Cohen MJ, Jones P, Osborne TZ. 2023. Changes in the spatial distribution of total phosphorus in sediment and water column of a shallow subtropical lake. Lake Reserv Manage. 39:213–230. In shallow lakes, interactions between bed sediment and the water column strongly influence the availability and transport of nutrients. Lake Okeechobee, in South Florida, is a eutrophic, shallow, polymictic lake that exhibits frequent mixing of the water column and resuspension of unconsolidated sediments. The objective of this study was to evaluate the temporal and spatial patterns of water and sediment characteristics and evaluate the linkage of sediment and water column total phosphorus (TP) concentrations. Spatiotemporal generalized additive models identified key periods during which both surface water total suspended solids (TSS) and TP increased, corresponding to hurricane and tropical storm activity. Our models identified regions with persistently greater water column concentrations than the lake average, indicating potential hot spots in resuspension processes and/or internal loading. Sediment bulk density (BD) and TP were inversely correlated, indicating that light, less dense sediments have greater TP concentrations, potentially contributing to the redistribution of sediments and P. An integrated evaluation of surface water and sediment characteristics using a generalized additive model revealed that sediment TP strongly influences water column TP concentrations, potentially explaining the spatial distribution of surface water TP concentrations. The area of the lake with sediment TP concentration ≤500 mg/kg is increasing while the low bulk density sediments area is decreasing, marking a potential improvement in sediment conditions. If this trend persists, it indicates that the lake is increasingly storing P in sediments that can resist entrainment, with significant implications for assessing the trajectory of lake restoration.

Carbon storage of Mangrove Sediments in Kibani, Eastern Niger Delta, Nigeria: Implications on the Environment

The amount of carbon stored in the mangrove sediments in Kibani area of Eastern Niger Delta was quantified. The objectives were to determine the sediment bulk density (SBD) and organic carbon concentration (%C), and estimate the amount of organic carbon stock in the top 100-cm depth of mangrove sediments and its carbon dioxide equivalent (CO2 e). A total of 312 sediment samples were collected from 24 different sampling points at seven locations. An open-cylindrical gouge auger of 100-cm length and 5-cm diameter was designed and fabricated for the purpose of sediment sampling. Field measurement, laboratory analysis, data presentation and result presentation followed the Guidelines for Forest Carbon Measurement, Monitoring and Reporting. The sediment type has a major control on the distribution of organic carbon. The lithology is a typical of mangrove forest and included peat and clay with pockets of sands. Sediment bulk density (SBD) and %C ranged from 0.95 g/cm3 to 1.97 g/cm3 (mean = 1.30 g/cm3 ) and 0.9 % to 33.6 % (mean = 12.48 %) respectively. Carbon stocks ranged from 24.12 ± 15.18 Mg C ha-1 to 3245.76 ± 15.18 Mg C ha-1 (mean value = 39.80 Mg ha-1) while CO2 e ranged 88.52 ± 15.18 Mg CO2 ha-1 to 11,911.94 ± 15.18 Mg CO2 ha-1 (mean = 2,348.07 ± 15.18 Mg CO2 ha-1). The total carbon stock of the study area was estimated at 947.78 ± 55.64 Gg C, equivalent to 3,478.36 ± 55.64 Gg CO2 . The study revealed that 732,595.71 ± 55.64 Mg of carbon dioxide is stored in the sediments. This amount could either be removed or released to the atmosphere through forest management practices and/or degradation to exacerbate climate change and /or mitigate it. It is recommended that Niger Delta region of Nigeria should be protected against all anthropogenic-related degradation which influence the amount of carbon storage.

Effect of Manila clam Ruditapes philippinarum on burial depth of eelgrass Zostera marina seeds and sediment porosity

Eelgrass Zostera marina is widely distributed in coastal areas of the Northern Hemisphere; however, its continuous population decline is increasingly serious. Besides exogenous stresses, there are endopathic reasons for the decline, e.g., its seedling establishment rate, which is affected by burial depth of seeds and sediment types, is extremely low. Studies have shown that eelgrass seedlings in bivalve distribution areas are easier to establish. Moreover, large-scale bottom sowing culture for bivalves affects the bulk density of sediments which affects seed germination and seedling establishment of eelgrass. This study sought to examine the relationship between sediment modification induced by Manila clams with different body size and stocking density and the burial depth of eelgrass seeds caused by clams. The porosity of surface sediments varied with Manila clam biomass and was mainly affected by clam body size. Clams with a weight of 2.56 ± 0.12 g created more porosity. Clams with a weight of 3.71 ± 0.10 g caused more porosity than clam with a weight of 7.47 ± 0.22 g did. The sand diving of Manila clams affected the distribution of eelgrass seeds in sediments at different depths. The moderately abundant and large Manila clams promoted the burial depth of eelgrass seeds in sediments, and this effect was more obvious at 1–3 cm near the burial depth of 2 cm which is conducive to germination of eelgrass seeds and seedlings establishment. Thus, based on the bioturbation of Manila clams, the efficiency of sowing methods to restore the eelgrass population can potentially be improved.

Thermal structure and sediment bulk density of the MV420 mud volcano on the continental slope of the Canadian Beaufort Sea and its implications for the plumbing system

The shallow migration path of mudflow of the mud volcano MV420 on the continental slope of the Canadian Beaufort Sea is investigated in terms of thermal and geotechnical characteristics. MV420 is a nearly flat topped active mud volcano that emits methane and fluidized mud. Its top is at a depth of water of 420 m, within the gas hydrate stability zone. During the summer 2017 IBRV Araon expedition, several measurements of marine heat flow were conducted in the area of juvenile mud mound/pond morphologies identified by multibeam bathymetry and backscatter intensity images obtained by the Monterey Bay Aquarium Research Institute’s autonomous underwater vehicle. The heat probe (a gravity corer with temperature, pressure, and tilt sensors) appeared to penetrate to a depth of >70 m below the seafloor. The sediment bulk density in the mudflow migration path of MV420, estimated by pressure change with controlled stretches of a winch cable, was 1.56 g/cm3. The subsurface temperature reaches up to 18°C and profiles show that the maximum local temperature occurs from 20 to 40 m below the seafloor, implying substantial vertical variability of geothermal gradients. Our finding of large positive and small negative geothermal gradients above and below the depth of the local temperature maximum may represent stagnant hot mud along the mudflow migration path, indicating a pulsative eruption of the mudflow. Gas hydrate is stable only within a few meter thick layer near the seafloor above the path because of the cold bottom water (0.4°C) covering the top of the mud volcano. Furthermore, the thermal conductivity of 0.939 W/m/K and the marine heat flow of 22.5 mW/m2 at a control site outside MV420 is estimated as the background heat flow in the slope, for which little data exists.

Blue carbon stock in sediments of mangroves and seagrass ecosystems at Southeast Sulawesi, Indonesia

AbstractCoastal ecosystems store approximately 80% of carbon in sediments, which could be emitted due to degradation and deforestation. Therefore, protecting and managing these ecosystems are crucial for climate change mitigation. This study aims to determine sediment bulk density (SBD), sediment organic carbon (SOC) concentration, and sediment carbon stocks (SCS) in mangroves and seagrasses, and to elucidate how the protection scheme impacts them. We collected four sediment cores of 100 cm each from protected and unprotected mangrove areas and three sediment cores of 80 cm each from unprotected and protected seagrass areas. The average SBD in mangroves was 0.48 g cm−3 in protected areas and 0.55 g cm−3 in unprotected areas, while the average SBD in seagrass was 0.79 g cm−3 in protected areas and 0.85 g cm−3 in unprotected areas. The average SOC in mangroves was 8.65% in protected areas and 8.63% in unprotected areas, while the average SOC in seagrass was 5.89% and 6.25% in protected and unprotected areas, respectively. The average SCS in the protected area was 381 and 363 Mg C ha−1 in the mangrove and seagrass ecosystems, respectively. On the other hand, the average SCS in the unprotected areas was 449 and 422 Mg C ha−1 in mangroves and seagrass ecosystems, respectively. Although we did not find any significant differences in SCS between protected and unprotected ecosystems regardless of ecosystem types, the conservation management of blue carbon ecosystems is still needed to avoid future carbon emissions.

Precision of mangrove sediment blue carbon estimates and the role of coring and data analysis methods.

Carbon accumulation in coastal wetlands is normally assessed by extracting a sediment core and estimating its carbon content and bulk density. Because carbon content and bulk density are functionally related, the latter can be estimated gravimetrically from a section of the core or, alternatively, from the carbon content in the sample using the mixing model equation from soil science. Using sediment samples from La Paz Bay, Mexico, we analyzed the effect that the choice of corer and the method used to estimate bulk density could have on the final estimates of carbon storage in the sediments. We validated the results using a larger dataset of tropical mangroves, and then by Monte Carlo simulation. The choice of corer did not have sizable influence on the final estimates of carbon density. The main factor in selecting a corer is the operational difficulties that each corer may have in different types of sediments. Because of the multiplication of errors in a product of two variables subject to random sampling error, when using gravimetric estimates of bulk density, the dispersion of the data points in the estimation of total carbon density rises rapidly as the amount of carbon in the sediment increases. In contrast, the estimation of total carbon density using only the carbon fraction as a predictor is very precise, especially in sediments rich in organic matter. This method, however, depends critically on the accurate estimation of the two parameters of the mixing model: the bulk density of pure peat and the bulk density of pure mineral sediment. The estimation of carbon densities in peaty sediments can be very imprecise when using gravimetric bulk densities. Estimating carbon density in peaty sediments using only the estimate of organic fraction can be much more precise, provided the model parameters are estimated with accuracy. These results open the door for simplified and precise estimates of carbon dynamics in mangroves and coastal wetlands.

Physicochemical Properties of Bottom Sediments in Maruba Dam Reservoir, Machakos, Kenya

Bottom sediments form an integral part of the aquatic ecosystem, where they serve as important sinks for contaminants. However, management options for bottom sediments require an analysis of the physical and chemical properties. Therefore, the aim of this study was to assess the physicochemical properties of bottom sediments in the Maruba dam reservoir in order to inform their potential use. The bottom sediments were obtained from three sampling points using a vibe-coring device. The samples were analyzed for grain size, sediment bulk density, pH, electrical conductivity, organic matter content, and nutrient content (nitrogen, phosphorous, and potassium) using standard laboratory procedures. The results of the study revealed that the bottom sediments were predominantly clay (56%). The mean pH value of the sediments was 6.63, which was found to be slightly acidic. The concentration of cations and anions in the bottom sediments was found to be quite high, with a mean value of 0.225 dS⋅m−1. The bottom sediments in the reservoir were found to be quite rich in the organic matter content (2.10%) and had a mean bulk density of 0.620 g·cm−3. The macronutrients (nitrogen, potassium, and phosphorous) had mean values of 0.12%, 0.46%, and 12.81 mg·kg−1, respectively. The study established that finely grained particles together with organic matter had a potential effect on the availability of macronutrients in bottom sediments. The concentration of the macronutrients of the bottom sediments evaluated in this study points to their potential use in agricultural activities or even in land reclamation.

Concurrent effects of flooding regimes and floodwater quality on sediment properties in a Yangtze River-connected floodplain wetland: Insights from field investigations during 2011–2020

Changes in flood regimes, floodwater quality, and macrophyte types may affect sediment characteristics post-flooding. However, few studies have attempted to unravel their complex influences in floodplain wetlands. From 2011 to 2020, the physical and chemical properties of surface layer sediment pre- and post-flooding was investigated through field surveys in the Dongting Lake wetland. Results indicated that the pre-flooding soil total phosphorus (TP) and total nitrogen (TN) exhibited an increasing trend during 2011–2020. Soil TP increased post-flooding relative to that pre-flooding. The changes in TN, sediment organic matter (SOM), sediment moisture content (SMC), and sediment bulk density (SBD) fluctuated over the years. The best-fitting multi-regression model demonstrated that the changes in sediment variables post-flooding showed a parabolic trajectory along the inundation duration (ID), except for SMC. Changes in soil properties post-flooding were negatively correlated with ID for sediment with a low IDs (<148 days). Meanwhile, changes in soil properties post flooding were positively correlated with ID for sediment with a high IDs (>193 days). Changes in SBD and SOM post-flooding were positively influenced by the TP content in the floodwater. These findings indicate that changes in the flooding regime, and water quality generated by anthropogenic disturbances such as the Three Gorges Dam significantly affect sediment properties, and subsequently influence the ecological functions of the Dongting Lake wetland.

Critical Shear Stress for Erosion of Sand-Mud Mixtures and Pure Mud

The erosion threshold of sand-mud mixtures is investigated by analyzing the momentum balance of a sand particle or a mud parcel in the mixture bed surface, and a formula for the critical shear stress of sand-mud mixtures is developed, which also applies for pure sand and mud. The developed formula suggests that the variation of the critical shear stress of sand-mud mixtures over mud content is mainly caused by the varying dry bulk density of the mud component in the mixture. The developed formula reproduces well the variation of the critical shear stress of sand-mud mixtures over mud content and can predict the critical shear stress of both sand-mud mixtures and pure mud in the process of consolidation. The developed formula promises to be convenient for application by relating the critical shear stress to mud content and the dry bulk density of sediment.

The contribution of sediment desiccation and rewetting process to eutrophication in the presence and absence of emergent macrophytes.

The purpose of current study was to investigate the effects of sediment desiccation on nutrient dynamics and eutrophication in wetlands during the presence or absence of wiry and sturdy rooted emergent macrophytes, based on the hypothesis that sediment desiccation negatively correlated with plants nutrient uptake abilities and positively with nutrients fluxes at sediment-water interface. Growth of four emergent macrophytes, including two wiry rooted plants, i.e., Alocasia cucullata and Aglaonema commutatum, and two sturdy rooted plants, i.e., Cannabis indica and Acorus calamus, were grown and investigated in dried-rewetted sediments (DS) and constantly wet sediments (WS), respectively, for 6 months. The findings revealed that sediment drying and rewetting process significantly decreased the diffusion of overlying nutrient into sediment and the particle size density, porosity, and nutrients' repository ability in DS treatments, while the sediment bulk density and mineralization of organic macronutrients increased. Compared to WS treatments, the DS treatments impaired plant growth, root biomass, shoot biomass, and stimulated higher fluxes of ammonium nitrogen ([Formula: see text]-N, 0.042-0.081 mg m - 2 d - 1) and phosphate (P[Formula: see text] 0.009-0.030 mg m-2 d-1) at sediment-water interface upon rewetting. The higher internal release of macronutrients and dissolved organic carbon (DOC) from DS led to the higher chlorophyll-a (Chl-a) concentrations (34.47-21.28 to 41.76-33.36 μg L-1) in their water column than in the water column of WS. The wiry rooted plants with higher root biomass displayed lower internal release of [Formula: see text]-N, PO43-P and DOC and water column Chl-a concentrations than the sturdy rooted plants in two sediment types. Root biomass of plants correlated positively with TN (63-87%) and TP (56-78%) removal percentages from WS and DS. These results demonstrated that sediment desiccation process reduced plant growth and enhanced internal loading of nutrients and consequently accelerated eutrophication in these wetlands.

Long‐term study reveals top‐down effect of crabs on a California salt marsh

AbstractConsumers can structure plant communities and may function as keystone species or ecosystem engineers. In salt marshes, the prevailing paradigm has shifted in recent decades from nearly complete focus on bottom‐up processes to inclusion of top‐down effects. Although the number of studies investigating top‐down control continues to climb, few experiments span multiple years, so temporal variability in or long‐term impacts of consumers have not been well characterized. In addition, while top‐down control has been found to be common in Western and Eastern Atlantic and Western Pacific salt marshes, our study is one of the first to experimentally consider top‐down control of salt marsh plants in the Eastern Pacific. We conducted a five‐year field experiment along eroding creekbank edges of a California salt marsh in which we manipulated densities of the shore crab, Pachygrapsus crassipes, and tracked marsh responses over time. Our results demonstrate that, through both consumption and engineering activities, this superabundant crab is regulating marsh vegetation and soil structure. Experimentally reducing crab abundance enhanced vegetation biomass and sediment bulk density. Moreover, root biomass and bulk density—factors known to increase marsh resilience to erosion and sea‐level rise—decreased linearly with increasing burrow density. Our long‐term study uniquely revealed that burrows can persist for years after crab abundances are reduced and that plant responses from grazer exclusions gradually strengthen over time, likely due to the relatively slow growth of woody perennial foundational plants. Since shore crabs are abundant throughout the marsh in most major estuaries within the range of the species (from Baja California, Mexico to Oregon, USA), we hypothesize that this species is exerting significant, yet underappreciated top‐down control and modifying the sediment properties of many West Coast salt marshes.

Sediment Bulk Density Effects on Benthic Macrofauna Burrowing and Bioturbation Behavior

Benthic macrofauna are a key component of intertidal ecosystems. Their mobility and behavior determine processes like nutrient cycling and the biogeomorphic development of intertidal flats. Many physical drivers of benthic macrofauna behavior, such as sediment grain size, have been well-studied. However, little is known about how sediment bulk density (a measure of sediment compaction and water content) affects this behavior. We investigated the effect of bulk density on the burrowing rate, burrowing depth, bioturbation activity, and oxygen consumption of bivalves (Limecola balthica, Scrobicularia plana, and Cerastoderma edule) and polychaetes (Hediste diversicolor and Arenicola marina) during a 29-day mesocosm experiment. We compared four sediment treatments consisting of two sediments of differing grain size classes (sandy and muddy) with two bulk densities (compact and soft). Overall, bulk density had a strong effect on benthic macrofauna behavior. Benthic macrofauna burrowed faster and bioturbation more intensely in soft sediments with low bulk density, regardless of grain size. In addition, L. balthica burrowed deeper in low bulk density sediment. Finally, we found that larger bivalves (both C. edule and S. plana) burrowed slower in compact sediment than smaller ones. This study shows that benthic macrofauna change their behavior in subtle but important ways under different sediment bulk densities which could affect animal-sediment interactions and tidal flat biogeomorphology. We conclude that lower bulk density conditions lead to more active macrofaunal movement and sediment reworking.

Onset of runaway fragmentation of salt marshes

Summary Salt marshes are valuable but vulnerable coastal ecosystems that adapt to relative sea level rise (RSLR) by accumulating organic matter and inorganic sediment. The natural limit of these processes defines a threshold rate of RSLR beyond which marshes drown, resulting in ponding and conversion to open waters. We develop a simplified formulation for sediment transport across marshes to show that pond formation leads to runaway marsh fragmentation, a process characterized by a self-similar hierarchy of pond sizes with power-law distributions. We find the threshold for marsh fragmentation scales primarily with tidal range and that sediment supply is only relevant where tides are sufficient to transport sediment to the marsh interior. Thus the RSLR threshold is controlled by organic accretion in microtidal marshes regardless of the suspended sediment concentration at the marsh edge. This explains the observed fragmentation of microtidal marshes and suggests a tipping point for widespread marsh loss.