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Abstract Bottom sediments in tidal estuaries influence organic matter and nutrient cycling, habitat suitability, and geomorphological processes. Characterizing the grain size distribution of bottom sediments is essential for predicting sediment transport, channel stability, and ecosystem health. However, this information can be challenging to acquire over large areas as traditional in situ approaches provide only point‐based observations that are spatially limited. This study addresses this limitation by applying the sediment balance equation to remotely sensed maps of total suspended solids concentration and numerical model outputs to derive a high‐resolution, spatially explicit sediment grain size distribution within a tidal channel of a New England mesotidal estuary. Results reveal a distinct gradient in sediment grain size, with coarse sediments near the inlet transitioning to finer sediments landward. Fine sand covers over 85% of the channel bottom, while medium and coarse sand occupy 14% and 1%, respectively. Peaks in settling velocity identify zones of sediment convergence controlled by tidal forcing and river inflows. The positive correlation between , estimated through the depth integrated suspended sediment continuity equation, and bottom grain size confirms the effectiveness of this approach for sediment classification in estuarine environments. 

Abstract Sediment budgets are diagnostic of saltmarsh resilience, yet quantifying net sediment fluxes remains constrained by limited, asynchronous observations of suspended sediment concentration (SSC) and water flux. Using high‐frequency observations from 15 U.S. saltmarsh stations, we combine phase folding and Monte Carlo simulations to assess whether sporadic sampling can reliably resolve net sediment exchange. We show that asynchronous SSC and water‐flux measurements spanning >8 full tidal cycles robustly identify sediment sources and sinks. High‐resolution optical satellite archives (Sentinel‐2) typically capture >30 tidal cycles in most saltmarshes, yielding 5%–41% uncertainty in net flux magnitude across systems. This accuracy is sufficient to distinguish saltmarshes functioning as sediment sources from those acting as sinks at regional to global scales, though local calibration could improve accuracy. Integrating optical remote sensing with emerging water‐flux technologies can therefore enable scalable assessments of tidal saltmarsh sediment budgets, providing a pathway for evaluating coastal resilience under accelerating sea‐level rise. 

Abstract Tropicalization is a global phenomenon where tropical or sub-tropical species expand their ranges poleward into temperate areas. We document the northern range expansion of a tropical species, the Atlantic mangrove fiddler crab, Leptuca thayeri, on the east coast of the United States beyond its historical northern range (Jacksonville, Florida). We used iNaturalist observations, personal observations, and surveys to establish its expanded range. Based on 31 iNaturalist and 17 personal observations, we found that its current northernmost limit occurs in Beaufort County, South Carolina, 307 km north of its historical limit. Leptuca thayeri expanded its range beyond mangroves and into salt marshes, where it was found among the saltmarsh plants Spartina alterniflora and Juncus roemerianus. This result suggests that L. thayeri is a habitat generalist. Sea surface temperatures along the coast of Georgia have warmed 1.15°C over 22 years (1998–2022) or 0.5°C per decade, on average. These rising ocean temperatures suggest that climate change is driving the range expansion of L. thayeri making it a climate migrant. Future work could examine the impacts of this burrowing crab on salt marshes. 

IntroductionSaltmarsh introduction has been widely implemented to restore ecosystem services and promote sedimentation in tidal mudflats, yet its effects on tidal network dynamics remain hard to predict. The interplay between saltmarsh extent and sediment availability in shaping long-term mudflat morphodynamics is not fully understood. MethodsWe develop a two-dimensional biomorphodynamic model to examine the individual and combined influences of saltmarsh presence and sediment availability on the evolution of tidal-flat channels. Results and discussionOur results demonstrate that sediment availability controls the long-term morphological change of mudflats, while the presence of saltmarshes exerts substantial short-term alterations in mudflat evolution. During the initial phase of saltmarsh introduction, vegetation promotes the development of tidal networks, characterized by channel elongation, narrowing and deepening. However, under higher sediment supply, saltmarshes restrict sediment deposition on landward and central mudflats compared to that on unvegetated flats. Furthermore, sediment availability primarily facilitates the extension of pre-existing channels, while saltmarshes play a dual role in both generating new channels and elongating existing ones. This distinction highlights the competing mechanisms driving channel network development. 

Abstract Protists and viruses dynamically alter the flow of mass and energy through microbial food webs via predation. Simple microbial food web models show that the addition of microbial predators can increase the primary production of a microbial community but only for some configurations of food web structure. Under the conjecture that systems self-organize to maximize energy dissipation, known as the maximum entropy production (MEP) principle, we developed an MEP-based model that predicts microbial food web structure, and we examine how food web structure differs when entropy production is maximized over short versus long timescales. The model design follows from an experimental system and uses a trait-based variational method to set trait values by maximizing entropy production over a specified interval of time. Model results show that short-term MEP optimization produces microbial communities that specialize in substrate preference and consumers that have fewer trophic levels than solutions based on long-term optimization that have substrate generalists and more trophic levels. Our MEP-based approach provides an alternative to food web structure synthesis that does not depend on assumptions of community stability. 

Abstract This study examines the seasonal and spatial variabilities of total alkalinity (TA) and dissolved inorganic carbon (DIC) concentrations and fluxes in the Parker River, part of the Plum Island estuary, the largest macro tidal saltmarsh system in northeastern US. Our investigation reveals that the Parker River estuary is a net source of TA and DIC, with outwelling fluxes ranging 8.9–13.2 mmol m⁻² d⁻¹for TA and 10.4–18.1 mmol m⁻² d⁻¹for DIC. Porewater fluxes from marshes contributed to 71% (55%–96%) and 59% (51%–78%) of the TA and DIC outwelling flux, respectively. Spatial heterogeneity is pronounced, with high‐salinity section porewater fluxes nearly three times higher than in low‐salinity section. TA fluxes peak in summer and are lowest in the spring, with the seasonal changes controlled by both hydrology and marsh phenology. The interplay of primary production and aerobic and anaerobic respiration governs carbon transformations in the estuary, leading to net DIC export and contributing to the acidification of adjacent coastal waters. Overall, approximately 20% saltmarsh net primary production is laterally exported to the coastal ocean as TA during 2023, providing potential long‐term carbon sinks comparable to about 40% of sediment organic carbon burial. Our research underscores the spatial and temporal variabilities in TA and DIC fluxes driven by biotic processes and the importance of considering both TA and DIC fluxes to fully understand the carbon sequestration potential of saltmarshes. 

Abstract Saltmarshes provide vital ecosystem services, including coastal protection and habitat for fisheries. While feedbacks influencing vertical sediment accretion in marshes are well‐documented, including those between relative sea level and primary production, relationships among nutrient cycles, flooding, and primary production remain less explored. This study presents a unique 30‐year data set from the North Inlet estuary, South Carolina, tracking monthly growth of the dominant macrophyte,Spartina alterniflora, and porewater concentrations of sulfide, ammonium, and phosphate. Our findings reveal correlated seasonal and decadal patterns of sulfides, nutrients, and plant growth, with periodicities linked to seasonal climate and decadal tidal cycles.S.alterniflorathrives in anaerobic sediments where sulfides are toxic byproducts of sulfate‐reducing bacteria (SRB) produced through the oxidation of organic acids. Dissolved organic compounds fluctuated seasonally in tandem with plant growth. Organic acids utilized by SRB are released as root exudates and produced via cellulosic biomass fermentation. SRB fix nitrogen and produce sulfides that solubilize phosphate, compensating for nutrients lost in drainage and rafting of aboveground biomass. By directly and indirectly producing the substrates,Spartinaeffectively regulates SRB activity, thereby orchestrating biogeochemical cycles and accumulating sulfides that inhibit competing species. Our results challenge the traditional view of sulfides as mere phytotoxins and saltmarshes as passive nutrient transformers. Instead, we propose a model whereinS.alterniflorasustains its growth and supports estuarine productivity by regulating its nitrogen and phosphorus supplies through a mutualistic relationship with SRB. 

Abstract Sea level rise and storm surges are salinizing coastal areas, transforming forests and agricultural fields into salt marshes. This study examines seasonal variations in soil and groundwater hydrology at the transition zone between salt marshes and upland areas along the Mid‐Atlantic coast of the USA. We apply Linear Discriminant Analysis to data sets of soil moisture, soil conductivity, soil temperature, water table depth, groundwater salinity, and groundwater temperature to identify and categorize patterns within hydrological data. We further used Canonical Correlation Analysis to determine relationships between vegetation characteristics (greenness and Basal Area (BA)), soil composition (fraction of clay and sand), and hydrological parameters (groundwater level, groundwater specific conductivity, soil water content, and soil conductivity). Our analysis indicates that storm surges increase spatial variations in seasonal subsurface hydrological conditions among sites, as each location responds differently to disturbance. This heightened inter‐site variability contrasts with previous findings of storm‐driven homogenization within individual sites. By summer, sites become more homogeneous as hydrological variability decreases, indicating that vegetation gradients along the marsh–forest ecotone are mainly governed by winter and spring conditions. Vegetation characteristics (BA and greenness) are more strongly associated with soil water than with groundwater characteristics. Vegetation greenness also correlates closely with soil water conditions in fall and winter, when site differences are more pronounced. Groundwater salinity and water table depth are related to clay content in the soil. This regional‐scale comparison highlights the hydrological variability of the marsh‐upland boundary, how it is influenced by soil characteristics, and its relation to vegetation cover, offering insights for effective adaptation strategies to mitigate salinization and preserve coastal biodiversity. 

Abstract Plastic litter is a globally pervasive pollutant. Storms are likely key drivers of plastic transport to oceans, but plastic transport during rising and falling limbs of storm hydrographs is rarely measured. Measurements of plastic movement throughout individual storms will improve watershed models of plastic dynamics. We used cameras to quantify macroplastic movement (i.e., particles > 5 mm) in rivers before, during, and after individual storms (N = 18) at 10 sites within three North American watersheds. Most storms showed no difference in macroplastic transport between rising and falling hydrograph limbs or evidence of hysteresis (transport rate range = 0–236 items/30 min). Total macroplastic exported during storm events was positively related to storm magnitude and was greatest at more urban sites. Thus, macroplastic transport during storms was driven by storm size and land use. The quantitative relationships between macroplastic movement and hydrology will improve discharge‐weighted calculations of macroplastic transport which can benefit modeling, monitoring, and mitigation efforts. Practitioner PointsMacroplastic particles (i.e, > 5 mm) are both retained in urban streams (e.g., in debris dams), and move downstream during baseflow and stormflow conditionsStorm flows are key periods of macroplastic transport: transport rates are higher on both rising and falling limbs of storm hydrographs relative to baseflow.The amount of macroplastics moving during storm flows is positively related to storm intensity.The predictive relationships generated between storm flow and macroplastic transport will improve estimates of annual export, and policies for macroplastic pollution reduction. 

Abstract Tidal freshwater wetlands are critical for removing or sequestering watershed‐derived nitrogen loads before they reach the coast, where they can lead to eutrophication. However, rising seas and increasing climate variability will alter important physicochemical parameters that control nitrogen generation (e.g., nitrogen fixation) and removal processes (e.g., denitrification) in these habitats. Furthermore, the frequency and timing of these changes could vary from short, finite pulses during a storm or drought to long‐term presses from sea level rise, which may differentially affect biogeochemical cycling. We used intact core mesocosms to examine how microbial community structure and nitrogen cycling changed in response to increased temperature and salinity under pulse and press disturbances. We found that net N₂flux rates, defined as the balance between nitrogen fixation, which adds nitrogen, and denitrification, which removes it, did not directionally change in response to stressor pulse or press. Instead, it became more variable under both disturbance regimes, underscoring the importance of both denitrification and nitrogen fixation in these systems. Nitrous oxide production rates, however, decreased and became more stable over time in the press scenario but remained highly variable in the pulse scenario. Under both pulse and press disturbance, both the overall and the active component of the microbial community changed, particularly in response to the salinity treatment. Although there was an overall community shift, core members of the microbiome capable of denitrification and nitrogen fixation persisted. Both pulses and presses of temperature and salinity changed the microbial communities of tidal freshwater wetlands, but a combination of microbial resistance and functional redundancy appears to allow important N cycling processes to persist. These findings provide valuable knowledge on the functional and structural potential of the nitrogen cycling microbial communities in tidal freshwater wetlands when facing future climate variability.