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Intertidal creeks (channel width <5 m) weave through salt marshes, delivering water, nutrients, and sediments into the marsh interior and affecting spatial heterogeneity in plant and animal distributions. Despite their global prevalence, creek connectivity, and the mechanisms controlling cross‐marsh hydrodynamics, remain poorly resolved. In this study, we measured flow and total suspended solids transport in three intertidal creeks within a confined drainage basin in a Georgia, USA salt marsh. We discovered that the effective drag is 3–12 times greater than bed drag, reaching levels similar to those observed in coral reefs. Furthermore, the drag between tidal flood and ebb phases differs, indicating an asymmetric drag. Analyses of along‐channel momentum reveal that pressure gradient O(10−3−10−2) m/s2 and friction O(10−3−10−2) m/s2 dominate creek momentum balance. Divergence in tidal and suspended solids transport between adjacent creeks revealed contrasting tidal transport asymmetries (i.e., flood or ebb dominated) within this confined basin. We discuss how these differences may alter the eco‐geospatial evolution of salt marshes and their response to sea‐level rise. Plain Language Summary: Salt marshes generate a valuable suite of ecological and economic services. Key to understanding their fate in relation to rising sea levels is understanding the eco‐physical dynamics of the creeks that weave through these marshes, as these features serve as major conduits of water, sediments, and biological material exchange. Models have been developed to explain how flow moves through creeks over one tidal cycle. However, there are limited field measurements resolving the dominant forms of drag controlling tidal fluxes through creeks, divergences in water flow and sediment transport between creeks, or creek‐creek circulations. This study applies field measurements from a Georgia salt marsh to address these knowledge gaps. We discover that adjacent creeks in a confined drainage basin are governed by form drag produced by flow interactions with the landscape and vegetation and exhibit profound heterogeneity in tidal flows and sediment transport. We highlight that this local‐scale heterogeneity likely plays a major role in altering the pattern and mechanisms by which salt marshes are responding to sea‐level rise. Key Points: Effective drag is ∼3–12 times greater than bed drag in intertidal creeks with channel widths less than 5 mWithin a confined drainage area, multiple creeks have differing flood/ebb durations, and opposite net water and suspended solids transport (i.e., net import or export)Tidal creek flows regularly to exchange and interact with one another [ABSTRACT FROM AUTHOR]

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Consumers 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. [ABSTRACT FROM AUTHOR]

Foundation species often interact with each other and co-create habitat upon which other species depend. Whether the presence of these facilitated species feeds back to mediate the growth and resilience of the foundation species themselves, and influence the strength of their interactions, remains poorly understood. In a 16-month field experiment in a southeastern US salt marsh, we tested how the overlapping presence of two foundation species, cordgrass (Spartina alterniflora) and ribbed mussels (Geukensia demissa), influences the abundance of facilitated species, specifically burrowing crabs (mainly Uca pugnax), and how crabs, in turn, affect each foundation species and their mutualistic interaction. Mussel aggregations enhanced crab abundance 3.9-fold, which in turn reduced both mussel and cordgrass growth and stifled cordgrass recovery after a simulated disturbance. Porewater and plant tissue analyses suggest crabs reduced cordgrass growth by reducing nitrogen availability, damaging roots, and potentially interfering with mussel deposition of nutrient-rich pseudofeces. A five-site field survey along 700 km southeastern US coastline revealed that cordgrass biomass and crab abundance are consistently higher in mussel aggregations. Furthermore, cordgrass biomass correlated negatively with crab abundance, supporting our experimental findings and the hypothesis that facilitated biota can negatively impact the foundation species upon which they depend. We anticipate that such negative, but non-lethal, feedbacks between foundation species and the biota they facilitate may be a common but overlooked phenomenon controlling foundation species growth and interactions in a wide range of ecosystems. [ABSTRACT FROM AUTHOR]

Although there is mounting evidence that biodiversity is an important and widespread driver of ecosystem multifunctionality, much of this research has focused on small-scale biodiversity manipulations. Hence, which mechanisms maintain patches of enhanced biodiversity in natural systems and if these patches elevate ecosystem multifunctionality at both local and landscape scales remain outstanding questions. In a 17 month experiment conducted within southeastern United States salt marshes, we found that patches of enhanced biodiversity and multifunctionality arise only where habitat-forming foundation species overlap—i.e. where aggregations of ribbed mussels (Geukensia demissa) form around cordgrass (Spartina alterniflora) stems. By empirically scaling up our experimental results to the marsh platform at 12 sites, we further show that mussels—despite covering only approximately 1% of the marsh surface—strongly enhance five distinct ecosystem functions, including decomposition, primary production and water infiltration rate, at the landscape scale. Thus, mussels create conditions that support the co-occurrence of high densities of functionally distinct organisms within cordgrass and, in doing so, elevate salt marsh multifunctionality from the patch to landscape scale. Collectively, these findings suggest that patterns in foundation species' overlap drive variation in biodiversity and ecosystem functioning within and across natural ecosystems. We therefore argue that foundation species should be integrated in our conceptual understanding of forces that moderate biodiversity–ecosystem functioning relationships, approaches for conserving species diversity and strategies to improve the multifunctionality of degraded ecosystems. [ABSTRACT FROM AUTHOR]

Formed at the confluence of marine and fresh waters, estuaries experience both the seaside pressures of rising sea levels and increasing storm severity, and watershed and precipitation changes that are shifting the quality and quantity of freshwater and sediments delivered from upstream sources. Boating, shoreline hardening, harvesting pressure, and other signatures of human activity are also increasing as populations swell in coastal regions. Given this shifting landscape of pressures, the factors most threatening to estuary health and stability are often uncertain. To identify the greatest contemporary threats to coastal wetlands and oyster reefs across the southeastern United States (Mississippi to North Carolina), we summarized recent population growth and land-cover change and surveyed estuarine management and science experts. From 1996 to 2019, human population growth in the region varied from a 17% decrease to a 171% increase (mean = +43%) with only 5 of the 72 SE US counties losing population, and nearly half growing by more than 40%. Individual counties experienced between 999 and 19,253 km2 of new development (mean: 5725 km2), with 1–5% (mean: 2.6%) of undeveloped lands undergoing development over this period across the region. Correspondingly, our survey of 169 coastal experts highlighted development, shoreline hardening, and upstream modifications to freshwater flow as the most important local threats facing coastal wetlands. Similarly, experts identified development, upstream modifications to freshwater flow, and overharvesting as the most important local threats to oyster reefs. With regards to global threats, experts categorized sea level rise as the most pressing to wetlands, and acidification and precipitation changes as the most pressing to oyster reefs. Survey respondents further identified that more research, driven by collaboration among scientists, engineers, industry professionals, and managers, is needed to assess how precipitation changes, shoreline hardening, and sea level rise are affecting coastal ecosystem stability and function. Due to the profound role of humans in shaping estuarine health, this work highlights that engaging property owners, recreators, and municipalities to implement strategies to improve estuarine health will be vital for sustaining coastal systems in the face of global change. [Display omitted] • Between 1996 and 2019, coastal counties in the southeastern US grew by −17%–171%. • Regionally, 2.6% of undeveloped lands were developed between 1996 and 2016. • Coastal experts cite development as greatest local threat to wetlands and oysters. • Experts cite sea level rise as greatest global threat to SE US wetlands. • Acidification and changing rainfall cited as key global threats to SE US oysters. [ABSTRACT FROM AUTHOR]