Geology topics

Joel A. Carr

Publications and source records attributed to Joel A. Carr.

At least 19 recordsLinked to original sources

USGS2026

Climate change is reshaping seagrass futures: Novel environments and opportunities along coastlines and across oceans

Seagrasses are a highly productive ecosystem that supports biodiversity and fisheries, protects coastlines, and buries globally significant carbon stocks. Because seagrasses are increasingly stressed by warmer, darker, and more variable waters, established management strategies may not guarantee future success. Here, we depict a global outlook for seagrass futures by synthesizing how climate and non-climate human pressures may restructure meadow composition, food webs, fisheries, and blue carbon services. Current seagrass range shifts, adaptations, and acclimations to novel environmental conditions preview future states: climate-tolerant species often become dominant post-disturbance, seeds can disperse hundreds of kilometers, and local management can facilitate habitat persistence and large-scale restoration. To boost climate resilience, we propose a three-part framework involving (1) expanding seagrass mariculture and planting, (2) detecting and predicting shifting stressor gradients and species distributions, and (3) managing for seagrasses tolerant of novel environments. Together, these strategies offer proactive pathways for cultivating climate-adapted seagrass meadows worldwide.

BioScience↗

USGS2026

Towards a refined definition of “tidal influence” for the coastal wetland sciences

The astronomical tide relates to lunar (and to a lesser extent solar) influences on rising and falling water bodies, and is a key control of many coastal processes. However, tides manifest along coastal margins in different ways to muddle application beyond Sir Isaac Newton’s first descriptions. Here, we argue that tide, or specifically “tidal,” has not been adequately defined for the coastal wetland sciences to facilitate contemporary interdisciplinary distinction for rapidly changing coastlines and future climate adaptation planning. We provide a brief history of the tidal concept, and we describe tides in a diversity of uncommonly recognized marginal coastal environments. Common astronomical tide manifestations are described through hydrographic examples, then expanded to include those tidal environments exposed additionally to fluvial, meteorological (wind), or groundwater influences. Our expanded definition of “tidal” is intended to be a forward-looking construct as environments will inevitably change with sea-level rise, water extraction, and anthropogenic flow alteration. Our definition of wetland tidal influence, which we broaden, includes those “wetlands affected by astronomic water level fluctuations within coastally restricted environments causing frequent or infrequent surface inundation or groundwater tidal variation that affects the biogeochemistry of the soil (e.g., ion exchange capacity, oxygen state, mineralization processes)”. Astronomic water level fluctuations are often amplified or dampened by non-astronomical events, such as wind. Expanding the definition of what is considered tidal by the scientific community is critical for improved understanding of coastal wetland function and the suite of ecosystem services they provide people and society, including climate mitigation opportunity, management, and adaptation.

Wetlands↗

USGS2026

Loss and transformation of coastal wetlands due to global change in the conterminous United States: Past, present, and future

Coastal wetlands are being transformed by global change, impacting the ecological and societal benefits provided by these ecosystems. Synthesizing knowledge of historical and expected future transformations in coastal wetlands can help inform forward-looking planning and stewardship efforts. Here, we review anticipated future ecological transformations in coastal wetlands of the conterminous United States and contrast them with past transformations. We examine trends at the national scale but focus in detail on the following six wetland-rich regions: (1) Everglades, (2) Mississippi River delta, (3) Chesapeake Bay, (4) San Francisco Estuary, (5) Puget Sound, and (6) Great Lakes. This review is among the first to integrate and compare coastal wetland transformations in the Great Lakes with their oceanic counterparts. Between 1850 and 1970, wetland losses across the nation were high because coastal wetlands were viewed as flooded wastelands needing to be drained, filled, or excavated to make room for other land uses. Over the last 50 years, growing public and scientific appreciation of the ecological and societal benefits provided by coastal wetlands has fostered tremendous gains in wetland restoration and legal protection. Looking to the future, climate change and accelerated sea-level rise have become critical threats to coastal wetlands and are emerging as major drivers of ecological loss and transformation. This review synthesizes regional knowledge regarding past, present, and future changes to help coastal scientists, environmental managers, and the public better anticipate and prepare for future coastal wetland transformations due to climate change, accelerated sea-level rise, land-use change, and other aspects of global change.

conterminous United States↗

USGS2026

Challenges and opportunities for national-scale projections of future coastal landscape change

Local to global scale projections of future coastal landscape change are essential to improve land and resource management decisions that aim to prepare for and reduce risk exposure to impending coastal hazards. However, the availability of actionable knowledge is often limited due to the complexity of drivers of change, their consequences, and uncertainties that span disciplines. Varying spatial and temporal scales of physical processes that occur on landscapes with differing levels of coastal development and modification complicate decision-making. This paper reviews advances in observational approaches, modeling, and quantification of uncertainty to support the next generation of coastal change projections. We also highlight transdisciplinary opportunities for making this information more accessible and useful to support decision-making in the coastal zone. This includes consideration of different levels of user engagement to address specific needs, as well as resources and mechanisms that can support knowledge transfer broadly. A central recommendation is the development of a coordinated framework to support research integration including three components: accounting for relevant drivers, processes, and feedbacks; working with users to identify their information needs; and generating meaningful projections and uncertainty. Actions that would support such frameworks include the evaluation of models through coordinated research approaches and multi-model comparisons; ongoing investments in observations and data-driven approaches necessary to understand this complex environment; expansion of modeling capabilities using novel techniques; prioritization of co-development efforts between researchers and users; and continued training and support for transdisciplinary research.

Alaska, California, Florida, New York↗

USGS2026

Carbon sequestration along a gradient of tidal marsh degradation in response to sea level rise

Tidal marshes are considered one of the world's most efficient ecosystems for belowground organic carbon sequestration and hence climate mitigation. Marsh systems are however also vulnerable to degradation due to climate-induced sea level rise, whereby marsh vegetation conversion to open water often follows distinct spatial patterns: levees (i.e. marsh zones < 10 m from tidal creeks) show lower vulnerability of vegetation conversion to open water than basins (i.e. interior marsh zones > 30 m from creeks). Here, we use sediment cores to investigate spatial variations in organic carbon accumulation rates (OCAR) in a microtidal system (Blackwater marshes, Maryland, USA): (1) across a gradient of marsh zones with increasing marsh degradation, assessed as increasing ratio of unvegetated versus vegetated marsh area and (2) by comparing levees versus basins. We show that OCAR is up to four times higher on marsh levees than in adjacent basins. The data suggest that this is caused by spatial variation in three processes: sediment accretion rate, vegetation productivity, and sediment compaction, which are all higher on levees. Additionally, OCAR was observed to increase with increasing degree of marsh degradation in response to sea level rise. We hypothesize this may be due to more soil waterlogging in more degraded marsh zones, which may decrease carbon decomposition. Our results highlight that tidal marsh levees, in a microtidal system, are among the fastest soil organic carbon sequestration systems on Earth, and that both levees and basins sustain their carbon accumulation rate along gradients of increasing marsh degradation in response to sea level rise.

Maryland↗

USGS2026

The contribution of a surge event to infilling in a barrier-enclosed estuary: Insights from field observations

Many estuaries worldwide face increasing sediment loading caused by catchment land use change and intensification, creating subsequent adverse effects on estuarine ecosystems. Extreme weather events can disproportionately alter sediment pathways and loading. Although storm-driven sediment exchange has been widely examined at open coasts and inlets, key transport mechanisms within constricted, sheltered estuaries remain understudied. This study presents an observational dataset capturing the impact of a 99th percentile water-level event (based on 20 years of records) on sediment transport pathways in a sheltered, barrier-enclosed estuary. This event, driven by a 3-day storm surge (>0.5 m) combined with a spring tide, was recorded during a 3-week field campaign. Sediment transport pathways and riverine contributions were analysed, and observations revealed substantial changes in suspended sediment concentrations increasing from 18 mg/l to 70 mg/l during the event. The elevated water levels and resulting pressure gradient at the constricted study site entrance caused by the storm surge increased local flood dominance. Combined with higher flow velocities and resuspension, the storm led to a sixfold increase in sediment import at the estuary entrance and a 600-fold increase in sediment flux to the upper estuary. The decoupling of peak suspended sediment concentrations from streamflow indicates that the resuspension of estuarine legacy sediment, rather than catchment inputs, dominated the system's response. These findings challenge assumptions about estuarine sediment budgets and emphasise that incorporating high water-level surge events into models can enhance the prediction of long-term estuarine evolution. Given projected increases in storm frequency under climate change, understanding these episodic but highly consequential sediment pulses can support the assessment of wetland resilience and inform estuarine management strategies.

Whangateau Estuary↗

USGS2025

Present-day vertical land motions (VLM) of the Chesapeake Bay region derived from robust network imaging of global navigation satellite system (GNSS) observations

The Chesapeake Bay region (defined as longitudes − 78° to -74° and latitudes 36.5° to 40°) experiences the highest rates of relative sea-level rise (RSLR) on the Atlantic Coast. Regional land subsidence influences RSLR, however quantified rates of vertical land motions (VLM) are inconsistent in published solutions. For 5 years from 2019 to 2023, new Global Navigation Satellite System (GNSS) campaign data were collected at over 60 sites across the Chesapeake Bay region annually. These data were processed and combined with continuous GNSS data (120 stations) from the region covering the same time-period using GAMIT-GLOBK to produce 3D velocities and their associated uncertainties. We use the Robust Network Imaging algorithm to interpolate GNSS-derived VLM to produce a new regional VLM solution of the Chesapeake Bay region. We find that land subsidence is ubiquitous throughout the region with rates varying from − 2.97 to -0.40 mm/yr. In major cities across the Chesapeake Bay region, VLM rates are − 1.1 ± 1.6 mm/yr (1-sigma) for Washington DC, -0.8 ± 1.4 mm/yr for Baltimore, MD, -2.4 ± 0.5 mm/yr for Ocean City, MD, and − 2.3 ± 1.0 mm/yr for Hampton, VA. When we compare our VLM rates with a geodetic-based solution from 1974, we observe meaningful shifts in the locations and rates of maximum subsidence. The results of this work underscore that regular monitoring of VLM and can be used to improve projections of relative sea-level changes as well as the associated coastal hazards for communities in the Chesapeake Bay region.

Delaware, Maryland, New Jeresy, Pennsylvania, Virg↗

USGS2025

A simple predictive model for salt marsh internal deterioration under sea-level rise and sediment deficits: Application to Chesapeake Bay

Salt marshes are dynamic biogeomorphic systems reliant on autochthonous and allochthonous input to maintain their three-dimensional configuration. Sea-level rise, subsidence, and sediment deficits can lead to submergence, open-water expansion, and ultimately loss of the vegetated marsh plain and associated ecosystem services. Widely used management-focused models focus on vegetation zonation in response to sea level but neglect sediment transport processes and geomorphic change. Process-based research models attempt to represent complex physical and biogeomorphic interactions but operate on spatiotemporal scales that are not directly transferable to restoration or management. Here we bridge these two paradigms and present a novel geomorphic model (UBMorph) based on the sediment-based lifespan concept that accounts for sea-level rise and open-water expansion to predict changes in salt marsh area in Chesapeake Bay. Model parameters such as surface accretion rate and elevation-to-areal loss fraction are selected using a separate, fully coupled biogeomorphic model (MarshMorpho2D) and the predicted lifespan is then compared with high marsh coverage from a zonation model (SLAMM). Across all of Chesapeake Bay, UBMorph estimates an overall loss of 404 km 2 (37%) of vegetated marsh area under a dynamic 3–12 mm/y sea-level rise scenario (between 2010 and 2110). We then demonstrate a management-focused application of UBMorph and SLAMM used in tandem, for developing both a marsh condition and restoration model of the Chesapeake Bay portion of Maryland. The restoration model, which includes hydrologic intervention and sediment placement actions, indicates that ~ 400 km 2 of marsh require either no intervention or low effort hydrologic intervention presently, whereas if no action is taken, over 700 km 2 will require high effort intervention by 2070. This synthesis of research models with management-focused decision models demonstrates a tangible advance in bridging the gap between process-based research and restoration needs.

Maryland, Virginia↗

USGS2025

Causal interpretations can be based on mechanistic knowledge

There exists a long-standing disconnect between statistical and mechanistic approaches to the development of causal understanding. Statistical approaches, which have dominated the literature, have focused on the need to obtain perfectly unbiased estimates of causal effects often using either experimental, quasi-experimental or other methods. Mechanistic approaches have instead focused on investigating how systems work by elucidating the structures and processes whereby variations in one system property can propagate to other system properties. Explicit references to ‘causal effects’ have tended to require adherence to statistical methods and standards, inadvertently downplaying the suitability of mechanistic knowledge for that purpose. It has been recently demonstrated that both mechanistic and statistical approaches can contribute to the long-term goal of developing causal knowledge and understanding. Proponents of statistical causal inference have seldom recommended that mechanistic evidence be relied upon to support causal interpretations. This paper provides a clear and thorough example where a causal interpretation can be supported based on mechanistic knowledge. Arguing for a causal interpretation based on knowledge of mechanisms has typically been an informal process and one that has thus far infrequently led to explicit declarations of causal knowledge by scientists. To overcome this problem, we illustrate a recently described procedure referred to as ‘causal knowledge analysis’ to summarize explicit support for causal interpretations. In this paper, we first clarify the basis of the long-standing disagreement by describing the crux of the problem as viewed from a statistical perspective and by describing how it can be overcome when there is sufficient mechanistic knowledge. We then offer a proof-of-concept example based on robust documentation and description of the mechanisms whereby plants causally regulate the responses of coastal marsh elevation to changes in sea level. Synthesis —The evidential requirements for declaring a relationship to be causal have been obscured until very recently, leading to a long neglect of this issue by scientists. Meanwhile, subject matter experts have accumulated a vast body of undeclared causal knowledge that we now need to recognize in order to position scientists as essential players in defending causal interpretations.

Journal of Ecology↗

USGS2025

Multi-model comparison of salt marsh longevity under relative sea-level rise

Understanding salt marsh resilience under increasing sea levels can inform for management decisions. We compared temporal projections from various wetland process-based models and a geospatially derived metric (i.e., marsh lifespan) to understand key considerations and uncertainties about salt marsh resilience when using these products for decision-making. The influences of lidar topographic correction and marsh surface sediment accretion were explored across a suite of relative sea level rise (RSLR) projections to assess differences in the timing and amount of habitat change for each modeling approach. All models were run for a small coastal wetland site located in the Chesapeake Bay, Maryland, USA, to assess potential change in marsh habitat, and timing of marsh loss due to RSLR. All modeling results agreed that marsh longevity was threatened by RSLR but they varied in the time of predicted marsh submergence between the years 2070 and 2100 depending on the initial marsh surface elevation and accretion rates. Models with similar accretion rates predicted similar years until marsh submergence. Removing a positive elevation bias from lidar surveys in densely vegetated marsh areas for these models resulted in onset of submergence ~ 7 years earlier. Because there are many tradeoffs to each model type, end users need to evaluate management questions, overall goals, the amount of effort involved in model parameterization, and the amount of uncertainty in the model that they are willing to accept.

Maryland↗

USGS2025

Ecological thresholds and transformations due to climate change: The role of abiotic stress

An ecological threshold is the point at which a comparatively small environmental change triggers an abrupt and disproportionately large ecological response. In the face of accelerating climate change, there is concern that abrupt ecosystem transformations will become more widespread as critical ecological thresholds are crossed. There has been ongoing debate, however, regarding the prevalence of ecological thresholds across the natural world. While ecological thresholds are ubiquitous in some ecosystems, thresholds have been difficult to detect in others. Some studies have even concluded that threshold responses are uncommon in the natural world and overly emphasized in the ecological literature. As ecologists who work in ecosystems chronically exposed to high abiotic stress, we consider ecological thresholds and ecosystem transformations to be critical concepts that can greatly advance understanding of ecological responses to climate change and inform ecosystem management. But quantifying ecological thresholds can be challenging, if not impossible, without data that are strategically collected for that purpose. Here, we present a conceptual framework built upon linkages between abiotic stress, climate-driven ecological threshold responses, and the risk of ecosystem transformation. We also present a simple approach for quantifying ecological thresholds across abiotic stress gradients. We hypothesize that climate-driven threshold responses are especially influential in ecosystems chronically exposed to high abiotic stress, where autotroph diversity is low and foundation species play a prominent ecological role. Abiotic conditions in these environments are often near physiological tolerance limits of foundation species, which means that small abiotic changes can trigger landscape-level ecological transformations. Conversely, the alleviation of stress near thresholds can allow foundation species to thrive and spread into previously inhospitable locations. We provide examples of this climate-driven threshold behavior from four high-stress environments: coastal wetlands, coral reefs, drylands, and alpine ecosystems. Our overarching aim in this review is to clarify the strong relationships between abiotic stress, climate-driven ecological thresholds, and the risk of ecosystem transformation under climate change.

Ecosphere↗

USGS2025

Distribution and disturbances of ditches across salt marshes of the Northeast U.S. with implications for management and restoration

Effective management of valuable coastal systems, such as salt marshes requires an understanding of the complex stressors influencing their continued threat of drowning. However, efforts to determine the effects of one potential stressor, ditches, have produced diverging results complicating management efforts. Ditches (linear trenches dug to drain salt marshes for agriculture and mosquito control) alter salt marsh hydrology, but their effects on widescale marsh function and degradation are poorly understood. We created a dataset of visible ditches and summarized ditch densities (length of ditches over area) for salt marshes of the Northeast U.S. to evaluate ditching against vulnerability metrics, including elevation and the unvegetated to vegetated marsh ratio (UVVR). We identified a scale dependency in which the larger/coarser the spatial scale of analysis, the greater the fraction of ditched salt marshes. Scale dependence explains discrepancies between previously determined ditch indices. In terms of effects on marsh vulnerability, relative elevation was not influenced by visible ditch presence. Ditch densities affected UVVR, exhibiting a multiple threshold behavior. When present at low densities, ditches have little effect on ponding; yet as ditch densities increase, UVVR (i.e., ponding) increases. The relationship between ditching and UVVR reverses at the highest ditch densities, with ponding substantially decreasing. The multiple threshold vulnerability response of Northeast salt marshes to the hydrologic influences imposed by ditching suggests restoration strategies should consider the degree of ditching rather than simply ditching presence.

Connecticut, Delaware, Maine, Maryland, Massachuse↗

USGS2024

Trends and environmental impacts of virtual water trade

Virtual water describes water embedded in the production of goods and offers meaningful insights about the complex interplay between water, trade and sustainability. In this Review, we examine the trends, major players, traded products and key drivers of virtual water trade (VWT). Roughly 20% of water used in global food production is traded virtually rather than domestically consumed. As such, agriculture dominates VWT, with livestock products, wheat, maize, soybean, oil palm, coffee and cocoa contributing over 70% of total VWT. These products are also driving VWT growth, the volume of which has increased 2.9 times from 1986 to 2022. However, the countries leading VWT contributions (with China, the United States, the Netherlands, Germany and India accounting for 34% of the global VWT in 2022) have remained relatively stable over time, albeit with China becoming an increasingly important importer. VWT can mitigate the effects of water scarcity and food insecurity, although there are concerns about the disconnect between consumers and the environmental impacts of their choices, and unsustainable resource exploitation. Indeed, approximately 16% of unsustainable water use and 11% of global groundwater depletion are virtually traded. Future VWT analyses must consider factors such as water renewability, water quality, climate change impacts and socioeconomic implications.

Nature Reviews Earth & Environment↗

USGS2024

Projecting mangrove forest resilience to sea-level rise on a Pacific Island: Species dynamics and ecological thresholds

Mangroves can increase their elevation relative to tidal flooding through biogeomorphic feedbacks but can submerge if rates of sea-level rise are too great. There is an urgent need to understand the vulnerability of mangroves to sea-level rise so local communities and resource managers can implement and prioritize actions. The need is especially pressing for small islands, which have been identified as an area of concern by the IPCC. We developed a generalizable modeling framework for tidal wetlands (WARMER-3) that accounts for species interactions and the belowground processes that dictate soil elevation building relative to sea levels. The model was calibrated with extensive field datasets, including accretion rates derived from 29 soil cores, over 300 forest inventory plots, water level, and elevation. The model included five mangrove tree species and was applied across seven regions around the Pacific Island of Pohnpei, Federated States of Micronesia, where mangrove forest is a critical ecosystem that supports subsistence living for local communities. We explored mangrove resilience and carbon accumulation under six sea-level rise scenarios. We also conducted an analysis to determine the sea-level rise rate threshold above which mangroves would be lost. The results suggest that Pohnpei mangroves can build their elevations relative to low and moderate rates of sea-level rise to prevent submergence, with limited loss of mangrove area through 2150. Under higher sea-level rise rates, however, forest elevation decreased substantially relative to mean sea level and there was extensive loss of mangrove area by that year. Regarding mangrove community composition, for all sea-level rise scenarios, the model predicted a change to increasing relative abundance of flood tolerant species and decreasing relative abundance of high-elevation species, which started to being realized by 2100. Variation in sediment supply, water levels, and elevation capital led to differential vulnerability around the island. We identified a threshold for Pohnpei mangroves where if local sea-level rise rates exceed 7.8 ± 2.2 mm/year they are projected to eventually submerge and be lost. Our modeling framework is novel by addressing both species interactions and critical belowground processes to better understand potential tidal ecosystem responses to sea-level rise.

Pohnpei↗

USGS2024

Rising seas could cross thresholds for initiating coastal wetland drowning within decades across much of the United States

Accelerated sea-level rise is an existential threat to coastal wetlands, but the timing and extent of wetland drowning are debated. Recent data syntheses have clarified future relative sea-level rise exposure and sensitivity thresholds for drowning. Here, we integrate these advances to estimate when and where rising sea levels could cross thresholds for initiating wetland drowning across the conterminous United States. Our results show that there is much spatial variation in relative sea-level rise rates, which impacts the potential timing and extent of wetlands crossing thresholds. High rates of relative sea-level rise along wetland-rich parts of the Gulf of Mexico and Atlantic coasts highlight areas where wetlands are already drowning or could begin to drown within decades, including large wetland landscapes within the Mississippi River delta, Greater Everglades, Chesapeake Bay, Texas, Georgia, and the Carolinas. Collectively, our results underscore the need to prepare for transformative coastal change.

Communications Earth & Environment↗

USGS2024

Microtopographic variation as a potential early indicator of ecosystem state change and vulnerability in salt marshes

As global climate change alters the magnitude and rates of environmental stressors, predicting the extent of ecosystem degradation driven by these rapidly changing conditions becomes increasingly urgent. At the landscape scale, disturbances and stressors can increase spatial variability and heterogeneity — indicators that can serve as potential early warnings of declining ecosystem resilience. Increased spatial variability in salt marshes at the landscape scale has been used to quantify the propagation of ponding in salt marsh interiors, but ponding at the landscape scale follows a state change rather than predicts it. Here, we suggest a novel application of commonly collected surface elevation table (SET) data and explore millimeter-scale marsh surface microtopography as a potential early indicator of ecosystem transition. We find an increase in spatial variability using multiple metrics of microtopographic heterogeneity in vulnerable salt marsh communities across the North American Atlantic seaboard. Increasing microtopographic heterogeneity in vulnerable salt marshes mirrored increasing trends in variance when a tipping point is approached in other alternative stable state systems — indicating that early warning signals of marsh drowning and ecosystem transition are observable at small-spatial scales prior to runaway ecosystem degradation. Congruence between traditional and novel metrics of marsh vulnerability suggests that microtopographic metrics can be used to identify hidden vulnerability before widespread marsh degradation. This novel analysis can be easily applied to existing SET records expanding the traditional focus on vertical change to additionally encapsulate lateral processes.

Atlantic Coast↗

USGS2024

Planning hydrological restoration of coastal wetlands: Key model considerations and solutions

The hydrological restoration of coastal wetlands is an emerging approach for mitigating and adapting to climate change and enhancing ecosystem services such as improved water quality and biodiversity. This paper synthesises current knowledge on selecting appropriate modelling approaches for hydrological restoration projects. The selection of a modelling approach is based on project-specific factors, such as costs, risks, and uncertainties, and aligns with the overall project objectives. We provide guidance on model selection, emphasising the use of simpler and less expensive modelling approaches when appropriate, and identifying situations when models may not be required for project managers to make informed decisions. This paper recognises and supports the widespread use of hydrological restoration in coastal wetlands by bridging the gap between hydrological science and restoration practices. It underscores the significance of project objectives, budget, and available data and offers decision-making frameworks, such as decision trees, to aid in matching modelling methods with specific project outcomes.

Science of the Total Environment↗

USGS2023

Biophysical drivers of coastal treeline elevation

Sea level rise is leading to the rapid migration of marshes into coastal forests and other terrestrial ecosystems. Although complex biophysical interactions likely govern these ecosystem transitions, projections of sea level driven land conversion commonly rely on a simplified “threshold elevation” that represents the elevation of the marsh-upland boundary based on tidal datums alone. To determine the influence of biophysical drivers on threshold elevations, and their implication for land conversion, we examined almost 100,000 high-resolution marsh-forest boundary elevation points, determined independently from tidal datums, alongside hydrologic, ecologic, and geomorphic data in the Chesapeake Bay, the largest estuary in the U.S. located along the mid-Atlantic coast. We find five-fold variations in threshold elevation across the entire estuary, driven not only by tidal range, but also salinity and slope. However, more than half of the variability is unexplained by these variables, which we attribute largely to uncaptured local factors including groundwater discharge, microtopography, and anthropogenic impacts. In the Chesapeake Bay, observed threshold elevations deviate from predicted elevations used to determine sea level driven land conversion by as much as the amount of projected regional sea level rise by 2050. These results suggest that local drivers strongly mediate coastal ecosystem transitions, and that predictions based on elevation and tidal datums alone may misrepresent future land conversion.

Delaware, Maryland, Virginia↗