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At least 1,639 records · Page 91Linked to original sources
USGS2022

Restoring aquatic habitats through dam removal

This report presents results from a four-year project (2018–2022) to document the effects of small, run-of-river dams and dam removal on water quality (stream temperature and dissolved oxygen (DO)), aquatic macroinvertebrates, and fishes. Temperature and DO are critical water quality parameters that shape biogeochemical processes and biotic assemblages in streams. Macroinvertebrate and fish assemblages can be reflective of habitat and water quality due to their diversity and sensitivity to high temperatures and low DO and are often used as indicators of ecosystem health (e.g., Clean Water Act Section 401). This study aimed to better explain the responses of these important ecological parameters to small dam removals, which may support a more comprehensive understanding of the benefits of restoration to aquatic ecosystems. We collected pre- and post-restoration water quality data and macroinvertebrate samples at 16 small dams in Massachusetts that have been removed (10 sites) or are currently being considered for removal (6 sites). General results from these monitoring efforts indicate that: ● 15 of 16 small dams increased impoundment water temperatures and warming persisted downstream at 11 of those sites, relative to upstream. Dam removal reduced summer impoundment warming at 7 of 10 removal sites and reduced downstream warming at 5 of 10 sites. These in-stream temperature improvements occurred within 5 years after dam removal. ● 13 of 16 small dams negatively impacted dissolved oxygen (DO) concentrations within the impoundments, but the magnitude of impact varied across sites. Negative impoundment DO impacts did not consistently translate downstream, and downstream responses to dam removal were generally minimal and variable across sites. Dam removal significantly reduced negative impoundment DO impacts within 1 year after removal at 7 of 10 sites, and sites with greater pre-removal impacts experienced the greatest magnitude of DO recovery after dam removal. ● Interannual variability in dam impacts on water quality across sites suggests periods of extreme weather (i.e., droughts or high precipitation) due to climate change may exacerbate adverse impacts from run-of-river dams. ● Macroinvertebrate assemblages within dam impoundments differed from assemblages in adjacent un-impounded stream sections and exhibited a loss of sensitive organisms (an average of 17% fewer). Dam removal led to more similar macroinvertebrate assemblages throughout most stream sections, and recovery of sensitive taxa occurred relatively quickly (1-3 years). ● Fish species richness increased upstream at 2 of 10 removal sites, suggesting potential increases in fish passage from downstream reaches. However, particular species, such as American Eel ( Anguilla rostrata ), exhibited both positive and negative responses to dam removal across study sites. Incorporating more sites with pre-and post-dam removal fish data could allow for better understanding factors explaining site-specific differences.

Massachusetts↗
USGS2024

Paleogene Earth perturbations in the US Atlantic Coastal Plain (PEP-US): Coring transects of hyperthermals to understand past carbon injections and ecosystem responses

The release of over 4500 Gt (gigatonnes) of carbon at the Paleocene–Eocene boundary provides the closest geological analog to modern anthropogenic CO 2 emissions. The cause(s) of and responses to the resulting Paleocene–Eocene Thermal Maximum (PETM) and attendant carbon isotopic excursion (CIE) remain enigmatic and intriguing despite over 30 years of intense study. CIE records from the deep sea are generally thin due to its short duration and slow sedimentation rates, and they are truncated due to corrosive bottom waters dissolving carbonate sediments. In contrast, PETM coastal plain sections along the US mid-Atlantic margin are thick, generally having an expanded record of the CIE. Drilling here presents an opportunity to study the PETM onset to a level of detail that could transform our understanding of this important event. Previous drilling in this region provided important insights, but existing cores are either depleted or contain stratigraphic gaps. New core material is needed for well-resolved marine climate records. To plan new drilling, members of the international scientific community attended a multi-staged, hybrid scientific drilling workshop in 2022 designed to maximize not only scientifically and demographically diverse participation but also to protect participants' health and safety during the global pandemic and to reduce our carbon footprint. The resulting plan identified 10 sites for drill holes that would penetrate the Cretaceous–Paleogene (K–Pg) boundary, targeting the pre-onset excursion (POE), the CIE onset, the rapidly deposited Marlboro Clay that records a very thick CIE body, and other Eocene hyperthermals. The workshop participants developed several primary scientific objectives related to investigating the nature and the cause(s) of the CIE onset as well as the biotic effects of the PETM on the paleoshelf. Additional objectives focus on the evidence for widespread wildfires and changes in the hydrological cycle, shelf morphology, and sea level during the PETM as well as the desire to study both underlying K–Pg sediments and overlying post-Eocene records of extreme hyperthermal climate events. All objectives address our overarching research question: what was the Earth system response to a rapid carbon cycle perturbation?

Scientific Drilling↗
USGS2025

Non-native bird populations respond differently to their environment and exhibit shifts in ecological niche limits across continents

Aim The degree to which species' niches remain stable over space and time–the niche conservatism hypothesis–is critical for predicting species' responses to environmental change. Tests of this hypothesis typically focus on changes in niche centroids and boundaries. An outstanding question is whether species' environmental associations differ within the interior of their niche space–that is, across the full range of occupied conditions–in original versus novel geographic space. Location Europe and North America. Time Period 1997–2018. Major Taxa Studied Birds. Methods We used over 400,000 observations collected over 22 years and across 28 countries to explore whether two common songbird species—European starling ( Sturnus vulgaris ) and house sparrow ( Passer domesticus ) – exhibit niche conservatism between their native European and non-native North American ranges. We tested for niche conservatism via (1) an ordination approach that quantified change in niche shape and boundaries, and (2) generalised linear mixed effects models to quantify how abundance varied with the interaction between continent and climate or land cover variables. Results The ordination analysis indicated that both European starling and house sparrow exhibited niche conservatism between Europe and North America. However, abundance models revealed continental differences in how the species responded to temperature and land cover. The abundance models also revealed that areas with wetter conditions that were occupied by both species in their native European ranges were available but unoccupied in their non-native North American ranges (i.e., niche unfilling). Main Conclusions Our work demonstrates that species can exhibit apparent consistency in niche boundaries but varied abundance responses to the environment within niche boundaries. Expanding the study of niche conservatism to explore changes both at the edge of and within niche boundaries would improve the ability to assess and predict species' invasion risk or sensitivity to ongoing global change.

Diversity and Distributions↗
USGS2012

Spring onset variations and trends in the continental United States: past and regional assessment using temperature-based indices

Phenological data are simple yet sensitive indicators of climate change impacts on ecosystems, but observations have not been made routinely or extensively enough to evaluate spatial and temporal patterns across most continents, including North America. As an alternative, many studies use weather-based algorithms to simulate specific phenological responses. Spring Indices (SI) are a set of complex phenological models that have been successfully applied to evaluate variations and trends in the onset of spring across the Northern Hemisphere’s temperate regions. To date, SI models have been limited by only producing output in locations where both the plants’ chilling and warmth requirements are met. Here, we develop an extended form of the SI (abbreviated SI-x) that expands their application into the subtropics by ignoring chilling requirements while still retaining the utility and accuracy of the original SI (now abbreviated SI-o). The validity of the new indices is tested, and regional SI anomalies are explored across the data-rich continental United States. SI-x variations from 1900 to 2010 show an abrupt and sustained delay in spring onset of about 4–8 d (around 1958) in parts of the Southeast and southern Great Plains, and a comparable advance of 4–8 d (around 1984) in parts of the northern Great Plains and the West. Atmospheric circulation anomalies, linked to large-scale modes of variability, exert modest but significant roles in the timing of spring onset across the United States on interannual and longer timescales. The SI-x are promising metrics for tracking spring onset variations and trends in mid-latitudes, relating them to relevant ecological, hydrological, and socioeconomic phenomena, and exploring connections between atmospheric drivers and seasonal timing.

International Journal of Climatology↗
USGS2017

Best practices for assessing forage fish fisheries-seabird resource competition

Worldwide, in recent years capture fisheries targeting lower-trophic level forage fish and euphausiid crustaceans have been substantial (∼20 million metric tons [MT] annually). Landings of forage species are projected to increase in the future, and this harvest may affect marine ecosystems and predator-prey interactions by removal or redistribution of biomass central to pelagic food webs. In particular, fisheries targeting forage fish and euphausiids may be in competition with seabirds, likely the most sensitive of marine vertebrates given limitations in their foraging abilities (ambit and gape size) and high metabolic rate, for food resources. Lately, apparent competition between fisheries and seabirds has led to numerous high-profile conflicts over interpretations, as well as the approaches that could and should be used to assess the magnitude and consequences of fisheries-seabird resource competition. In this paper, we review the methods used to date to study fisheries competition with seabirds, and present “best practices” for future resource competition assessments. Documenting current fisheries competition with seabirds generally involves addressing two major issues: 1) are fisheries causing localized prey depletion that is sufficient to affect the birds? (i.e., are fisheries limiting food resources?), and 2) how are fisheries-induced changes to forage stocks affecting seabird populations given the associated functional or numerical response relationships? Previous studies have been hampered by mismatches in the scale of fisheries, fish, and seabird data, and a lack of causal understanding due to confounding by climatic and other ecosystem factors (e.g., removal of predatory fish). Best practices for fisheries-seabird competition research should include i) clear articulation of hypotheses, ii) data collection (or summation) of fisheries, fish, and seabirds on matched spatio-temporal scales, and iii) integration of observational and experimental (including numerical simulation) approaches to establish connections and causality between fisheries and seabirds. As no single technique can provide all the answers to this vexing issue, an integrated approach is most promising to obtain robust scientific results and in turn the sustainability of forage fish fisheries from an ecosystem perspective.

Fisheries Research↗
USGS2016

Coherent late-Holocene climate-driven shifts in the structure of three Rocky Mountain lakes

Large-scale atmospheric pressure centers, such as the Aleutian and Icelandic Low, have a demonstrated relationship with physical lake characteristics in contemporary monitoring studies, but the responses to these phenomena are rarely observed in lake records. We observe coherent changes in the stratification patterns of three deep (>30 m) lakes inferred from fossil diatom assemblages as a response to shifts in the location and intensity of the Aleutian Low and compare these changes with similar long-term changes observed in the δ 18 O record from the Yukon. Specifically, these records indicate that between 3.2 and 1.4 ka, the Aleutian Low shifted westward, resulting in an increased frequency of storm tracks across the Pacific Northwest during winter and spring. This change in atmospheric circulation ultimately produced deeper mixing in the upper waters of these three lake systems. Enhanced stratification between 4.5 and 3.3 ka and from 1.3 ka to present suggests a strengthened Aleutian Low and more meridional circulation.

Montana, Wyoming↗
USGS2017

Case studies of riparian and watershed restoration in the southwestern United States—Principles, challenges, and successes

Globally, rivers and streams are highly altered by impoundments, diversions, and stream channelization associated with agricultural and water delivery needs. Climate change imposes additional challenges by further reducing discharge, introducing variability in seasonal precipitation patterns, and increasing temperatures. Collectively, these changes in a river or stream’s annual hydrology affects surface and groundwater dynamics, fluvial processes, and the linked aquatic and riparian responses, particularly in arid regions. Recognizing the inherent ecosystem services that riparian and aquatic habitats provide, society increasingly supports restoring the functionality of riparian and aquatic ecosystems. Given the wide range in types and scales of riparian impacts, approaches to riparian restoration can range from tactical, short-term, and site-specific efforts to strategic projects and long-term collaborations best pursued at the watershed scale. In the spirit of sharing information, the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center convened a workshop June 23-25, 2015, in Flagstaff, Ariz. for practitioners in restoration science to share general principles, successful restoration practices, and discuss the challenges that face those practicing riparian restoration in the southwestern United States. Presenters from the Colorado River and the Rio Grande basins, offered their perspectives and experiences in restoration at the local, reach and watershed scale. Outcomes of the workshop include this Proceedings volume, which is composed of extended abstracts of most of the presentations given at the workshop, and recommendations or information needs identified by participants. The organization of the Proceedings follows a general progression from local scale restoration to river and watershed scale approaches, and finishes with restoration assessments and monitoring.

Open-File Report↗
USGS2024

Human-induced range expansions result in a recent hybrid zone between sister species of ducks

Landscapes are consistently under pressure from human-induced ecological change, often resulting in shifting species distributions. For some species, changing the geographical breadth of their niche space results in matching range shifts to regions other than those in which they are formally found. In this study, we employ a population genomics approach to assess potential conservation issues arising from purported range expansions into the south Texas Brush Country of two sister species of ducks: mottled ( Anas fulvigula ) and Mexican ( Anas diazi ) ducks. Specifically, despite being non-migratory, both species are increasingly being recorded outside their formal ranges, with the northeastward and westward expansions of Mexican and mottled ducks, respectively, perhaps resulting in secondary contact today. We assessed genetic ancestry using thousands of autosomal loci across the ranges of both species, as well as sampled Mexican- and mottled-like ducks from across overlapping regions of south Texas. First, we confirm that both species are indeed expanding their ranges, with genetically pure Western Gulf Coast mottled ducks confirmed as far west as La Salle county, Texas, while Mexican ducks recorded across Texas counties near the USA–Mexico border. Importantly, the first confirmed Mexican × mottled duck hybrids were found in between these regions, which likely represents a recently established contact zone that is, on average, ~100 km wide. We posit that climate- and land use-associated changes, including coastal habitat degradation coupled with increases in artificial habitats in the interior regions of Texas, are facilitating these range expansions. Consequently, continued monitoring of this recent contact event can serve to understand species’ responses in the Anthropocene, but it can also be used to revise operational survey areas for mottled ducks.

Genes↗
USGS2015

Structure, diversity, and biophysical properties of old-growth forestsin the Klamath region, USA

The diverse old-growth forests in Klamath region of northern California and southern Oregon provide valuable ecosystem services (e.g., maintaining watersheds, wildlife habitat, recreation), but may be vulnerable to a wide range of stressors, including invasive species, disrupted disturbance regimes, and climatic change. Yet our understanding of how forest structure in the Klamath region relates to the current physical environment is limited. Here we provide present-day benchmarks for old-growth forest structure across a climatic gradient ranging from coastal to dry interior sites. We established 16 large (1 ha) forest plots where all stems > 5 cm in diameter were identified to species and mapped. Climate across these sites was highly variable, with estimated actual evapotranspiration correlated to several basic measures of forest structure, including plot basal area, stem size-class inequality, tree species diversity and, to a lesser extent, tree species richness. Analyses of the spatial arrangement of stems indicated a high degree of non-uniformity, with 75% of plots showing significant stem clumping at small spatial scales (0 to 10 m). Downscaled predictions of future site water balance suggest changes will be dominated by rapidly increasing climatic water deficit (D, a biologically meaningful index of drought). While these plots give a picture of current conditions, continued monitoring of these stands is needed to describe forest dynamics and to detect forest responses to ongoing and future stressors.

Klamath region↗
USGS2016

A landscape-scale assessment of above- and belowground primary production in coastal wetlands: Implications for climate change-induced community shifts

Above- and belowground production in coastal wetlands are important contributors to carbon accumulation and ecosystem sustainability. As sea level rises, we can expect shifts to more salt-tolerant communities, which may alter these ecosystem functions and services. Although the direct influence of salinity on species-level primary production has been documented, we lack an understanding of the landscape-level response of coastal wetlands to increasing salinity. What are the indirect effects of sea-level rise, i.e., how does primary production vary across a landscape gradient of increasing salinity that incorporates changes in wetland type? This is the first study to measure both above- and belowground production in four wetland types that span an entire coastal gradient from fresh to saline wetlands. We hypothesized that increasing salinity would limit rates of primary production, and saline marshes would have lower rates of above- and belowground production than fresher marshes. However, along the Northern Gulf of Mexico Coast in Louisiana, USA, we found that aboveground production was highest in brackish marshes, compared with fresh, intermediate, and saline marshes, and belowground production was similar among all wetland types along the salinity gradient. Multiple regression analysis indicated that salinity was the only significant predictor of production, and its influence was dependent upon wetland type. We concluded that (1) salinity had a negative effect on production within wetland type, and this relationship was strongest in the fresh marsh (0–2 PSU) and (2) along the overall landscape gradient, production was maintained by mechanisms at the scale of wetland type, which were likely related to plant energetics. Regardless of wetland type, we found that belowground production was significantly greater than aboveground production. Additionally, inter-annual variation, associated with severe drought conditions, was observed exclusively for belowground production, which may be a more sensitive indicator of ecosystem health than aboveground production.

Estuaries and Coasts↗
USGS2021

Simulation of water-table response to sea-level rise and change in recharge, Sandy Hook unit, Gateway National Recreation Area, New Jersey

The Sandy Hook Unit, Gateway National Recreation Area (hereafter Sandy Hook) in New Jersey is a 10-kilometer-long spit visited by thousands of people each year who take advantage of the historical and natural resources and recreational opportunities. The historical and natural resources are threatened by global climate change, including sea-level rise (SLR), changes in precipitation and groundwater recharge, and changes in the frequency and severity of coastal storms. Fresh groundwater resources are important to the ecosystems of Sandy Hook. The Bayside Holly Forest, one of only two known old-growth American holly ( Ilex opaca ) maritime forests, is particularly vulnerable to global climate change because of the proximity of the water table to land surface in low-lying areas and the potential for saltwater intrusion and inundation. The shallow groundwater-flow system on Sandy Hook is dominated by recharge from precipitation, fresh groundwater discharge to evapotranspiration (ET), discharge to surface seeps, and submarine groundwater discharge (groundwater discharging directly to the ocean). A three-dimensional groundwater-flow model that simulates the shallow groundwater-flow system and interaction with surrounding saltwater boundaries was constructed to simulate multi-density groundwater flow, treating the freshwater/saltwater transition zone as a sharp interface that represents the half-seawater surface. Groundwater-flow simulations completed for this study include a Baseline scenario, three SLR scenarios (0.2, 0.4, and 0.6 meter [m]), two Recharge scenarios—a 10-percent Increased Recharge scenario and a 10-percent Decreased Recharge scenario—and a scenario with 0.6 m of SLR and 10-percent increase in recharge. The Recharge scenarios indicate the system is not sensitive to a 10-percent increase or decrease in recharge from the Baseline scenario. In the SLR scenarios, SLR causes the water table to rise, resulting in increased fresh groundwater discharge to ET and seeps, and reduced submarine discharge compared to the Baseline scenario. The increased discharge to ET and seeps causes the magnitude of water-table rise to be less than that of SLR, which in turn causes the thickness of the freshwater lens to thin, reducing the depth to the half-seawater surface. Water-table rise associated with SLR diminishes the thickness of the unsaturated zone; comparing the Baseline and the 0.6-m SLR scenarios, the area where the simulated water table is above land surface increases by 50.6 hectares, from about 0.9 to 7.4 percent of the land area of Sandy Hook. Areas where the simulated water table is above land surface are likely to be emergent wetlands and contain freshwater if they are tens of meters or more from the shoreline. The steady-state simulations indicate that the percentage of land where the half-seawater surface is less than 9 m below the water table increases from about 2.5 percent (20 hectares) to about 9 percent (74 hectares) with 0.6 m of SLR. In low-lying areas close to the Sandy Hook Bay shoreline, the half-seawater surface is simulated to be as much as 20 m closer to the water table with SLR of 0.6 m. Transient salinization, if any, of shallow groundwater from increased frequency or severity of storm-driven inundation is not included in the analysis. Natural resources on Sandy Hook, particularly the Bayside Holly Forest, may be adversely affected by the rising water table associated with SLR. Freshwater emergent wetlands may increase in area at the expense of other ecosystem assemblages occurring in or on the edges of low-lying enclosed depressions. Cultural resources close to the water table, such as existing basements of structures, may be adversely affected.

New Jersey↗
USGS1995

Regression models of monthly water-level change in and near the Closed Basin Division of the San Luis Valley, south-central Colorado

The Bureau of Reclamation is developing a water-resource project, the Closed Basin Division, in the San Luis Valley of south-central Colorado that is designed to salvage unconfined ground water that currently is discharged as evapotranspiration. The water table in and near the 130,000-acre Closed Basin Division area will be lowered by an annual withdrawal of as much as 100,000 acre-feet of ground water from the unconfined aquifer. The legislation authorizing the project limits resulting drawdown of the water table in preexisting irrigation and domestic wells outside the Closed Basin Division to a maximum of 2 feet. Water levels in the closed basin in the northern part of the San Luis Valley historically have fluctuated more than 2 feet in response to water-use practices and variation of climatically controlled recharge and discharge. Declines of water levels in nearby wells that are caused by withdrawals in the Closed Basin Division can be quantified if water-level fluctuations that result from other water-use practices and climatic variations can be estimated. This study was done to evaluate water-level change at selected observation wells in and near the Closed Basin Division. Regression models of monthly water-level change were developed to predict monthly water-level change in 46 selected observation wells. Predictions of monthly water-level change are based on one or more of the following: elapsed time, cosine and sine functions with an annual period, streamflow depletion of the Rio Grande, electrical use for agricultural purposes, runoff into the closed basin, precipitation, and mean air temperature. Regression models for five of the wells include only an intercept term and either an elapsed-time term or terms determined by the cosine and sine functions. Regression models for the other 41 wells include 1 to 4 of the 5 other variables, which can vary from month to month and from year to year. Serial correlation of the residuals was detected in 24 of the regression models. These models also include an autoregressive term to account for serial correlation in the residuals. The adjusted coefficient of determination (Ra2) for the 46 regression models range from 0.08 to 0.89, and the standard errors of estimate range from 0.034 to 2.483 feet. The regression models of monthly water- level change can be used to evaluate whether post-1985 monthly water-level change values at the selected observation wells are within the 95-percent confidence limits of predicted monthly water-level change.

Water-Resources Investigations Report↗
USGS2002

Climate change impacts on U.S. coastal and marine ecosystems

Increases in concentrations of greenhouse gases projected for the 21st century are expected to lead to increased mean global air and ocean temperatures. The National Assessment of Potential Consequences of Climate Variability and Change (NAST 2001) was based on a series of regional and sector assessments. This paper is a summary of the coastal and marine resources sector review of potential impacts on shorelines, estuaries, coastal wetlands, coral reefs, and ocean margin ecosystems. The assessment considered the impacts of several key drivers of climate change: sea level change; alterations in precipitation patterns and subsequent delivery of freshwater, nutrients, and sediment; increased ocean temperature; alterations in circulation patterns; changes in frequency and intensity of coastal storms; and increased levels of atmospheric CO 2 . Increasing rates of sea-level rise and intensity and frequency of coastal storms and hurricanes over the next decades will increase threats to shorelines, wetlands, and coastal development. Estuarine productivity will change in response to alteration in the timing and amount of freshwater, nutrients, and sediment delivery. Higher water temperatures and changes in freshwater delivery will alter estuarine stratification, residence time, and eutrophication. Increased ocean temperatures are expected to increase coral bleaching and higher CO 2 levels may reduce coral calcification, making it more difficult for corals to recover from other disturbances, and inhibiting poleward shifts. Ocean warming is expected to cause poleward shifts in the ranges of many other organisms, including commercial species, and these shifts may have secondary effects on their predators and prey. Although these potential impacts of climate change and variability will vary from system to system, it is important to recognize that they will be superimposed upon, and in many cases intensify, other ecosystem stresses (pollution, harvesting, habitat destruction, invasive species, land and resource use, extreme natural events), which may lead to more significant consequences.

Estuaries↗
USGS2024

The anatomy of a drought in the upper San Francisco Estuary: Water quality and lower-trophic responses to multi-year droughts

Multi-year droughts are important and impactful features of California’s Mediterranean climate and can fundamentally affect the water quality and the ecosystem response of the San Francisco Estuary (Estuary) and the Sacramento-San Joaquin Delta (Delta). This study assesses data collected by long-term monitoring programs over the past 46 years (1975-2021) to evaluate how water quality in the Estuary changes during multi-year droughts. We found that multi-year droughts alter multiple physical and chemical parameters in the Estuary, increasing water temperature, salinity, water clarity, and nutrient levels. This trend was consistent across regions and seasons, with few exceptions. Increases in these parameters during droughts are likely caused by reduced Delta inflows that intensified in each successive dry year because of reduced precipitation and reservoir releases. Droughts did not substantially effect on tidal velocities within the Estuary, which remained mostly consistent across wet and dry periods. Trends in chlorophyll concentrations during droughts were more nuanced with higher concentrations occurring in the South-Central Delta region and during the winter and spring. Together, these results paint a picture of drought in the Estuary as warm, clear, high in nutrients, with patchy phytoplankton blooms (as indexed by chlorophyll), all of which have implications for higher trophic levels. Considering that droughts are expected to increase in frequency and intensity in California with climate change, it is imperative to understand the effects of multi-year droughts on the water quality conditions of the Estuary when making water management decisions.

California↗
USGS2023

Soil salinity and water level interact to generate tipping points in low salinity tidal wetlands responding to climate change

Low salinity tidal wetlands (LSTW) are vulnerable to sea level rise and saltwater intrusion, thus their carbon sequestration capacity is threatened. However, the thresholds of rapid changes in carbon dynamics and biogeochemical processes in LSTW due to changes in hydroperiod and salinity regime remain unclear. In this study, we examined the effects of soil porewater salinity and water level on changes in net primary productivity (NPP) and greenhouse gas fluxes [GHG: methane (CH 4 ), nitrous oxide (N 2 O), and carbon dioxide (CO 2 )] in LSTW using a wetland biogeochemistry model, Tidal Freshwater Wetland Denitrification and Decomposition (TFW-DNDC). TFW-DNDC was run with a series of combinations of soil salinities (0.1, 0.5, 1, 2, 4, 6, 8, 10 psu) and water levels relative to soil surface (-30, -20, -10, -5, 0, 5, 10, 20, 30 cm) for tidal forest and oligohaline marsh sites along the Savannah River and Waccamaw River, USA. Our results indicate that soil salinity and water level have antagonistic effects on CH 4 emissions and synergistic effects on CO 2 release. A soil salinity of 2-3 psu is the tipping point for the ecosystem level functional changes (e.g., NPP and CH 4 emissions) in LSTW. There are negative and nonlinear responses (NPP and CH 4 emission) to soil salinity. Furthermore, a soil water level from 10 cm below to 10 cm above the surface is a critical range in which biogeochemical processes respond strongly to hydrological changes. The presence of nonlinear tipping points in LSTW has large implications for understanding and predicting the effects of climate change on coastal wetland blue carbon storage and ecosystem dynamics.

Georgia, South Carolina↗
USGS2020

Rapid peat development beneath created, maturing mangrove forests: Ecosystem changes across a 25-year chronosequence

Mangrove forests are among the world’s most productive and carbon‐rich ecosystems. Despite growing understanding of factors controlling mangrove forest soil carbon stocks, there is a need to advance understanding of the speed of peat development beneath maturing mangrove forests— especially in created and restored mangrove forests that are intended to compensate for ecosystem functions lost during mangrove forest conversion to other land uses. To better quantify the rate of soil organic matter development beneath created, maturing mangrove forests, we measured ecosystem changes across a 25‐year chronosequence. We compared ecosystem properties in created, maturing mangrove forests to adjacent natural mangrove forests. We also quantified site‐specific changes that occurred between 2010 and 2016. Soil organic matter accumulated rapidly beneath maturing mangrove forests as sandy soils transitioned to organic‐rich soils (peat). Within 25 years, a 20‐cm deep peat layer developed. The time required for created mangrove forests to reach equivalency with natural mangrove forests was estimated as: (1) < 15 years for herbaceous and juvenile vegetation; (2) ~55 years for adult trees; (3) ~25 years for the upper soil layer (0‐10 cm); and (4) ~45‐80 years for the lower soil layer (10‐30 cm). For soil elevation change, the created mangrove forests were equivalent to or surpassed natural mangrove forests within the first five years. A comparison to chronosequence studies from other ecosystems indicates that the rate of soil organic matter accumulation beneath maturing mangrove forests may be among the fastest globally. In most peatland ecosystems, soil organic matter formation occurs slowly (centuries, millennia); however, these results show that mangrove peat formation can occur within decades. Peat development, primarily due to sub‐surface root accumulation, enables mangrove forests to sequester carbon, adjust their elevation relative to sea level, and adapt to changing conditions at the dynamic land‐ocean interface. In the face of climate change and rising sea levels, coastal managers are increasingly concerned with the longevity and functionality of coastal restoration efforts. Our results advance understanding of the pace of ecosystem development in created, maturing mangrove forests, which can improve predictions of mangrove forest responses to global change and ecosystem restoration.

Florida↗
USGS2019

Climatic controls on the distribution of foundation plant species in coastal wetlands of the conterminous United States: Knowledge gaps and emerging research needs

Foundation plant species play a critical role in coastal wetlands, often modifying abiotic conditions that are too stressful for most organisms and providing the primary habitat features that support entire ecological communities. Here, we consider the influence of climatic drivers on the distribution of foundation plant species within coastal wetlands of the conterminous USA. Using region-level syntheses, we identified 24 dominant foundation plant species within 12 biogeographic regions, and we categorized species and biogeographic regions into four groups: graminoids, mangroves, succulents, and unvegetated. Literature searches were used to characterize the level of research directed at each of the 24 species. Most coastal wetlands research has been focused on a subset of foundation species, with about 45% of publications directed at just one grass species—Spartina alterniflora. An additional 14 and 8% have been directed, respectively, at two mangrove species—Rhizophora mangle and Avicennia germinans. At the national scale, winter temperature extremes govern the distribution of mangrove forests relative to salt marsh graminoids, and arid conditions can produce hypersaline conditions that increase the dominance of succulent plants, algal mats, and unvegetated tidal flats (i.e., salt flats, salt pans) relative to graminoid and mangrove plants. Collectively, our analyses illustrate the diversity of foundation plant species in the conterminous USA and begin to elucidate the influence of climatic drivers on their distribution. However, our results also highlight critical knowledge gaps and identify emerging research needs for assessing climate change impacts. Given the importance of plant-mediated processes in coastal wetland ecosystems, there is a pressing need in many biogeographic regions for additional species- and functional group-specific research that can be used to better anticipate coastal wetland responses to rising sea levels and changing temperature and precipitation regimes.

Estuaries and Coasts↗
USGS2023

Simulating the migration dynamics of juvenile salmonids through rivers and estuaries using a hydrodynamically driven enhanced particle tracking model

Juvenile salmonids migrate hundreds of kilometers from their natal streams to mature in the ocean. Throughout this migration, they respond to environmental cues such as local water velocities and other stimuli to direct and modulate their movements, often through heavily modified riverine and estuarine habitats. Management strategies in an uncertain future of climate change and altered land use regimes depend heavily on being able to reliably predict their ocean entry timings, route use, and survival rates through rivers and estuaries. We developed a spatially-explicit agent-based model of fish movement in response to hydrodynamic flows that uses movement dynamics gleaned from multi-dimensional tracking datasets of acoustically tagged juveniles moving through an urbanized, branched tidal estuary. We demonstrate how such models can be calibrated, and we apply it to the Sacramento-San Joaquin Delta in Central California. The quality of the out-of-sample validation of the model to predict juvenile salmon survival and route selection indicates that the model is versatile and flexible enough to be used in novel hydroclimatological conditions.

California↗