The impacts of mangrove range expansion on wetland ecosystem services in the southeastern United States: Current understanding, knowledge gaps, and emerging research needs.

Climate change is transforming ecosystems and affecting ecosystem goods and services. Along the Gulf of Mexico and Atlantic coasts of the southeastern United States, the frequency and intensity of extreme freeze events greatly influence whether coastal wetlands are dominated by freeze-sensitive woody plants (mangrove forests) or freeze-tolerant grass-like plants (salt marshes). In response to warming winters, mangroves have been expanding and displacing salt marshes at varying degrees of severity in parts of north Florida, Louisiana, and Texas. As winter warming accelerates, mangrove range expansion is expected to increasingly modify wetland ecosystem structure and function. Because there are differences in the ecological and societal benefits that salt marshes and mangroves provide, coastal environmental managers are challenged to anticipate the effects of mangrove expansion on critical wetland ecosystem services, including those related to carbon sequestration, wildlife habitat, storm protection, erosion reduction, water purification, fisheries support, and recreation. Mangrove range expansion may also affect wetland stability in the face of extreme climatic events and rising sea levels. Here, we review the current understanding of the effects of mangrove range expansion and displacement of salt marshes on wetland ecosystem services in the southeastern United States. We also identify critical knowledge gaps and emerging research needs regarding the ecological and societal implications of salt marsh displacement by expanding mangrove forests. One consistent theme throughout our review is that there are ecological trade-offs for consideration by coastal managers. Mangrove expansion and marsh displacement can produce beneficial changes in some ecosystem services, while simultaneously producing detrimental changes in other services. Thus, there can be local-scale differences in perceptions of the impacts of mangrove expansion into salt marshes. For very specific local reasons, some individuals may see mangrove expansion as a positive change to be embraced, while others may see mangrove expansion as a negative change to be constrained.

Meta-analysis of salt marsh vegetation impacts and recovery: a synthesis following the Deepwater Horizon oil spill.

Marine oil spills continue to be a global issue, heightened by spill events such as the 2010 Deepwater Horizon spill in the Gulf of Mexico, the largest marine oil spill in US waters and among the largest worldwide, affecting over 1,000 km of sensitive wetland shorelines, primarily salt marshes supporting numerous ecosystem functions. To synthesize the effects of the oil spill on foundational vegetation species in the salt marsh ecosystem, Spartina alterniflora and Juncus roemerianus, we performed a meta-analysis using data from 10 studies and 255 sampling sites over seven years post-spill. We examined the hypotheses that the oil spill reduced plant cover, stem density, vegetation height, aboveground biomass, and belowground biomass, and tracked the degree of effects temporally to estimate recovery time frames. All plant metrics indicated impacts from oiling, with 20-100% maximum reductions depending on oiling level and marsh zone. Peak reductions of ~70-90% in total plant cover, total aboveground biomass, and belowground biomass were observed for heavily oiled sites at the marsh edge. Both Spartina and Juncus were impacted, with Juncus affected to a greater degree. Most plant metrics had recovery time frames of three years or longer, including multiple metrics with incomplete recovery over the duration of our data, at least seven years post-spill. Belowground biomass was particularly concerning, because it declined over time in contrast with recovery trends in most aboveground metrics, serving as a strong indicator of ongoing impact, limited recovery, and impaired resilience. We conclude that the Deepwater Horizon spill had multiyear impacts on salt marsh vegetation, with full recovery likely to exceed 10 years, particularly in heavily oiled marshes, where erosion may preclude full recovery. Vegetation impacts and delayed recovery is likely to have exerted substantial influences on ecosystem processes and associated species, especially along heavily oiled shorelines. Our synthesis affords a greater understanding of ecosystem impacts and recovery following the Deepwater Horizon oil spill, and informs environmental impact analysis, contingency planning, emergency response, damage assessment, and restoration efforts related to oil spills.

Do small-scale saltmarsh planting living shoreline projects enhance coastal functionality? A case study in the Northern Gulf of Mexico.

Human coastal occupation often leads to the degradation of the structural properties and environmental functions of natural coastlines. . Much research has been done on the cost-effectiveness of various living shorelines designs, however more work is needed for simple, small-scale designs that are typically adopted in waterfront residential or recreational properties. To contribute to this gap, we planted small-scale plots of black needlerush (Juncus roemerianus) in two sites, one in a residential property and another one in a recreational property in the Northern Gulf of Mexico that experienced significant wave energy. Plots were planted at two different densities (50% or 100% initial cover) or left unplanted (controls) and, along with monitoring the evolution of the planted salt marsh, we measured a number of functional metrics including soil slope, abundance of nekton within and in front of the plots, and cover of submerged aquatic vegetation (SAV) in front of the plots monthly over two years. In one of the sites plant cover decreased precipitously, and in the other site we did not observe any significant changes in plant cover over time (i.e. the initial 50% and 100% plantings remained at that level throughout the experiment) despite protecting the planted salt marsh with coir logs. We did not find any changes in soil slope or nekton abundance between planted and control plots. SAV growth was restrained in front of planted plots in relation to control plots, possibly due to deleterious impacts by the coir logs. Overall, the results suggest the protection against wave energy attained in this experiment is insufficient for adequate saltmarsh establishment and growth, thereby encountering decreasing or stationary plant density and no significant differences in soil slope or nekton abundance between planted and non-planted plots. Our results indicate the adoption of small-scale saltmarsh planting to reduce erosion and enhance coastal functionality needs to ensure that wave energy is sufficiently dampened for adequate saltmarsh growth and, concomitantly, the conceived saltmarsh protection mechanism does not negatively impact adjacent SAV.

Large-scale variation in wave attenuation of oyster reef living shorelines and the influence of inundation duration.

One of the paramount goals of oyster reef living shorelines is to achieve sustained and adaptive coastal protection, which requires meeting ecological (i.e., develop a self-sustaining oyster population) and engineering (i.e., provide coastal defense) targets. In a large-scale comparison along the Atlantic and Gulf coasts of the United States, the efficacy of various designs of oyster reef living shorelines at providing wave attenuation was evaluated accounting for the ecological limitations of oysters with regard to inundation duration. A critical threshold for intertidal oyster reef establishment is 50% inundation duration. Living shorelines that spent less than one-half of the time (<50%) inundated were not considered suitable habitat for oysters, however, were effective at wave attenuation (68% reduction in wave height). Reefs that experienced >50% inundation were considered suitable habitat for oysters, but wave attenuation was similar to controls (no reef; ~5% reduction in wave height). Many of the oyster reef living shoreline approaches therefore failed to optimize the ecological and engineering goals. In both inundation regimes, wave transmission decreased with an increasing freeboard (difference between reef crest elevation and water level), supporting its importance in the wave attenuation capacity of oyster reef living shorelines. However, given that the reef crest elevation (and thus freeboard) should be determined by the inundation duration requirements of oysters, research needs to be refocused on understanding the implications of other reef parameters (e.g., width) for optimizing wave attenuation. A broader understanding of the reef characteristics and seascape contexts that result in effective coastal defense by oyster reefs is needed to inform appropriate design and implementation of oyster-based living shorelines globally.

Challenges and opportunities for sustaining coastal wetlands and oyster reefs in the southeastern United States.

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.

Restoring Fringing Tidal Marshes for Ecological Function and Ecosystem Resilience to Moderate Sea-level Rise in the Northern Gulf of Mexico.

Tidal marshes are increasingly vulnerable to degradation or loss from eutrophication, land-use changes, and accelerating sea-level rise, making restoration necessary to recover ecosystem services. To evaluate effects of restoration planting density and sea-level rise on ecosystem function (i.e., nitrogen removal), we restored three marshes, which differed in elevation, at Weeks Bay National Estuarine Research Reserve, Alabama, USA and planted them with Juncus roemerianus sods at 0, 25, 50, 75, or 100% initial cover. We simulated future sea level using passive weirs that increased flooding during low tide. Because additional species emerged shortly after transplantation, we also tested for treatment effects on community structure. In all marshes, species richness increased following restoration, regardless of treatments, while relative abundances of new species tended to increase with increasing initial cover. Plant percent cover increased with increasing initial cover in all marshes, with similar vegetated cover at 50, 75, and 100% after 3 years in the highest elevation marsh. Porewater dissolved inorganic nitrogen concentrations ([DIN]) decreased with increasing initial cover in all marshes, and were significantly lower in 50, 75, and 100% treatments than 0 or 25% after 1 year. Furthermore, [DIN] was similarly low among 50, 75, and 100% treatments when elevation capital was highest. These results suggest that intermediate initial cover (50%) can recover plant cover and promote nitrogen removal when elevation capital is adequate at relatively lower labor and material costs than planting at higher cover, thereby maximizing restoration outcomes in the face of low to moderate sea-level rise.

Ecological effects of non-native species in marine ecosystems relate to co-occurring anthropogenic pressures.

Predictors for the ecological effects of non-native species are lacking, even though such knowledge is fundamental to manage non-native species and mitigate their impacts. Current theories suggest that the ecological effects of non-native species may be related to other concomitant anthropogenic stressors, but this has not been tested at a global scale. We combine an exhaustive meta-analysis of the ecological effects of marine non-native species with human footprint proxies to determine whether the ecological changes due to non-native species are modulated by co-occurring anthropogenic impacts. We found that non-native species had greater negative effects on native biodiversity where human population was high and caused reductions in individual performance where cumulative human impacts were large. On this basis we identified several marine ecoregions where non-native species may have the greatest ecological effects, including areas in the Mediterranean Sea and along the northwest coast of the United States. In conclusion, our global assessment suggests coexisting anthropogenic impacts can intensify the ecological effects of non-native species.

Eliciting expert judgment to inform management of diverse oyster resources for multiple ecosystem services.

This study presents the most comprehensive set of ecosystem service provision estimates for diverse oyster-based resources to date. We use expert elicitation methods to derive estimates of five ecosystem services provided by oysters: oyster harvest (as indicated by oyster density), improved water quality (net nitrogen assimilation), shoreline protection (net erosion), and other fish habitat (blue crab and red drum density). Distributions are estimated for three distinct resources: on-bottom production, off-bottom farms, and non-harvested restoration/conservation efforts, under twelve distinct scenarios according to varying environmental conditions (eutrophication, sedimentation, and salinity regimes). Our expert-derived estimates of ecosystem services provide useful comparisons across oyster resources of both expected ecosystem service delivery levels and the amount of variation in those levels. These estimates bridge an information gap regarding relative performance of diverse oyster resources along multiple dimensions and should serve as a useful guide for resource managers facing competing interests.

An evaluative tool for rapid assessment of derelict vessel effects on coastal resources.

Derelict vessels impact coastal and estuarine habitats, fisheries resources, are aesthetically unappealing, and may be a hazard to navigation and recreation. The Government Accountability Office estimated in 2013 over 5600 derelict vessels existed throughout the coastal United States. Considering the large number of derelict vessels present in coastal areas, effective tools are needed to assess the environmental damage exerted by derelict vessels and aid in management strategies for their removal. After carefully reviewing regulations, we developed a 100-point scoring rubric (DVET) to evaluate damage by derelict vessels to natural resources with minimal field effort. The DVET's ability to rapidly assess a derelict vessel's impact on surrounding natural resources was confirmed with additional rigorous sampling and suggest environmental enhancement following vessel removal. The DVET shows promise for informing derelict vessel removal strategies, although more work is needed to quantify environmental benefits of derelict vessel removal and establish guidelines for removal prioritization.

Effects of Stormwater Pipe Size and Rainfall on Sediment and Nutrients Delivered to a Coastal Bayou.

Pollutants discharged from stormwater pipes can cause water quality and ecosystem problems in coastal bayous. A study was conducted to characterize sediment and nutrients discharged by small and large (< 20 cm and >20 cm in internal diameters, respectively) pipes under different rainfall intensities (< 2.54 cm and > 2.54 cm, respectively). Results showed that large pipes had greater discharge than small pipes. Pollutants concentrations did not vary by pipe size. Large pipes had greater loads of TSS (138.2 vs. 24.0 mg/s), NO3(-) (5.54 vs. 2.74 mg/s), and NH4(+) (0.39 vs. 0.19 mg/s) than small pipes. Neither discharge nor constituents varied by rainfall events. Pipe size may be a useful metric for estimating loads to a system. Nutrient reduction efforts should be directed to reducing the dissolved nutrient pools, while stormwater management efforts should be directed to reducing pipe freshwater discharge volumes that drive constituent loads.

Groundwater nitrogen processing in Northern Gulf of Mexico restored marshes.

Groundwater nitrogen processing was examined in a restored black needlerush (Juncus roemerianus) marsh to assess its potential for removing land-derived nitrogen pollution. Two restoration designs, one initially planted at 50% cover (half density plots) and the other one at 100% cover (full density plots), were compared with non-vegetated controls. The introduction via groundwater of a NO3(-) solution with a conservative tracer (Br(-)) and labeled isotopically ((15)N) allowed calculation of nitrogen removal in the plots following two methods. The first method used changes in the ratio [NOx]:[Br(-)] as the groundwater plume traveled through the plot, and the second method relied on balancing (15)N input with (15)N export. Both methods showed ≈97% of the N from the simulated groundwater plume was removed (i.e. not delivered to the open waters of the adjacent estuary) in vegetated plots and ≈86% was removed in non-vegetated controls. The most dominant routes of N removal from the introduced solution were N2 production and assimilation into macrophyte biomass, which were similar in magnitude for the vegetated plots, whereas N2 production dominated in the unvegetated plots. The majority of N removed from the introduced solution occurred in the first 30 cm the solution traveled in the vegetated treatments. In addition, ambient porewater concentrations of dissolved inorganic nitrogen (DIN) were similar between full and half density plots, but lower than the non-vegetated control (≈8.5× and 7.5×), suggesting full and half density plots removed more DIN than non-vegetated plots. These results suggest that restoring marshes by planting 50% of the area may be a more cost-effective restoration design in terms of mitigating land-derived nutrient pollution than planting 100% of the area since it requires less effort and cost while removing similar quantities of N.

Reconstructed life history metrics of the iconic seagrass Posidonia oceanica (L.) detect localized anthropogenic disturbance signatures.

Substantial losses of the seagrass Posidonia oceanica have initiated investigations into localized resilience declines related to anthropogenic disturbances. In this study, we determined reconstructed shoot age and interannual growth metrics can detect anthropogenic impact effects on P. oceanica production. Interannual rhizome vertical growth, leaf production, and demographics of shoots collected from sewage and trawling impacted areas were examined using mixed effects modeling. Detected impact effects were specific to the type of impact, manifesting as an older-skewed age distribution of sewage outfall shoots and reduced vertical growth and reduced leaf production of trawling site shoots. A stress event period was also detected for all shoots >5 years old, with trawling impacted shoots indicating little recovery. Reconstructed age and growth metrics are simple to measure, incorporate multiple years of in situ shoot development, and are advantageous for identification of declining P. oceanica resilience prior to catastrophic losses.

The blue carbon of southern southwest Atlantic salt marshes and their biotic and abiotic drivers.

Coastal vegetated ecosystems are acknowledged for their capacity to sequester organic carbon (OC), known as blue C. Yet, blue C global accounting is incomplete, with major gaps in southern hemisphere data. It also shows a large variability suggesting that the interaction between environmental and biological drivers is important at the local scale. In southwest Atlantic salt marshes, to account for the space occupied by crab burrows, it is key to avoid overestimates. Here we found that southern southwest Atlantic salt marshes store on average 42.43 (SE = 27.56) Mg OC·ha-1 (40.74 (SE = 2.7) in belowground) and bury in average 47.62 g OC·m-2·yr-1 (ranging from 7.38 to 204.21). Accretion rates, granulometry, plant species and burrowing crabs were identified as the main factors in determining belowground OC stocks. These data lead to an updated global estimation for stocks in salt marshes of 185.89 Mg OC·ha-1 (n = 743; SE = 4.92) and a C burial rate of 199.61 g OC·m-2·yr-1 (n = 193; SE = 16.04), which are lower than previous estimates.

The NOAA NCEI marine microplastics database.

Microplastics (<5 mm) pollution is a growing problem affecting coastal communities, marine ecosystems, aquatic life, and human health. The widespread occurrence of marine microplastics, and the need to curb its threats, require expansive, and continuous monitoring. While microplastic research has increased in recent years and generated significant volumes of data, there is a lack of a robust, open access, and long-term aggregation of this data. The National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information (NCEI) now provides a global open access to marine microplastics data on an easily discoverable and accessible GIS web map and data portal ( https://www.ncei.noaa.gov/products/microplastics ). The objective of this data portal is to develop a repository where microplastics data are aggregated, archived, and served in a user friendly, consistent, and reliable manner. This work contributes to NCEI's efforts towards data standardization, integration, harmonization, and interoperability among national and international collaborators for monitoring global marine microplastics. This paper describes the NOAA NCEI global marine microplastics database, its creation, quality control procedures, and future directions.

Effects of short-term sediment nutrient enrichment and grazer (Neritina Reclivata) removal on sediment microalgae in a shallow eutrophic estuary (Alabama, USA).

The olive snail (Neritina reclivata) is ubiquitous in tropical and sub-tropical systems of the Gulf of Mexico, however its impacts on sediment microalgae have been little studied. Many coastal systems around the world are being eutrophied due to human activities, and seemingly they will continue to be eutrophied to a further extent in the future. Exploring the single and combined impacts of further nutrient enrichment and grazing by the olive snail on sediment microalgae in such eutrophic systems is an important question for our understanding and management of these systems. Here we examine the effects of short-term nutrient enrichment and grazing by the olive snail N. reclivata on sediment microalgal biomass and composition in a shallow eutrophic estuary (Weeks Bay, Alabama, USA) of the Northern Gulf of Mexico. For this, we performed a series of factorial experiments adding or not nutrients and removing or not the snail, for a total of four treatments in each experiment: ambient grazing, ambient nutrients; ambient grazing, increased nutrients; no grazing, ambient nutrients; and no grazing, increased nutrients. We did not find any significant impact of nutrient addition in any of the eight short-term (i.e. four days) experiments carried out. Impacts by the snail were minor; we only found a decrease in biomass due to snail grazing in one of the eight experiments, and no impacts on microalgal (i.e. diatom) composition. High ambient nutrient concentrations in the sediment porewater and low snail abundances on the sediment could explain these findings. Our results suggest that ephemeral, short-term nutrient pulses into eutrophic coastal systems of the Northern Gulf of Mexico, such as Weeks Bay (Alabama, USA), should not greatly affect the abundance of sediment microalgae, even though those pulses occur in well-lit areas. The results further suggest the snail N. reclivata is not a major control of sediment microalgal populations in the subtidal sedimentary areas studied. Our findings contrast with the results of past work in sediments with well-lit and nutrient poor conditions, or sediments with high densities of other snail grazers. In conjunction this and other investigations indicate that the response of sediment microalgae to nutrient enrichment and modified grazer abundance depends to a large extent on the initial levels of nutrient availability and grazing before the system is altered.

Decoupled effects (positive to negative) of nutrient enrichment on ecosystem services.

Eutrophication is a widespread phenomenon that disrupts natural ecosystems around the globe. Despite the general recognition that ecosystems provide many services and benefits to humans, little effort has been made to address how increasing anthropogenic eutrophication affects those services. We conducted a field experiment to determine the effect of nutrient enrichment on five ecological services provided by a model coastal system, a shallow seagrass community near Mobile Bay, Alabama (USA): (1) the provision of shelter for fauna; (2) the quality of food provided to first-order consumers; (3) quantity of food provision to first-order consumers and O2/CO2 exchange; (4) producer carbon and nitrogen storage, and (5) water clarity. The results showed a severe negative impact on seagrass density and biomass, which greatly reduced the structural complexity of the community and provision of shelter to fauna. Water clarity and the standing stock of producer carbon were reduced in the fertilized area in comparison with the control area. In contrast, nutrient addition did not affect in any consistent way the total quantity of food available for first-order consumers, the net exchange of O2/CO2, or the standing stock of producer nitrogen in the community. The nutritional quality of the food available for first-order consumers increased with fertilization. These results show that the impacts of nutrient enrichment on the services provided by natural systems may be disparate, ranging from negative to positive. These findings suggest that management policies for anthropogenic eutrophication will depend on the specific ecosystem service targeted. In the case of shallow seagrass beds, the loss of biogenic habitat and drastic impacts on commercially important fauna may be sufficiently alarming to warrant rigorous control of coastal eutrophication.

Uncertainty propagation in an ecosystem nutrient budget.

New aspects and advancements in classical uncertainty propagation methods were used to develop a nutrient budget with associated uncertainty for a northern Gulf of Mexico coastal embayment. Uncertainty was calculated for budget terms by propagating the standard error and degrees of freedom. New aspects include the combined use of Monte Carlo simulations with classical error propagation methods, uncertainty analyses for GIS computations, and uncertainty propagation involving literature and subjective estimates of terms used in the budget calculations. The methods employed are broadly applicable to the mathematical operations employed in ecological studies involving step-by-step calculations, scaling procedures, and calculations of variables from direct measurements and/or literature estimates. Propagation of the standard error and the degrees of freedom allowed for calculation of the uncertainty intervals around every term in the budget. For scientists and environmental managers, the methods developed herein provide a relatively simple framework to propagate and assess the contributions of uncertainty in directly measured and literature estimated variables to calculated variables. Application of these methods to environmental data used in scientific reporting and environmental management will improve the interpretation of data and simplify the estimation of risk associated with decisions based on ecological studies.

Herbivory patterns along the intertidal gradient of Juncus roemerianus salt marshes.

Grasshopper herbivory can vary substantially among locations within a salt marsh or among marshes, but its variability along the marsh intertidal gradient (extending from the shoreline to the upland fringing forest) is not well reported. Previous papers have shown that grasshopper herbivory may affect nutrient processes in salt marsh ecosystems, but how such effects are tied up to the intensity of herbivory and how they vary spatially is poorly known. To help address these gaps, we evaluated whether grasshopper herbivory intensity and herbivore abundance together with other plant characteristics (such as total leaf length, plant live and dead biomass, plant nutrient content and plant nutrient standing stocks) varied along the intertidal gradient of two black needlerush marshes in the Northern Gulf of Mexico. Our results show that in one marsh grazing intensity decreased from the shoreline to the forest tree line, but in the other there was similar grazing intensity across the entire intertidal gradient. None of the measured plant characteristics followed the differences in herbivory found along the intertidal gradient and between salt marshes. We also found that, in the salt marsh with decreasing herbivory towards the upland edge, the combination of herbivory, plant nutrient content and plant nutrient standing stocks suggest two different functional zones along the intertidal gradient, one of nutrient availability and recycling near the shoreline and another one of nutrient inmobilization near the upland fringing forest. In concert, the results suggest that grasshopper herbivory intensity may vary along the intertidal gradient in some marshes, but not in others. In turn, spatial differences in herbivory along the intertidal gradient, if they occur, may influence nutrient processes, such as recycling and storage, leading to associated spatial differences in nutrient dynamics in the salt marsh.

Quantifying and addressing the prevalence and bias of study designs in the environmental and social sciences.

Building trust in science and evidence-based decision-making depends heavily on the credibility of studies and their findings. Researchers employ many different study designs that vary in their risk of bias to evaluate the true effect of interventions or impacts. Here, we empirically quantify, on a large scale, the prevalence of different study designs and the magnitude of bias in their estimates. Randomised designs and controlled observational designs with pre-intervention sampling were used by just 23% of intervention studies in biodiversity conservation, and 36% of intervention studies in social science. We demonstrate, through pairwise within-study comparisons across 49 environmental datasets, that these types of designs usually give less biased estimates than simpler observational designs. We propose a model-based approach to combine study estimates that may suffer from different levels of study design bias, discuss the implications for evidence synthesis, and how to facilitate the use of more credible study designs.