Estuarine Research Research Articles

Benthic Community Metrics Track Hydrologically Stressed Mangrove Systems

Mangrove restoration efforts have increased in order to help combat their decline globally. While restoration efforts often focus on planting seedlings, underlying chronic issues, including disrupted hydrological regimes, can hinder restoration success. While improving hydrology may be more cost-effective and have higher success rates than planting seedlings alone, hydrological restoration success in this form is poorly understood. Restoration assessments can employ a functional equivalency approach, comparing restoration areas over time with natural, reference forests in order to quantify the relative effectiveness of different restoration approaches. Here, we employ the use of baseline community ecology metrics along with stable isotopes to track changes in the community and trophic structure and enable time estimates for establishing mangrove functional equivalency. We examined a mangrove system impacted by road construction and recently targeted for hydrological restoration within the Rookery Bay National Estuarine Research Reserve, Florida, USA. Samples were collected along a gradient of degradation, from a heavily degraded zone, with mostly dead trees, to a transition zone, with a high number of saplings, to a full canopy zone, with mature trees, and into a reference zone with dense, mature mangrove trees. The transition, full canopy, and reference zones were dominated by annelids, gastropods, isopods, and fiddler crabs. Diversity was lower in the dead zone; these taxa were enriched in 13C relative to those found in all the other zones, indicating a shift in the dominant carbon source from mangrove detritus (reference zone) to algae (dead zone). Community-wide isotope niche metrics also distinguished zones, likely reflecting dominant primary food resources (baseline organic matter) present. Our results suggest that stable isotope niche metrics provide a useful tool for tracking mangrove degradation gradients. These baseline data provide critical information on the ecosystem functioning in mangrove habitats following hydrological restoration.

Prevalence and Genetic Diversity of Deer Tick Virus (Powassan Virus, Lineage II) in Ixodes scapularis Ticks in Five Habitats at a Nature Reserve in Southern Maine, United States.

Deer tick virus (DTV), also known as Powassan virus lineage II, is a rising health concern due to increased recognition as a cause of human encephalitis. Since European tick-borne encephalitis virus persists in nature in enzootic foci (i.e., higher prevalence rates in small pockets in nature), we sought to determine whether DTV is also focally maintained in relation to habitat type, to better understand factors leading to human risk of exposure. From 2018 to 2021, questing Ixodes scapularis ticks were collected from five habitats at the Wells National Estuarine Research Reserve (WNERR) in Wells, ME: forest with invasive vegetation in the understory, edge, shrub, forest with native vegetation in the understory, and open field. Deer tick virus prevalence was greater in adult ticks (2.0%) than in nymphs (0.5%). Deer tick virus prevalence in adult ticks collected from forest with invasive vegetation was 3.2% compared to 0 to 1.7% in other habitat types. A hot spot analysis revealed a higher number of infected adults collected per hour on one of the transects located in forest with invasive vegetation. Phylogenetic analysis of 37 full-length DTV genomes sequenced in this study revealed four major clades from the WNERR, and there was high genetic diversity within each transect, suggesting frequent, short-range dispersal between habitats. Analysis of DTV sequences from other New England counties and states also indicated long-distance dispersal to and/or from the WNERR. This study provides preliminary evidence that DTV is focal and that the risk of encountering DTV-infected ticks in forest with invasive vegetation may be greater than in other habitat types.

Investigating concentrations and sources of polycyclic aromatic hydrocarbons in South and Central Texas bays and estuaries along the Gulf of Mexico, USA

Polycyclic aromatic hydrocarbons (PAHs) are among the most widespread organic contaminants in the environment, and anthropogenic activities can produce PAHs through a variety of pyrogenic or petrogenic means. Knowing the concentrations and sources of PAHs helps evaluate ecosystem health and manage natural resources. In this study, 16 US Environmental Protection Agency priority PAHs were analyzed in water and sediment samples collected from September 2021 to September 2023 in four bay systems along the south and central Texas coast, which are a hotpot of crude oil transportation in the United States. Our results indicated that the total concentration of PAHs ranged from 1.9 to 8.3 ng/mL in surface waters (< 0.5 m) and from 520 to 1257 ng/g in surface sediments (top 5 cm). Grain size analysis revealed that the sediment was dominated by silt (4 - 63 μm), followed by clay (< 4 μm) and sand (> 63 μm) fractions. Both organic carbon and clay content were shown to play a significant role in controlling the PAH content in sediments. Diagnostic ratios indicated that PAHs were primarily sourced via pyrolytic processes, such as the combustion of fossil fuels. Additional sampling at Port Bay, a shallow, secondary bay in the Mission-Aransas National Estuarine Research Reserve, implicated a strong role of resuspension in the distribution and composition of PAHs in the bay systems studied. Overall, these data offer insights into the concentration levels and sources of PAHs in this key region housing oil production and transportation in the United States.

20-year water quality analysis reveals spatial variability and long-term changes at North Carolina's Masonboro Island National Estuarine Research Reserve

Urbanization impacts, especially stormwater runoff and pollutant discharge, pose ecological threats to coastal ecosystems globally. Long-term environmental monitoring datasets and assessments are used to quantify anthropogenic impacts on ecosystem health. This study synthesized twenty years (2002–2021) of water quality data from the National Estuarine Research Reserve System-wide Monitoring Program at the understudied Masonboro Island Reserve (North Carolina, United States) and provided an analytical methodology that can be applied to other Reserves and monitoring programs world-wide. Time series and correlation analyses of biological, chemical, and physical water quality parameters were used to identify spatiotemporal trends of nutrient dynamics and phytoplankton biomass at three stations within a 5-km stretch of the Atlantic Intracoastal Waterway. Heterogenous water quality conditions were observed between stations, especially for chlorophyll a, and were influenced by the relative proximity to marine waters, human developments, and urban tidal creeks. Long-term trends indicated significant water quality changes (i.e., increased chlorophyll a, reduced pH and salinity) related to anthropogenic influences. An unexpected and rapid shift in nutrient dynamics occurred around 2007, marked by significantly decreased ammonium and orthophosphate concentrations, which considerably reduced DIN/DIP ratios. This positive change was potentially influenced by sewage infrastructure improvements and stormwater wetland construction, removing waste-derived nutrients that previously entered the ecosystem. Impacts of major storms were also observed with weeks of hypoxia and low salinity following Hurricane Florence. These findings emphasized the value of long-term trend analyses of monitoring program data at multiple sites and provided insights into the significant water quality impacts of urbanization and benefits from wetland protection. This study further encourages the evaluation of long-term water quality monitoring datasets in estuarine environments globally.

Exploring source-specific ecological risks of PAHs near oil platforms in the Yellow River Estuary, Bohai Sea

The Yellow River Estuary (YRE) is one of highly remarkable regions profoundly impacted by human activities, with numerous oil platforms dispersed throughout. In this area, offshore oil exploitation may pose significant ecological risks. To comprehensively evaluate the quantitative impacts of oil field exploitation on the marine coastal ecosystem, this study investigated the occurrence, sources, and ecological risks associated with 16 polycyclic aromatic hydrocarbons (PAHs) in seawater and sediment near oil platforms in the YRE. We found that 1) The concentrations of PAHs decreased from the surface seawater to sediments; 2) The ecological risk level of PAHs in seawater exceeded that in sediments; 3) terrestrial sources (combustion), rather than offshore oil drilling activities, significantly influenced regional ecological risks through processes of atmospheric deposition and surface runoff. These findings provide essential data for future estuarine research efforts while supporting mitigation measures aimed at addressing marine environmental pollution related to oil production activities.

Nutrient concentration, loads and retention in a semiarid micro-estuary: The relative contribution of baseflow and flood events

Estuaries are a significant source of nutrients to the marine environment. The magnitude of this source is a function of nutrients load reaching the estuary and removal (attenuation) within estuaries. Most estuarine research is conducted in large estuaries, which do not reflect the processes in small estuaries in urban and semi-arid regions where flood water is a substantial portion of the annual discharge and the estuarine baseflow is often low and dominated by wastewater. To improve the understanding of nutrient attenuation and load into the Mediterranean, we conducted high-resolution nutrient sampling in the eutrophic Alexander micro-estuary as a test case. We sampled once per month during baseflows (years 2014–2019) and hourly during floods (years 2016–2018). The concentrations of inorganic nutrients (phosphorous (P) and nitrogen (N)) were extremely high during baseflows. Dissolved ammonium and particulate P were the only nutrients that were in the estuary (by 55 % and 30 %, respectively). Floods were rare, occurring ~4 % of the time, but contributed 62 % of the annual water discharge of the Alexander micro-estuary (14.7 ± 3.8 106 m3 y−1). The concentration of all dissolved nutrients decreased during floods but was higher than expected (DIN 584 ± 50 μmol L−1, phosphate 21 ± 2 μmol L−1), accounting for 42 % and 55 % of the overall annual DIN (123.5 ± 44.9-ton yr−1) and P (6.7 ± 1.9 ton yr−1) loads to sea, respectively. The N:P ratios were 16 and 34 during baseflow and flood events, respectively. Previously, nutrient loads were calculated by multiplying baseflow-measured concentrations by the total water volume of baseflow and floods. Our calculations, based on high-resolution sampling, revealed lower annual loads of P and N to the sea that were 56 % and 89 % of previous estimates, which is a considerable difference in an oligotrophic system such as the eastern Mediterranean.

Effects of the Aguirre Power Plant on the Marine Soundscape: a New Mangrove Function in Jobos Bay National Estuarine Research Reserve, Puerto Rico: a Preliminary Study

Effects of the Aguirre Power Plant on the Marine Soundscape: a New Mangrove Function in Jobos Bay National Estuarine Research Reserve, Puerto Rico: a Preliminary Study

Biological and Physical Controls on Multidecadal Acidification in a Eutrophic Estuary

AbstractEstuaries support ecologically and economically important resources that are vulnerable to ocean acidification from rising anthropogenic CO2. However, complex local processes in estuaries complicate and may disguise long‐term pH trends. For example, terrestrial nutrient runoff and coastal upwelling may exacerbate pH variability and declines. We investigated eutrophication impacts on acidification in a central California estuary, Elkhorn Slough, which receives high nutrient loads from intensive surrounding agriculture and upwelling of the California Current System. We examined drivers of acidification including nutrients, ecosystem metabolism, and upwelling by modeling pH trends over 20 years using a Generalized Additive Mixed Model at four sites from the National Estuarine Research Reserve Systemwide Monitoring Program and collected additional water samples to calculate aragonite saturation. Our models revealed acidification trends over two decades which were more pronounced near the marine inlet. Near the marine inlet, high nutrient levels and lower buffering were associated with the greatest rate of acidification in the estuary, which was four times greater than the trend from anthropogenic CO2 alone. Compared to fully tidal sites, a tidally restricted site showed diminished pH declines over time because of higher mean pH and aragonite saturation levels, but greater diel and seasonal variability associated with cycles of ecosystem metabolism and tidal range. Therefore, short‐term drops of saturation are threats to acidity in this location. The effects of enhanced seasonal cycles or long‐term trends in different zones of the estuary have implications for monitoring estuaries with a temporal frequency and scale to capture coastal acidification risks.

Using vulnerability assessment to characterize coastal protection benefits provided by estuarine habitats of a dynamic intracoastal waterway.

The existence of coastal ecosystems depends on their ability to gain sediment and keep pace with sea level rise. Similar to other coastal areas, Northeast Florida (United States) is experiencing rapid population growth, climate change, and shifting wetland communities. Rising seas and more severe storms, coupled with the intensification of human activities, can modify the biophysical environment, thereby increasing coastal exposure to storm-induced erosion and inundation. Using the Guana Tolomato Matanzas National Estuarine Research Reserve as a case study, we analyzed the distribution of coastal protection services-expressly, wave attenuation and sediment control-provided by estuarine habitats inside a dynamic Intracoastal waterway. We explored six coastal variables that contribute to coastal flooding and erosion-(a) relief, (b) geomorphology, (c) estuarine habitats, (d) wind exposure, (e) boat wake energy, and (f) storm surge potential-to assess physical exposure to coastal hazards. The highest levels of coastal exposure were found in the north and south sections of the Reserve (9% and 14%, respectively) compared to only 4% in the central, with exposure in the south driven by low wetland elevation, high surge potential, and shorelines composed of less stable sandy and muddy substrate. The most vulnerable areas of the central Reserve and main channel of the Intracoastal waterway were exposed to boat wakes from larger vessels frequently traveling at medium speeds (10-20 knots) and had shoreline segments oriented towards the prevailing winds (north-northeast). To guide management for the recently expanded Reserve into vulnerable areas near the City of Saint Augustine, we evaluated six sites of concern where the current distribution of estuarine habitats (mangroves, salt marshes, and oyster beds) likely play the greatest role in natural protection. Spatially explicit outputs also identified potential elevation maintenance strategies such as living shorelines, landform modification, and mangrove establishment for providing coastal risk-reduction and other ecosystem-service co-benefits. Salt marshes and mangroves in two sites of the central section (N-312 and S-312) were found to protect more than a one-quarter of their cross-shore length (27% and 73%, respectively) from transitioning to the highest exposure category. Proposed interventions for mangrove establishment and living shorelines could help maintain elevation in these sites of concern. This work sets the stage for additional research, education, and outreach about where mangroves, salt marshes, and oyster beds are most likely to reduce risk to wetland communities in the region.

Changes in inundation drive carbon dioxide and methane fluxes in a temperate wetland

Wetlands cycle carbon by being net sinks for carbon dioxide (CO2) and net sources of methane (CH4). Daily and seasonal temporal patterns, dissolved oxygen (DO) availability, inundation status (flooded or dry/partially flooded), water depth, and vegetation can affect the magnitude of carbon uptake or emissions, but the extent and interactive effects of these variables on carbon gas fluxes are poorly understood. We characterized the linkages between carbon fluxes and these environmental and temporal drivers at the Old Woman Creek National Estuarine Research Reserve (OWC), OH. We measured diurnal gas flux patterns in an upstream side channel (called the cove) using chamber measurements at six sites (three vegetated and three non-vegetated). We sampled hourly from 7 AM to 7 PM and monthly from July to October 2022. DO concentrations and water levels were measured monthly. Water inundation status had the most influential effect on carbon fluxes with flooded conditions supporting higher CH4 fluxes (0.39 μmol CH4 m−2 s−1; −1.23 μmol CO2 m−2 s−1) and drier conditions supporting higher CO2 fluxes (0.03 μmol CH4 m−2 s−1; 0.86 μmol CO2 m−2 s−1). When flooded, the wetland was a net CO2 sink; however, it became a source for both CH4 and CO2 when water levels were low. We compared chamber-based gas fluxes from the cove in flooded (July) and dry (August) months to fluxes measured with an eddy covariance tower whose footprint covers flooded portions of the wetland. The diurnal pattern of carbon fluxes at the tower did not vary with changing water levels but remained a CO2 sink and a CH4 source even when the cove where we performed the chamber measurements dried out. These results emphasize the role of inundation status on wetland carbon cycling and highlight the importance of fluctuating hydrologic patterns, especially hydrologic drawdowns, under changing climatic conditions.

Using Podcasts to Bring National Estuarine Research Reserves into the Classroom

In secondary and undergraduate classroom settings, there are significant challenges to exposing students to concepts related to Earth sciences and the environment. These challenges are exacerbated when conveying lessons about geographic areas with limited access, such as the ocean and coastlines. It is now more important than ever for environmental education to improve and adapt to our changing world. Educators may have the opportunity to bring these subjects to life by using media content such as podcasts to introduce students to new places and the scientists, managers, and educators who work in those spaces. Although education and public information sharing are part of the mission of the National Estuarine Research Reserve System (NERRS), there are few pathways for bringing reserves and the research, monitoring, and conservation associated with them into schools that are distant from NERRS sites. To address this educational gap, an instructor survey was conducted, and 43 responses were analyzed to identify best practices for secondary school and higher education lesson formats and assessment strategies. Based on this feedback, an introductory podcast series called “NERR or Far: The Reserves Are Where You Are” was created. This educational series is designed to increase access to regional conservation information and foster stewardship of our coastal resources.

Cultivating commitment: how cultural ecosystem services affect visitor loyalty attitudes and intention-to-return in parks and protected areas

ABSTRACT Parks and protected areas (PPAs) are crucial for providing society with essential ecosystem services, encompassing both tangible and intangible benefits derived from healthy ecosystems. While previous research has predominantly focused on material-based ecosystem services, limited attention has been devoted to the impact of non-material cultural ecosystem services (CES), such as recreation and cultural-historic aspects, on visitor loyalty. Visitor loyalty refers to an individual’s commitment and willingness to repeatedly visit a PPA. CES can significantly shape these attitudes, as many visitors are drawn to specific PPAs due to their cultural significance or scenic beauty. This study investigated how CES influences visitor loyalty attitudes and intention-to-return at the Great Bay National Estuarine Research Reserve (GBE). In the summer of 2022, data were collected from 645 GBE visitors using a population sampling approach. Structural equation modeling analyses revealed a robust link between various CES factors, visitor loyalty attitudes, and intention-to-return. For instance, findings suggest that education and the sense of place exert a strong and consistent influence on visitor attitudes regarding referrals, financial support, volunteerism, and advocacy. Additionally, positive visitor attitudes toward referrals strongly and independently predicted their overall intent to revisit the GBE. This research contributes to the advancement of both CES and visitor loyalty frameworks, offering empirical insights for natural resource managers. By acknowledging the significance and trade-offs associated with CES elements, like education and the sense of place, resource managers can enhance visitor loyalty and secure the long-term sustainability of natural resources worldwide.

Stark differences in spatial gradients of Eastern oyster (Crassostrea virginica) productivity in two Florida, USA, estuaries

The eastern oyster (Crassostrea virginica) is a foundation species in U.S. Atlantic and Gulf coast estuaries because of its numerous ecosystem services and importance to local fisheries. Concerns about the persistence and sustainable harvesting of oysters in the state of Florida is growing due to environmental factors (e.g., increased predation, salinity, and disease) and increased harvesting pressure. Thus, we need to understand which estuary locations are best suited for conservation or restoration efforts. Here, we use a combination of field experiments and modeling to assess the potential oyster productivity at sites throughout two estuaries at the same latitude but in different ocean basins: Apalachicola Bay (Gulf of Mexico coast) and the Guana Tolomato Matanzas National Estuarine Research Reserve (GTM, Atlantic coast). Our population model used the per-recruit approach from fisheries science, allowing us to combine field data on growth and mortality to estimate productivity (measured as lifetime spawning output) in an elegant way, without requiring assumptions about unknown larval supply processes. Subtidal Apalachicola Bay oysters exhibit an expected pattern, with greater productivity in the inner estuary nearer the river mouth and predation reducing productivity in the seaward direction. In contrast, intertidal GTM oysters do not exhibit strong spatial gradients, with little effect of predators and slightly higher productivity near the river mouth; intertidal oysters in Apalachicola exhibited similar productivity and lack of predator effect. We attribute these differences in oyster productivity to differences in tidal exposure, the predator community, and hydrology of oyster reefs at the two study sites. Thus, we would give diverging recommendations for spatial prioritization of restoration and conservation efforts. Additionally, these results lead us to caution against generalized statements about spatial patterns of oyster productivity within estuaries without consideration of the predation context.

A Rigorous Derivation of the Water Mass Transformation Framework, the Relation between Mixing and Diasurface Exchange Flow, and Links to Recent Theories in Estuarine Research

Abstract In this paper we present the analytical derivation of a local water mass transformation (WMT) framework for an individual water column. We exactly formulate the mapping of the governing equations from geopotential coordinates to an arbitrary tracer space. Unique definitions for the local effective vertical diasurface fluxes are given. In tracer space we derive new relations between the local diatracer fluxes and the mixing per tracer class. The key relation between the effective vertical diatracer velocity and the mixing per tracer class directly formulates how the overturning circulation is linked to local tracer variance dissipation. Horizontal integration of the governing equations in tracer space and the relations between the diatracer quantities finally recovers the well-known integral WMT formulations.

How Does Human Activity Shape the Largest Estuarine Bay of the Pearl River Estuary, South China (1964–2019)

The morphological changes in an estuarine bay are affected by fluvial and oceanic dynamics, as well as human activities. Human activity has increased considerably in recent years, especially in Lingding Bay of the Pearl River Estuary. Based on mass measured bathymetric data and remote sensing images, morphological changes in Lingding Bay were examined and its long-term morphological evolution from 1964 to 2019 was studied using GIS method and EOF methods. The water area of Lingding Bay gradually decreased through this period due to shore reclamation and the evolutionary characteristics of the underwater topography were different before and after 2007 due to changes in the intensity of human activities. From 1964 to 2007, the water depth and volume of Lingding Bay decreased slightly and the bay experienced a slow silting process with the geomorphic pattern of “three shoals and two troughs” under low-intensity human activity. From 2007 to present, high-intensity sand-dredging activities in the bay have led to considerable deepening and a significant increase in water volume in the East Trough and Middle Shoal areas. The amount of sediment loss caused by the sand-dredging activities after 2007 far exceeded the amount of sediment deposition over the past four decades prior to 2007. Therefore, even if the sand-dredging activities had been banned, the eroded parts of Lingding Bay (i.e., East Trough and Middle Shoal) may not recover in a short time due to the small sediment load from the Pearl River. These recent morphological changes in Lingding Bay may bring about challenges for estuary regulation, disaster control, environmental protection, and the operational safety of the nearby ports and channels. Consequently, the subsequent evolution of the bay requires further research. This will enrich the scientific work for estuarine and coastal research and be conducive to revealing the interaction mechanisms between humans and nature, guiding sustainable development, estuarine disaster control, and promoting interdisciplinary innovation in estuarine research.

Marsh migration and beyond: A scalable framework to assess tidal wetland resilience and support strategic management

Tidal wetlands are critical but highly threatened ecosystems that provide vital services. Efficient stewardship of tidal wetlands requires robust comparative assessments of different marshes to understand their resilience to stressors, particularly in the face of relative sea level rise. Existing assessment frameworks aim to address tidal marsh resilience, but many are either too localized or too general, and few directly translate resilience evaluations to recommendations for management strategies. In response to the deficiencies in existing frameworks, we identified a set of metrics that influence overall marsh resilience that can be assessed at any spatial scale. We then developed a new comprehensive assessment framework to rank relative marsh resilience using these metrics, which are nested within three categories. We represent resilience as the sum of results across the three metric categories: current condition, adaptive capacity, and vulnerability. Users of this framework can add scores from each category to generate a total resilience score to compare across marshes or take the score from each category and refer to recommended management actions we developed based on expert elicitation for each combination of category results. We then applied the framework across the contiguous United States using publicly available data, and summarized results at multiple spatial scales, from regions to coastal states to National Estuarine Research Reserves to finer scale marsh units, to demonstrate the framework’s value across these scales. Our national analysis allowed for comparison of tidal marsh resilience across geographies, which is valuable for determining where to prioritize management actions for desired future marsh conditions. In combination, the assessment framework and recommended management actions function as a broadly applicable decision-support tool that will enable resource managers to evaluate tidal marshes and select appropriate strategies for conservation, restoration, and other stewardship goals.

SHELL HASH: A NATURE-BASED SOLUTION FOR BEACHFRONT COASTAL RESILIENCE?

Based upon examination of beach profiles measured in June, 2016 (i.e. pre-Hurricane Matthew), November, 2016 (post-Matthew), and in September, 2017 (post Hurricane Irma) it was found that the 6.8 km of natural beaches in the Guana, Tolomato, Matanzas National Estuarine Research Reserve (GTM), extending from State range monuments R-46 to R-67 in St. Johns County, FL, experienced essentially no erosion of the primary dune, whereas dunes and bluffs in the developed beaches further south in St. Johns County were severely eroded and many beachfront homes experienced heavy damage

Controls on Surface and Downcore Sedimentary Organic Matter in a Constructed Oyster Reef

Oysters provide a suite of important ecosystem services, and recent research shows that oyster restoration rapidly enhances sedimentary organic carbon deposition. In 2012, an oyster reef enhancement project began in the GTM National Estuarine Research Reserve in Northeast FL, USA. We analyzed the spatial and downcore variability in sedimentary organic matter (OM) and particle sizes in the intertidal zone between the reefs and the marsh, along with oyster reef characteristics, to better understand physical and/or biological influences on sediment. Our data indicate that OM in the top 20 cm of sediment cores was negatively correlated with reef age. Similar decreases in particles <63 μm suggest remobilization of sediment, likely driven by the degradation of the reef structure over its approximately 9-year lifetime. Likewise, a survey of surface sediments showed that adjacent reef structural metrics were the best predictor of sediment OM and particle size. These results highlight the importance of reef structure as a control on sedimentary organic carbon deposition and stability in areas where physical energy is relatively high. This result is discussed in the context of implications for carbon budgets and biogeochemical ecosystem services of oysters as a part of living shorelines.

Uncertainties in wetland methane-flux estimates.

The wetland at Old Woman Creek National Estuarine Research Reserve, with flux tower US-OWC, which is characterized by high methane fluxes, high spatial heterogeneity, dynamic hydrology and water level fluctuations, and high lateral transport of dissolved organic carbon and nutrients, embodies many of the challenges in modeling methane fluxes.

Increased salinity decreases annual gross primary productivity at a Northern California brackish tidal marsh

Tidal marshes sequester 11.4–87.0 Tg C yr−1 globally, but climate change impacts can threaten the carbon capture potential of these ecosystems. Tidal marshes occur across a wide range of salinity, with brackish marshes (0.5–18 ppt (parts per thousand)) dominating global tidal marsh extents. A diverse mix of freshwater- and saltwater-tolerant plant and microbial communities has led researchers to predict that carbon cycling in brackish wetlands may be less sensitive to changes in salinity than fresh- or saltwater wetlands. Rush Ranch, a well-monitored brackish tidal wetland of the San Francisco Bay National Estuarine Research Reserve, experiences highly variable annual salinity regimes. Within a five-year period (2014–2018), Rush Ranch experienced particularly extreme drought-induced salinization during the 2014 and 2015 growing seasons. During drought years, tidal channel salinity rose from a 15 year baseline of 4.7 ppt to growing season peaks of 10.3 ppt and 12.5 ppt. Continuous eddy covariance data from 2014 to 2018 demonstrate that during drought summers, gross primary productivity (GPP) decreased by 24%, whereas ecosystem respiration remained similar among all five years. Stepwise linear regression revealed that salinity, not air temperature or tidal height, was the dominant driver of annual GPP. A random forest model trained to predict GPP based on environmental data from low salinity years (i.e. naive to salinization) significantly over predicted GPP in drought years. When growing season salinities were doubled, annual estimates of net ecosystem exchange of CO2 decreased by up to 30%. These results provide ecosystem-scale evidence that increased salinity influences CO2 fluxes dominantly through reductions in GPP. This relationship provides a starting point for incorporating the effect of changes in salinity in wetland carbon models, which could improve wetland carbon forecasting and management for climate resilience.