Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional ...carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here we construct such a budget for eastern North America using historical data, empirical models, remote sensing algorithms, and process‐based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they, respectively, make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters, and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling.
Plain Language Summary
A carbon budget for a particular site or region describes the inputs and outputs of carbon to that site or region as well as the processes that change carbon from one form to another. A carbon budget is needed to fully understand many important issues facing coastal waters. We constructed the carbon budget for coastal waters of eastern North America. We found that about 60% of the carbon entering the domain is from rivers and about 40% is from the atmosphere, while about 80% of the carbon leaving the domain goes to the open ocean and about 20% is buried. Transfers of carbon from wetlands to estuaries and from estuaries to the ocean were as important as transfers of carbon at the domain boundaries. Tidal wetlands and estuaries were found to be important to the carbon budget despite making up only 2.4 and 8.9% of the study domain area, respectively. This study shows that coastal carbon budgets should explicitly consider tidal wetlands, estuaries, shelf waters, and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling.
Key Points
Tidal wetlands, estuaries, and shelf waters each contribute substantially to the carbon budget of eastern North American coastal waters
Study region net ecosystem production, atmospheric uptake, and burial are 20.2 ± 4.4, 5.1 ± 2.4, and 2.5 ± 0.7 Tg C yr−1, respectively
Net lateral carbon fluxes between tidal wetlands, estuaries, and shelf waters are large terms in the carbon budget of eastern North American coastal waters
Coastal dunes are often the first and only line of physical defense for communities subjected to damaging storms and waves. Planting vegetation on them has been proposed as one way to increase their ...protective capacity, but it is unknown how dune plant architecture reduces erosion. We conducted wave flume and field experiments to address this question and found that dune plants primarily reduced erosion by attenuating wave swash and run up bores with their stems and leaves, while their roots initially enhanced erosion through uprooting. After excavation, the roots also attenuated waves and reduced erosion. We then sampled the biophysical attributes of a broad distribution of plants, and found that herbaceous non-Graminoid (non-grass) species that inhabited the lowest latitudes and most seaward zones had the most efficient structures for erosion reduction. Our results suggest that there is a fundamental tradeoff in the ability of dune plant species to respond to hydrodynamic versus Aeolian processes, based on the relative allocation of aboveground versus belowground biomass. Through the combination of flume experiments, field survey, and meta-analysis, our findings show that vegetation provides on average ∼1.6 factor of safety over bare sand across a wide range of latitudes in the northern hemisphere - translating into a reduction of wave run up erosion by approximately 40% for dunes.
Human encroachment on the coasts is extensive and expected to increase over the coming decades. This proximity to the sea is coupled with potentially more frequent strong tropical cyclones and ...eustatic sea level rise, and thus human life, property and infrastructure are threatened. In this scenario, it is vital to find the means to maintain or increase the resilience and resistance of coastal zones. As an alternative to engineering solutions, ecosystem-based coastal defence strategies have been recommended as better and more sustainable solutions, although it has not been clearly demonstrated that they really work. In 24 wave flume experiments, we studied the effects of four densities of vegetation cover (none, low, medium, high) on the movement of sediment along two beach–dune profiles (with and without a berm), under three storm conditions (mild, moderate and intense). Erosion regimes of collision and overwash were observed in the dune profiles with a berm, whereas swash and overwash regimes were observed when no berm was present. In the profiles with a collision regime, vegetation decreased the amount of erosion. For both profiles, in the tests with the largest wave conditions, vegetation prevented overwash and thereby erosion of the landward side of the dune. Erosion of the dune face was reduced by vegetation, particularly with a strong storm, when Iribarren numbers were greater. In summary, vegetation reduced net erosion on the dune face, regardless of the wave conditions, the morphology of the beach–dune profile, or the mode of erosion.
•Vegetation can help reduce beach and coastal dune erosion as well as shoreline retreat.•Dune vegetation increases and maintains the resilience of coastal zones.•Vegetation is an alternative ecosystem based solution.
This study challenges the paradigm that salt marsh plants prevent lateral wave-induced erosion along wetland edges by binding soil with live roots and clarifies the role of vegetation in protecting ...the coast. In both laboratory flume studies and controlled field experiments, we show that common salt marsh plants do not significantly mitigate the total amount of erosion along a wetland edge. We found that the soil type is the primary variable that influences the lateral erosion rate and although plants do not directly reduce wetland edge erosion, they may do so indirectly via modification of soil parameters. We conclude that coastal vegetation is best-suited to modify and control sedimentary dynamics in response to gradual phenomena like sea-level rise or tidal forces, but is less well-suited to resist punctuated disturbances at the seaward margin of salt marshes, specifically breaking waves.
Tidal wetlands are among the most valuable ecosystems in the world in terms of ecosystem service value, and the strategic management of their carbon resources is an important part of climate change ...mitigation. Tidal wetland soils contain relatively high quantities of soil organic carbon (SOC); yet there is little information about whether this SOC can be predicted based on easily identifiable environmental factors. We investigated how tidal wetland SOC density was distributed across the continental United States among various coastal locations, estuarine typologies, vegetation types, water regimes, and management regimes and found that knowledge of a wetland's coastal affiliation provided the most differentiation (whether on the East Coast, West Coast, or Gulf Coast). We then sought to identify whether SOC density was correlated with 47 different environmental variables and found that knowledge based on latitude and precipitation explained ~46% of the variance in SOC density for wetlands on the West Coast and Gulf Coast, respectively. Several other geographic, oceanic, terrestrial, and atmospheric factors were cross‐correlated along these axes, including oceanic salinity, temperature, and average catchment elevation. For the U.S. East Coast, SOC density was not simple to explain and was likely dependent on a wide range of interacting and complex factors. This synthetic work can provide a better understanding of how changing environmental and social conditions can lead to enhanced or degraded carbon sequestration rates in times of rapid global change.
Key Points
The geographic location of a tidal wetland best explains the differentiation in SOC, due to distinct gradients in environmental conditions
For wetlands on U.S. West and Gulf coasts, 46% of the variance in SOC was related to gradients in latitude and precipitation
For the U.S. East Coast, the variation in SOC was not easy to explain but rather was dependent on range of interacting factors
This technical note presents empirically-derived values for biophysical attributes of several commonly occurring wetland plant species, including plant stem diameter and tapering, plant clump and ...stem spacing statistics, biomass, Young's modulus of elasticity, and bending strength. These parameters can be used to more realistically configure plant canopies in numerical and laboratory studies to further our understanding of wave attenuation by wetlands.
We examined total mercury (Hg) distributions in sediments from the Penobscot River and estuary, Maine, a site of extensive Hg releases from HoltraChem (1967–2000). Our objectives were to quantify: ...(1) bottom sediment Hg inventories (upper ~1m; 50–100 y); (2) sediment accumulation rates; and (3) contemporary Hg fluxes to bottom sediments; by sampling the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR) and the Penobscot estuary (ES). Hg was rapidly distributed here, and the cumulative total (9.28 metric tons) associated with sediments system-wide was within the range released (6–12 metric tons). Evidence of sediment/Hg remobilization was observed in cores primarily from the PBR, and to a lesser extent the ES, whereas cores from MM, most of the OR, the ES, and half from the PBR exhibited sharp peaks in Hg concentrations at depth, followed by gradual decreases towards the surface. Based on background PBR sediment Hg concentrations (100ngg−1), “elevated” (300ngg−1), or “highly elevated” (600ngg−1) Hg concentrations in sediments, and resulting inventories, we assessed impact levels (“elevated”≥270, or “highly elevated”≥540mgm−2). 71% of PBR stations had “elevated”, and 29% had “highly elevated” Hg inventories; 45% of MM stations had “elevated”, and 27% had “highly elevated” inventories; 80% of OR stations had “elevated” inventories only; and 17% of ES stations had “elevated” inventories only. Most “highly elevated” stations were located within 8km of HoltraChem, in MM, in the PBR, and in the OR. Near-surface sediments in the OR, PBR and MM were all “highly elevated”, while those in the ES were “elevated”, on average. Mean Hg fluxes to bottom sediments were greatest in the OR (554), followed by the PBR (469), then MM (452), and finally the ES (204ngcm−2y−1).
Simple contour map (kriging) showing the distributions of total sedimentary Hg inventories (ngcm−2) throughout the Penobscot system. Display omitted
•Total Hg was rapidly distributed and deposited throughout this system.•The calculated cumulative total sedimentary Hg (9.28 metric tons) throughout the system falls within the range of total Hg (6-12 metric tons) believed to have been released from HoltraChem.•Differences between distributions of total Hg inventories, near-surface (upper 3 cm) total Hg concentrations, and contemporary total Hg fluxes show that total Hg is being redistributed throughout the system.•Mean, near-surface (upper 3 cm) total Hg concentrations are greatest in the Orland River (1,120 ng g-1) > Penobscot River (815 ng g-1) > Mendall Marsh (673 ng g-1) > Penobscot Estuary (526 ng g-1).•Hg(o) values at different sites were similar, though individual total Hg profiles were heterogeneous.
Estuaries represent the primary linkage between the terrestrial and marine carbon cycles, and estuarine processing of riverine and coastal carbon plays a disproportionately large role in the global ...carbon cycle relative to the small areal extent of the estuarine environments. However, knowledge of the rate of organic carbon deposition and burial in estuarine sediments is lacking at regional scales. Data on surficial total organic carbon, linear sedimentation, and bulk density of estuarine sediments were compiled and categorized via a cluster analysis in order to estimate carbon deposition within the contiguous United States (CONUS). The cluster analysis broadly grouped estuaries by geography, but exceptions to geographic clustering highlighted differences within regions. A transfer function from deposition to burial based on linear sedimentation rate was used to estimate burial efficiency, and thus the rate of carbon burial within each cluster. We estimate organic carbon deposition rates within CONUS estuarine sediments to be 161 121–217, 95% confidence g C/m2/yr with a burial efficiency estimated at 38 34–42, 95% confidence %, which yields a long‐term burial rate of 64 44–97, 95% confidence g C/m2/yr. Spatially integrated organic deposition and burial rates are 11.3 8.5–15.2, 95% confidence and 4.5 3.1–6.8, 95% confidence Tg C/yr, respectively. Our findings allow a more thorough understanding of coastal carbon cycling, which is critical for both management purposes as well as for the assessment of the role of estuaries in past and future climate change.
Plain Language Summary
Estuaries are diverse ecosystems located at the coastal mouth of rivers or within embayments, where terrestrial and marine environments meet. Estuaries tend to be highly productive and provide many critical ecosystem services for their communities. Carbon‐based material delivered primarily by rivers or produced by aquatic organisms is deposited within the sediments of estuaries. Organisms in sediments consume some of the deposited material and respire it principally as carbon dioxide. However, a portion is buried within deeper sediments and is removed from the contemporary carbon cycle. The rate of material deposited and ultimately buried can influence the function of the estuary as well as its relation to communities and the adjacent coastal ecosystems. On longer time scales (many thousands of years), this removal can also influence the global carbon cycle. We grouped estuaries based on their physical and chemical characteristics and estimated sediment deposition and burial rates for the contiguous United States, where we found that approximately 40% of deposited material was buried within sediments.
Key Points
Sedimentation processes in estuaries within the contiguous United States are estimated to deposit 11.3 8.5–15.2, 95% confidence Tg of carbon per year
Remineralization liberates a significant portion of deposited carbon, yielding centennial‐scale burial of 4.5 3.1–6.8, 95% confidence Tg carbon per year
Estuaries of the contiguous United States are classified into six clusters based on geospatial and biogeochemical characteristics using k‐means analysis
We mapped tidal wetland gross primary production (GPP) with unprecedented detail for multiple wetland types across the continental United States (CONUS) at 16‐day intervals for the years 2000–2019. ...To accomplish this task, we developed the spatially explicit Blue Carbon (BC) model, which combined tidal wetland cover and field‐based eddy covariance tower data into a single Bayesian framework, and used a super computer network and remote sensing imagery (Moderate Resolution Imaging Spectroradiometer Enhanced Vegetation Index). We found a strong fit between the BC model and eddy covariance data from 10 different towers (r2 = 0.83, p < 0.001, root‐mean‐square error = 1.22 g C/m2/day, average error was 7% with a mean bias of nearly zero). When compared with NASA's MOD17 GPP product, which uses a generalized terrestrial algorithm, the BC model reduced error by approximately half (MOD17 had r2 = 0.45, p < 0.001, root‐mean‐square error of 3.38 g C/m2/day, average error of 15%). The BC model also included mixed pixels in areas not covered by MOD17, which comprised approximately 16.8% of CONUS tidal wetland GPP. Results showed that across CONUS between 2000 and 2019, the average daily GPP per m2 was 4.32 ± 2.45 g C/m2/day. The total annual GPP for the CONUS was 39.65 ± 0.89 Tg C/year. GPP for the Gulf Coast was nearly double that of the Atlantic and Pacific Coasts combined. Louisiana alone accounted for 15.78 ± 0.75 Tg C/year, with its Atchafalaya/Vermillion Bay basin at 4.72 ± 0.14 Tg C/year. The BC model provides a robust platform for integrating data from disparate sources and exploring regional trends in GPP across tidal wetlands.
Key Points
We created the Blue Carbon (BC) model, which mapped the Gross Primary Production (GPP) of all tidal wetlands within the continental United States
The BC model provides maps of tidal wetland GPP at sub‐250 m scales and at 16‐day intervals for the years 2000‐2019
The average daily GPP per m2 was 4.32 ± 2.45 g C/m2/day, and the total annual GPP for the continental United States was 39.65 ± 0.89 Tg C/year
Active growth faults contribute to the subsidence of deltaic basins around the world and can significantly augment local rates of relative sea level rise, which can inundate and drown wetlands. ...Little information exists on the temporal frequency and spatial magnitude of motion along these faults. Our objective was to quantify fault activity and displacement at multiple time scales in a salt marsh wetland on the East Matagorda Peninsula, Texas. We present evidence of this activity that includes remotely sensed aerial imagery, LiDAR data, ground penetrating radar data, shallow seismic data, lithostratigraphic and biostratigraphic evidence from core data, and sub-centimeter GPS and survey monitoring. The results support the interpretation that the Matagorda fault is currently active, and has been active in the past. Subsurface data to depths of ~150m displays evidence of disrupted strata at the fault plane, as well as thicker strata on the downthrown side of the fault. In the shallow subsurface down to ~3m of depth, vertical displacement in stratigraphic markers and surface deformation exhibits a sinusoidal pattern that runs perpendicular to the fault plane, which we interpret to represent fault-propagation folding. Maximum throw on this fault is estimated at ~0.75m over the last ~40–50years. We also recorded a vertical drop of −0.208m in the span of a single year, at a location close to the fault on its downthrown side. Records of yearly elevation change also show evidence of ongoing fault-propagation folding. We conclude that whereas the surface at the Matagorda fault moves intermittently up and down at the yearly time scale, fault displacement manifests as a sinusoidal pattern of displacement and deformation which has been integrated into the stratigraphic record at decadal-to-millennial time scales. This study presents a rare look into active motion and details its effects on the modern evolution of unconsolidated sedimentary surfaces.
► We recorded active growth fault displacement, deformation, and propagation folding. ► The magnitude of active displacement that we recorded was up to −0.208m per year. ► We reconcile the past stratigraphic record with modern day motion.
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