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
The Ocean Model Intercomparison Project (OMIP) focuses on the physics and biogeochemistry of the ocean component of Earth system models participating in the sixth phase of the Coupled Model ...Intercomparison Project (CMIP6). OMIP aims to provide standard protocols and diagnostics for ocean models, while offering a forum to promote their common assessment and improvement. It also offers to compare solutions of the same ocean models when forced with reanalysis data (OMIP simulations) vs. when integrated within fully coupled Earth system models (CMIP6). Here we detail simulation protocols and diagnostics for OMIP's biogeochemical and inert chemical tracers. These passive-tracer simulations will be coupled to ocean circulation models, initialized with observational data or output from a model spin-up, and forced by repeating the 1948-2009 surface fluxes of heat, fresh water, and momentum. These so-called OMIP-BGC simulations include three inert chemical tracers (CFC-11, CFC-12, SF subscript 6) and biogeochemical tracers (e.g., dissolved inorganic carbon, carbon isotopes, alkalinity, nutrients, and oxygen). Modelers will use their preferred prognostic BGC model but should follow common guidelines for gas exchange and carbonate chemistry. Simulations include both natural and total carbon tracers. The required forced simulation (omip1) will be initialized with gridded observational climatologies. An optional forced simulation (omip1-spunup) will be initialized instead with BGC fields from a long model spin-up, preferably for 2000 years or more, and forced by repeating the same 62-year meteorological forcing. That optional run will also include abiotic tracers of total dissolved inorganic carbon and radiocarbon, CTabio and 14CTabio, to assess deep-ocean ventilation and distinguish the role of physics vs. biology. These simulations will be forced by observed atmospheric histories of the three inert gases and CO2 as well as carbon isotope ratios of CO2. OMIP-BGC simulation protocols are founded on those from previous phases of the Ocean Carbon-Cycle Model Intercomparison Project. They have been merged and updated to reflect improvements concerning gas exchange, carbonate chemistry, and new data for initial conditions and atmospheric gas histories. Code is provided to facilitate their implementation.
Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon‐cycle Model ...Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse‐resolution ocean circulation models. The model mean (±1σ) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr−1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation‐based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep‐ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep‐ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross‐model means of several biogeochemical metrics show better agreement with observation‐based estimates when restricted to those models that best simulate deep‐ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.
We evaluate the hypothesis that sea‐level rise over the second half of the 20th century has led to detectable increases in Chesapeake Bay salinity. We exploit a simple, statistical model that ...predicts monthly mean salinity as a function of Susquehanna River flow in 23 segments of the main stem Chesapeake Bay. The residual (observed minus modeled) salinity exhibits statistically significant linear (p < 0.05) trends between 1949 and 2006 in 13 of the 23 segments of the bay. The salinity change estimated from the trend line over this period varies from −2.0 to 2.2, with 10 of the 13 cells showing positive changes. The mean and median salinity changes over all 23 cells are 0.47 and 0.72; over the 13 cells with significant trends they are 0.71 and 1.1. We ran a hydrodynamic model of the bay under present‐day and reduced sea level conditions and found a bay‐average salinity increase of about 0.5, which supports the hypothesis that the salinity residual trends have a significant component due to sea‐level rise. Uncertainties remain, however, due to the spatial and temporal extent of historical salinity data and the infilling of the bay due to sedimentation. The salinity residuals also exhibit interannual variability, with peaks occurring at intervals of roughly 7 to 9 years, which are partially explained by Atlantic Shelf salinity, Potomac River flow and the meridional component of wind stress.
The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). In this second phase of ...the Regional Carbon Cycle Assessment and Processes (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap‐filled observation‐based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ∼60% larger in models (−0.72 vs. −0.44 PgC year−1, 1998–2018, coastal ocean extending to 300 km offshore or 1,000 m isobath with area of 77 million km2). We attribute most of this model‐product difference to the seasonality in sea surface CO2 partial pressure at mid‐ and high‐latitudes, where models simulate stronger winter CO2 uptake. The coastal ocean CO2 sink has increased in the past decades but the available time‐resolving observation‐based products and models show large discrepancies in the magnitude of this increase. The global coastal ocean is a major source of N2O (+0.70 PgCO2‐e year−1 in observational product and +0.54 PgCO2‐e year−1 in model median) and CH4 (+0.21 PgCO2‐e year−1 in observational product), which offsets a substantial proportion of the coastal CO2 uptake in the net radiative balance (30%–60% in CO2‐equivalents), highlighting the importance of considering the three greenhouse gases when examining the influence of the coastal ocean on climate.
Plain Language Summary
The coastal ocean regulates greenhouse gases. It acts as a sink of carbon dioxide (CO2) but also releases nitrous oxide (N2O) and methane (CH4) into the atmosphere. This synthesis contributes to the second phase of the Regional Carbon Cycle Assessment and Processes (RECCAP2) and provides a comprehensive view of the coastal air‐sea fluxes of these three greenhouse gases at the global scale. We use a multi‐faceted approach combining gap‐filled observation‐based products and ocean biogeochemical models. We show that the global coastal ocean is a net sink of CO2 in both observational products and models, but the coastal uptake of CO2 is ∼60% larger in models than in observation‐based products due to model‐product differences in seasonality. The coastal CO2 sink is strengthening but the magnitude of this strengthening is poorly constrained. We also find that the coastal emissions of N2O and CH4 counteract a substantial part of the effect of coastal CO2 uptake in the atmospheric radiative balance (by 30%–60% in CO2‐equivalents), highlighting the need to consider these three gases together to understand the influence of the coastal ocean on climate.
Key Points
We synthesize air‐sea fluxes of CO2, nitrous oxide and methane in the global coastal ocean using observation‐based products and ocean models
The coastal ocean CO2 sink is 60% larger in ocean models than in observation‐based products due to systematic differences in seasonality
Coastal nitrous oxide and methane emissions offset 30%–60% of the CO2 coastal uptake in the net radiative balance
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
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
In this paper, direct torque and flux control of interior permanent magnet synchronous motor has been described in the stator flux field-oriented reference frame based on adaptive input-output ...feedback linearization (AIOFL) control method. The proposed control method does not need to know the motor two-axis inductances, the rotor permanent magnetic flux linkages, and the rotor position. However, the knowledge of the stator resistance is mandatory and that is estimated by AIOFL method. In practice, the rotor speed is approximately estimated by using the derivation of the stator flux vector angle in the stator stationary reference frame. The experimental and simulation results presented in this paper show the effectiveness and capability of the proposed control approach.
Geometric and topological methods are applied to significance testing in the wavelet domain. A geometric test was developed for assigning significance to pointwise significance patches in local ...wavelet spectra, i.e., contiguous regions of significant wavelet power coefficients with respect to some noise model. This geometric significance test was found to produce results similar to an existing areawise significance test while being more computationally flexible and efficient. The geometric significance test can be readily applied to pointwise significance patches at various pointwise significance levels in wavelet power and coherence spectra. The geometric test determined that features in wavelet power of the North Atlantic Oscillation (NAO) are indistinguishable from a red-noise background, suggesting that the NAO is a stochastic, unpredictable process, which could render difficult the future projections of the NAO under a changing global system. The geometric test did, however, identify features in the wavelet power spectrum of an El Niño index (Niño 3.4) as distinguishable from a red-noise background. A topological analysis of pointwise significance patches determined that holes, deficits in pointwise significance embedded in significance patches, are capable of identifying important structures, some of which are undetected by the geometric and areawise tests. The application of the topological methods to ideal time series and to the time series of the Niño 3.4 and NAO indices showed that the areawise and geometric tests perform similarly in ideal and geophysical settings, while the topological methods showed that the Niño 3.4 time series contains numerous phase-coherent oscillations that could be interacting nonlinearly.
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