The carbon cycle of the coastal ocean is a dynamic component of the global carbon budget. But the diverse sources and sinks of carbon and their complex interactions in these waters remain poorly ...understood. Here we discuss the sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide during post-industrial times. Continued human pressures in coastal zones will probably have an important impact on the future evolution of the coastal ocean's carbon budget.
Feedbacks between flooding and plant growth that help to stabilize marshes against rising sea level are being investigated in estuaries at Plum Island, Massachusetts, and North Inlet, South Carolina. ...Net annual primary production of the marsh grassSpartina alterniflorahas been quite variable through the years, and correlates positively with sea level during the growing season at both sites. The elevation of the marsh surface relative to mean high water determines the duration of flooding, or hydroperiod, that in turn affects plant growth. The effect of flooding was tested experimentally using an in situ bioassay to simulate growth at different relative elevations. At North Inlet, we found a parabolic response to relative elevation, with clear evidence of minimum and maximum vertical limits and an optimal elevation for growth. The Plum Island bioassay provided evidence of the super-optimal side of the growth curve. In both marshes, the responses ofS. alterniflorato rising sea level, at their current elevations, are consistent with the bioassay results. This growth curve is important because it defines suboptimal elevations that are unstable for marshes and super-optimal elevations that are stable. Instability results when an increase in sea level decreases primary production, leading to declines in mineral sedimentation and sediment organic matter accretion. Conversely, stability results when rising sea level stimulates primary production, leading to increased sedimentation and organic matter accretion. There also has been interannual variability in the maximum standing biomass (a proxy for productivity) of another marsh grass,Spartina patens, but no significant correlation has been found with sea level, possibly due to methodological limitations. Finally, bothSpartinaspecies responded positively to nitrogen and have remained highly productive for 13 years of fertilization at Plum Island and 30 years at North Inlet.
Oceanic dissolved organic carbon (DOC) constitutes one of the largest pools of reduced carbon in the biosphere. Estimated DOC export from the surface ocean represents 20% of total organic carbon flux ...to the deep ocean, which constitutes a primary control on atmospheric carbon dioxide levels. DOC is the carbon component of dissolved organic matter (DOM) and an accurate quantification of DOM pools, fluxes and their controls is therefore critical to understanding oceanic carbon cycling. DOC export is directly coupled with dissolved organic nitrogen and phosphorus export. However, the C:N:P stoichiometry (by atoms) of DOM dynamics is poorly understood. Here we study the stoichiometry of the DOM pool and of DOM decomposition in continental shelf, continental slope and central ocean gyre environments. We find that DOM is remineralized and produced with a C:N:P stoichiometry of 199:20:1 that is substantially lower than for bulk pools (typically >775:54:1), but greater than for particulate organic matter (106:16:1—the Redfield ratio). Thus for a given mass of new N and P introduced into surface water, more DOC can be exported than would occur at the Redfield ratio. This may contribute to the excess respiration estimated to occur in the interior ocean. Our results place an explicit constraint on global carbon export and elemental balance via advective pathways.
The large areal extent of hypoxia in the northern Gulf of Mexico has been partially attributed to substantial nitrogen (N) loading from the Mississippi River basin, which is driven by multiple ...natural and human factors. The available water quality monitoring data and most of the current models are insufficient to fully quantify N load magnitude and the underlying controls. Here we use a process‐based Dynamic Land Ecosystem Model to examine how multiple factors (synthetic N fertilizer, atmospheric N deposition, land use changes, climate variability, and increasing atmospheric CO2) have affected the loading and delivery of total nitrogen (TN) consisting of ammonium and nitrate (dissolved inorganic N) and total organic nitrogen from the Mississippi River basin during 1901–2014. The model results indicate that TN export during 2000–2014 was twofold larger than that in the first decade of twentieth century: Dissolved inorganic N export increased by 140% dominated by nitrate; total organic nitrogen export increased by 53%. The substantial enrichment of TN export since the 1960s was strongly associated with increased anthropogenic N inputs (synthetic N fertilizer and atmospheric N deposition). The greatest export of TN was in the spring. Although the implementation of N reduction has been carried out over the past three decades, total N loads to the northern Gulf of Mexico have not decreased significantly. Due to the legacy effect from historical N accumulation in soils and riverbeds, a larger reduction in synthetic N fertilizer inputs as well as improved N management practices are needed to alleviate ocean hypoxia in the northern Gulf of Mexico.
Key Points
Over the period 1901‐2014, DIN and TON export from the Mississippi River basin increased by 85% and 60%, respectively. The Ohio River basin was largest contributor to the increase among seven subbasins
Synthetic N fertilizer application was the dominant contributor to increases in the export of DIN (70%) and TON (40%) since the 1970s
The highest DIN and TON export occurred in the spring, accounting for 39% and 36% of the total export from the MRB during 1990‐2014
► Vegetated coastal systems, including seagrass beds, mangroves, and intertidal marshes currently sequester 67–215TgCyr−1, globally. ► There is considerable uncertainty in the magnitude of C storage ...because of the difficulty in extrapolating rates with very high spatial variability and we lack accurate and precise measures of the coverage of these systems in many parts of the world. ► The magnitude of C sequestration by these systems is declining and will continue to decline primarily because of direct human conversion of these systems to other uses and anthropogenic activities that affect global climate and the quality of river runoff. ► A decline in rates of C sequestration by vegetated coastal systems over time will contribute to ever increasing levels of CO2 in the atmosphere and hence climate change.
Coastal vegetated wetlands have recently been identified as very important global C sinks but vulnerable to degradation by direct human alteration of their habitats. While their expanse is small globally, areal rates of C burial, or sequestration, are among the highest of Earth's ecosystems. There is considerable uncertainty in the magnitude of total global sequestration in these systems for two reasons: poor estimates of their global areas and high variability and uncertainty in areal rates of burial between systems. The magnitude of C burial in vegetated coastal systems has been decreasing rapidly over the past century due primarily to human disturbances such as dredging, filling, eutrophication, and timber harvest. These systems continue to be lost globally at rates ranging from 1% to 7% annually. We find that climate change including global warming, human engineering of river systems, continued agricultural expansion, and sea level rise will also negatively impact C burial of coastal vegetated wetlands. A decrease in global C burial in these systems will ultimately exacerbate CO2 emissions, and further contribute to climate change in the future.
Urbanization, an important driver of climate change and pollution, alters both biotic and abiotic ecosystem properties within, surrounding, and even at great distances from urban areas. As a result, ...research challenges and environmental problems must be tackled at local, regional, and global scales. Ecosystem responses to land change are complex and interacting, occurring on all spatial and temporal scales as a consequence of connectivity of resources, energy, and information among social, physical, and biological systems. We propose six hypotheses about local to continental effects of urbanization and pollution, and an operational research approach to test them. This approach focuses on analysis of "megapolitan" areas that have emerged across North America, but also includes diverse wildland-to-urban gradients and spatially continuous coverage of land change. Concerted and coordinated monitoring of land change and accompanying ecosystem responses, coupled with simulation models, will permit robust forecasts of how land change and human settlement patterns will alter ecosystem services and resource utilization across the North American continent. This, in turn, can be applied globally.
Between the land and ocean, diverse coastal ecosystems transform, store, and transport material. Across these interfaces, the dynamic exchange of energy and matter is driven by hydrological and ...hydrodynamic processes such as river and groundwater discharge, tides, waves, and storms. These dynamics regulate ecosystem functions and Earth's climate, yet global models lack representation of coastal processes and related feedbacks, impeding their predictions of coastal and global responses to change. Here, we assess existing coastal monitoring networks and regional models, existing challenges in these efforts, and recommend a path towards development of global models that more robustly reflect the coastal interface.
With sea level rise accelerating and sediment inputs to the coast declining worldwide, there is concern that tidal wetlands will drown. To better understand this concern, sources of sediment ...contributing to marsh elevation gain were computed for Plum Island Sound estuary, MA, USA. We quantified input of sediment from rivers and erosion of marsh edges. Maintaining elevation relative to the recent sea level rise rate of 2.8 mm yr−1 requires input of 32,299 MT yr−1 of sediment. The input from watersheds is only 3,210 MT yr−1. Marsh edge erosion, based on a comparison of 2005 and 2011 LiDAR data, provides 10,032 MT yr−1. This level of erosion is met by <0.1% of total marsh area eroded annually. Mass balance suggests that 19,070 MT yr−1 should be of tidal flat or oceanic origin. The estuarine distribution of 14C and 13C isotopes of suspended particulate organic carbon confirms the resuspension of ancient marsh peat from marsh edge erosion, and the vertical distribution of 14C‐humin material in marsh sediment is indicative of the deposition of ancient organic carbon on the marsh platform. High resuspension rates in the estuarine water column are sufficient to meet marsh accretionary needs. Marsh edge erosion provides an important fraction of the material needed for marsh accretion. Because of limited sediment supply and sea level rise, the marsh platform maintains elevation at the expense of total marsh area.
Plain Language Summary
Tidal marshes in the Plum Island Sound estuary have been gaining elevation over the past 100 years at about the same rate as sea level rise (SLR), but there is concern that they will drown and disappear if rates of SLR increase substantially due to CO2 emissions and climate change. What are the sediment sources enabling elevation gain? Rivers were not the primary source, providing less than 10% of elevation gain needs. Marsh edge erosion is much more important—providing over 30% of needs. We estimate that the remainder comes from the ocean or erosion of tidal flats. If the ocean is a major source, these marshes might be able to maintain elevation throughout the 21st century even if SLR greatly accelerates. However, if erosion of tidal flats is the primary source of sediments, the future outlook is less favorable because the more tidal flat erosion increases, the more edge erosion will also increase. The net result will be a loss of marsh area in this system. The dynamics we found for Plum Island Sound are likely to be occurring globally, as declining sediment inputs from rivers, and increasing rates of SLR due to climate change are worldwide phenomenon. Loss of marshes will have a major impact on the resilience of coastal communities landward of the marshes and the productivity of most coastal fisheries.
Key Points
Sediment input from rivers (3,210 MT yr−1) and marsh edge erosion (10,032 MT yr−1) provides only 39% of the sediment required for marshes to maintain elevation relative to SLR
The marsh platform has been able to maintain its relative elevation at the expense of total marsh area
Sediment inputs from the ocean or from erosion of tidal flats are likely an important factor in the mineral sediment budget of the system and together must contribute 19,070 MT yr−1 in order for the marsh to accrete at its current rate of 2.8 mm yr−1
Although the hypoxia formation in the Gulf of Mexico is predominantly driven by increased riverine nitrogen (N) export from the Mississippi-Atchafalaya River basin, it remains unclear how ...hydroclimate extremes affect downstream N loads. Using a process-based hydro-ecological model, we reveal that over 60% of the land area of the Basin has experienced increasing extreme precipitation since 2000, and this area yields over 80% of N leaching loss across the region. Despite occurring in ~9 days year
−1
, extreme precipitation events contribute ~1/3 of annual precipitation, and ~1/3 of total N yield on average. Both USGS monitoring and our modeling estimates demonstrate an approximately 30% higher annual N load in the years with extreme river flow than the long-term median. Our model suggests that N load could be reduced by up to 16% merely by modifying fertilizer application timing but increasing contribution of extreme precipitation is shown to diminish this potential.
Over the past 40 years, precipitation extremes have become more important for delivering N to the Gulf of Mexico, according to simulations with a hydro-ecological model. This is likely to diminish the effectiveness of alternative N use practices
Climate change has caused shifts in species' ranges and extinctions of high-latitude and altitude species. Most cold-tolerant evergreen broadleaved woody plants (shortened to cold-evergreens below) ...are rare species occurring in a few sites in the alpine and subalpine zones in the Korean Peninsula. The aim of this research is to 1) identify climate factors controlling the range of cold-evergreens in the Korean Peninsula; and 2) predict the climate change effects on the range of cold-evergreens. We used multimodel inference based on combinations of climate variables to develop distribution models of cold-evergreens at a physiognomic-level. Presence/absence data of 12 species at 204 sites and 6 climatic factors, selected from among 23 candidate variables, were used for modeling. Model uncertainty was estimated by mapping a total variance calculated by adding the weighted average of within-model variation to the between-model variation. The range of cold-evergreens and model performance were validated by true skill statistics, the receiver operating characteristic curve and the kappa statistic. Climate change effects on the cold-evergreens were predicted according to the RCP 4.5 and RCP 8.5 scenarios. Multimodel inference approach excellently projected the spatial distribution of cold-evergreens (AUC = 0.95, kappa = 0.62 and TSS = 0.77). Temperature was a dominant factor in model-average estimates, while precipitation was minor. The climatic suitability increased from the southwest, lowland areas, to the northeast, high mountains. The range of cold-evergreens declined under climate change. Mountain-tops in the south and most of the area in the north remained suitable in 2050 and 2070 under the RCP 4.5 projection and 2050 under the RCP 8.5 projection. Only high-elevations in the northeastern Peninsula remained suitable under the RCP 8.5 projection. A northward and upper-elevational range shift indicates change in species composition at the alpine and subalpine ecosystems in the Korean Peninsula.
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