Article

# Coastal wetlands, sea level, and the dimensions of geomorphic resilience

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## Abstract

Geomorphic system resilience is often perceived as an intrinsic property of system structure and interactions but is also related to idiosyncratic place and history factors. The importance of geographical and historical circumstances makes it difficult to generate categorical statements about geomorphic resilience. However, network-based analyses of system structure can be used to determine the dynamical stability (= resilience) based on generally applicable relationships and to determine scenarios of stability or instability. These provide guidelines for assessing place and history factors to assess resilience. A model of coastal wetlands is analyzed, based on interactions among relative sea level, wetland surface elevation, hydroperiod, vegetation, and sedimentation. The system is generally (but not always) dynamically unstable and non-resilient. Because of gradients of environmental factors and patchy distributions of microtopography and vegetation, a coastal wetland landscape may have extensive local variations in stability/resilience and in the key relationships that trigger instabilities. This is illustrated by a case study where dynamically unstable fragmentation is found in two nearby coastal wetlands in North Carolina's Neuse River estuary—Otter Creek Mouth and Anderson Creek. Neither is keeping pace with relative sea level rise, and both show unstable state transitions within the wetland system; but locally stable relationships exist within the wetland systems.

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... g., Phillips, 1986;Nyman et al., 1994). Recent studies confirm this common phenomenon (Phillips, 2018a(Phillips, , 2018bSchoolmaster et al., 2018;Stagg et al., 2019;Wu, 2019;Schepers et al., 2020). Marsh loss is often caused or influenced by other factors, particularly human impacts, independently of or in conjunction with SLR. ...
... The Neuse estuary is experiencing the effects of rising sea-level and coastal submergence, reflected in a net loss of wetlands, estuarine shoreline erosion, and creation of "ghost forests" (standing dead trees killed by waterlogging and/or higher salinity) (Bellis et al., 1975;Brinson et al., 1995;Moorhead and Brinson, 1995;Cowart et al., 2011;Kopp et al., 2015;Eulie et al., 2017;Phillips, 2018aPhillips, , 2018bTaillie et al., 2019;Gunderson et al., 2021). The FETZ exhibits several geomorphic signatures typical of the leading edge of coastal submergence, as described by Phillips (2022b). ...
... This pattern of marsh loss, which includes interior conversion to open water as well as fringe erosion, has been observed in other Neuse River estuarine marshes (Phillips, 2018a(Phillips, , 2018b. Once interior conversion (fragmentation or disaggregation) is initiated, the open water areas are often expanded. ...
Article
Determining effects of climate change on landscapes involves numerous uncertainties. This paper presents and illustrates a protocol for climate attribution of landscape responses. The major steps are ascertaining potential climate-related responses, establishing plausibility for a climatic influence, identifying alternative or additional causes, testing possible climate and non-climate causes, and interpreting the role of climate and climate change in the landscape response. The protocol is based on existing practice in the historical and interpretive branches of Earth and ecological sciences, and explicitly considers negative (non-confirmatory) results for climate and other factors. The protocol is applied to the conversion of brackish marsh to open water in the upper Neuse River estuary, North Carolina. Conversion since at least the mid twentieth century can be attributed to relative sea-level rise, driven primarily by general climate warming, with no supporting evidence for any additional or alternative drivers. The only other factor with supporting evidence is human modification in the form of ditches, around which conversion was concentrated, though marsh loss also occurred in unditched portions. Rapid recent marsh loss is attributable to Hurricane Florence (2018), particularly the storm surge. Weak positive inferential support exists for a role of climate change in the storm, but aspects of the storm’s impact not linked to climate are more important for the marsh conversions. Overall, the landscape response can be linked to climate, exacerbated by direct human impacts of marsh ditching, and strongly influenced by local place factors and the specific storm track. Recent and ongoing climate change is a significant factor, but not paramount, in determining the landscape response. The Neuse River case study is not unusual—and is probably typical—in identifying a combination of climate and other factors strongly influencing landscape response.
... Complex nonlinear dynamics have been discussed specifically in the context of coastal geomorphology by, e.g., Phillips (1992Phillips ( , 1999Phillips ( , 2018, Baas (2002), Eslami-Andergoli et al. (2015), Kirwan et al. (2012Kirwan et al. ( , 2016, Wang and Temmerman (2013), and Raposa et al. (2016). Nonlinearity allows for complex behaviors not possible in linear systems. ...
... Phillips, 1986Phillips, , 1989. At two small coastal wetland sites in North Carolina (within the study area of this paper), Phillips (2018) found dynamically unstable state transitions to be prevalent in geomorphic response to sea-level rise, and that dynamical stability (resilience) may vary within sub- environments within a single wetland. Phillips (2018) also highlighted the key role of local geographical and historical contingencies in con- trolling or influencing geomorphic responses. ...
... • Conversion of wetlands to open water and wetlands "climbing" adjacent uplands (Phillips, 1986(Phillips, , 1997(Phillips, , 2018Moorhead and Brinson, 1995;Riggs and Ames, 2003). ...
Article
Relative sea-level rise (SLR) raises geomorphic base levels, displaces salt water and tidal or backwater effects inland, and changes the hydrology of aquatic and upland environments. On an all-other-things-being equal basis, we can predict some transitions associated with SLR. However, in real coastal landscapes, all other things are not equal. Factors other than sea-level influence geomorphic, hydrological, and ecological processes and controls, environmental interactions often complicate or obscure process-response relationships, and local disturbances may interrupt or overprint them. In this study relationships among coastal environments in North Carolina, USA were investigated as they respond to changes in multiple environmental gradients driven by relative sea level rise, to determine the extent to which the spatial complexity of landscape response can be explained by environmental gradients. Spatial adjacency graphs reflecting observed patterns of contiguity were derived empirically , and five key environmental gradients related to relative sea-level were identified (elevation, hydro-period, salinity, vegetation, and process regime). The spectral radius of the spatial adjacency graph indicates a complex system that on the landscape level cannot be described or modeled based on linear gradients or suc-cessional relationships. Yet, spectral graph theory measures show that the complexity of the system can be fully explained, in the aggregate, by the five identified gradients, despite some redundancy of information therein. This indicates that coastal responses to SLR should be assessed based on multiscalar, nested environmental gradients rather than a single advancing front of change or linear sequence.
... Some consider resistance an intrinsic component of resilience, especially where resistance is a dynamical property derived from traditional engineering and economic ideas about stability [32]. Many geomorphologists, however, consider resilience and resistance to be distinct properties of geomorphic systems [89,90], where resistance is the ability of a geomorphic system to withstand or absorb a change or disturbance with minimal alteration, and resilience is the ability of the system to recover toward its pre-disturbance state [91]. By this definition, resistance is a capacity exerted before the system is perturbed; resilience can be measured after the perturbation has occurred. ...
... Salt marshes in microtidal regimes are particularly sensitive to a reduction in sediment supply under increasing rates of sea level rise, but salt marshes in macrotidal regimes are more resilient to high rates of sea level rise and/or reduced sediment supply [175,176]. Resilience may be an intrinsic property of system structure and interactions, but is nonetheless related to, if not controlled by, site-specific geographical and historical circumstances [91,172,174], further complicating any categorical statements about resilience in geomorphic systems. ...
Article
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The concept of resilience has taken root in the discourse of environmental management, especially regarding Building with Nature strategies for embedding natural physical and ecological dynamics into engineered interventions in developed coastal zones. Resilience is seen as a desirable quality, and coastal management policy and practice are increasingly aimed at maximising it. Despite its ubiquity, resilience remains ambiguous and poorly defined in management contexts. What is coastal resilience? And what does it mean in settings where natural environmental dynamics have been supplanted by human-dominated systems? Here, we revisit the complexities of coastal resilience as a concept, a term, and a prospective goal for environmental management. We consider examples of resilience in natural and built coastal environments, and offer a revised, formal definition of coastal resilience with a holistic scope and emphasis on systemic functionality: “Coastal resilience is the capacity of the socioeconomic and natural systems in the coastal environment to cope with disturbances, induced by factors such as sea level rise, extreme events and human impacts, by adapting whilst maintaining their essential functions.” Against a backdrop of climate change impacts, achieving both socioeconomic and natural resilience in coastal environments in the long-term (>50 years) is very costly. Cost trade-offs among management aims and objectives mean that enhancement of socioeconomic resilience typically comes at the expense of natural resilience, and vice versa. We suggest that for practical purposes, optimising resilience might be a more realistic goal of coastal zone management.
... Interpreted in this way, the geomorphic transformations such as net marsh loss that sometimes occur as marshes respond to coastal submergence (e.g., Phillips, 2018c;2018d) are evidence of a failure to develop spillway and secondary storage capacity rapidly enough. ...
Article
Concentrated or preferential flow patterns occur at all scales in hydrologic systems. They shape, and are shaped by, geomorphic and pedologic patterns and structures. Preferential flow patterns in surface channel networks and dual-porosity subsurface flow systems are a way of achieiving maximum efficiency, as predicted by dissipative systems, constructal, network evolution, percolation, and ecohydrological theories. These all converge on the same predictions and interpretations of preferential flow, which satisfactorily answer “why” these patterns form and persist. However, as geomorphic and hydrologic systems have no intentionality or agency, and thus no ability to actively seek improved efficiency, how these systems evolve is an open question. I propose an emergent explanation based on five phenomena. First, concentrated flows form due to principles of gradient and resistance selection. Second, positive feedback reinforces the concentrated preferential flow paths and their relationship to potential moisture storage zones. Third, intersecting flow paths form networks. Fourth, the expansion of concentrated flow paths and networks is limited by thresholds of flow needed for channel, macropore, or conduit growth and maintenance. This results in a “store and pour” flow system that can retain water during dry periods and transport it efficiently during wet periods. These systems survive provided they develop “spillway” and/or secondary storage mechanisms to accommodate excess water inputs. Finally, store-and-pour systems are maintained (selected for) because they are often stable. Store-and-pour structures are advantageous for flow systems, and for vegetation and ecosystems. These entities cannot actively pursue goals, and no laws dictate evolution toward such patterns. Their development is an emergent phenomenon and their persistence a matter of selection, i.e., survival of the most stable.
... With the sea-level rise, knowing which marshes are most vulnerable allows for the prioritization of restoration and conservation efforts, minimizing future impacts to estuarine systems. Therefore, understanding the evolution process of wetlands driven by geomorphic restrictions and human activities is of great significance for formulating long-term wetland management strategies and revealing the temporal and spatial characteristics of wetland evolution 25,26 . ...
Article
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... Some observational studies have also noted the loss of marshes to colonization by upland woody vegetation along the Virginia coast (Zinnert et al., 2019)-a dynamic observed in our model predictions given that as much as 65% of marsh converted to the barrier interior geomorphic setting in the No Intervention scenario. Ultimately, the response of marsh ecosystems is also highly dependent on sediment availability and other localized characteristics (van Belzen et al., 2017;Kirwan & Megonigal, 2013;Kirwan & Murray, 2007;Philips, 2017). Such context-specific information would need to be included with an explicit consideration of marsh evolution and back-barrier shoreline migration to generate high-confidence forecasts of marsh persistence. ...
Article
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Forecasting biogeomorphological conditions for barrier islands is critical for informing sea‐level rise (SLR) planning, including management of coastal development and ecosystems. We combined five probabilistic models to predict SLR‐driven changes and their implications on Fire Island, New York, by 2050. We predicted barrier island biogeomorphological conditions, dynamic landcover response, piping plover (Charadrius melodus) habitat availability, and probability of storm overwash under three scenarios of shoreline change (SLC) and compared results to observed 2014/2015 conditions. Scenarios assumed increasing rates of mean SLC from 0 to 4.71 m erosion per year. We observed uncertainty in several morphological predictions (e.g., beach width, dune height), suggesting decreasing confidence that Fire Island will evolve in response to SLR as it has in the past. Where most likely conditions could be determined, models predicted that Fire Island would become flatter, narrower, and more overwash‐prone with increasing rates of SLC. Beach ecosystems were predicted to respond dynamically to SLR and migrate with the shoreline, while marshes lost the most area of any landcover type compared to 2014/2015 conditions. Such morphological changes may lead to increased flooding or breaching with coastal storms. However—although modest declines in piping plover habitat were observed with SLC—the dynamic response of beaches, flatter topography, and increased likelihood of overwash suggest storms could promote suitable conditions for nesting piping plovers above what our geomorphology models predict. Therefore, Fire Island may offer a conservation opportunity for coastal species that rely on early successional beach environments if natural overwash processes are encouraged.
... Resilience is usually regarded as a system's capacity to absorb external forcing shocks, or adaption to such extreme events that the system can retain a majority of its functionality, while reworking toward full functionality over a longer term than the forcing event's time-line and its memory effects operate. Coastal resilience is heavily dependent on context (especially temporal domains) and place (spatial remit), such that resilience analyses tend to define the conceptual (i.e., awareness of resilience as a system), while there is only limited functionality noted in the general macro-behavior of coastal sub-systems (Phillips, 2018;Kombiadou et al., 2019). In the case of coastal morphodynamics, the last decade has witnessed an increased number of extreme storm events impinging on the western coasts of Britain, Ireland and France (Dodet et al., 2019) by which variable macro-resolution of shoreline resilience has been examined Scott et al., 2016;Burvingt et al., 2017). ...
Chapter
Gravel beaches occupy a dominantly reflective morphodynamic domain, which has so far been relatively less explored compared to that of their sandier counterparts. The dynamics of gravel beachfaces are considered in terms of step and berm evolution and their contribution to profile variation. Extensions of gravel profile types are considered in terms of relative gravel to sand volumes under the profile. The use of sediment assemblages and their association with both process dynamics and microscale morphology are presented via the concept of mosaics, which is considered to be the best approach toward better definition of microscale gravel beach morphodynamic regimes. The introduction of a specific numerical model (XBeach-G) to consider gravel beach response to extreme events is proving fruitful. There are however limited morphodynamic links with gravel barrier evolution, though the mosaics as a statement of long-term barrier stability might be considered as a likelihood statement of potential future change.
... Tidal flats are found primarily along river and creek banks, especially in upper CB. Marshes can be converted to tidal flats through drowning and/or animal disturbances, such as foraging by muskrats and nutria (Beckett et al., 2016;Phillips, 2018). Continued loss of elevation ultimately results in conversion of tidal flats to open water, and so both tidal flats and marshes are especially vulnerable to sea-level rise (SLR). ...
Chapter
Critical habitats that have been impacted by anthropogenic pressures in Chesapeake Bay (CB) and the northern Adriatic Sea (NAS) include seagrass beds and tidal marshes (both systems), oyster reefs (CB), coral reefs (NAS), beaches (CB), and coastal lagoons (NAS), all of which support important ecological services. Major anthropogenic pressures include excess nutrient loading (eutrophication), coastal development, climate change (sea‐level rise and increases in water temperature), invasive species, and overfishing. Acidification (driven by both climate change and eutrophication) likely impacts calcareous organisms more in CB than in the NAS. While the rapid loss of oyster bars during the 20th century was primarily due to overfishing and disease, loss of oxygenated habitat also contributed. These pressures are likely to persist, and may become greater in the future, yet there are signs of recovery. The CB seagrass beds began a comeback in the mid‐1980s after decades of decline from eutrophication, mainly driven by anthropogenic nitrogen reductions. In the NAS, eutrophication in lagoons led to episodic bottom‐water anoxia and a shift from benthic to pelagic primary production. Eutrophication and overgrazing by sea urchins contributed to the decline of the NAS brown algal forests. Following nutrient reductions over the past two decades, there has been a slight recovery of canopy‐forming algae along the Croatian Istrian peninsula and the Slovenian coastline.
... A socio-ecological system in this context is understood as a biogeophysical unit and its associated social actors and institutions (Glaser et al., 2008). River deltas face a multitude of challenges, such as anthropogenic water, soil, and air pollution (Kuenzer et al., 2014b(Kuenzer et al., , 2014aRenaud and Kuenzer, 2012;Renaud et al., 2013), a decline of biodiversity and ecosystem health (Hossain et al., 2016;Uzoekwe and Achudume, 2011), land subsidence (Higgins et al., 2013(Higgins et al., , 2014, and especially in recent decades, climate change-driven sea level rise (Auerbach et al., 2015;Dasgupta et al., 2009;Ericson et al., 2006;Phillips, 2018). Sea level rise is also one of the main drivers of salinity intrusion in deltas (i.e. the influx of saltwater into areas that are usually not exposed to high levels of salinity), which poses one of the most existential threats to delta systems (Rahman et al., 2019;Zhang and Zhao, 2010). ...
Article
River deltas and estuaries are disproportionally-significant coastal landforms that are inhabited by nearly 600 M people globally. In recent history, rapid socioeconomic development has dramatically changed many of the World's mega deltas, which have typically undergone agricultural intensification and expansion, land-use change, urbanization, water resources engineering and exploitation of natural resources. As a result, mega deltas have evolved into complex and potentially vulnerable socio-ecological systems with unique threats and coping capabilities. The goal of this research was to establish a holistic understanding of threats, resilience, and adaptation for four mega deltas of variable geography and levels of socioeconomic development, namely the Mekong, Yellow River, Yangtze, and Rhine deltas. Compiling this kind of information is critical for managing and developing these complex coastal areas sustainably but is typically hindered by a lack of consistent quantitative data across the ecological, social and economic sectors. To overcome this limitation, we adopted a qualitative approach, where delta characteristics across all sectors were assessed through systematic expert surveys. This approach enabled us to generate a comparative assessment of threats, resilience, and resilience-strengthening adaptation across the four deltas. Our assessment provides novel insights into the various components that dominate the overall risk situation in each delta and, for the first time, illustrates how each of these components differ across the four mega deltas. As such, our findings can guide a more detailed, sector specific, risk assessment or assist in better targeting the implementation of risk mitigation and adaptation strategies.
... Drainage basin geology and local coastal dynamics determine whether sediment is retained within the intertidal region. Local geomorphology contributes significantly towards resilience as the structure of a wetland ecosystem influences resistance to, or recovery from, a disturbance (Phillips, 2017a). The responses of mangrove and salt marsh ecosystems to sea-level rise are therefore not uniform between different regions, and variability exists between sites within the same mangrove forest or salt marsh platform Rogers et al., 2013;Passeri et al., 2015). ...
Technical Report
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The aims of this project were described in the original proposal document as follows: a) To determine the extent of blue carbon ecosystems in South Africa and estimate blue carbon storage using the IPCC assessment methods. b) To quantify the loss of blue carbon habitats and associated ecosystem services. c) To predict the responses of blue carbon ecosystems to climate change in the form of sea-level rise and increased global temperatures. The following tasks were developed to achieve the project aims: Assess the extent of blue carbon habitats and their associated ecosystem services in South Africa. Quantify changes in blue carbon habitats and ecosystem services over time. Directly quantify blue carbon storage in mangrove, salt marsh, and seagrass habitats at a representative study site. Compare carbon storage between mangrove and salt marsh habitats at the mangrove distributional range limit. - Measure surface elevation change in mangrove and salt marsh habitats and relate this response to sea-level rise threats. Predict changes in mangrove distribution along the South African coastline in response to rising temperatures and changes in precipitation regimes associated with climate change. Review the implications of climate change on salt marsh along the South African coastline. Determine the viability of a carbon offset mechanism for South Africa’s blue carbon ecosystems.
... Likewise, the sand percentages are correlated with salinity values or oceanic influence. It has been described that sand predominates in areas subject to significant energy from estuarine waves or with greater erosion [77]. ...
Article
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Coastal wetlands are ecosystems that provide multiple benefits to human settlements; nonetheless, they are seriously threatened due to both a lack of planning instruments and human activities associated mainly with urban growth. An understanding of their functioning and status is crucial for their protection and conservation. Two wetlands with different degrees of urbanization, Rocuant-Andalién (highly urbanized) and Tubul-Raqui (with little urbanization), were analyzed using temperature, salinity, dissolved oxygen, pH, turbidity, granulometry, fecal coliform, and macroinvertebrate assemblage variables in summer and winter. In both wetlands marked seasonality in salinity, temperature and sediment texture classification, regulated by oceanic influence and changes in the freshwater budget, was observed. In the Rocuant-Andalién wetland, the increases in pH, dissolved oxygen, gravel percentage, and coliform concentration were statistically significant. Urbanization generated negative impacts on macroinvertebrate assemblage structure that inhabit the wetlands; greater richness and abundance (8.5 times greater) were recorded in the Tubul-Raqui wetland than in the more urbanized wetland. The multivariate statistical analysis reflects the alteration of these complex systems.
... Resistance is a more dominant property, perhaps as a consequence of sea-level rise underway. Phillips (2018a) also suggested that the conditions for high resilience to develop might be less dynamically favored. In this study, high resilience developed only within circumscribed regions of topographic state space. ...
Article
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We compared two biogeomorphic models that postulate how vegetation is intertwined in the response and recovery of barrier island dunes. Each model was developed in a separate coastal region using different methods. Both relied on simple elevational representations of topography. By comparing topographies among more islands of these two regions and by linking multiple representations of topographic pattern to resistance and resilience, we provide a synthesis that shows the validity of both models and the consequences of reifying one over the other. Using airborne LiDAR, topographic metrics based on point, patch, and gradient representations of topography were derived for fifty-two sites across eleven islands along the Georgia Bight and Virginia. These seventeen metrics were categorized in terms of resistance and resilience to disturbance from storm-forced high water levels and overwash. Resistance refers to intrinsic properties that directly counter expressions of power from disturbance. Resilience refers to the degrees of freedom to adjust and adapt to disturbance. Using a cross-scale data modeling approach, these data were visualized as topographic state space using multidimensional scaling. In this state space, similarity in topography as well as resistance and resilience were inferred through a site's position along low-dimension axes representing geomorphic resistance and high-dimension axes representing the spatial landscape properties of biogeomorphic resilience. The two models overlap in how they account for barrier dune resistance and resilience along the U.S. south Atlantic coast. Islands of the Georgia Bight have a propensity for higher resistance and resilience. The Virginia islands have lower resistance and resilience. Key Words: barrier islands, biogeomorphology, cross-scale structure, dunes, resilience.
... Salt marshes occupy a narrow elevation range, where wetland plants drown if inundated excessively and are replaced by upland species if inundated insufficiently (Raposa et al., 2016), which makes the ratio of accretionary upbuilding to relative SL rise critical to their survival (Phillips, 2018). Coastal wetlands will struggle to keep pace with SL rise if sediment supplies are not sufficient (Nicholls and Cazenave, 2010), whereas, if a basin has an abundant and continuous influx of external sediment, then it will be able to maintain its morphology and reach a stable state (Carrasco et al., 2016). ...
Article
Resilience has been used over a wide range of scientific fields and often ambiguously, causing confusion over terminology and concepts and giving rise to distinct interpretations and misconceptions, even within the same scientific discipline. Starting by providing clarifications and definitions of the main terminology and key principles of ecological resilience theory, we pass on to expressing them through geomorphic dimensions of barrier islands. Three distinct environments (beach, dune, marsh) are proposed as the panarchical levels of analysis, along with potential feedbacks between them and geomorphic dimensions that can express the changes of the stability landscape. Morphological changes induced by storms and subsequent recovery are transferred to stability landscapes, over a range of storm impacts and recovery. We postulate that post-perturbation recovery should not be restricted to regaining pre-disturbance barrier dimensions, but should be viewed in terms of reorganisation and adaptation, accounting for maintaining the existence of functions, or the ability of the system to regain them. The proposed scheme and dimensions are tested using geomorphological data from barrier response to distinct disturbances, over different temporal scales that range from event to multi-decadal ones. The case of a barrier island migrating landwards is conceptualised in terms of alternative states and thresholds arising during the process and related phases and changes to the adaptive cycle. The methodology and approach presented is a step towards more holistic views of geomorphic systems' resilience that we hope will contribute to furthering interdisciplinary understanding and cooperation in the area of sustainability and resilience of natural systems.
... natural or anthropogenic drivers) (cf. Millennium Ecosystem Assessment, 2005a; Phillips, 2018). An essential first step in adopting a landscape perspective is to incorporate knowledge of these geomorphic process-landform relationships into wetland classification schemes. ...
Article
Wetland classification has become a primary tool to characterize and inventory wetland landscapes, but wetlands are difficult to classify because they straddle the terrestrial and aquatic boundary and occur in a variety of hydroclimatic and topographic settings. Presently, many ecological wetland classification schemes are focused on the ‘hydrogeomorphic’ unit, which attempts to account for the physical setting of a wetland. In many cases topographic terms (e.g. flats, slopes) rather than geomorphological terms (e.g. oxbow, floodplain) are used to characterize landforms, and little attempt is made to characterize the process-landform relationships within wetland landscapes. The current misrepresentation of product geomorphology (i.e. topographic rather than landform description) and underrepresentation of process geomorphology (i.e. lacking process-landform relationships) means that many current wetland classification schemes represent an incomplete and static attempt to characterize geomorphologically dynamic wetland landscapes. Here, we use examples from wetlands in the drylands of Africa, Australia, and North America to identify the capacity for adjustment (i.e. form and timescale of adjustment) of wetland landforms and we relate this capacity to the geomorphological concepts of sediment connectivity and landform sensitivity. We highlight how geomorphological insights into process-landform relationships and timescales of landform adjustment can add value to wetland classification efforts, with important implications for wetland management and ecosystem service delivery. We submit that geomorphology has a much larger role to play in wetland characterization and can enhance existing wetland classification schemes. More participation by the geomorphology community in wetland science and more awareness by the ecology community in recognizing and characterizing wetlands as dynamic landscapes will facilitate more effective wetland research and management.
... There are a variety of wetland types including coastal and inland wetlands (Mitsch et al., 2009). Coastal wetlands are mainly influenced by alternate floods and ebbs tides from the ocean whereas, inland wetlands are not affected by the ocean tides and are several miles inland (Mitsch et al., 2009;Phillips, 2018). Inland wetlands are found in most parts of the United States and include peat lands, freshwater swamps and marshes. ...
Article
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The understanding of inland wetlands' distribution and their level of vulnerability is important to enhance management and conservation efforts. The aim of the study was to map inland wetlands and assess their distribution pattern and vulnerability to natural and human disturbances such as climate change (temperature increase) and human activities by the year 2080. Inland wetland types i.e. forested/shrub, emergent and open water bodies were classified and mapped using maximum likelihood standard algorithm. The spatial distribution pattern of inland wetlands was examined using average nearest neighbor analysis. A weighted geospatial vulnerability analysis was developed using variables such as roads, land cover/ land use (developed and agricultural areas) and climate data (temperature) to predict potentially vulnerable inland wetland types. Inland wetlands were successfully classified and mapped with overall accuracy of about 73 percent. Clustered spatial distribution pattern was found among all inland wetland types with varied degree of clustering. The study found about 13 percent of open water bodies, 11 percent of forested/shrub and 7 percent of emergent wetlands potentially most vulnerable to human and natural stressors. This information could be used to improve wetland planning and management by wetland managers and other stakeholders.
Article
The fluvial‐estuarine transition zone (FETZ) of the Neuse River, North Carolina features a river corridor that conveys flow in a complex of active, backflooded, and high‐flow channels, floodplain depressions, and wetlands. Hydrological connectivity among these occurs at median discharges and stages, with some connectivity at even lower stages. Water exchange can occur in any direction, and at high stages the complex effectively stores water within the valley bottom and eventually conveys it to the estuary along both slow and more rapid paths. The geomorphology of the FETZ is unique compared to the estuary, or to the fluvial reaches upstream. It has been shaped by Holocene and contemporary sea‐level rise, as shown by signatures of the leading edge of encroaching backwater effects. The FETZ can accommodate extreme flows from upstream, and extraordinary storm surges from downstream (as illustrated by Hurricane Florence). In the lower Neuse—and in fluvial‐to‐estuary transitions of other coastal plain rivers—options for geomorphological adaptation are limited. Landscape slopes and relief are low, channels are close to base level, sediment inputs are low, and banks have high resistance relative to hydraulic forces. Limited potential exists for changes in channel depth,width, or lateral migration. Adaptations are dominated by formation of multiple channels, water storage in wetlands and floodplain depressions, increased frequency of overbank flow (compared to upstream), and adjustments of roughness via vegetation, woody debris, multiple channels, and flow through wetlands.
Article
In September 2018 Hurricane Florence had severe impacts on the lower Neuse River and Neuse estuary, North Carolina, despite the fact that it was a minor storm in terms of traditional indicators of storm intensity. The storm was consistent with recent trends and predictions of tropical cyclone activity driven by Anthropocene climate warming. However, its impacts in the Neuse area were also conditioned by idiosyncratic aspects of the geographic setting and the synoptic situation. Geomorphic changes examined here include erosion of estuarine shoreline bluffs, geomorphic transformations of small freshwater swamps, and effects on the river and floodplain upstream of the estuary. The shoreline changes caused by Florence were unique with respect to previous tropical cyclones and ongoing episodic erosion, due to the extraordinarily high and unusually long duration of storm surge. Transformations of the “ravine swamps”—mainly associated with deposition of >0.6 m of sand on organic muck and open water surfaces—were similarly unprecedented. Despite high river discharges (third highest on record) and the high storm surge, fluvial impacts in the lower river and fluvial-estuarine transition zone were minimal. This is attributable to the morphology of the channel-floodplain system, adapted to Holocene sea-level rise and preserved by wetlands protection programs. The large area of the storm, slow forward movement, and extreme rainfall of Florence are likely indicative of a “new normal” with respect to tropical cyclones in the region. However, the geomorphic impacts in the lower Neuse were largely determined by particulars of the Neuse estuary and Florence's storm track. An exception is the limited impacts on the lower fluvial portion of the river and the fluvial-estuarine transition zone, where there exists a complex mosaic of channels and flowing wetlands capable of accommodating extreme discharges.
Article
Superior geographical location and profound cultural accumulation create a unique natural and humanistic environment for administrative place names, which is of great significance to explore the relationship between place names and geographical environment, and the role of place names and the relationship between people and land. In this paper, from the perspective of geography, the etymological types of administrative place names are sorted out and analysed, and the natural environment elements such as climate, topography, hydrology, biology and soil constitute the entirety of the geographical environment. The landscape pattern classification system is established according to the dominant factors, combining the topographical and human factors and the relationship between topographical and landform. The geographical environment factors of administrative place names are mainly divided into natural geographical factors and human geographical factors. Based on the scale principle of landscape ecological classification theory, combined with qualitative and quantitative analysis of information entropy, decomposition classification and aggregation classification are adopted. Finally, the digital information processing technology was used to integrate and process the data, and the spatial analysis module of Arc GIS was used to classify the quantitative value of the index of place name density and analyse the landscape features.
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Coastal wetlands are currently threatened by human drivers such as agriculture, infrastructure development, and urban sprawl. Pressures on these ecosystems disturb their morphology and biogeochemical cycles, resulting in the degradation of ecosystem services. However, little has been done to understand the coastal wetland response to identify efficient conservation and mitigation strategies. Along the Peruvian coast, wetlands present a diversity of landscapes that face similar threats and pressures; however, the ecosystem response in each one may be different. This study describes a methodology to assess the environmental impacts on ecosystem services based on the understanding of geomorphic features and the status of the Eten coastal wetland, located in northern Peru. The methodology combines the application of open-source GIS tools and the collection of field data to characterize the geomorphic settings as well as to also analyze the changes in river morphology, land use, water quality, and aquatic biota. Based on this information the main threats and pressures on the Eten wetland are defined and related to impacts on ecosystem services using a cause-effect model. The main results indicated that the river plays a vital role in defining the landscape and wetland functions. Moreover, the biological diversity of aquatic habitats is disturbed by hydraulic structures and agricultural activities, and human land use modifies the natural landscape, thus affecting supporting and regulation services such as water regulation.
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Coastal wetland vegetation is at risk of severe degradation at multiple scales due to multiple stresses. Although numerous studies have emphasized the importance of scale dependence, few studies have quantitatively explored how temporal-spatial scales affect vegetation coverage (VC) in coastal wetlands. To identify the effect path of scale on the driving mechanisms of VC, we constructed a structural equation model (SEM) to quantify the temporal-spatial scale dependence of the driving mechanisms of VC in coastal wetlands in China at three spatial scales (national, regional and local) mainly over a 19-year period (2000–2018, including the initial, middle and later stages of vegetation succession). The results showed that at all spatial scales, natural factors were the strongest drivers of VC at the later stage of succession (14–39 a). With reduced spatial scales, soil and topography (only middle stage, 9–11 a) played more important roles at the middle and later stages, whereas human factors had the opposite effect. The common medial factors of the driving mechanisms underlying VC were topography and soil in all scale models. The across-scale model explained 21.8% and 40.4% of the elevation and pH, respectively. The temporal-spatial scale dependences of topography and soil were stronger than those of other variables. Our findings highlight that temporal-spatial scales mainly controlled the driving mechanisms underlying VC via topography and soil in coastal wetlands. This study is the first to quantitatively reveal the effect path of scale on the driving mechanisms underlying VC in coastal wetlands.
Chapter
Thresholds are ubiquitous in Earth surface systems and fundamental to landscape evolution. They occur at the level of process mechanics, and at the broader level of landscape system states, and may be fuzzy or crisp in their occurrence and/or the ability to measure or define them. Five main types of thresholds occur: force vs. resistance, storage capacities, relative rates of linked processes, saturation and depletion effects, and limiting factors. Tipping points and regime shifts are types of thresholds that occur at the landscape level (or broader) and are abrupt. As virtually all landscape systems are strongly influenced by thresholds, and many are threshold-dominated, landscapes and ESS are nonlinear, opening up possibilities for complex phenomena that do not occur in linear systems. One of these is dynamical instability, which can be both a consequence (via nonlinearity) and a cause of thresholds. Instability is a cause of thresholds in the case of system-level meta-thresholds. These are shifts in the positive or negative effects, or in the relative magnitudes, of interactions within the system. They can result in switches between dynamically stable and unstable modes, often manifested as convergent or divergent evolution.
Chapter
An approach to landscape and Earth surface system evolution is outlined based on the inseparability of landform, soil, and ecosystem development, versus the traditional semi-independent treatment of geomorphic, ecological, pedological, and hydrological phenomena. Key themes are the coevolution of biotic and abiotic components of the environment; selection whereby more efficient and/or durable structures, forms, and patterns are preferentially formed and preserved; and the interconnected role of laws, place factors, and history. Existing conceptual frameworks for evolution of geomorphic, soil, ecological, and hydrological systems are reviewed and contrasted with the integrated approach.
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Context Coastal landscapes evolve in response to sea-level rise (SLR) through a variety of geologic processes and ecological feedbacks. When the SLR rate surpasses the rate at which these processes build elevation and drive lateral migration, inundation is likely. Objectives To examine the role of land cover diversity and composition in landscape response to SLR across the northeastern United States. Methods Using an existing probabilistic framework, we quantify the probability of inundation, a measure of vulnerability, under different SLR scenarios on the coastal landscape. Resistant areas—wherein a dynamic response is anticipated—are defined as unlikely (p < 0.33) to inundate. Results are assessed regionally for different land cover types and at 26 sites representing varying levels of land cover diversity. Results Modeling results suggest that by the 2050s, 44% of low-lying, habitable land in the region is unlikely to inundate, further declining to 36% by the 2080s. In addition to a decrease in SLR resistance with time, these results show an increasing uncertainty that the coastal landscape will continue to evolve in response to SLR as it has in the past. We also find that resistance to SLR is correlated with land cover composition, wherein sites containing land cover types adaptable to SLR impacts show greater potential to undergo biogeomorphic state shifts rather than inundating with time. Conclusions Our findings support other studies that have highlighted the importance of ecological composition and diversity in stabilizing the physical landscape and suggest that flexible planning strategies, such as adaptive management, are particularly well suited for SLR preparation in diverse coastal settings.
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When topography is incorporated into models of barrier dune dynamical states, how it is represented determines the dynamical properties inferred. Bottom-up representations rely on elevation and localized biogeomorphic modification. Top-down representations incorporate constraints imposed by the spatial patterns of topography. These spatial patterns emerge from island morphological context and the extent localized biogeomorphic processes can expand and structure the larger landscape. We compared topographies across 30 sites among seven barrier islands of the Virginia (U.S.A) coast to gauge the importance of elevation, the bottom-up variable often weighted most in dune biogeomorphic models, relative to top-down patch and continuous surface landscape representations of topography. LiDAR-derived digital elevation models of each site were characterized with non-metric multidimensional scaling to assess how these bottom-up and top-down metrics structured dune topographic variability. Multiple response permutation procedures gauged the strength of topographic differences among sites grouped according to island morphology versus groupings defined by clustering of topographic metrics. Elevation was the dominant metric structuring topography for these low relief islands. Spatial structure was weakly developed. Topographic differences were more robust when based on clusters defined largely by elevational properties rather than by island or island morphological type. For the Virginia barrier islands, storm inputs may more directly shape topography and override landscape-extent top-down spatial structure. The dominance of elevation suggests that resistance may be the more relevant dynamical property for this coast. Properties like resilience may be greater on higher islands with longer storm-free intervals in which biogeomorphic elements can configure relief and act as recursive top-down controls.
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River networks are typically treated as conduits of fixed discharge conveyance capacity in flood models and engineering design, despite knowledge that alluvial channel networks adjust their geometry, conveyance, planform, extent and drainage density over time in response to shifts in the magnitude and frequency of streamflows and sediment supply. Consistent relationships between modes of climate variability conducive to wetter-/drier-than-average conditions and changes in channel conveyance have never been established, hindering geomorphological prediction over interannual to multidecadal timescales. This paper explores the relationship between river channel conveyance/geometry and three modes of climate variability (the el niño-Southern oscillation, Atlantic Multidecadal oscillation, and Arctic Oscillation) using two-, five-and ten-year medians of channel measurements, streamflow, precipitation and climate indices over seven decades in 67 United States rivers. We find that in two thirds of these rivers, channel capacity undergoes coherent phases of expansion/contraction in response to shifts in catchment precipitation and streamflow, driven by climate modes with different periodicities. Understanding the sensitivity of channel conveyance to climate modes would enable better river management, engineering design, and flood predictability over interannual to multidecadal timescales.
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Chilika a shallow brackish lagoon, India, is shrinking for sediment surplus budget. South Mahanadi deltaic branches i.e. Daya and Bhargavi terminate at the southwest swamps of the Lagoon. The annual average salinity of the lake was depleted from 22.31 ppt (1957-58) to 8.5 ppt. (1999-2000) as the mixing process of saline and fresh water was influenced from 1995. Trepidation of conversion of Chilika to a atrophied fresh water lake due to blooming population and their hydrologic interventions like Kolleru lake in (India), Aral Lake (Uzbekistan) was apprehended by 1950's and was alarming by 1999 when the shallow inlet(s) shifted extreme north. The shallow mud flats of lean salinity were reclaimed further for agriculture. The ecology and biodiversity degraded with substantial pecuniary loss to the lagoon dependents. and Gabkund cut with weir (2014) were made to the hydraulic system. The deteriorating health, perturbed biodiversity and declined ecosystem of the lagoon has forced to have a comparative study of the various morphologic changes passed over the Chilika with time. The meteorological, hydrologic, and the salinity study of the lagoon area for the period 1990 to 2016 have shown changes. Topographic study using GIS is developed by collecting data from Glovis Classic (Google) and the interpretation is done using ERDAS 9.2 software for various geomorphic features (1984 and 2017) before and after the current anthropogenic interventions and compared with previous studies.
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Mangrove forest responses to sea-level rise (SLR) are likely misestimated in many studies because current assessment methods often assume a constant rate of surface elevation change along the intertidal profile. Most studies also neglect the ecogeomorphic feedback process known to accelerate soil building with SLR. Here, an empirical, dynamic model that accounts for the above processes is constructed for the intertidal zone of a mangrove bay in Hainan, China. The model allows us to explore the response of a tide-dominated mangrove forest to different rates of SLR. Rod surface elevation table-marker horizon (RSET-MH) data (2015–2018) indicate rather high rates of surface accretion (13.3–57.9 mm yr ⁻¹ ) and surface elevation changes (7.9–35.5 mm yr ⁻¹ ). Surface accretion and surface elevation change decreased exponentially with increasing ground elevation. Our model showed that SLR (at rates of 4 and 16 mm yr ⁻¹ ) would not lead to the loss of mangrove cover but that it could alter mangrove species zonation along the intertidal profile by 2100 due to a rich sediment supply and ecogeomorphic feedback. Importantly, rapid SLR and landward barriers can pose a considerable threat to the high-intertidal community by causing increased inundation of the high-intertidal zone, subsequent shifts in species zones, and the loss of mangrove biological and structural diversity along the intertidal profile. Overall, this study predicts probabilities of habitat suitability for mangrove species under SLR and expands our current understanding of the impacts of SLR on tide-dominated mangrove forests in sediment-rich systems. Incorporation of extensive, long-term RSET data into this kind of dynamic model could contribute to the better prediction of mangrove responses to SLR at large scales in the future.
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Hydroperiod has numerous ecological function and effective methods for calculating hydroperiod would be valuable for different ecological or biological studies. However, accurate computation of hydroperiod remain challenging mainly due to absence of accurate mathematical formulation. Additionally, computing hydroperiod is complex due to the daily fluctuating water levels from tides, wind, river flows, and other meteorological events. This research presents quantitative formulations for three components of hydroperiod (flooding depth, frequency, and duration) directly from the physical dynamics of water surface movement in a tidal wetland. A set of National Oceanic and Atmospheric Administration (NOAA) tide gauges along the U.S. West, East, and Gulf coasts are selected to demonstrate the application of the new formulations. The computational results in terms of duration are compared with Morris non-dimensional depth (D) of hydroperiod. The comparison indicates that D is a linear-triangular approximation of sinusoid tide curve and it would generally be a good estimate of the duration component of hydroperiod within the tidal range and the accuracy of D depends on geographic location of the stations that determine the tidal regimes. Whereas the proposed formulation is not limited to variations in tide regimes and can be used as a powerful tool to determine hydroperiod.
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This book provides a theory to overcome the problem of identifying the principles behind the interdependence of different aspects of nature. Climate, vegetation, geology, landforms, soils, hydrology, and other environmental factors are all linked. Many scientists agree that there must be some general principles about the way in which earth surface systems operate, and about the ways in which the interactions of the biosphere, lithosphere, hydrosphere, and atmosphere manifest themselves. Yet there may be inherent limits on our ability to understand and isolate these interactions using traditional reductionist science. The argument of this book is that the simultaneous presence of order and chaos reflects fundamental, common properties of earth surface processes and systems. It shows how and why this is the case, with examples ranging from evolutionary and geological times scales to microscale examinations of process mechanics.
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Tidal marshes and the ecosystem services they provide may be at risk from sea-level rise (SLR). Tidal marsh resilience to SLR can vary due to differences in local rates of SLR, geomorphology, sediment availability and other factors. Understanding differences in resilience is critical to inform coastal management and policy, but comparing resilience across marshes is hindered by a lack of simple, effective analysis tools. Quantitative, multi-metric indices are widely employed to inform management of benthic aquatic ecosystems, but not coastal wetlands. Here, we develop and apply tidal marsh resilience to sea-level rise (MARS) indices incorporating ten metrics that contribute to overall marsh resilience to SLR. We applied MARS indices to tidal marshes at 16 National Estuarine Research Reserves across the conterminous U.S. This assessment revealed moderate resilience overall, although nearly all marshes had some indication of risk. Pacific marshes were generally more resilient to SLR than Atlantic ones, with the least resilient marshes found in southern New England. We provide a calculation tool to facilitate application of the MARS indices to additional marshes. MARS index scores can inform the choice of the most appropriate coastal management strategy for a marsh: moderate scores call for actions to enhance resilience while low scores suggest investment may be better directed to adaptation strategies such as creating opportunities for marsh migration rather than attempting to save existing marshes. The MARS indices thus provide a powerful new approach to evaluate tidal marsh resilience and to inform development of adaptation strategies in the face of SLR.
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Unlabelled: The interplay between storms and sea level rise, and between ecology and sediment transport governs the behavior of rapidly evolving coastal ecosystems such as marshes and barrier islands. Sediment deposition during hurricanes is thought to increase the resilience of salt marshes to sea level rise by increasing soil elevation and vegetation productivity. We use mesocosms to simulate burial of Spartina alterniflora during hurricane-induced overwash events of various thickness (0-60 cm), and find that adventitious root growth within the overwash sediment layer increases total biomass by up to 120%. In contrast to most previous work illustrating a simple positive relationship between burial depth and vegetation productivity, our work reveals an optimum burial depth (5-10 cm) beyond which burial leads to plant mortality. The optimum burial depth increases with flooding frequency, indicating that storm deposition ameliorates flooding stress, and that its impact on productivity will become more important under accelerated sea level rise. Our results suggest that frequent, low magnitude storm events associated with naturally migrating islands may increase the resilience of marshes to sea level rise, and in turn, slow island migration rates. Synthesis: We find that burial deeper than the optimum results in reduced growth or mortality of marsh vegetation, which suggests that future increases in overwash thickness associated with more intense storms and artificial heightening of dunes could lead to less resilient marshes.
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Coastal marshes are considered to be among the most valuable and vulnerable ecosystems on Earth, where the imminent loss of ecosystem services is a feared consequence of sea level rise. However, we show with a meta-analysis that global measurements of marsh elevation change indicate that marshes are generally building at rates similar to or exceeding historical sea level rise, and that process-based models predict survival under a wide range of future sea level scenarios. We argue that marsh vulnerability tends to be overstated because assessment methods often fail to consider biophysical feedback processes known to accelerate soil building with sea level rise, and the potential for marshes to migrate inland.
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The magnitude of storm events is only one determinant of their geomorphic effects. The timing and sequence of storms and the initial conditions of the shoreline also exert important controls on the geomorphic impacts. Two category three hurricanes struck coastal North Carolina during the summer of 1996, Bertha in July and Fran in September, with similar wind velocities and storm surges in the Neuse River estuary. There was little evidence of wave attack on the faces of shoreline bluffs along the Neuse estuary following Bertha, but the same bluffs experienced retreat of three to 12 meters following Fran. The dominant long-term process of slope retreat is mass wasting. Bertha removed toeslope sedimentary aprons and woody debris, which absorbed the majority of the wave energy. Thus, when the second storm arrived, waves came into direct contact with the unconsolidated bluffs and initiated slope failures by undermining the lower bluffs. The effects of the the hurricanes illustrate the importance of event sequencing and initial conditions rather than energy or magnitude in determining shoreline response, and the important interactions between slope (mass wasting) and coastal processes in causing bluff retreat along the Neuse.
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Coastal responses to sea level rise (SLR) include inundation of wetlands, increased shoreline erosion, and increased flooding during storm events. Hydrodynamic parameters such as tidal ranges, tidal prisms, tidal asymmetries, increased flooding depths and inundation extents during storm events respond non-additively to SLR. Coastal morphology continually adapts towards equilibrium as sea levels rise, inducing changes in the landscape. Marshes may struggle to keep pace with SLR and rely on sediment accumulation and the availability of suitable uplands for migration. Whether hydrodynamic, morphologic or ecologic, the impacts of SLR are inter-related. To plan for changes under future sea levels, coastal managers need information and data regarding the potential effects of SLR to make informed decisions for managing human and natural communities. This review examines previous studies that have accounted for the dynamic, nonlinear responses of hydrodynamics, coastal morphology and marsh ecology to SLR by implementing more complex approaches rather than the simplistic “bathtub” approach. These studies provide an improved understanding of the dynamic effects of SLR on coastal environments and contribute to an overall paradigm shift in how coastal scientists and engineers approach modeling the effects of SLR, transitioning away from implementing the “bathtub” approach. However, it is recommended that future studies implement a synergetic approach that integrates the dynamic interactions between physical and ecological environments to better predict the impacts of SLR on coastal systems. Full text available: http://onlinelibrary.wiley.com/doi/10.1002/2015EF000298/full
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Chronosequences are a fundamental tool for studying and representing change in Earth surface systems. Increasingly, chronosequences are understood to be much more complex than a simple monotonic progression from a starting point to a stable end-state. The concept of path stability is introduced here as a measure of chronosequence robustness; i.e., the degree to which developmental trajectories are sensitive to disturbances or change. Path stability is assessed on the basis of the largest Lyapunov exponent (λ1) of an interaction matrix consisting of positive, negative, or zero entries based on whether existence of a given system state or stage promotes or facilitates (positive), prevents or inhibits (negative), or has no significant effect on transitions to another state. Analysis of several generic chronosequence structures represented as signed, directed, unweighted graphs indicates five general cases: Path-stable reversible progressions (λ1 < 0); neutrally path-stable irreversible progressions (λ1 = 0); path unstable with very low divergence (0 < λ1 < 1); path unstable with low divergence (λ1 = 1); and complex multiple pathways (λ1 > 1). Path stability is probably relatively rare in chronosequences due to the directionality inherent in most of them. A complex soil chronosequence on the lower coastal plain of North Carolina was analyzed as described above, yielding λ1 = 0.843, indicating very low divergence. This outcome is consistent with pedological interpretations, and derives largely from the presence of self-limiting early stages, and a few highly developed states that inhibit retrogression back to many of the earlier stages. This kind of structure is likely to be common in pedological and hydrological sequences, but this suggestion requires further testing.
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Applications of graph theory have proliferated across the academic spectrum in recent years. Whereas geosciences and landscape ecology have made rich use of graph theory, its use seems limited in physical geography, and particularly in geomorphology. Common applications of graph theory — analyses of connectivity, path or transport efficiencies, subnetworks, network structure, system behaviour and dynamics, and network optimization or engineering — all have uses or potential uses in geomorphology and closely related fields. In this paper, we give a short introduction to graph theory and review previous geomorphological applications or works in related fields that have been particularly influential. Network-like geomorphic systems can be classified into nonspatial or spatially implicit system components linked by statistical/causal relationships and spatial units linked by some spatial relationship, for example by fluxes of matter and/or energy. We argue that, if geomorphic system properties and behaviour (e.g., complexity, sensitivity, synchronisability, historical contingency, connectivity etc.) depend on system structure and if graph theory is able to quantitatively describe the configuration of system components, then graph theory should provide us with tools that help in quantifying system properties and in inferring system behaviour.
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Focusing on factors that cause relative sea-level (RSL) rise to differ from the global mean, we evaluate RSL trajectories for North Carolina, United States, in the context of tide gauge and geological sea-level proxy records spanning the last $\mathord{\sim}$11,000 years. RSL rise was fastest ($\mathord{\sim}$7 mm/yr) during the early Holocene and decreased over time. During the Common Era before the 19th century, RSL rise ($\mathord{\sim}$0.7 to 1.1 mm/yr) was driven primarily by glacio-isostatic adjustment, dampened by tectonic uplift along the Cape Fear Arch. Ocean/atmosphere dynamics caused centennial variability of up to $\mathord{\sim}$0.6 mm/yr around the long-term rate. It is extremely likely (probability $P = 0.95$) that 20th century RSL rise at Sand Point, NC, (2.8 $\pm$ 0.5 mm/yr) was faster than during any other century in $\geq2,900$ years. Projections based on a fusion of process models, statistical models, expert elicitation and expert assessment indicate that RSL at Wilmington, NC, is very likely ($P = 0.90$) to rise by 42--132 cm between 2000 and 2100 under the high-emissions RCP 8.5 pathway. Under all emission pathways, 21st century RSL rise is very likely ($P > 0.90$) to be faster than during the 20th century. Because sea level responds slowly to climate forcing, RSL rise in North Carolina to 2050 varies by <6 cm between pathways. Due to RSL rise, under RCP 8.5, the current `1-in-100 year' flood is expected at Wilmington in $\mathord{\sim}$30 of the 50 years between 2050-2100.
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Abrupt ecosystem regime shifts from one state to another can occur in response to environmental change (such as climate/sea level change). Detecting an approaching tipping point may help management to adapt to or mitigate the effects of catastrophic change. Intertidal wetlands are one of the most vulnerable ecosystems, faced with climate, sea level and anthropogenic changes. Early warning indicators of regime shifts that may be evident include slowing recovery rates from perturbation, increased autocorrelation and variance, changing skewness and self-organised patchiness. We examine these indicators using intertidal examples and discuss the limitations. Managers cannot adapt to or mitigate the effects of state shifts over tipping points if there is no way to detect early warning signals. This detection is highly dependent on system-specific modelling and requires understanding of alternate stable states theory and its application in large, complex ecosystems.
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Low-elevation bare intertidal flats and high-lying vegetated marshes are the main components of intertidal areas of estuaries, deltas and coastal embayments. Large-scale transitions between them have been reported worldwide. Because vegetated marshes provide significant services to coastal societies, predicting transitions between vegetated and unvegetated states is of widespread importance. Previous theoretical and modeling work highlighted the potential bistable nature of intertidal elevations, with low-elevation bare flats and high-elevation vegetated marshes being two alternative stable states. However, empirical evidence of this bistable condition is limited. In this study, we tested empirically the hypothesis that bare flats and vegetated marshes can be considered as alternative stable landscape states with the occurrence of rapid catastrophic shifts between them. We analyzed historical records of intertidal elevation surveys and aerial pictures from the macrotidal current-dominated Western Scheldt estuary (SW Netherlands). We found (1) a bimodal distribution of intertidal elevations corresponding to either a completely bare state or a densely vegetated state. (2) The shift from bare to vegetated state is accompanied with a relatively rapid shift in elevation, i.e., the mean accretion rate during the shift is 2 to 8 times larger than during the equilibrium state. (3) A threshold elevation could be identified above which the shift from bare to vegetated state has a high chance to occur. Hence, our results demonstrate the abrupt nonlinear shift between low-lying bare flats and high-elevation vegetated marshes, suggesting that the occurrence of catastrophic shifts between alternative stable states is indeed a potential mechanism in intertidal systems.
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A positive relationship between interannual sea level and plant growth is thought to stabilize many coastal landforms responding to accelerating rates of sea level rise. Numerical models of delta growth, tidal channel network evolution, and ecosystem resilience incorporate a hump-shaped relationship between inundation and plant primary production, where vegetation growth increases with sea level up to an optimum water depth or inundation frequency. In contrast, we use decade-long measurements of Spartina alterniflora biomass in seven coastal Virginia (USA) marshes to demonstrate that interannual sea level is rarely a primary determinant of vegetation growth. Although we find tepid support for a hump-shaped relationship between aboveground production and inundation when marshes of different elevation are considered, our results suggest that marshes high in the intertidal zone and low in relief are unresponsive to sea level fluctuations. We suggest existing models are unable to capture the behavior of wetlands in these portions of the landscape, and may underestimate their vulnerability to sea level rise because sea level rise will not be accompanied by enhanced plant growth and resultant sediment accumulation.
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The spatial and temporal complexity of earth surface processes and landforms and the presence of deterministic chaos in many fundamental physical processes provide reasons to suspect chaos in geomorphic systems. A method is presented to assess the likelihood of chaotic behavior in a geomorphic system. The method requires identification of the fundamental system components, their positive, negative, or negligible influences on each other, and the relative strength or magnitudes of these links. Based on this information, the method can classify geomorphic systems as stable and nonchaotic, unstable and potentially chaotic, or unstable and generally chaotic. Positive, self-enhancing feedback is a key diagnostic of the likelihood of chaotic behavior. A sample application of the method to the problem of coastal marsh response to sea level rise is provided, which shows the marsh to be unstable. If changes in vegetation cover are partly dependent on vegetation density, the system is generally chaotic if marsh vegetation exhibits self-enhancing feedback (for example, seed source effects) and potentially chaotic if vegetation exhibits self-limiting feedback (competitive effects). The attractors controlling the chaotic dynamics represent states of pronounced erosion/drowning or accretion/expansion.
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Controls of process-response relationships in physical geography vary with scale. Two approaches to this problem are identified. The fixed-scale approach studies process-response relationships within particular scale contexts. The spatial-analytic approach seeks first to determine the spatial structure of the pattern of variability of a process (such as shore erosion) to discover the important scale(s) of variation. Controls operating at this scale have the greatest relative importance at the broadest scale considered. This approach is applied to shoreline erosion along the New Jersey shore of Delaware Bay. Geostatistical and fractal analysis indicates that variability of erosion rates is high, the alongshore pattern complex, and the scale of variability local. This indicates that despite significant long-range differences in erosion rates, short-range, local factors are more important in determining differences in erosion rates than are long-range factors such as the shape of the estuary. A number of variables related to shore erosion were examined using hierarchical analysis of variance. Linking these results to analysis of erosion patterns shows two major factors accounting for alongshore differences in erosion rates: a complex pattern of differential resistance related to marsh fringe morphology and a crenulated, irregular shoreline configuration affecting shoreline exposure to wave energy.
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Vegetation change in response to restriction of the normal tidal prism of six Connecticut salt marshes is documented. Tidal flow at the study sites was restricted with tide gates and associated causeways and dikes for purposes of flood protection, mosquito control, and/or salt hay farming. One study site has been under a regime of reduced tidal flow since colonial times, while the duration of restriction at the other sites ranges from less than ten years to several decades. The data indicate that with tidal restriction there is a substantial reduction in soil water salinity, lowering of the water table level, as well as a relative drop in the marsh surface elevation. These factors are considered to favor the establishment and spread ofPhragmites australis (common reed grass) and other less salt-tolerant species, with an attendant loss ofSpartina-dominated marsh. Based on detailed vegetation mapping of the study sites, a generalized scheme is presented to describe the sequence of vegetation change from typicalSpartina- toPhragmites-dominated marshes. The restoration of thesePhragmites systems is feasible following the reintroduction of tidal flow. At several sites dominated byPhragmites, tidal flow was reintroduced after two decades of continuous restriction, resulting in a marked reduction inPhragmites height and the reestablishment of typical salt marsh vegetation along creekbanks. It is suggested that large-scale restoration efforts be initiated in order that these degraded systems once again assume their roles within the salt marsh-estuarine ecosystem.
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Phragmites australis occurs extensively along undisturbed salt-marsh shorelines of Delaware Bay. The species has been considered indicative of human disturbance when found in estuarine marshes in the USA. It is suggested that geomorphic processes associated with coastal submergence provide an analog of human disturbances which can enable Phragmites australis to become established naturally. Deposition of sand bodies (or rafted debris) can suppress existing vegetation and allow Phragmites to become established. Subsequently, even if the sand or debris is moved, erosional truncation of the intertidal profile can inhibit recolonization by the original dominant shoreline species, Spartina alterniflora.
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Competitive interactions among marsh plant species are mediated by the influence of the vegetation on sediment accretion and modifications of the relative elevation of the marsh surface. A model described here demonstrates some of the feedbacks between physical processes like sediment accretion and biological processes such as those that determine species zonation patterns. Changes in geomorphology, primary productivity and the spatial distribution of plant species are explained by competitive interactions and by interactions among the tides, biomass density, and sediment accretion that regulate the elevation of intertidal wetlands toward an equilibrium with mean sea level (MSL). This equilibrium is affected positively (relative elevation of the marsh surface increases) by the biomass density of emergent, salt marsh macrophytes and negatively by the rate of sea-level rise (SLR). It was demonstrated that a dominant, invading species is able to modify its environment, raising the elevation of the habitat, to exclude competitively inferior species, a process I refer to as geomorphological displacement. However, the outcome depends on a number of variables including the rate of sea-level rise and the distributions of the species across the intertidal gradient. The model predicts that a marsh will evolve toward alternative stable states, depending on the rate of sea-level rise and the species' fundamental and realized distributions within the intertidal zone.
Book
Intuitive ideas of stability in dynamics of a biological population, community, or ecosystem can be formalized in the framework of corresponding mathematical models. These are often represented by systems of ordinary differential equations or difference equations. Matrices and Graphs covers achievements in the field using concepts from matrix theory and graph theory. The book effectively surveys applications of mathematical results pertinent to issues of theoretical and applied ecology. The only mathematical prerequisite for using Matrices and Graphs is a working knowledge of linear algebra and matrices. The book is ideal for biomathematicians, ecologists, and applied mathematicians doing research on dynamic behavior of model populations and communities consisting of multi-component systems. It will also be valuable as a text for a graduate-level topics course in applied math or mathematical ecology.
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The relationship between erosion and shoreline configuration was investigated along the New Jersey shore of Delaware Bay. During a period of intense erosion and shoreline retreat the irregularity and complexity of the shoreline configuration, as measured by the fractal dimension, increased significantly. -from Author
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Since the period of agriculture began about 300 years ago, sedimentation rates have greatly increased, resulting in an alluvial progradational sequence advancing over the older sediment. Much of the older marshland has been buried by a modern forested flooplain and new marshland has appeared down-stream from the older marshes as a result of estuarine shoaling. -from Author
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Spatial heterogeneity and the domain of scale over which such heterogeneity is observed are fundamental topics in hiogeography. Focusing upon two facets of heterogeneity, or spatial point pattern andspatial texture (i.e. arrangement of patches andgaps), this study quantified how these facets vary across scales on the Skallingen salt marsh, Denmark. We established a 10 m × 10 m plot with 2500 cells (20 cm × 20 cm) inside. Each cell was surveyed for the presence of vascular plants. Among 12 species identified, we analyzed the spatial pattern of six species that had a probability of occurrence greater than 1%. To examine point pattern and texture, Ripley's K-function and lacunarity analysis were used, respectively. There was a gradient in K and lacunarity values in an order of Halimione portulacoi-des, Plantago maritima, Limonium vulgare, Puccinellia maritima, Triglochin maritima, and Salicornia herbacea. At all scales of our interest (< 5 m), H. portulacoides was randomly distributed due to its frequent occurrence (> 90%), while S. herbacea showed a strong clustering with extensive gaps. Correspondingly, lacunarity values were smallest for H. portulacoides and greatest for S. herbacea. The domain of scale where a lacunarity curve approaches zero was about 1.6 m for most species except for H. portulacoides and S. herbacea with smaller and larger domains, respectively. This domain variability among species was illustrated as a scale of 2.56 m2 (i.e. 1.6 m × 1.6 m) in the curve of the species-area relationship. This study makes a quantitative contribution to the body of emerging novel theory that emphasizes scale-dependent complexity in salt marsh ecosystems.
Article
A salt marsh responds in diverse ways to a rising sea level. A major factor is its ability to maintain surface elevations with respect to the mean high water level. Other influences include local submergence rates, sedimentation rates, density and composition of the indigenous flora, and type and intensity of cultural modifications. If the relative rate of sea level rise reaches catastrophic proportions (>10 mm yr-1), substantial reductions in wetland area and a corresponding increase in open water habitats is projected. -from Authors
Article
This study aimed to determine the extent of geomorphic change resulting from the catastrophic flood of 2011 in the Lockyer Valley in southeast Queensland and to place these impacts within a history of geomorphic adjustment. Aerial photographs dated from 1933 to 2011, parish maps and historical on-ground photographs dating from 1865 to 1966 were examined for evidence of geomorphic adjustment since European settlement in the first half of the nineteenth century. Eleven forms of geomorphic adjustment were identified in three categories; erosional, depositional, and reorganisational. Only 26% of the Lockyer Creek channel length has been affected by some form of geomorphic adjustment since European settlement. Most of this adjustment was localised and dominated by reorganisation of geomorphic unit assemblages within the macrochannel and sediment deposition on floodplains. No wholesale river change in the form of lateral migration or avulsion has occurred, and the river’s morphology has remained relatively characteristic over time (i.e., morphology remains relatively uniform in a reach-averaged sense). Geomorphic responses to extreme flooding has been minor, and the geomorphic effectiveness of floods in this system (including the 2011 flood) has been limited over the last several hundred years. The system is likely still adjusting to past flooding events that ‘set’ the morphology of the current system (i.e., the macrochannel). A form of event resilience has resulted in this system such that it is less prone to geomorphic adjustment during events than would normally be considered geomorphically effective. As a result, antecedent controls on macrochannel presence and capacity are considered to be first-order controls on contemporary forms and processes in this system. Work is required to test whether the resilience of this system will hold in the future, with more extreme episodes of flooding predicted to occur in this region under future climate change.
Article
Intuitive ideas of stability in dynamics of a biological population, community or ecosystem can be formalized in the framework of corresponding mathematical models. These are often represented by systems of ordinary differential equations or difference equations. This book covers achievements in the field using concepts from matrix theory and graph theory. It effectively surveys applications of methematical results pertinent to issues of theoretical and applied ecology. The only mathematical prerequisite to using this text is a working knowledge of linear algebra and matrices.
Article
The literature often holds that, in salt marshes, surface elevation mediates the depth, duration, and frequency of submergence, thereby constituting the fundamental factor of plant species distribution and most other environmental variables. However, such an elevation-centered view has not been fully tested in a temporal sense; it is still unclear whether elevation is also a significant control on the rate of changes in species composition over time. In the Skallingen salt marsh of the Danish Wadden Sea, this question was evaluated along two elevation gradients where distinct physical and ecological processes operate: a gradient across a marsh platform and the other across creek bars. The rate of vegetation dynamics was measured as the Euclidean distance between two positions of the same plot, each representing two different points in time, in a two-dimensional diagram produced by nonmetric multidimensional scaling. Results showed that the rate of vegetation dynamics did not show any significant relationships with surface elevation across either marsh platform or tidal creeks (R 2 less than 0.04). This suggests that, other than elevation, some biological factors, such as the presence of keystone species and the initial species composition, control patterns of vegetation change in the marsh. This logic leads to a point that hydrological effects (e.g., inundation frequency and duration), often represented by surface elevation, are not necessarily overriding factors of rates of changes in species composition in backbarrier marshes like Skallingen. The conventional elevation-centered perspective may be an oversimplification of the biological and environmental variability of salt marshes.
Article
Evidence is provided in support of Pethick's hypothesis that salt pans may develop on tidal salt marshes by suppression of surface vegetation (Pethick 1974). Such pans have formed in Spartina twonsendii (s.l.) marshland in Australia as a result of localized waterlogging, Spartina failure in the shade cast by mangroves, and the deposition of tidal wrack. Some pans have developed into pond holes, characterized by near vertical or undercuty sides. The evidence suggests that the majority of pans and pond holes formed at an early stage of marsh development will persist throughout the sequence of marsh maturation. As additional pans can be expected to form during this period, pan density is likely to increase with marsh age.
Article
The widely accepted view that salt pans are formed during the initial colonization of the marsh surface is examined using a large sample of salt pans from the North Norfolk coast salt marshes. A multiple regression model suggests that pan density is positively related to marsh height and negatively to distance from the marsh/sea edge. Since marsh height increases with time this model does not agree with the initial hypothesis. Further investigation of the regression model, however, indicates the dangers inherent in assigning causal mechanisms, for the independent variables are shown to be merely limiting conditions of pan density. Examination of these fators allows an alternative hypothesis for pan formation to be postulated: pans may be formed by the erosion of bare patches in the existing marsh vegetation, caused by rafts of vegetation debris brought in by high tides and deposited on the marsh surface.
Article
Long-term eustatic sea-level variation has been recognized as a primary factor affecting the hydrological and geomorphic dynamics of salt marshes. However, recent studies suggest that wind waves influenced by atmospheric oscillations also may play an important role in many coastal areas. Although this notion has been conceptually introduced for the Wadden Sea, no modeling attempts have been made yet. As a proof of concept, this study developed a simulation model using the commercially available STELLA® software, based on long-term data on water level and sedimentation collected at a back-barrier marsh on the Skallingen peninsula in Denmark. In the model, the frequency (number year–1) of wind-driven extreme high water level (HWL) events (>130 cm Danish Ordnance Zero) was simulated in terms of the North Atlantic Oscillation (NAO) index. Then, surface accretion (cm year–1) and submergence duration (h year–1) were simulated for the period 1933–2007. The model showed good performances: simulated rates of surface accretion and simulated durations of submergence decreased from 1950 to 1980, the point at which the NAO shifted from its negative to its positive phase, and increased thereafter. Despite continuous increases in surface elevation, increases in simulated submergence duration were apparently due to wind-driven HWL events, which generally increased in frequency after 1980. These findings for the Danish Wadden Sea add to the growing body of evidence that the role of atmospheric oscillations—e.g., the NAO—as drivers of wind-generated water level variations merits more attention in assessing the impact of climate change on coastal marshes.
Article
Most of the coastal wetlands of the South Atlantic region of the United States are expected to diminish in size in response to increasing human population growth and accelerating rates of rising sea level. after examination of the distribution of wetlands, elevation contours, estimates of surface slope, soil types, and peat deposits on the peninsula, current models were considered unsuited for wetlands of the Albemarle-Pamlico peninsula of North Carolina. Some unusual features of this peninsula are low elevation (56% of total area
Article
Long-term variation of mean sea level has been considered the primary exogenous factor of vegetation dynamics in salt marshes. In this study, we address the importance of short-term, wind-induced rise of the sea surface in such biogeographic changes. There was an unusual opportunity for examining field data on plant species frequency, sea-level variation, and sedimentation acquired from the Skallingen salt marsh in Denmark since the 1930s. The environmental and floristic history of Skallingen was summarized as (1) continuous sea-level rise with temporal variability (2.3–5.0 mm yr), (2) continuous sedimentation with spatial variability (2.0–4.0 mm yr), (3) increased frequency of over-marsh flooding events, and (4) contemporary dominance of Halimione portulacoides, indicating little progressive succession toward a later phase. Conventionally, recent eustatic sea-level rise was believed to drive the increased frequency of flooding and such retarded succession. Skallingen, however, has showed more or less equilibrated yearly rates between sea-level rise and surface accretion. This implies that the long-term, gradual sea-level rise alone might not be enough to explain the increased inundation frequency across the marsh. Here, we suggest an alternative chain: Recent trends in the North Atlantic Oscillation index toward its positive phase have led to increased storminess and wind tides on the ocean surface, resulting in increased frequency, duration, and depth of submergence, and hence, waterlogging of marsh soils, which has retarded ecological succession. To conclude, we stress the need for a multitemporal perspective that recognizes the significance of short-term sea-level fluctuations nested within long-term trends.
Article
Ecological network analysis allows for an investigation of the structural and functional interconnectedness in ecosystems. Typically, these interactions are seen to comprise a food web of “who eats whom”, but more generally applies to the transfer of energy-matter within the biotic and abiotic ecosphere. This web of transactions can be depicted as a digraph or an adjacency matrix in which the presence of direct transactions are represented as a 1 and no transactions as 0. Each transaction between system components leads to an overall network structural pattern. These structures cluster into different categories or regimes based on their cyclic nature. This paper demonstrates threshold effects of the placement or removal of links, such that certain changes essentially keep the structure in the same regime whereas others shift it to another regime in a non-linear manner.
Article
The response of coastal marshes to relative sea-level rise depends upon their ability to maintain their relative elevation through sedimentation. However, sediment supply and vegetation factors are in turn susceptible to alterations in marsh hydrology and flooding. Assessment of the viability of a marsh to survive various sea-level rise scenarios may best be achieved through ecosystem simulation modelling, which can incorporate the interconnections between physical processes and marsh sedimentation. -after Author
Article
Chronostratigraphic approaches to coastal geomorphology frequently include consideration of salt marsh deposits as indicators of past sea-level positions. Continuous horizons of such deposits can be used to infer that salt marshes were keeping pace with local rates of relative sea-level rise (RSLR). Rates of past accumulation, estimated using dating techniques, are then used to hindcast the rate of sea-level rise in that area. Estimates of contemporary sea-level rise rates are often derived from tide gauge records. This approach allows identification of subdecadal variations in mean water level. Accumulation rates of both organic and inorganic sediments can also be derived at these time scales and studies from many coastal marshes demonstrate the episodic nature of inorganic sediment deposition. The frequency and spacing of these events does not necessarily coincide with periods of increased local sea level. In addition, short-term increases in sea level could result in marsh deterioration as soils become excessively waterlogged. A conceptual model of changes in geomorphic and ecological processes contributing to marsh sustainability during the Holocene has been developed for the Mississippi delta plain (MDP). The survival of some marshes in this area, despite high rates of subsidence, indicates that the combined effect of organic and inorganic accumulation processes can be adequate to sustain coastal marshes in the face of sea-level rise.
Article
ARTICLE I NFO During the past 40 years colluvial and alluvial deposits have been used in Brazil as good indicators of regional landscape sensitivity to Quaternary environmental changes. In spite of the low resolution of most of the continental sedimentary record, geomorphology and sedimentology may favor palaeoenvironmental interpretation when supported by independent proxy data. This paper presents results obtained from pedostratigraphic sequences, in near-valley head sites of southern Brazilian highlands, based on geomorphologic, sedimentologic, micromorphologic, isotopic and palynologic data. Results point to environmental changes, with ages that coincide with Marine Isotopic Stages (MIS) 5b; 3; 2 and 1. During the late Pleistocene, although under temperatures and precipitation lower than today, the local record points to relatively wet local environments, where shallow soil-water saturated zones contributed to erosion and sedimentation during periods of climatic change, as during the transition between MIS 2 and MIS 1. Late Pleistocene events with ages that coincide with the Northern Hemisphere Younger Dryas are also depicted. During the mid Holocene, slope-wash deposits suggest a climate drier than today, probably under the influence of seasonally contrasted precipitation regimes. The predominance of overland flow-related sedimentary deposits suggests an excess of precipitation over evaporation that influenced local palaeohydrology. This environmental condition seems to be recurrent and explains how slope morphology had influenced pedogenesis and sedimentation in the study area. Due to relative sensitiveness, resilience and short source-to-sink sedimentary pathways, near-valley head sites deserve further attention in Quaternary studies in the humid tropics.
Article
High rates of wave-induced erosion along salt marsh boundaries challenge the idea that marsh survival is dictated by the competition between vertical sediment accretion and relative sea-level rise. Because waves pounding marshes are often locally generated in enclosed basins, the depth and width of surrounding tidal flats have a pivoting control on marsh erosion. Here, we show the existence of a threshold width for tidal flats bordering salt marshes. Once this threshold is exceeded, irreversible marsh erosion takes place even in the absence of sea-level rise. This catastrophic collapse occurs because of the positive feedbacks among tidal flat widening by wave-induced marsh erosion, tidal flat deepening driven by wave bed shear stress, and local wind wave generation. The threshold width is determined by analyzing the 50-y evolution of 54 marsh basins along the US Atlantic Coast. The presence of a critical basin width is predicted by a dynamic model that accounts for both horizontal marsh migration and vertical adjustment of marshes and tidal flats. Variability in sediment supply, rather than in relative sea-level rise or wind regime, explains the different critical width, and hence erosion vulnerability, found at different sites. We conclude that sediment starvation of coastlines produced by river dredging and damming is a major anthropogenic driver of marsh loss at the study sites and generates effects at least comparable to the accelerating sea-level rise due to global warming.
Article
Salt marshes are coastal ecosystems characterized by high biodiversity and rates of primary productivity, providing fundamental ecosystem services. Salt-marsh ecosystems are important indicators of environ-mental change as the dynamics are governed by interacting physical and biological processes, whose intertwined feedbacks critically affect the evolution. Settling deposition of inorganic sediment allows the platform to reach a threshold elevation for vegetation encroachment; the presence of vegetation then intensifies rates of accretion, thus, enhancing the resilience of marshes to increasing rates of sea level rise (SLR). The results from a two-dimensional numerical model, accounting for biotic and geomorphic processes, show that different morphological evolutionary regimes are followed depending on marsh biological processes. The average marsh elevation within the tidal frame decreases with increasing rates of SLR, decreasing availability of sediment, and decreasing productivity of vegetation. The spatial variability in platform elevations increases with increasing rates of SLR, increasing availability of sediment, and decreasing productivity of vegetation. Supply-limited settings tend to develop uniform marsh surface elevations, whereas supply-rich settings tend to develop patterns of sedimentation where large heterogeneities in marsh surface elevations occur. The complexity observed in tidal geomorphological patterns is deemed to arise from the mutual influence of biotic and abiotic components. The fate of tidal landforms and their possible geomorphological restoration should, thus, be addressed through approaches which explicitly incorporate bio-morphodynamic processes.
Article
Coastal marsh loss in Louisiana is attributed to plane dieback caused by processes that stress vegetation, and a common landscape pattern is broken marsh that expands at the expense of surrounding unbroken marsh. We tested the hypothesis that vegetation is more stressed in broken marsh than in adjacent unbroken marsh, as indicated by vegetation aboveground biomass, species diversity and soil Eh, on transects that extended from broken marsh to unbroken marsh at Marsh Island, Louisiana. Soil Eh, vegetation above-ground biomass and species diversity did not differ between broken marsh and unbroken marsh, and above-ground biomass was similar to that reported from other marshes. Thus, we rejected the hypothesis that marsh loss is related to vegetation stress. Two factors were related to vegetation vigour: soil drainage and soil bulk density. Surprisingly, significant soil drainage occurred in broken marsh but not in unbroken marsh. Above-ground biomass of the dominant plant, Spartina patens (Aiton) Muhl., was lowest where soil bulk density was less than 0-08 gcm−3, which illustrated the importance of mineral matter accumulation in submerging coastal marshes. The mechanism of marsh loss appeared to be erosion below the living root zone, as indicated by the vertical and often undercut marsh-water interface, and by the separation of sod clasts. This is different from more rapid marsh loss associated with plant stress which we observed in other Louisiana marshes only 135 km away, indicating that marsh loss mechanisms can vary spatially even within a relatively small region.
Article
Extensive storage of upper-basin Piedmont sediment and apparent low sediment supply to streams in lower-basin Coastal Plain areas generates questions as to the source of alluvium in lower reaches of rivers of the U.S. Atlantic drainage. This was investigated on the Neuse River, North Carolina, using a mineralogical indicator of sediment source areas. The utility of mica flakes for discriminating between Piedmont and non-Piedmont sources of sediment in the lower Coastal Plain reaches of the Neuse was established on the basis of an examination of the U.S. National Soils Database and of 26 soil surveys of the North Carolina Coastal Plain. From the Neuse River estuary to 48 km upstream there are no mica flakes in floodplain soils or in river bank and channel shelf sediments. Mica flakes become more common upstream. This suggests that a very small proportion of the sediment eroded in the Piedmont portion of the watershed is delivered to the river mouth. The small amounts which presumably do reach the lower Coastal Plain are so diluted by Coastal Plain-derived alluvium that no Piedmont origin can be discerned. This demonstrates a dominantly Coastal Plain source and underscores the importance of storage and discontinuous transport in fluvial sediment systems. More importantly, results suggest that upper- and lower-basin sediment dynamics are not only non-linearly related, but may be virtually decoupled.
Article
Complexity theory highlights scale-dependent feedback mechanisms as an explanation for regular spatial patterning in ecosystems. To what extent scale-dependent feedback clarifies spatial structure in more complex, non-regular systems remains unexplored so far. We report on a scale-dependent feedback process generating patchy landscapes at the interface of intertidal flats and salt marshes. Here, vegetation was characterized by Spartina anglica tussocks, surrounded by erosion gullies. To demonstrate the presence of a scale-dependent feedback, we determined if vegetation induced habitat modification resulted in local facilitation and large scale-inhibition of plant growth. Field surveys revealed that larger tussocks have deeper gullies, suggesting that gully erosion is caused by increased water flow around tussocks. This was confirmed by flume experiments, showing that feedback effects vary with current velocity and water depth. Transplantation of small Spartina units inside and just outside present tussocks revealed that the growth of Spartina transplants compared to transplant growth on bare sediment was higher within the raised Spartina tussocks, but lower in the gully just outside Spartina tussocks, providing clear evidence of scale-dependent feedback. Our results emphasize that scale-dependent feedback is a more general explanation for spatial complexity in ecosystems than previously considered.
Article
Six distinct plant zones were identified within a mesohaline tidal marsh in the Cape Fear Estuary, North Carolina. USA. All six vegetative zones were found within an 18-cm portion of the 1.35-m tidal range. Aerial photographs show that these six zones have existed within the marsh for the past 20 years. A monotypicJuncus roemerianus stand occupied soils with the highest salinity porewater (17 ppt), while stands dominated (>90%) by eitherScirpus robustus orTypha angustifolia were found associated with the least saline soil water (7 ppt) in areas of the marsh least flooded by tidal waters.Spartina cynosuroides dominated areas of the marsh at lowest elevations. In general, Eh was highest in theJuncus zone and lowest in theSpartina alterniflora zone. Four of the six vegetative zones represented distinct physical and chemical environments and could be statistically separated via canonical discriminate analyses. We suggest that established vegetation may be an accurate analog for specific hydrogeomorphic conditions.
Article
In this paper we provide a conceptual model to examine changes in ecosystem state during the transition from terrestrial forest to shallow estuarine environments for coastal mainland marshes at the Virginia Coast Reserve (VCR), United States of America. Ecosystem states are characterized by plant community dominants and soil/sediment characteristics. The five states considered are upland or wetland forest, organic high marsh, intertidal mineral low marsh, autotrophic benthic with or without submersed aquatic vascular plants, and heterotrophic benthic (estuarine bottom). Transitions between states are described from the perspective of a fixed forest location undergoing transition from one ecosystem state to another. Rising sea level is acknowledged as the master variable that forces the process of change overall. Each state is hypothesized to have self-maintaining properties and thus is resistant to change from rising sea level; alternatively, transitions between states are facilitated by disturbance or exposure to acute stress. For change to occur, resistance must be overcome by events that are more abrupt than rising sea level and that appear as accentuated pulsings, which result in another self-maintaining and resistnnt state. Such events facilitate plant species replacement and alter sediment conditions. Mechanisms responsible for causing a state to cross a threshold are unique for each transition type and include brackish-water intrusion (osmotic stress and sulfide toxicity), tidal creek encroachment (redistribution of sediments), erosive currents and waves (resuspension of sediments, which increases light extinction), and increasing water depth (leads to greater bottom shading). Field experiments relevant to scales at which pulsings occur are not abundant in coastal marshes.
Article
The tidal energy subsidy hypothesis postulates that the high primary productivity of coastal salt marshes is the result of an energy subsidy provided by the tides. The sediment component of this subsidy is especially important in contributing to the elevation increase of the marsh surface, a process essential for the sustainability of salt marshes during periods of sea level rise. This research tested the hypothesis that sediment subsidies have an ameliorating effect on sea level rise-induced impacts to salt marsh vigor. We assessed the plant structural and soil physico-chemical responses to different intensities of sediment subsidy in a salt marsh experiencing a high rate of relative sea level rise. Sediments were hydraulically dredged with a high fluid to solids ratio (85%:15%) from the Gulf of Mexico and dispersed into a Spartina alterniflora dominated salt marsh. Approximately 2 years after this fluid-sediment-enrichment, maximum sediment elevation did not exceed 30 cm above ambient and both plant cover and aboveground biomass responded positively. Sediment subsidy increased soil mineral matter, and, in turn, soil fertility and marsh elevation, and thereby reduced nutrient deficiency, flooding, and interstitial sulfide stresses. Thus, sediment subsidy generated a more favorable environment for plant growth and potentially, marsh sustainability.
Article
The existence and function of tidally dominated and predominantly allochthonous marshes are ultimately contingent upon the operation of hydrodynamic and sedimentary processes within constraints imposed by the available accommodation space and sediment supply. This paper re-interprets published data relating to contemporary vertical marsh growth and sea-level rise in the context of the conceptual model relating elevation, sedimentation, sea-level rise, sediment supply and tidal range. This analysis is supported by numerical mass balance modelling of the equivalent parameter space and of the sensitivity of marsh hydroperiod and sedimentation to sea-level and sediment supply forcing. The effect of autocompaction on the translation of sedimentation into elevation change is also considered.Parameter space modelling provides a framework for the interpretation of field data and affords indicative insights into marsh resilience to change. It is argued that the assessment of marsh response to external environmental forcing should be based not on empirical comparisons of sedimentation versus sea-level rise but on the estimation of sediment supply, and the efficiency with which this is depleted by deposition, as metrics of marsh resilience. This implies a shift towards more intensive process studies aimed at elucidating more fully the linkages between tidal marshes and adjacent estuarine and coastal systems.Model results also indicate significant variability in marsh sedimentation associated with 18.6 yr tidal modulation and meteorological processes at short-term scales. Such variability further complicates the interpretation of sedimentation or elevation change data obtained from monitoring programmes of short duration. Longer-term monitoring is of value, however, as a means of identifying important mechanisms of climate and sediment supply forcing that may contribute to the formation and maintenance of marsh sedimentary sequences.
Article
Coastal marshes accrete vertically in response to sea-level rise and subsidence. Inadequate accretion and subsequent conversion of coastal marshes to open water generally is attributed to inadequate mineral sedimentation because mineral sedimentation is widely assumed to control accretion. Using 137Cs dating to determine vertical accretion, mineral sedimentation, and organic matter accumulation, we found that accretion varied with organic accumulation rather than mineral sedimentation across a wide range of conditions in coastal Louisiana, including stable marshes where soil was 80% mineral matter. These results agreed with previous research, but no mechanism had been proposed to explain accretion via vegetative growth. In an exploratory greenhouse experiment, we found that flooding stimulated root growth above the marsh surface. These results indicated the need for additional work to determine if flooding controls accretion in some marshes by stimulating root growth on the marsh surface, rather than by mineral accumulation on the marsh surface. Restoration or management that focus on mineral sedimentation may be ineffective where a relationship between accretion and mineral sedimentation is assumed rather than tested.
Article
Purposes of this study were to determine the relative importance of organic matter and mineral matter to marsh vertical accretion and to determine if insufficient vertical accretion was a factor in land loss in a Terrebonne Basin (Louisiana, USA) marsh. Cs-137 dating indicated that vertical accretion (0.98 cm yr-1) was extremely rapid relative to other marshes, but insufficient to counter submergence (1.38 cm yr-1). Mineral and organic matter accumulation were similar to that in other marshes. Variation in vertical accretion was accounted for by variation in organic matter accumulation rather than mineral matter accumulation; thus, inadequate vertical accretion resulted from inadequate organic matter accumulation. Inadequate organic matter accumulation (593 g m-2 yr-1) was attributed to inadequate plant production resulting primarily from flooding stresses; inadequate mineral matter accumulation (1629 g m-2 yr-1) was attributed to inadequate mineral matter availability. These data suggest that this marsh is threatened by a positive feedback loop of plant flooding stress and inadequate vertical accretion. Inadequate plant growth limits vertical accretion, which further increases flooding and decreases plant production. Thus plant production, which has previously been considered only in the context of trophic dynamics, also partly determines the degree of submergence some coastal and estuarine marshes will tolerate.