Browsing by Subject "Salt marsh"

At a given time, tidal landforms inhabit one of three alternate elevation-determined stable states: salt marsh, sub-tidal platform, or tidal flat. The balance between soil production and sea level rise controls transitions between states. Due to increasing anthropogenic inputs to the carbon and nitrogen cycles, CO2 and nutrient enrichment rates are rising. What effect will this have on the evolution of the tidal landform? The present thesis recognizes that (1) soil production depends on halophyte biomass, (2) nutrient enrichment promotes a shift in biomass production from below- to aboveground thus increasing potential for sediment trapping, and (3) elevated CO2 causes an increase in total biomass production. Through use of point- and one-dimensional models, the present thesis finds that under constant suspended sediment levels equal to 20 kg/m3, (1) nutrient enrichment decreases accretion and increases suspended sediment requirements necessary to maintain accessibility of the salt marsh state, (2) elevated CO2 increases accretion decreases suspended sediment requirements necessary to maintain accessibility of the salt marsh state, and (3) the increase in accretion affected by CO2 addition is greater in magnitude than the decrease affected by nutrient enrichment. Thus we can infer that in a future scenario including increasing CO2, nutrient enrichment, and decreasing suspended sediment concentration, the enhancement effect of CO2 will dominate and a net increase in accretion will occur.

Coastal ecosystems provide a wide range of services including protecting the mainland from the destructive effects of storms, nutrient cycling, water filtration, nurseries for fish and crustaceans, and carbon sequestration. These zones are threatened by human impacts and climate change through more frequent intense storms and sea level rise with a projected increase of up to 16 mm/yr for the last two decades of the 21st century. However, it is not fully understood what mechanisms control the formation and degradation of these landforms, and determine their resilience to environmental change. In this work, I highlight the role of various physical characteristics and environmental parameters that contribute to the formation and stability of coastal environments.

First, I develop and use remote sensing analyses to quantitatively characterize coastal dune eco-topographic patterns by simultaneously identifying the spatial distribution of topographic elevation and vegetation biomass in order to understand the coupled dynamics of vegetation and coastal dunes. LiDAR-derived leaf area index and hyperspectral-derived normalized difference vegetation index patterns yield vegetation distributions at the whole-system scale which are in agreement with each other and with field observations. LiDAR-derived concurrent quantifications of biomass and topography show that plants more favorably develop on the landward side of the foredune crest and that the foredune crestline marks the position of an ecotone, which is interpreted as the result of a sheltering effect sharply changing local environmental conditions. The findings reveal that the position of the foredune crestline is a chief ecomorphodynamic feature resulting from the two-way interaction between vegetation and topography.

Next, to shed light on the vertical depositional dynamics of salt marshes in response to sea level rise, I investigate the hypothesis that competing effects between biomass production and aeration/decomposition determine an approximately spatially constant contribution of soil organic matter (SOM) to total accretion. I use concurrent observations of SOM and decomposition rates from marshes in North Carolina. The results are coherent with the notion that SOM does not significantly vary in space and suggest that this may be the result of an at least partial compensation of opposing trends in biomass productivity and decomposed organic matter. The analyses show that deeper soil layers are characterized by lower decomposition rates and higher stabilization factors than shallower layers, likely because of differences in inundation duration. However, overall, decomposition processes are sufficiently rapid that the labile material in the fresh biomass is completely decomposed before it can be buried and stabilized. The findings point to the importance of the fraction of initially refractory material and of the stabilization processes in determining the final distribution of SOM within the soil column.

Finally, I develop a process-based model to evaluate the relative role of watershed, estuarine, and oceanic controls on salt marsh depositional/erosional dynamics and define how these factors interact to determine salt marsh resilience to environmental change at the estuary scale. The results show that under some circumstances, vertical depositional dynamics can lead to transitions between salt marsh and tidal flat equilibrium states that occur much more rapidly than marsh/tidal flat boundary erosion or accretion could. Additionally, the analyses reveal that river inputs affect the existence and extent of marsh/tidal flat equilibria by both modulating exchanges with the ocean (by partially “filling” the basin) and by providing suspended sediment.

Recent experiments and comparative surveys in Southern US salt marshes revealed that a common larval trematode parasite, Parorchis acanthus, generated a trophic cascade that protected foundational marsh plants (Spartina alterniflora) from drought-associated overgrazing by suppressing the per capita grazing impacts of its host, the marsh periwinkle (Littoraria irrorata). While it is clear that parasites can play a positive role in mediating marsh ecosystem response to disturbance, there is still little known about the context dependency of this interaction, the role of definitive avian hosts in regulating parasite prevalence, and whether other commonly-occurring parasites may also modify processes that underpin ecosystem stability. The purpose of this project was to extend the current understanding of the roles played by parasites and their hosts in mediating marsh ecosystem stability. A field manipulation of Littoraria density in which infection prevalence with Parorchis acanthus was held at a constant value revealed that these parasites yielded positive impacts on Spartina aboveground biomass at middling densities of snails, but the positive effects of parasites were negligible at both low, and high snail densities. Surveys of drought-impacted marshes revealed that birds – the definitive hosts for trematode that infect Littoraria – congregated within die-off areas and that increased bird usage of die-off areas was associated with increased trematode parasitism in snails within grazer fronts, decreased per capita grazing rates of snails, and proportionate decreases in ecosystem die-off rate. Multi-site bird exclusion and mechanistic field studies experimentally confirmed that birds increased ecosystem resistance to drought-driven die-off by acting as the dispersive vectors for parasites that suppress Littoraria grazing. Finally, we explored how the trematode Cercaria opaca in ribbed mussels (Geukensia demissa) influenced the facultative mutualism between Guekensia and Spartina – an interaction that underlies marsh ecosystem resilience to drought-associated die-off. A field manipulation using experimentally infected mussels revealed that mutualistic benefits to Spartina decreased with increasing infection intensity in mussels. Subsequent mechanistic experiments demonstrated that increasing infection with C. opaca decreased mussel biodeposit production, the functional trait underlying mutualistic benefits to Spartina. Additionally, increasing parasite load was associated with decreased strength of both shells and byssal attachments, potentially explaining the relatively higher predation on heavily infected mussels in our field study. A survey of five North Carolina salt marshes revealed that infection intensity in mussels increased with proximity to die-off areas, indicating that C. opaca could influence marsh recovery following die-off events. Taken together, these results underscore the importance of parasitism’s influence on Southern salt marsh ecosystem stability and more generally show that parasites can be major arbiters of community structure and functioning.

Low-lying coastal landforms such as barrier islands and river deltas are attractive sites for human habitation and infrastructure. They are also highly vulnerable to both climate change impacts such as rising sea levels or increases in storm intensity and anthropogenic impacts such as changes in sediment supply. In this dissertation I aim to improve understanding of some of the primary drivers of the evolution of low-lying coastal landforms over varying space (1-100s km) and time (decadal to millennial) scales. I focus in Chapter 2 on the influence of shoreline curvature and resulting gradients in alongshore sediment transport on shoreline change; in Chapter 3 on the influence of wave-edge erosion on back-barrier marsh resilience; and in Chapters 4 and 5 on the cohesive effects of vegetation on river deltas.

Sandy coastlines, often associated with low-lying barrier islands that are highly vulnerable to sea level rise and storms, can experience high rates of shoreline change. However, they also attract human habitation, recreation, and infrastructure. Previous research to understand and quantify contributions to shoreline erosion has considered factors such as grain size, underlying geology, regional geography, sea level rise, and anthropogenic modifications. Shoreline curvature is often not considered in such analyses, but subtle shoreline curvature (and associated alongshore variation in relative offshore wave angles) can result in gradients in net alongshore transport which can cause significant erosion or accretion. In Chapter 2, we conducted a spatially extensive analysis of the correlation between shoreline curvature and shoreline change rates for the sandy shorelines of the US East and Gulf coasts. For wave-dominated, sandy coasts where nourishment and shoreline stabilization do not dominate the shoreline change signal, we find a significant negative correlation between shoreline curvature and shoreline change rates over decadal to centurial and 1-5 km temporal and spatial scales. This indicates that some of the coastal erosion observed in these areas can be explained by the smoothing of subtle shoreline curvature by gradients in alongshore transport. In other settings, this signal can be obscured by tidal, anthropogenic, or geologic processes which also influence shoreline erosion. While limited in practical application to long, sandy shorelines with limited human stabilization, these results have widespread implications for the inclusion of shoreline curvature as an important variable in modelling and risk assessment of long-term coastal erosion on sandy, wave-dominated coastlines.

The marshes and bays in the back-barrier environment between barrier islands and the mainland can also experience wave-driven erosion, and their dynamics are coupled to those of barrier islands. Previous results show that overwash provides an important sediment source to back-barrier marshes, sustaining a narrow marsh state under conditions in which marsh drowning would otherwise occur. In Chapter 3, I expand the coupled barrier island-marsh evolution model GEOMBEST+ to explore the effects of wind waves on back-barrier marshes. I find that the addition of marsh-edge erosion leads to wider, more resilient marshes and that horizontal erosion of the marsh edge is a more efficient sediment source than vertical erosion of the marsh surface as it drowns. Where marshes and bays are vertically keeping up with sea level, and the net rate of sediment imported to (or exported from) the basin is known, the rate of marsh-edge erosion or progradation can be predicted knowing only the present basin geometry, sea-level rise rate, and the net rate of sediment input (without considering the erosion or progradation mechanisms). If the rate of sediment input/export is known, this relationship applies whether sediment exchange with the open ocean is negligible (as in basins dominated by riverine sediment input), or is significant (including the loss of sediment remobilized by waves in the bay). Analysis of these results reveals that geometry and stratigraphy can exert a first order control on back-barrier marsh evolution and on the marsh-barrier island system as a whole, and provides new insights into the resilience of back-barrier marshes and on the interconnectedness of the barrier-marsh system.

Coastal wetlands such as marshes are also an important component of river deltas. Like barrier islands, these low-lying landscapes are both attractive to human settlement (providing fertile farmland, fisheries, hydrocarbon reserves, and many other services) and prone to hazards such as flooding and land loss. Delta evolution is governed by complex interactions between coastal, marine, and fluvial processes, many of which are still not well understood. In Chapters 4 and 5, I use the delta-building model DeltaRCM to explore the effects of vegetation, specifically its ability to introduce cohesion, on delta morphology and the dynamics of delta distributary networks. The use of this rule-based model allows me to simplify vegetation dynamics and effects in order to enhance the clarity of potential insights into which processes or interactions may be most important in the context of vegetation as a cohesive agent.

Cohesive sediment exerts a significant influence on delta evolution, increasing shoreline rugosity and decreasing channel mobility. Vegetation has been assumed to play a similar role in delta evolution, but its cohesive effects have not been explicitly studied. In Chapter 4, I use DeltaRCM to directly explore two cohesive effects of vegetation: decreasing lateral transport and increasing flow resistance. I find that vegetation and cohesive sediment do alter delta morphology and channel dynamics in similar ways (e.g. more rugose shorelines, deeper, narrower, less mobile channels), but that vegetation may have additional implications for deltaic sediment retention and stratigraphy, by confining flow and sand in channels. My results suggest that sediment composition is a first-order control on delta morphology but vegetation has a stronger influence on channel mobility timescales. To fully understand the cohesive influences acting on a delta, the influence of vegetation should be considered in addition to fine sediment.

In Chapter 5, I explore the cohesive effects of vegetation on delta evolution under different environmental conditions. The dynamics and evolution of deltas and their channel networks are controlled by interactions between a number of factors, including water and sediment discharge, cohesion from fine sediment and vegetation, and sea level rise rates. Vegetation’s influence on the delta is likely to be significantly impacted by other environmental factors. For example, increasing sea level or sediment discharge increases aggradation rates on the delta, and may result in sediment transport processes such as deposition and erosion, both of which can kill vegetation, happening more rapidly than vegetation growth. I conduct two sets of experiments; in the first, I explore the interactions between vegetation and sea level rise rate, and in the second, between vegetation and rate of sediment and water discharge. As expected, I find that sea level rise decreases vegetation’s ability to stabilize channels but that vegetation can still exert a strong influence on the delta at low rates of sea level rise. This limit appears to be higher for channel dynamics than delta morphology, supporting the findings of Chapter 4. In addition, I propose two new insights into delta evolution under different discharge conditions with and without vegetation. First, without vegetation, I observe a shift in avulsion dynamics with increasing water discharge: from a few active channels supplemented by overbank flow and undergoing episodic avulsion (with low discharge) to many active channels experiencing frequent local and partial avulsions (with high discharge). Second, with vegetation, increased sediment discharge and associated aggradation results in more frequent switching of the dominant channels, but also prevents vegetation from establishing in non-dominant channels, resulting in more frequent channel reoccupation and therefore in channel network planform stability. These insights have important implications for understanding the distribution of water, sediment, and nutrients on deltas in the face of future changes in climate, human modifications of fluxes of sediment and water to the coast, and especially for restored or engineered deltas with controlled water or sediment discharges.