Currents and Sediment Transport in Mangrove Forests
TL;DR: A field study of the tidal currents, cohesive sediment dynamics and transport of organic carbon in a highly vegetated mangrove swamp was carried out at Middle Creek, Cairns, Australia as discussed by the authors.
Abstract: A field study of the tidal currents, cohesive sediment dynamics and transport of organic carbon in a highly vegetated mangrove swamp was carried out at Middle Creek, Cairns, Australia. The interaction of tidal currents and the vegetation generated jets, eddies and zones of stagnant waters which were numerically modelled. A high value of the Manning friction coefficient (n=0·1) was derived by the dense vegetation. About 80% of the suspended sediment brought in from coastal waters at spring flood tide was trapped in the mangroves, corresponding to about 10–12 kg of sediment m−1creek length/spring tide, resulting in a rise of the substrate by about 0·1 cm year−1. The selective trapping of clay was caused by flocculation of the finer particles in the mangroves. There was an indication of a slight inwelling of organic carbon. Creek water was readily differentiated from mangrove water by large differences in the molecular weight distribution of the dissolved organic carbon.
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TL;DR: Mangroves are woody plants that grow at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity, extreme tides, strong winds, high temperatures and muddy, anaerobic soils, creating unique ecological environments that host rich assemblages of species.
Abstract: Mangroves are woody plants that grow at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity, extreme tides, strong winds, high temperatures and muddy, anaerobic soils. There may be no other group of plants with such highly developed morphological and physiological adaptations to extreme conditions.
Because of their environment, mangroves are necessarily tolerant of high salt levels and have mechanisms to take up water despite strong osmotic potentials. Some also take up salts, but excrete them through specialized glands in the leaves. Others transfer salts into senescent leaves or store them in the bark or the wood. Still others simply become increasingly conservative in their water use as water salinity increases Morphological specializations include profuse lateral roots that anchor the trees in the loose sediments, exposed aerial roots for gas exchange and viviparous waterdispersed propagules.
Mangroves create unique ecological environments that host rich assemblages of species. The muddy or sandy sediments of the mangal are home to a variety of epibenthic, infaunal, and meiofaunal invertebrates Channels within the mangal support communities of phytoplankton, zooplankton and fish. The mangal may play a special role as nursery habitat for juveniles of fish whose adults occupy other habitats (e.g. coral reefs and seagrass beds).
Because they are surrounded by loose sediments, the submerged mangroves' roots, trunks and branches are islands of habitat that may attract rich epifaunal communities including bacteria, fungi, macroalgae and invertebrates. The aerial roots, trunks, leaves and branches host other groups of organisms. A number of crab species live among the roots, on the trunks or even forage in the canopy. Insects, reptiles, amphibians, birds and mammals thrive in the habitat and contribute to its unique character.
Living at the interface between land and sea, mangroves are well adapted to deal with natural stressors (e.g. temperature, salinity, anoxia, UV). However, because they live close to their tolerance limits, they may be particularly sensitive to disturbances like those created by human activities. Because of their proximity to population centers, mangals have historically been favored sites for sewage disposal. Industrial effluents have contributed to heavy metal contamination in the sediments. Oil from spills and from petroleum production has flowed into many mangals. These insults have had significant negative effects on the mangroves.
Habitat destruction through human encroachment has been the primary cause of mangrove loss. Diversion of freshwater for irrigation and land reclamation has destroyed extensive mangrove forests. In the past several decades, numerous tracts of mangrove have been converted for aquaculture, fundamentally altering the nature of the habitat. Measurements reveal alarming levels of mangrove destruction. Some estimates put global loss rates at one million ha y−1, with mangroves in some regions in danger of complete collapse. Heavy historical exploitation of mangroves has left many remaining habitats severely damaged.
These impacts are likely to continue, and worsen, as human populations expand further into the mangals. In regions where mangrove removal has produced significant environmental problems, efforts are underway to launch mangrove agroforestry and agriculture projects. Mangrove systems require intensive care to save threatened areas. So far, conservation and management efforts lag behind the destruction; there is still much to learn about proper management and sustainable harvesting of mangrove forests.
Mangroves have enormous ecological value. They protect and stabilize coastlines, enrich coastal waters, yield commercial forest products and support coastal fisheries. Mangrove forests are among the world's most productive ecosystems, producing organic carbon well in excess of the ecosystem requirements and contributing significantly to the global carbon cycle. Extracts from mangroves and mangrove-dependent species have proven activity against human, animal and plant pathogens. Mangroves may be further developed as sources of high-value commercial products and fishery resources and as sites for a burgeoning ecotourism industry. Their unique features also make them ideal sites for experimental studies of biodiversity and ecosystem function. Where degraded areas are being revegetated, continued monitoring and thorough assessment must be done to help understand the recovery process. This knowledge will help develop strategies to promote better rehabilitation of degraded mangrove habitats the world over and ensure that these unique ecosystems survive and flourish.
1,568 citations
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...This corresponds to 10-12 kg of sediment m-1 on each spring tide and could produce sediment accretions of 0.1 cm y-1 (Furukawa et al., 1997)....
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TL;DR: In this article, the potential benefits of conservation, restoration and use of marine vegetated habitats for coastal protection and climate change mitigation are assessed, and the potential benefit of using these habitats in eco-engineering solutions for coast protection is discussed.
Abstract: Marine vegetated habitats occupy a small fraction of the ocean surface, but contribute about 50% of the carbon that is buried in marine sediments. In this Review the potential benefits of conservation, restoration and use of these habitats for coastal protection and climate change mitigation are assessed. Marine vegetated habitats (seagrasses, salt-marshes, macroalgae and mangroves) occupy 0.2% of the ocean surface, but contribute 50% of carbon burial in marine sediments. Their canopies dissipate wave energy and high burial rates raise the seafloor, buffering the impacts of rising sea level and wave action that are associated with climate change. The loss of a third of the global cover of these ecosystems involves a loss of CO2 sinks and the emission of 1 Pg CO2 annually. The conservation, restoration and use of vegetated coastal habitats in eco-engineering solutions for coastal protection provide a promising strategy, delivering significant capacity for climate change mitigation and adaption.
1,239 citations
TL;DR: Knowing on mangrove carbon dynamics has improved considerably in recent years, but there are still significant gaps and shortcomings, and relevant research directions are suggested.
1,018 citations
Cites background from "Currents and Sediment Transport in ..."
...…from the water column likely depends on a range of factors such as the particle size, salinity, tidal pumping and the areal extent of the intertidal zone (e.g., Wolanski, 1995), but can be very high: 15–44% (Victor et al., 2004), 30–60% (Kitheka et al., 2002), and up to 80% (Furukawa et al., 1997)....
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TL;DR: In this article, the authors review the state of knowledge of mangrove vulnerability and responses to predicted climate change and consider adaptation options, based on available evidence, of all the climate change outcomes, relative sea level rise may be the greatest threat to mangroves.
952 citations
TL;DR: In this paper, the authors conduct a literature review and a small meta-analysis of wave attenuation data, and find overwhelming evidence in support of established theory that mangrove and salt marsh vegetation afford context-dependent protection from erosion, storm surge, and potentially small tsunami waves.
Abstract: For more than a century, coastal wetlands have been recognized for their ability to stabilize shorelines and protect coastal communities. However, this paradigm has recently been called into question by small-scale experimental evidence. Here, we conduct a literature review and a small meta-analysis of wave attenuation data, and we find overwhelming evidence in support of established theory. Our review suggests that mangrove and salt marsh vegetation afford context-dependent protection from erosion, storm surge, and potentially small tsunami waves. In biophysical models, field tests, and natural experiments, the presence of wetlands reduces wave heights, property damage, and human deaths. Meta-analysis of wave attenuation by vegetated and unvegetated wetland sites highlights the critical role of vegetation in attenuating waves. Although we find coastal wetland vegetation to be an effective shoreline buffer, wetlands cannot protect shorelines in all locations or scenarios; indeed large-scale regional erosion, river meandering, and large tsunami waves and storm surges can overwhelm the attenuation effect of vegetation. However, due to a nonlinear relationship between wave attenuation and wetland size, even small wetlands afford substantial protection from waves. Combining man-made structures with wetlands in ways that mimic nature is likely to increase coastal protection. Oyster domes, for example, can be used in combination with natural wetlands to protect shorelines and restore critical fishery habitat. Finally, coastal wetland vegetation modifies shorelines in ways (e.g. peat accretion) that increase shoreline integrity over long timescales and thus provides a lasting coastal adaptation measure that can protect shorelines against accelerated sea level rise and more frequent storm inundation. We conclude that the shoreline protection paradigm still stands, but that gaps remain in our knowledge about the mechanistic and context-dependent aspects of shoreline protection.
828 citations
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...2007) and promote sediment settling (Leonard and Luther 1995; Furukawa et al. 1997; Mudd et al. 2010)....
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...These hydrodynamic impacts tend to reduce sediment erosion (Le Hir et al. 2007) and promote sediment settling (Leonard and Luther 1995; Furukawa et al. 1997; Mudd et al. 2010)....
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References
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TL;DR: In this paper, high-frequency (5 Hz) in situ measurements of flow speed were collected in Spartina alternijlora, Juncus roemerianus, and Di-tichlis spicata canopies using hot-film anemometry sensor arrays.
Abstract: The transport of particulate and dissolved matter on the surface of coastal marshes is controlled by the hydrodynamic characteristics of over-marsh flows. High-frequency (5 Hz) in situ measurements of flow speed were collected in Spartina alternijlora, Juncus roemerianus, and Di:-tichlis spicata canopies using hot-film anemometry sensor arrays. These data indicate that mean flow speed, turbulence intensity, and the shape of the vertical speed profile are influenced by variations in plant morphology and stem density. Mean flow speed and turbulence intensity are inversely related to stem density and to distance from the creek edge. Flow energies decrease by about one order of magnitude when flows encounter the vegetated marsh surface and continue to decrease as vegetation density increa,ses. Turbulent flow energy also decays exponentially with increasing distance from the creek edge. Reductions in flow speed coupled with energy decay provide a hydrologic mechanism for sediment deposition patterns commonly observed in marsh systems. Suspended matter transport is also affected by plant-flow interactions. Vertical flow structure is strongly influenced by canopy morphology (plant type and plant shape). Plant-flow interactions result in vertical speed profiles whose shapes deviate from the logarithmic profile typical in free-stream conditions and in the development of transitional flow regimes (i.e. neither laminar nor fully turbulent).
490 citations
Journal Article•
TL;DR: In this article, high-frequency (5 Hz) in situ measurements of flow speed were collected in Spartina alternijlora, Juncus roemerianus, and Di-tichlis spicata canopies using hot-film anemometry sensor arrays.
Abstract: The transport of particulate and dissolved matter on the surface of coastal marshes is controlled by the hydrodynamic characteristics of over-marsh flows. High-frequency (5 Hz) in situ measurements of flow speed were collected in Spartina alternijlora, Juncus roemerianus, and Di:-tichlis spicata canopies using hot-film anemometry sensor arrays. These data indicate that mean flow speed, turbulence intensity, and the shape of the vertical speed profile are influenced by variations in plant morphology and stem density. Mean flow speed and turbulence intensity are inversely related to stem density and to distance from the creek edge. Flow energies decrease by about one order of magnitude when flows encounter the vegetated marsh surface and continue to decrease as vegetation density increa,ses. Turbulent flow energy also decays exponentially with increasing distance from the creek edge. Reductions in flow speed coupled with energy decay provide a hydrologic mechanism for sediment deposition patterns commonly observed in marsh systems. Suspended matter transport is also affected by plant-flow interactions. Vertical flow structure is strongly influenced by canopy morphology (plant type and plant shape). Plant-flow interactions result in vertical speed profiles whose shapes deviate from the logarithmic profile typical in free-stream conditions and in the development of transitional flow regimes (i.e. neither laminar nor fully turbulent).
464 citations
TL;DR: Topographically generated fronts affect the distribution of sediments, and they aggregate waterborne eggs, larvae, and plankton, which influences the distribution and density of benthic assemblages and of pelagic secondary and tertiary predators.
Abstract: Headlands, islands, and reefs generate complex three-dimensional secondary flows that result in physical and biological fronts. Mixing and diffusion processes near these reefs and headlands are quite different from these processes in the open sea, and classical advection-diffusion models that were developed for the open sea are not valid near shore. Topographically generated fronts affect the distribution of sediments, and they aggregate waterborne eggs, larvae, and plankton. This aggregation influences the distribution and density of benthic assemblages and of pelagic secondary and tertiary predators.
425 citations
TL;DR: In this paper, it was found that the drag coefficient is related to the Reynolds number and the vegetation length scale LE, which is a function of the projected area of mangrove vegetation and the volume of the vegetation.
Abstract: Field studies of tidal flows in largely pristine mangrove swamps suggestthat the momentum equation simplifies to a balance between the water surfaceslope and the drag force. The controlling parameter is the vegetation lengthscale LE, which is a function of the projected area ofmangrove vegetation and the volume of the vegetation. The value ofLE varies greatly with mangrove species and water depth. It isfound that the drag coefficient is related to the Reynolds number Re definedusing LE. The drag coefficient decreases with increasingvalues of Re from a maximum value of 10 at low value of Re (<104), and converges towards 0.4 for Re < 5 ×104.
294 citations
Book•
27 Jan 1994
TL;DR: Physical Oceanographic Processes of the Great Barrier Reef as discussed by the authors is the first comprehensive volume describing the water circulation and its influence in controlling the distribution of marine life on the great Barrier Reef of Australia.
Abstract: Physical Oceanographic Processes of the Great Barrier Reef is the first comprehensive volume describing the water circulation and its influence in controlling the distribution of marine life on the Great Barrier Reef of Australia The book uses exhaustive field and numerical studies to show how the influence of the salient topography occurs at all scales
271 citations
"Currents and Sediment Transport in ..." refers background in this paper
...The tides along the Queensland coast have a strong diurnal inequality (Wolanski, 1994), so that, as in this case, rates of rise and fall of sea level are commonly different from one another....
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