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An Integrated Hydrodynamic-Marsh Model with Applications in Fluvial, Marine, and Mixed Estuarine Systems
Title
An Integrated Hydrodynamic-Marsh Model with Applications in Fluvial, Marine, and Mixed Estuarine Systems.
Creator
Alizad, Karim, Hagen, Scott, Medeiros, Stephen, Wang, Dingbao, Weishampel, John, University of Central Florida
Abstract / Description
Coastal wetlands experience fluctuating productivity when subjected to various stressors. One of the most impactful stressors is sea level rise (SLR) associated with global warming. Research has shown that under SLR, salt marshes may not have time to establish an equilibrium with sea level and may migrate landward or become open water. Salt marsh systems play an important role in the coastal ecosystem by providing intertidal habitats and food for birds, fish, crabs, mussels, and other animals...
Show moreCoastal wetlands experience fluctuating productivity when subjected to various stressors. One of the most impactful stressors is sea level rise (SLR) associated with global warming. Research has shown that under SLR, salt marshes may not have time to establish an equilibrium with sea level and may migrate landward or become open water. Salt marsh systems play an important role in the coastal ecosystem by providing intertidal habitats and food for birds, fish, crabs, mussels, and other animals. They also protect shorelines by dissipating flow and damping wave energy through an increase in drag forces. Due to the serious consequences of losing coastal wetlands, evaluating the potential future changes in their structure and distribution is necessary in order for coastal resource managers to make informed decisions. The objective of this study was to develop a spatially-explicit model by connecting a hydrodynamic model and a parametric marsh model and using it to assess the dynamic effects of SLR on salt marsh systems within three National Estuarine Research Reserves (NERRs) in the Northern Gulf of Mexico. Coastal salt marsh systems are an excellent example of complex interrelations between physics and biology, and the resulting benefits to humanity. In order to investigate salt marsh productivity under projected SLR scenarios, a depth integrated hydrodynamic model was coupled to a parametric marsh model to capture the dynamic feedback loop between physics and biology. The hydrodynamic model calculates mean high water (MHW) and mean low water (MLW) within the river and tidal creeks by harmonic analysis of computed tidal constituents. The responses of MHW and MLW to SLR are nonlinear due to localized changes in the salt marsh platform elevation and biomass productivity (which influences bottom friction). Spatially-varying MHW and MLW are utilized in a two-dimensional application of the parametric Marsh Equilibrium Model to capture the effects of the hydrodynamics on biomass productivity and salt marsh accretion, where accretion rates are dependent on the spatial distribution of sediment deposition in the marsh. This model accounts both organic (decomposition of in-situ biomass) and inorganic (allochthonous) marsh platform accretion and the effects of spatial and temporal biomass density changes on tidal flows. The coupled hydro-marsh model, herein referred to as HYDRO-MEM, leverages an optimized coupling time step at which the two models exchange information and update the solution to capture the system's response to projected linear and non-linear SLR rates.Including accurate marsh table elevations into the model is crucial to obtain meaningful biomass productivity projections. A lidar-derived Digital Elevation Model (DEM) was corrected by incorporating Real Time Kinematic (RTK) surveying elevation data. Additionally, salt marshes continually adapt in an effort to reach an equilibrium within the ideal range of relative SLR and depth of inundation. The inputs to the model, specifically topography and bottom roughness coefficient, are updated using the biomass productivity results at each coupling time step to capture the interaction between the marsh and hydrodynamic models.The coupled model was tested and validated in the Timucuan marsh system, located in northeastern Florida by computing projected biomass productivity and marsh platform elevation under two SLR scenarios. The HYDRO-MEM model coupling protocol was assessed using a sensitivity study of the influence of coupling time step on the biomass productivity results with a comparison to results generated using the MEM approach only. Subsequently, the dynamic effects of SLR were investigated on salt marsh productivity within the three National Estuarine Research Reserves (NERRs) (Apalachicola, FL, Grand Bay, MS, and Weeks Bay, AL) in the Northern Gulf of Mexico (NGOM). These three NERRS are fluvial, marine and mixed estuarine systems, respectively. Each NERR has its own unique characteristics that influence the salt marsh ecosystems. The HYDRO-MEM model was used to assess the effects of four projections of low (0.2 m), intermediate-low (0.5 m), intermediate-high (1.2 m) and high (2.0 m) SLR on salt marsh productivity for the year 2100 for the fluvial dominated Apalachicola estuary, the marine dominated Grand Bay estuary, and the mixed Weeks Bay estuary. The results showed increased productivity under the low SLR scenario and decreased productivity under the intermediate-low, intermediate-high, and high SLR. In the intermediate-high and high SLR scenarios, most of the salt marshes drowned (converted to open water) or migrated to higher topography. These research presented herein advanced the spatial modeling and understanding of dynamic SLR effects on coastal wetland vulnerability. This tool can be used in any estuarine system to project salt marsh productivity and accretion under sea level change scenarios to better predict possible responses to projected SLR scenarios. The findings are not only beneficial to the scientific community, but also are useful to restoration, planning, and monitoring activities in the NERRs. Finally, the research outcomes can help policy makers and coastal managers to choose suitable approaches to meet the specific needs and address the vulnerabilities of these three estuaries, as well as other wetland systems in the NGOM and marsh systems anywhere in the world.
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Date Issued
2016
Identifier
CFE0006523, ucf:51360
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006523
Identifying inundation-driven effects among intertidal Crassostrea virginica in a commercially important Gulf of Mexico estuary
Title
Identifying inundation-driven effects among intertidal Crassostrea virginica in a commercially important Gulf of Mexico estuary.
Creator
Solomon, Joshua, Walters, Linda, Weishampel, John, Quintana-Ascencio, Pedro, Hagen, Scott, Wang, Dingbao, University of Central Florida
Abstract / Description
Sea level rise and changing storm frequency and intensity resulting from climate change create tremendous amounts of uncertainty for coastal species. Intertidal species may be especially affected since they are dependent on daily inundation and exposure. The eastern oyster Crassostrea virginica is an economically and biologically important sessile intertidal species ranging from Canada to the Gulf of Mexico. Declines and changes in distribution of oyster populations has forced commercial...
Show moreSea level rise and changing storm frequency and intensity resulting from climate change create tremendous amounts of uncertainty for coastal species. Intertidal species may be especially affected since they are dependent on daily inundation and exposure. The eastern oyster Crassostrea virginica is an economically and biologically important sessile intertidal species ranging from Canada to the Gulf of Mexico. Declines and changes in distribution of oyster populations has forced commercial harvesting to spread from subtidal to intertidal reefs. We investigated the potential responses of intertidal C. virginica to sea level rise, and the response of larval settlement to sedimentation which is likely to increase with higher water levels and storm frequency. Inundation was used as a proxy for sea level rise. We hypothesized four possible outcomes for intertidal oyster reefs as a result of changes in inundation due to sea level rise: (a) intertidal reefs become subtidal and remain in place, (b) intertidal reefs will be lost, (c) intertidal reefs migrate shoreward upslope and remain intertidal, and (d) intertidal reefs will grow in elevation and remain intertidal. To test the plausibility of these four outcomes, oyster ladders were placed at two sites within Apalachicola Bay, Florida, USA. Ladders supported oyster recruitment mats at five heights within the range of intertidal elevations. The bottom-most mat was placed near mean low tide, and the top mat near mean high tide to investigate the effect of tidal inundation time on C. virginica. Sediment traps were attached to ladders with openings at equal elevation to the oyster mats. Ladders were deployed for one year starting in June 2012, and again in June 2013, during peak oyster recruitment season. Monthly for six months during year one, sediment was collected from traps, dried to constant weight and weighed to obtain a monthly average for total sediment at each elevation. At the end of one year, oyster mats were collected from the field and examined for the following responses: live oyster density, mean oyster shell length of live oysters, mean oyster shell angle of growth relative to the benthos, and mean number of sessile competitors. We used AICc to identify the most plausible models using elevation, site, and year as independent variables.Oyster density peaked at intermediate inundation at both sites (maximum 1740 oysters per m2), it decreased slightly at the mean low tide, and sharply at the mean high tide. This response varied between years and sites. Mean oyster shell length peaked near mean low tide (6.7 cm), and decreased with increasing elevation. It varied between years and sites. Oyster shell angle of growth relative to the benthos showed a quadratic response for elevation; site but not year affected this response. Sessile competitor density also showed a quadratic response for elevation and varied between sites and years. Barnacles were the primary spatial competitor reaching densities of up to 28,328 barnacles per m2. Total monthly sedimentation peaked at the lowest elevations, and varied by site, with an order of magnitude difference between sites. Sediment increased with decreasing elevation.Outcomes a, c, and d were found to be viable results of sea level rise, ruling out complete loss of intertidal reefs. Outcome (a) would be associated with decrease in oyster density and increase in oyster length. Outcome (c) would require the laying of oyster cultch upslope and shoreward of current intertidal reefs, as well as the removal of any hard armoring or development. Outcome (d) remained possible, but is the least likely requiring a balance between sedimentation, oyster angle of growth, and recruitment. This should be further investigated. A laboratory experiment was designed to test relative impact of varying sediment grain sizes on settlement of C. virginica larvae. Previous studies showed that suspended solids resulted in decreased larval settlement when using mixed sediment grain sizes. Predicted storm levels and hurricane levels of total suspended solids were used in flow tanks. Sediment from the field experiment was sieved into seven size classes, the most common five of which were used in the experiment since they represented 98.8% of total mass. Flow tanks were designed and built that held 12 aged oyster shells, instant ocean saltwater, and sediment. Oyster larvae were added to the flow tanks and allowed one hour to settle on shells. Each run utilized one of the five size classes of sediment at either a high or low concentration. Following the one-hour settlement period, oyster shells were removed from the flow tank and settled larvae were counted under a dissecting microscope. Settlement was standardized by settlement area using Image J. AICc model selection was performed and the selected model included only grain size, but not concentration. A Tukey's post hoc test differentiated (<)63 ?m from 500 (-) 2000 ?m, with the (<) 63 (&)#181;m grain size having a negative effect on oyster larval settlement. This indicates that the smaller grain sizes of suspended solids are more detrimental to oyster larval settlement than larger grain sizes. The oyster ladder experiment will help resource managers predict and plan for oyster reef migration by cultch laying, and or associated changes in oyster density and shell length if shoreward reef growth is not allowed to occur. The laboratory experiment will help to predict the impacts of future storms on oyster larval recruitment. Together this information can help managers conserve as much remaining oyster habitat as possible by predicting future impacts of climate change on oysters.
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Date Issued
2015
Identifier
CFE0005717, ucf:50132
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005717

Pages