Vertical density stratification often plays an important role in the formation and expansion of coastal hypoxic zones through its effect on near-bed circulation and vertical oxygen flux. However, the ...impact of future climate change on estuarine circulation is widely unknown. Here, we developed and calibrated a three-dimensional hydrodynamic model for Pensacola Bay, a shallow subtropical estuary in the northeastern Gulf of Mexico. Model simulations based on years 2013–2017 were applied to examine changes in salinity, temperature, and density under future climate scenarios, including increased radiative forcing (IR) and temperature (T), increased freshwater discharge (D), sea level rise (SLR), and wind intensification (W). Simulations showed that the impacts of climate change on modeled state variables varied over time with external forcing conditions. The model demonstrated the potential for sea level rise and increased freshwater discharge to episodically increase vertical density gradients in the Bay. However, increased wind forcing destabilized vertical gradients, at times reducing the spatial extent and duration of stable stratification. For time periods with low freshwater discharge, moderate increases in wind speed (10%) can destabilize density gradients strengthened by increased discharge (10%) and sea level rise (0.48 m). In contrast, destruction of strong density gradients that form near the mid-Bay channel following flood events requires stronger wind forcing. These results highlight the importance of considering natural variability in freshwater and wind forcing, as well as local phenomena that are generally unresolved by global climate models.
•Estuarine stratification can affect the development of summertime benthic hypoxia.•Impacts of climate change on stratification and water quality are widely unknown.•Sea level rise and increased freshwater inputs will likely enhance stratification.•Intensification of cross-shore winds may destruct stable vertical density gradients.
The seasonal occurrence of deep-water hypoxia in western Long Island Sound (LIS) has been documented for decades by water quality cruise surveys and fixed mooring buoys. While previous studies have ...focused on factors modulating bottom dissolved oxygen (DO) at subtidal timescales, here we analyze continuous timeseries data from a moored buoy during summers 2021 and 2022 to examine factors controlling high-frequency fluctuations in surface and bottom DO at diurnal and semidiurnal timescales. Fluctuations in surface DO at diurnal timescales are associated with biological production, while fluctuations in bottom DO near semidiurnal timescales are associated with horizontal advection of DO by tides from the upper East River tidal strait into western LIS. Results from timeseries analysis are supported by weekly cruise surveys that resolve horizontal and vertical DO gradients in the western narrows. However, inferences regarding the duration of hypoxia during a given summer vary across datasets in part because weekly survey data do not resolve dominant timescales of variability within a particular summer. While prior studies have illustrated the importance of nutrient loading, stratification, and wind in controlling the development of hypoxia, the results presented here demonstrate the role of tidal advection in modulating hypoxia in far western LIS. Despite stronger stratification in 2021, the duration of hypoxia was 11.1 days shorter compared to 2022 in part due to greater advection of DO by tidal currents that intermittently increased bottom DO near the buoy. Furthermore, five-year averaged hypoxic area in the western narrows has increased since 2017, which highlights the spatially variable response of DO to nutrient load reductions. Future analysis of hypoxia in LIS should focus on leveraging high-frequency information contained in continuous datasets to improve estimates of hypoxia based on less temporally resolved water quality surveys.
The sustainability of marshes adjacent to coastal bays is driven by the exchange of sediment across the marsh-bay boundary, where edge erosion commonly leads to lateral marsh loss and enhanced ...vertical accretion. The timing and patterns of sediment deposition on salt marshes adjacent to larger bodies of water such as coastal bays, however, differ from those on better-studied tidal creek marshes primarily owing to the importance of wind-waves. We combined field measurements and modeling to examine controls on suspended sediment concentrations and fluxes on a tidal flat (tidal range of 1.2 m) and rates of sediment deposition on the adjacent marsh at a site on the Eastern Shore of Virginia. Suspended sediment concentrations over tidal flats were strongly controlled by waves. Yet, storm winds sufficient to drive large resuspension events often coincided with peak tidal elevations that were too low to flood the marsh, which was oriented away from the wind directions most favorable for storm surge, thereby restricting storm-driven, episodic sediment delivery to the marsh. Winds also drove wide variability in the direction of surface currents near the marsh edge when water depths were high enough to flood the marsh. Nevertheless, our results show that sediment in the upper water column over the tidal flat was effectively transported across the marsh edge during flooding tides. A sediment deposition model developed to investigate the combined effects of vegetation and wave action on depositional patterns predicted that waves displace maximum deposition inland from the marsh edge, consistent with measured deposition at the study site. Marsh deposition was sensitive to inundation frequency as well as the concentration of sediment in water flooding the marsh, underscoring the importance of nontidal controls on water surface elevation, such as meteorological effects (e.g., storm surge) and sea level rise. Whereas short-term increases in marsh inundation enhance deposition, sea level rise that results in deeper average water depths over the tidal flats decreases deposition if marsh elevation is rising in step with sea level.
In shallow estuaries, fluctuations in bottom dissolved oxygen (DO) at diel (24 h) timescales are commonly attributed to cycles of net production and respiration. However, bottom DO can also be ...modulated by physical processes, such as tides and wind, that vary at or near diel timescales. Here, we examine processes affecting spatiotemporal variations in diel-cycling DO in Escambia Bay, a shallow estuary along the Gulf of Mexico. We collected continuous water quality measurements in the upper and middle reaches of the Bay following relatively high (> 850 m
3
s
−1
) and low (< 175 m
3
s
−1
) springtime freshwater discharge. Variations in diel-cycling amplitude over time were estimated using the continuous wavelet transform, and correlations between DO and biophysical processes at diel timescales were examined using wavelet coherence. Our results reveal that freshwater discharge modulated inter-annual variations in the spatial extent and duration of summertime hypoxia through its effect on vertical density stratification. In the absence of strong stratification (> 15 kg m
−3
), vertical mixing by tropic tides and sea breeze enhanced diel fluctuations in deeper areas near the channel, while in shallower areas the largest fluctuations were associated with irradiance. Our findings suggest that processes affecting diel-cycling DO in the bottom layer can vary over a relatively short spatial extent less than 2 km and with relatively small changes in bottom elevation of 1 m or less. Implications for water quality monitoring were illustrated by subsampling DO timeseries, which demonstrates how low-frequency measurements may misrepresent water quality in estuaries where diel-cycling DO is common. In these systems, adequate assessment of hypoxia and its aquatic life impacts requires continuous measurements that capture the variation in DO at diel timescales.
Water markets as a response to scarcity DEBAERE, Peter; RICHTER, Brian D; DAVIS, Kyle Frankel ...
Water policy,
01/2014, Letnik:
16, Številka:
4
Journal Article
Recenzirano
Existing water governance systems are proving to be quite ineffective in managing water scarcity, creating severe risk for many aspects of our societies and economies. Water markets are a relatively ...new and increasingly popular tool in the fight against growing water scarcity. They make a voluntary exchange possible between interested buyers and sellers of water rights. This paper presents direct evidence from seven water markets around the globe to document key economic and ecological challenges and achievements of water markets with respect to water scarcity. We specifically approach water markets as localized cap-and-trade systems, similar to those for carbon emissions. We examine whether water use remains within the set limits on use of water rights (i.e., under the cap), the degree to which water markets help protect the health of ecosystems and species, and whether (as predicted by economic theory) the explicit pricing of water is accompanied by improving efficiency, as less productive water users decide to sell water to more productive water users.
The loss of functional and accreting coral reefs reduces coastal protection and resilience for tropical coastlines. Coral restoration has potential for recovering healthy reefs that can mitigate ...risks from coastal hazards and increase sustainability. However, scaling up restoration to the large extent needed for coastal protection requires integrated application of principles from coastal engineering, hydrodynamics, and ecology across multiple spatial scales, as well as filling missing knowledge gaps across disciplines. This synthesis aims to identify how scientific understanding of multidisciplinary processes at interconnected scales can advance coral reef restoration. The work is placed within the context of a decision support framework to evaluate the design and effectiveness of coral restoration for coastal resilience. Successfully linking multidisciplinary science with restoration practice will ensure that future large‐scale coral reef restorations maximize protection for at‐risk coastal communities.
Seafloor topography affects a wide range of physical and biological processes; therefore, collapsing the three‐dimensional structure of the bottom to roughness metrics is a common challenge in ...studies of marine systems. Here we assessed the properties captured by metrics previously proposed for the seafloor, as well as metrics developed to characterize other types of rough surfaces. We considered three classes of metrics: properties of the bottom elevation distribution (e.g., standard deviation), length scale ratios (e.g., rugosity), and metrics that describe how topography varies with spatial scale (e.g., Hölder exponents). The metrics were assessed using idealized topography and natural seafloor topography data from airborne lidar measurements of a coral reef. We illustrate that common roughness metrics (e.g., rugosity) can have the same value for topographies that are geometrically very different, limiting their utility. Application of the wavelet leaders technique to the reef data set demonstrates that the topography has a power law scaling behavior, but it is multifractal so a distribution of Hölder exponents is needed to describe its scaling behavior. Using principal component analysis, we identify three dominant modes of topographic variability, or ways metrics covary, among and within reef zones. Collectively, the results presented here show that coral reef topography is both multiscale and multifractal. While individual metrics that capture specific topography properties relevant to a given process may be suitable for some studies, many applications will require a set of metrics that includes statistics that capture how topography varies with spatial scale.
Key Points
Individually, common roughness metrics (rugosity and standard deviation) do not quantify horizontal scales of variability, limiting utility
For multiscale natural topography, it is important to consider elevation distribution moments, length scale ratios, and power law scaling
A principal component analysis illustrates how multiple metrics covary and identifies dominant modes of topographic variability
In shallow water systems like coral reefs, bottom friction is an important term in the momentum balance. Parameterizations of bottom friction require a representation of canopy geometry, which can be ...conceptualized as an array of discrete obstacles or a continuous surface. Here, we assess the implications of using obstacle‐ and surface‐based representations to estimate geometric properties needed to parameterize drag. We collected high‐resolution reef topography data using a scanning multibeam sonar that resolved individual coral colonies within a set of 100‐m2 reef patches primarily composed of mounding Porites corals. The topography measurements yielded 1‐cm resolution continuous surfaces consisting of a single elevation value for each position in a regular horizontal grid. These surfaces were analyzed by (1) defining discrete obstacles and quantifying their properties (dimensions, shapes), and (2) computing properties of the elevation field (root mean square (rms) elevations, rms slopes, spectra). We then computed the roughness density (i.e., frontal area per unit plan area) using both analysis approaches. The obstacle and surface‐based estimates of roughness density did not agree, largely because small‐scale topographic variations contributed significantly to total frontal area. These results challenge the common conceptualization of shallow‐water canopies as obstacle arrays, which may not capture significant contributions of high‐wavenumber roughness to total frontal area. In contrast, the full range of roughness length scales present in natural reefs is captured by the continuous surface representation. Parameterizations of bottom friction over reef topography could potentially be improved by representing the contributions of all length scales to total frontal area and drag.
Plain Language Summary
Coral reefs depend on water motion for survival. At the same time, their complex structure exerts friction on the overlying flow and thereby affects water movement across reefs. Prediction of flow over reefs requires a model that captures the interaction between reef topography and the flow. This is typically done with a drag law that requires specification of the frontal area of topography perpendicular to the flow per unit horizontal area. Quantifying frontal area is problematic because reef topography has many length scales. Here we consider the effect that representing reefs either as discrete obstacle arrays (e.g., cylinders) or as continuous rough surfaces (e.g., represented by spectra) has on estimates of frontal area and drag. Our results show that the obstacle representation misses the contribution of small length scales while the surface approach captures all length scales, which contribute significantly to the total frontal area of natural reefs. This suggests that the many different topographic length scales should be considered in models of friction for flow across reefs.
Key Points
For reef topography, which is multiscale, a wide range of length scales contribute substantially to the frontal area per unit plan area
Representing reef topography as an obstacle array does not capture the substantial contribution of small length scales to total frontal area
Method is proposed for computing roughness density from local surface slopes that includes contributions from all topographic length scales
In shallow water systems like coral reefs, bottom friction is often a significant part of the overall momentum balance. The frictional effects of the bottom on the flow are in part determined by the ...structure of the topography, which varies over orders of magnitude in spatial scale. Predicting spatial and temporal patterns of water motion depends on adequately capturing the relevant properties of the topography. However, representing and quantifying the complex, heterogeneous structure of coral reefs using measures of roughness or geometry remains a challenge.Many roughness metrics have been proposed to relate seafloor structure to biological and physical processes. In Chapter 1, we assess the properties captured by one-dimensional roughness metrics previously proposed for the seafloor, as well as metrics developed to characterize other types of rough surfaces. We consider three classes of metrics: properties of the bottom elevation distribution (e.g., standard deviation), length scale ratios (e.g., rugosity), and metrics that describe how topography varies with spatial scale (e.g., Hölder exponents). We evaluate these metrics using idealized topography and natural seafloor topography of a reef lagoon system from airborne lidar measurements. The analyses illustrate that common metrics of bathymetric roughness (e.g., rugosity) can have the same value for topographies that are geometrically very different, thus limiting their utility. Application of the wavelet leaders technique to the reef dataset demonstrates that the topography has a power law scaling behavior, but it is multifractal so a distribution of Hölder exponents is needed to describe its scaling behavior. Using principal component analysis, we identify three dominant modes of topographic variability, or ways metrics covary, among and within reef zones. While individual roughness metrics that capture specific topography properties relevant to a given process may be suitable for some studies, for many applications, adequately parameterizing bathymetric roughness will require a set of metrics.For reefs where the roughness layer takes up a large fraction of the water column, parameterizations of bottom friction require a representation of three-dimensional canopy geometry. In Chapter 2, we assess the implications of using obstacle- and surface-based representations to estimate geometric properties of coral colonies needed to parameterize drag. We collected high-resolution topography data using a scanning multi-beam sonar that resolved individual coral colonies within a set of 100 m2 reef patches primarily composed of mounding Porites corals. The topography measurements yielded 1-cm resolution gridded surfaces consisting of a single elevation value for each position in a regular horizontal grid. These surfaces were analyzed by (1) defining discrete obstacles and quantifying their properties (dimensions, shapes), and (2) computing properties of the elevation field (rms elevations, rms slopes, spectra). We then computed the roughness density (i.e., frontal area per unit plan area) using both analysis approaches. The obstacle and surface-based estimates of roughness density did not agree, largely because small-scale topographic variations contributed significantly to total frontal area. These results challenge the common conceptualization of shallow-water canopies as obstacle arrays, which may not capture significant contributions of high-wavenumber roughness to total frontal area. In contrast, the full range of roughness length scales present in natural reefs is captured by the continuous surface representation. Parameterizations of drag could potentially be improved by considering the distribution of frontal area across length scales.Collectively, the results presented in Chapters 1 and 2 show that coral reef topography is both multiscale and multifractal. However, there is a limited understanding of the effects of the structural complexity on water motion around individual and groups of corals. In Chapter 3, we present detailed hydrodynamic measurements from the same shallow reef sites for which we quantified reef geometry (Chapter 2). Using these measurements, we compare spatial and temporal variations in flow patterns across three sites: (1) a high relief site with waves; (2) a low relief site with waves; and (3) a high relief site without waves. Our observations suggest that the flow is likely unidirectional and current dominated over much of the backreef. These measurements also show that flow variations at different frequencies have different spatial patterns. At low frequencies, flow variations follow the spatial pattern of wakes. The lack of coherent structure in wave band variations can be explained by the distribution of orbital excursion length to colony diameter (Keulegan-Carpenter number), which is typically less than 2π, thus wakes do not form behind elements. Variations at high frequencies were up to two times larger in the canopy than upstream. In the future, these observations could be compared to computational models of flow at the sites, which would allow us to better understand mechanisms controlling frequency-dependent spatial patterns, as well as the importance of colony and patch-scale processes for reef and regional scale circulation.
High-throughput screening has become a mainstay of small-molecule probe and early drug discovery. The question of how to build and evolve efficient screening collections systematically for cell-based ...and biochemical screening is still unresolved. It is often assumed that chemical structure diversity leads to diverse biological performance of a library. Here, we confirm earlier results showing that this inference is not always valid and suggest instead using biological measurement diversity derived from multiplexed profiling in the construction of libraries with diverse assay performance patterns for cell-based screens. Rather than using results from tens or hundreds of completed assays, which is resource intensive and not easily extensible, we use high-dimensional image-based cell morphology and gene expression profiles. We piloted this approach using over 30,000 compounds. We show that small-molecule profiling can be used to select compound sets with high rates of activity and diverse biological performance.
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