Deep Estuary Research Articles

Estuarine nutrient pollution impact reduction assessment through euphotic zone avoidance/bypass considerations

The feasibility of reducing nutrient pollution impact by redirecting the effluent to depths below the euphotic zone was investigated for the deep estuarine Puget Sound region of the Salish Sea in the Pacific Northwest of America. The hypothesis tested was that the thickness of the outflow layer in deep estuaries may be greater than the euphotic zone depth, allowing a fraction of nutrients to be exported out passively through the layers immediately below. The euphotic zone depth in Puget Sound varies from 8 to 25 m while the depth of the outflow layer can reach up to ≈ 60 m. Outfall relocation strategies were tested on 99% of the anthropogenic nutrient loads currently delivered to Puget Sound. The impact was quantified using the previously established biophysical Salish Sea Model, using gross primary production and exposure to low dissolved oxygen (DO) levels as the metric (< 2 mg/L for hypoxia and < 5 mg/L for impairment). Eliminating nutrient pollution (above natural) from rivers and wastewater reduced hypoxia exposure by 8.1% and 11.2%, respectively. Relocating the outfalls to deeper waters resulted in improvements, but only in the sill-less sub-basins such as Whidbey, where hypoxia and DO impairment exposure decreased (7.9% and 6.8%, respectively). The presence of multiple sills and circulation cells in Puget Sound resulted in increased exposure and rendered nutrient bypass goals unfeasible as originally envisioned. However, an alternate nutrient export pathway was identified through bottom exchange flow out of Puget Sound via Whidbey Basin and Deception Pass. An unexpected reduction in the exchange outflow magnitude (≈ 4%) due additional (22%) freshwater discharged to the estuary bottom was also noted. The potential loss in circulation strength due to rerouting of natural surface freshwater through submerged deep-water outfalls is identified as a new unforeseen anthropogenic impact.

Measuring and modeling diel oxygen dynamics in a shallow hypereutrophic estuary: Implications of low oxygen exposure on aquatic life

Hypoxia, or low dissolved oxygen (DO) is a common outcome of excess nitrogen and phosphorus delivered to coastal waterbodies. Shallow and highly productive estuaries are particularly susceptible to diel-cycling hypoxia, which can exhibit DO excursions between anoxia (DO ≤1 mg L−1) and supersaturated concentrations within a day. Shallow estuaries exhibiting diel-cycling hypoxia are understudied relative to larger and deeper estuaries, with very few mechanistic models that can predict diel oxygen dynamics. We utilized continuous monitoring data and the Coastal Generalized Ecosystem Model (CGEM) coupled with an Environmental Fluid Dynamics Code (EFDC) hydrodynamic model to simulate diel DO dynamics in Weeks Bay, AL. Low oxygen conditions ranging from anoxia to DO ≤4 mg L−1 were consistently observed and simulated in the lower water column for periods of minutes to >11 h. High frequency observations and model simulations also identified significant vertical gradients in near bottom DO that varied as much as 0.8 to 3.1 mg L−1 within 0.4 m from the bottom. This spatiotemporal variability presents unique challenges to adequately quantify DO dynamics and the potential exposure of aquatic life to low oxygen conditions. Our results demonstrate the need for detailed measurements to adequately quantify the complex DO dynamics in shallow estuaries. We also demonstrate that simulation models can be successfully applied to evaluate diel oxygen dynamics in complex estuarine environments when calibrated with fine time scale data and effective parameterization of water column and benthic metabolic processes.

Microplankton Interactions with Decadal-Scale Nutrient Enrichment in a Deep Estuary, with Implications for Eutrophication—Related Ecosystem Stressors

Long-term (decadal) records of microplankton provide insights into how lower trophic levels of coastal ecosystems respond to nutrient enrichment, over and above shorter-term variability. We used a 15-year seasonal census in the Firth of Thames, a deep, nutrient-enriched estuary in northeastern Aotearoa/New Zealand, to determine microplankton responses to enrichment. Kendall trend analyses showed that dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen were enriched by 99% and 34%, respectively, over 15 years, while phosphorus changed little. Larger phytoplankton (> 2 µm) increased by 46%, including 57% increases by diatoms (mainly large centrics with 93% increase) and nanoflagellates (151% increase). Dinoflagellates decreased by 46%, such that the community shifted from dinoflagellate to diatom/nanoflagellate dominance. Within phytoplankton ≤ 2 µm, picoprokaryotes increased by 369%, while picoeukaryotes changed little. Among microheterotrophs (< 200 µm), bacteria increased by 89%, and small oligotrichs increased by 53%. Trend analyses and multivariate general additive modelling showed that microplankton biomass responded primarily to increased DIN over the 15-year period and secondarily to stratification variation at shorter time scales. The changed biomasses and community composition are explained as responses to increased N:P and food-web interactions. Deleterious changes included increased toxic Pseudo-nitzschia abundance and potentially reduced nutritional quality of the phytoplankton community for grazers. The increased N and larger diatoms indicated potential for increased deposition to sediments, possibly explaining previous observations of lowered denitrification in the Firth during the time series period. The results indicated a continuation of enrichment the Firth has received over decades, with implications for expression of ecosystem stressors of acidification and hypoxia.

Differential Responses of Eelgrass and Macroalgae in Pacific Northwest Estuaries Following an Unprecedented NE Pacific Ocean Marine Heatwave

The 2013-2016 northeast Pacific Ocean marine heatwave (MHW) had myriad impacts on marine communities, but little is known about how this event affected coastal estuaries. We examined the extent to which elevated temperatures associated with the offshore MHW were observed in four estuaries (Willapa Bay, Washington and Netarts Bay, Yaquina Bay, and Coos Bay, Oregon) and the responses of macrophytes at marine dominated sites within these estuaries. Records of eelgrass (Zostera marina), ulvoid macroalgae (Ulva and Enteromorpha spp.), and environmental characteristics, including water temperature, were analyzed over three periods: before (2006-2010), during (2015-2016), and after (2017-2019) the MHW. During the MHW, all four estuaries experienced “estuarine heatwaves” (anomalous warm water events) at monitoring stations and there was an associated decline in macroalgae biomass. In northern estuaries (Willapa and Netarts bays) where water temperatures are normally higher and estuaries are shallower, aboveground eelgrass biomass declined and did not recover for at least three years after the MHW. In southern estuaries (Yaquina and Coos bays), where water temperatures are normally colder and estuaries are deeper, the MHW was associated with a neutral, or temporarily positive, change in aboveground eelgrass biomass. Our analysis supports the hypothesis that upwelling intensity and estuarine depth determined the severity of MHW impacts on macrophytes at marine dominated sites, and that the colder and deeper estuaries in the south may serve as refuges for eelgrass but not macroalgae. We suggest that estuary-specific responses were predictable given the drivers of macrophyte abundance at marine dominated sites, and that baseline ocean upwelling and estuarine temperature can be used to anticipate macrophyte responses at marine dominated sites to MHWs in the future.

Sea-level-rise-induced threats depend on the size of tide-influenced estuaries worldwide

The effects of sea-level rise on the future morphological functioning of estuaries are largely unknown because tidal amplitudes will change due to combined deepening of the estuary mouth and shifting amphidromic points at sea. Fluvial sediment supply is also globally decreasing, which hampers infilling necessary to maintain elevation relative to sea level. Here we model 36 estuaries worldwide with varying sizes, shapes and hydrodynamic characteristics, and find that small shallow estuaries and large deep estuaries respond in opposite ways to sea-level rise. Large estuaries are threatened by sediment starvation and therefore loss of intertidal area, particularly if tidal amplitude decreases at the mouth. In contrast, small estuaries face enhanced flood risks and are more sensitive to tidal amplification on sea-level-rise-induced deepening. Estuary widening can partly mitigate adverse effects. In large estuaries, expanded intertidal areas increase tidal prism and available erodible sediment for adaptation, whereas it slightly reduces tidal amplification in small estuaries. Small shallow estuaries face enhanced flood risk under climate change because of sea-level-rise-induced tidal amplification. In contrast, large deep estuaries are threatened by sediment starvation and therefore a loss of intertidal area. Both cases can potentially be mitigated by estuary widening.

HISTORY OF THE LOWER VOLGA PALEO-DELTAS FORMATION

The Paleo-Volga River valley existed within the present Lower Volga basin during the last 600-700 ka. Periodically its lower parts were transformed into a long and deep ingressive estuary. Over the last 16 ka there have been 6 marine (estuarine) phases within the Volga-Akhtuba valley corresponding to transgressive phases of the Late Khvalyn and New Caspian ages. There are pronounced traces of the 4 transgressive-regressive stages of the Late Khvalyn and New Caspian ages in the present Volga-Akhtuba floodplain topography, correlated with the three generations of ancient floodplains, whose age are decreasing downstream from 16 to 1.5 ka.

From global 137Cs to determinations of the sedimentation rate in deep lakes and sea estuaries

Sedimentation rate in deep lakes and estiaries of the Baltic, Pechora, and Laptev Seas was determined using global 137Cs as a sedimentogenesis marker. The sedimentation rate in Lake Ladoga and Levinson- Lessing Lake was 0.3–3 mm year−1, and that in estuaries of the Baltic, Pechora, and Laptev Seas was 0.74–1.76, 1.0, and 3.3–5.0 mm year−1, respectively. At a sedimentation rate of >1–2 mm year–1, the concentration peaks of global and “Chernobyl” 137Cs in the bulk of the bottom sediments do not overlap, and the sedimentation rate is estimated for each kind of nuclides separately. At a sedimentation rate of <1 mm year−1, “Chernobyl” 137Cs + 137Cs from dumping into sea mask in the bottom sediments the subsurface maximum of the global 137Cs concentration and prevent the sedimentation rate determination. The use of 137Cs for sedimentation rate determinations is appropriate for the poorly studied shelf zone of Russian Arctic seas.

Drivers of Change in Shallow Coastal Photic Systems: An Introduction to a Special Issue

Coastal ecosystems are characterized by relatively deep, plankton-based estuaries and much shallower systems where light reaches the bottom. These latter systems, including lagoons, bar-built estuaries, the fringing regions of deeper systems, and other systems of only a few meters deep, are characterized by a variety of benthic primary producers that augment and, in many cases, dominate the production supplied by phytoplankton. These “shallow coastal photic systems” are subject to a wide variety of both natural and anthropogenic drivers and possess numerous natural “filters” that modulate their response to these drivers; in many cases, the responses are much different from those in deeper estuaries. Natural drivers include meteorological forcing, freshwater inflow, episodic events such as storms, wet/dry periods, and background loading of optically active constituents. Anthropogenic drivers include accelerated inputs of nutrients and sediments, chemical contaminants, physical alteration and hydrodynamic manipulation, climate change, the presence of intensive aquaculture, fishery harvests, and introduction of exotic species. The response of these systems is modulated by a number of factors, notably bathymetry, physical flushing, fetch, sediment type, background light attenuation, and the presence of benthic autotrophs, suspension feeding bivalves, and fringing tidal wetlands. Finally, responses to stressors in these systems, particularly anthropogenic nutrient enrichment, consist of blooms of phytoplankton, macroalgae, and epiphytic algae, including harmful algal blooms, subsequent declines in submerged aquatic vegetation and loss of critical habitat, development of hypoxia/anoxia particularly on short time scales (i.e., “diel-cycling”), fish kills, and loss of secondary production. This special issue of Estuaries and Coasts serves to integrate current understanding of the structure and function of shallow coastal photic systems, illustrate the many drivers that cause change in these systems, and synthesize their varied responses.

Relative importance of pelagic and sediment respiration in causing hypoxia in a deep estuary

Oxygen depletion in the 100‐m thick bottom layer of the deep Lower St. Lawrence Estuary is currently thought to be principally caused by benthic oxygen demand overcoming turbulent oxygenation from overlying layers, with pelagic respiration playing a secondary role. This conception is revisited with idealized numerical simulations, historical oxygen observations and new turbulence measurements. Results indicate that a dominant sediment oxygen demand, over pelagic, is incompatible with the shape of observed oxygen profiles. It is further argued that to sustain oxygen depletion, the turbulent diffusivity in the bottom waters should be ≪10−4 m2 s−1, consistent with direct measurements but contrary to previous model results. A new model that includes an Arrhenius‐type function for pelagic respiration and a parameterization for turbulence diffusivity is developed. The model demonstrates the importance of the bottom boundary layer in reproducing the shape of oxygen profiles and reproduces to within 14% the observed change in oxygen concentration in the Lower St. Lawrence Estuary. The analysis indicates that turbulent oxygenation represents about 8% of the sum of sediment and pelagic oxygen demand, consistent with the low turbulent oxygenation required to maintain oxygen depletion. However, contrary to previous hypotheses, it is concluded that pelagic oxygen demand needs to be five time larger than sediment oxygen demand to explain hypoxia in the 100‐m thick bottom layer of the Lower St. Lawrence Estuary.

Transverse structure of tidal and residual flow and sediment concentration in estuaries

An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped estuarine cross-sections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model. The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter space considered (average tidal velocity amplitude, 0.1–1 m s − 1 and maximum water depth, 10–60 m). Its skills are lower for along-channel residual flows if nonlinearities are moderate to high (strong tides in deep estuaries) and for transverse flows and residual sediment concentrations if the Ekman number is small (weak tides in deep estuaries). An important new aspect of the analytical model is the incorporation of tidal variations in the across-channel density gradient, causing a double circulation pattern in the transverse flow during slack tides. The gradient also leads to a new tidally rectified residual flow component via net advection of along-channel tidal momentum by the density-induced transverse tidal flow. The component features landward currents in the channel and seaward currents over the slopes and is particularly effective in deeper water. It acts jointly with components induced by horizontal density differences, Coriolis-induced tidal rectification and Stokes discharge, resulting in different along-channel residual flow regimes. The residual across-channel density gradient is crucial for the residual transverse circulation and for the residual sediment concentration. The clockwise density-induced circulation traps sediment in the fresher water over the left slope (looking up-estuary in the northern hemisphere). Model results are largely consistent with available field data of well-mixed estuaries.

Influence of Lateral Advection on Residual Currents in Microtidal Estuaries

Abstract The influence of nonlinear lateral advection on estuarine exchange flow is examined with a scaling analysis and eight groups of idealized numerical experiments. Nonlinear lateral advection is related to the linkage between lateral circulation and the lateral shear of the along-estuary flow. The relative contribution of lateral advection to the overall dynamics of a microtidal estuary is found to be a function of width and depth, and of vertical mixing. Lateral advection is dynamically important in narrow and deep estuaries, particularly under relatively weak vertical mixing. The relative importance of lateral advection and the earth’s rotation on estuarine dynamics can be evaluated in terms of the nondimensional Rossby and Ekman numbers (Ro and Ek). Lateral advection is most effective at large Ro and small Ek and is negligible at small Ro and large Ek. As expected, the earth’s rotation is most significant at small Ro and Ek, and is negligible at large Ro and Ek. Under the influence of lateral advection and the earth’s rotation, the lateral structure of estuarine exchange flows is a function of Ro and Ek. In some instances, the exchange flow is vertically sheared and in others it is laterally sheared. Classical estuarine dynamics, which yields vertically sheared exchange flows, occurs at intermediate Ro and large Ek. The main role of lateral advection is to reduce lateral variability of estuarine exchange flow and generate a vertically sheared, two-layer exchange flow structure.

Adapting the light · biomass ( BZI) models of phytoplankton primary production to shallow marine ecosystems

Several authors have reported a strong linear relationship between daily phytoplankton production and the product of chlorophyll biomass, photic depth, and incident irradiance for a variety of estuaries. This “light · biomass” ( BZ pI o ) formulation has been proposed as an alternative to traditional mechanistic approaches for computing phytoplankton production in numerical estuarine models. One limitation to their application in shallow systems is that the BZ pI o models have been developed in relatively deep estuaries where light does not reach the bottom. We propose a nonlinear correction factor to adapt the BZ pI o relationship to shallow systems where light does reach the bottom. Our function takes into account variations in incident irradiance, attenuation coefficient for light, photosynthetic efficiency, and maximum rate of photosynthesis. A series of correction polynomials are proposed for various ranges of incident irradiance, and are integrated into a single multiple polynomial which applies across all irradiance levels. Our new correction factor was tested against a 14C-based productivity dataset from shallow stations in Narragansett Bay, RI and an O 2-based dataset from shallow (1.1 m) lagoon mesocosms at the University of Rhode Island. Results showed that our polynomials accurately correct BZ pI o -predicted rates of production in shallow water columns. Application of our correction factor to a series of shallow water productivity datasets from the literature together with theoretical calculations show how significant the shallow water correction can be, especially in very shallow water columns with low turbidity.

Community shifts, alternative stable states, biogeochemical controls and feedbacks in eutrophic coastal lagoons: a brief overview

(1) The succession of primary producer communities in coastal lagoons is analysed in the light of the regime shift theory. Pristine coastal lagoons are considered to be dominated by extensive meadows of seagrass species, which are assumed to take advantage of nutrient supply from sediments. An increasing nutrient input is thought to favour phytoplankton and/or epiphytic micro-, macroalgae as well as opportunistic ephemeral macroalgae that coexist with seagrasses. In the latest stages of this succession, the imbalance of phosphorus to nitrogen ratio can favour macroalgal, cyanobacteria and/or picoplankton blooms, often causing dystrophy. (2) The primary causes of shifts and succession in the macrophyte community are nutrient loadings, mainly nitrogen, as well as changes in coastal hydrology or interactions between them. To some extent, in very shallow choked lagoons, benthic vegetation is mainly controlled by loading rates, while in open deep estuaries hydromorphological factors predominate. (3) External stressors/perturbations cause an amplification in benthic biogeochemical processes, e.g. wide variations in primary productivity and dark respiration, with large oscillations in oxygen and sulphide concentrations. Altered biogeochemical processes can determine positive feedbacks inducing a shift from pristine to altered macrophyte communities, which in turn amplify the perturbation until the shift becomes irreversible. (4) Macrophyte typology, organic matter composition and sedimentary geochemistry are primary factors in controlling feedbacks and shifts. For example, the sedimentary buffering capacity of iron controls sulphide and phosphates, while nitrogen cycling is mainly controlled by primary producers - microbial process interactions. (5) The alternative states which occur through the transition from pristine to modified primary producer communities can also be viewed as a sequence of stable states with different degrees of embedded information and with different ecological functions. Copyright © 2008 John Wiley & Sons, Ltd.

Wind-driven sediment suspension controls light availability in a shallow coastal lagoon

Light availability is critically important for primary productivity in coastal systems, yet current research approaches may not be adequate in shallow coastal lagoons. Light attenuation in these systems is typically dominated by suspended sediment, while light attenuation in deeper estuaries is often dominated by phytoplankton. This difference in controls on light attenuation suggests that physical processes may exert a greater influence on light availability in coastal lagoons than in deeper estuaries. Light availability in Hog Island Bay, a shallow coastal lagoon on the eastern shore of Virginia, was determined for a summer and late fall time period with different wind conditions. We combined field measurements and a process-based modeling approach that predicts sediment suspension and light availability from waves and currents to examine both the variability and drivers of light attenuation. Total suspended solids was the only significant predictor of light attenuation in Hog Island Bay. Waves and currents in Hog Island Bay responded strongly to wind forcing, with bottom stresses from wind driven waves dominant for 60% of the modeled area for the late fall period and 24% of the modeled area for the summer period. Higher wind speeds in late fall than in summer caused greater sediment suspension (41 and 3 mg l−1 average, respectively) and lower average (spatial and temporal) downwelling light availability (32% and 55%, respectively). Because of the episodic nature of wind events and the spatially variable nature of sediment suspension, conventional methods of examining light availability, such as fair-weather monitoring or single in situ recorders, do not adequately represent light conditions for benthic plants.

Modeling Flushing and Mixing in a Deep Estuary

The distribution of constituents from discharges in estuaries can be determined only to the detail with which the time-varying velocity field is known. Deep estuaries can exhibit complex vertical and horizontal circulation structures depending on tides, freshwater inflows, density stratification, and geometry. A numerical, time-varying, three-dimensional hydrodynamic and transport model is applied to the Gastineau Channel, a deep estuary in Alaska, to determine the dilution of material discharged from a submerged outfall into the Channel. The ability of the model to produce realistic flow fields is first confirmed by comparing computed velocities to measured velocities for stratified and unstratified conditions. Methods are then shown for determining the flushing rate of the estuary and the distribution of dilution throughout the estuary for these same conditions.

The effect of wave‐current interaction on tidally forced estuarine circulation

Reductions in the velocity and surface elevation of tidal flows that have been observed in the Delaware Bay, a coastal plain estuary on the eastern coast of the United States, correlate well with local wind events. While there are several mechanisms that could account for this phenomenon, we consider hydrodynamic wave‐current interaction to be a reasonable explanation. In order to theoretically model the interaction between tides and surface gravity waves, and eliminate the need for a reference velocity required by the Grant and Madsen (1979) wave‐current interaction model, we propose a process‐oriented model that couples the mechanism of wave‐current interaction directly to the current dynamics of the tidal flow. The tidal model uses an eddy viscosity concept to parameterize the turbulent stresses and further characterizes the eddy viscosity in terms of a flow dependent shear velocity. Within the bottom boundary layer of the surface gravity wave, the turbulent stresses are also parameterized by an eddy viscosity that is coupled to the shear velocity of the tidal model. Results of the coupled model indicate, that as expected, wave‐current interaction increases the bottom friction felt by the tidal flow and usually reduces the volume flux. However, if the physical bottom roughness is sufficiently small (less than 30 cm) then it is possible to enhance the volume flux in a long, deep estuary, contrary to expectations. A simple one dimensional, linearized friction model shows this potentially dramatic effect can be attributed to an increase in the bottom friction that then tunes the estuary closer to its natural resonance. Obviously an estuary at resonance maximizes the volume flux that must pass through the estuary mouth. The effect of wave‐current interaction is to provide a possible means of increasing the bottom friction and enhancing the volume flux.

The influence of density gradients on the transport of solutes in a deep estuary

This paper reports on an investigation into the lower Forth estuary where a combination of hydrody namic, dye and salinity measurement techniques have been employed to quantify the effects of temporally varying vertical and transverse density gradients on the diffusion and dispersion processes. It is concluded that the interaction of velocity shear and weak longitudinal salinity induced density gradient produces a complex and profound effect on estuarine solute mixing processes.

A rate for the scavenging of fine particles by macroaggregates in a deep estuary

234Th activity profiles in Puget Sound have been studied using a model that incorporates reversible exchanges between dissolved, fine particulate, and macroaggregate Th reservoirs. Macroaggregate settling is made responsible for the downward flux of Th and the vertical gradients of activity in measured profiles. Least squares fits of model to data yield rates/time scales for the exchange processes involved. Fine‐particle scavenging by macroaggregates is found to occur with a time scale of 4–6 days over a large range of macroaggregate settling speeds, ws. Macroaggregate‐disaggregation time scales are 1–4 days when ws is 100 m/d. Rates of sorption and remobilization characterizing the exchange between dissolved and fine‐particulate forms of the isotope cannot be individually identified from these data, but acceptable model values include those measured in the laboratory. Rates of sorption that depend on particulate concentrations which increase to the seafloor result in profiles of dissolved Th having above‐bottom maxima. Based on inferred exchange rates, the residence time for fine particles introduced at the surface of this deep (∼200 m) estuary is estimated to be 11–16 days when ws = 100 m/d.

Covariance Analysis of Chlorophyll Distribution in the Sundays River Estuary, Eastern Cape

Summary An analysis of covariance performed on chlorophyll a distribution data from the Sundays River estuary identified five persistent water masses with significantly different chlorophyll a contents. These corresponded to different hydrodynamic regions within the estuary. The relationship between salinity and chlorophyll a was used to identify a transition zone between the low-salinity upper estuary and the brackish riverine inflow. Chlorophyll a concentrations ranged from <6 μg 1−1 near the mouth to >100 μg 1−1 in the middle and upper reaches. High variance of chlorophyll a data in this region was the result of temporal chlorophyll a fluctuations in two time scales. The first related to the daily ebb/tide and the second to the lunar spring/neap tidal cycle. Unlike its importance in larger and deeper estuaries, light played only a minor role in the horizontal distribution of chlorophyll a. The presence of a chlorophyll a maximum in the turbid low salinity region is explained in terms of the relatively constant photic depth/mixed depth ratio along the estuary and the high residence time of water in this region.

Coastal-Trapped Waves in Continuously Stratified Channels. Part I: Numerical Description of Behavioral Properties

Abstract A numerical model which solves the linearized, quasi-geostrophic potential vorticity equation is used to describe free subinertial coastal-trapped wave (CTW) properties in channels with variable depth in the offshore direction, and with surface intensified buoyancy frequency profiles N2(z) as found in large and deep estuaries. The variational form of the pressure equation reveals that CTWs propagate slower in channels as compared to those on continental shelves, due to the bottom slope reversal between the channel shores. Significant phase speed reduction occurs when the effective channel stratification parameter in O(1) or higher. Without coastal walls, phase speeds of long topographic waves increase with channel width and vertical averaged N2, but decrease with channel concavity and N2 surface intensification. Bottom-trapped motion is enhanced in channels relative to that on continental shelves, again due the bottom slope reversal. In the presence of coastal walls, hybrid waves are less affecte...