AccessScience from McGraw-Hill Education Page 1 of 12 www.accessscience.com

Estuarine oceanography

Contributed by: Daniel G. MacDonald, K. R. Dyer Publication year: 2019

The study of the physical, chemical, biological, and geological characteristics of estuaries. An estuary is a semienclosed coastal body of water which has a free connection with the sea and within which the seawater is measurably diluted by freshwater derived from land drainage. Many characteristic features of estuaries extend into the coastal areas beyond their mouths; and because the techniques of measurement and analysis are similar, the field of estuarine oceanography is often considered to include the study of some coastal waters which, by the above definition, are not strictly estuaries. Also, semienclosed bays and lagoons exist in which evaporation is equal to or exceeds freshwater inflow, so that the salt content either is equal to that of the sea or exceeds it. Hypersaline lagoons have been termed negative estuaries, whereas those with precipitation and river inflow equaling evaporation have been called neutral estuaries. Positive estuaries, in which river inflow and precipitation exceed evaporation, form the majority.

Topographic classification

Embayments are the result of fairly recent changes in sea level. During the Pleistocene ice age, much of the seawater was locked up in continental ice sheets, and the sea surface stood about 100 m (330 ft) below its present level. In areas not covered with ice, the rivers incised their valleys to this base level. During the ensuing Flandrian Transgression, when the sea level rose at about 1 m (3.3 ft) per century, these valleys became inundated. Much of the variation in form of the resulting estuaries depends on the volumes of sediment that the river or the nearby coastal erosion has contributed to fill the valleys.

Where river flow and sediment discharge were high, the valleys have become completely filled and even built out into deltas. Generally, deltas are best developed in areas where the tidal range is small and where the currents cannot easily redistribute the sediment the rivers introduce. They occur mainly in tropical and subtropical areas where river discharge is seasonally very high. The distributaries, or passes, of the delta are generally shallow, and often the shallowest part is a sediment bar at the mouths of the distributaries. The Mississippi and the Niger are examples of this type of delta.

Where sediment discharge was less, the estuaries are unfilled, although possibly they are still being filled. These are drowned river valleys or coastal plain estuaries, and they still retain the topographic features of river valleys, having a branching, dendritic, though meandering, outline and a triangular cross section, and widening regularly toward the mouth, which is often restricted by spits. River discharge tends to be reasonably steady throughout the year, and sediment discharge is generally small. These estuaries occur in areas of high tidal range, where the AccessScience from McGraw-Hill Education Page 2 of 12 www.accessscience.com

currents have helped to keep the estuaries clear of sediment. They are typical of temperate regions such as the east coast of North America and northwestern Europe, examples being the Chesapeake Bay system, the Thames, and the Gironde. See also: COASTAL LANDFORMS.

In areas where glaciation was active, the river valleys were overdeepened by glaciers and fiords were created. A characteristic of these estuaries is the rock bar or sill at the mouth that can be as little as a few tens of meters deep. Inside the mouth, however, the estuaries can be at least 600 m (1800 ft) deep and can extend hundreds of kilometers inland. Fiords are typical of Norway and the Canadian Pacific coast. See also: FIORD.

Another estuarine type is called the bar-built estuary. These are formed on low coastlines where extensive lagoons have narrow connecting passages or inlets to the sea. Within the shallow lagoons, the tidal currents are small, but the deep inlets have higher currents. Again, a sediment bar is generally present across the entrance. In tropical areas, the lagoons can be hypersaline during the hot season. They are typical of the southern United States and of parts of Australia.

Estuaries are ephemeral features since great alterations can be wrought by small changes in sea level. If the present ice caps were to melt, the sea level would rise an estimated 30 m (99 ft), and the effect on the form and distribution of estuaries would be drastic.

Physical structure and circulation

Within estuaries, the river discharge interacts with the seawater, and river water and seawater are mixed by the action of tidal motion, by wind stress on the surface, and by the river discharge forcing its way toward the sea. The difference in salinity between river water and seawater—about 35 parts per thousand—creates a difference in density of about 2%. Even though this difference is small, it is sufficient to cause horizontal pressure gradients within the water which affect the way it flows. Density differences caused by temperature variations are comparatively smaller. Salinity is consequently a good indicator of estuarine mixing and the patterns of water circulation. Obviously, there are likely to be differences in the circulation within estuaries of the same topographic type which are caused by differences in river discharge and tidal range. The action of wind on the water surface is an important mixing mechanism in shallow estuaries, particularly in lagoons; but generally its effect on estuarine circulation is only temporary, although it can produce considerable variability and thus make interpretation of field observations difficult. See also: SEAWATER.

Salt-wedge estuaries. Freshwater, being less dense than seawater, tends to flow outward over the surface of seawater, which penetrates as a salt wedge along the bottom into the estuary (Fig. 1). This creates a vertical salinity stratification, with a narrow zone of sharp salinity change, called a halocline, between the two water masses, which can reach 30 parts per thousand in 0.5 m (1.5 ft). If the sea is tideless, the water in the salt wedge is almost motionless. However, if the surface layer flowing toward the sea has a sufficiently high velocity, turbulent mixing can occur through a mechanism known as Kelvin-Helmholtz instability, which is a process where the AccessScience from McGraw-Hill Education Page 3 of 12 www.accessscience.com

WIDTH:BFig. 1 Diagrams of mixing in estuaries. (a) Salt-wedge type. (b) Partially mixed type. (c) Well-mixed type. (d) Fiord.

denser salt water is drawn up into the overflowing freshwater in a coherent “rolled-up” pattern. Ultimately these instabilities break down completely, and the salt water is mixed entirely into the overlying fresh-water mass, increasing the salinity of the upper layer, before eventually being discharged to the ocean. This and similar processes are sometimes referred to as entrainment, which results in a net loss of fluid from the salt wedge. Consequently, for this loss to be replaced, there must be a compensatory flow of salt water toward the head, or landward portion, of the estuary within the salt wedge, but of a magnitude much less than that of the flow in the surface layer. There is a considerable velocity gradient near the halocline as a result of the friction between the two layers. Consequently, the position of the salt wedge will change according to the magnitude of the flow in the surface layer, that is, according to the river discharge. The Mississippi River is an example of a salt-wedge estuary. When the flow in the Mississippi is low, the salt wedge extends more than 160 km (100 mi) inland, but with high discharge the salt wedge extends only a mile or so above the river mouth. Some bar-built estuaries, in areas of restricted tidal range and at times of high river discharge, as well as deltas, are typical salt-wedge types. AccessScience from McGraw-Hill Education Page 4 of 12 www.accessscience.com

Partially mixed estuaries. When tidal movements are appreciable, the whole mass of water in the estuary moves up and down with a tidal periodicity of about 12.5 h. Considerable friction occurs between the bed of the estuary and the tidal currents, and causes turbulence. The turbulence tends to mix the water column more thoroughly than occurs in salt-wedge estuaries, although little is known of the relationship of the exchanges to the salinity and velocity gradients. However, the turbulent mixing not only mixes the salt water into the fresher surface layer but also mixes the fresher water downward. This causes the salinity to decrease toward the head of the estuary in the lower layer and also to increase progressively toward the sea in the surface layer. As a consequence, the vertical salinity gradient is considerably less than that in salt-wedge estuaries. In the surface, seaward-flowing layer, the river discharge moves toward the sea; but because the salinity of the water has been increased by mixing during its passage down the estuary, the discharge at the mouth can be several times the river discharge. To provide this volume of additional water, the compensating inflow must also be much higher than that in the salt-wedge estuary. The velocities involved in these movements are only on the order of a few centimeters per second, but the tidal velocities can be on the order of 100 centimeters per second. Consequently, the only way to evaluate the effect of turbulent mixing on the circulation pattern is to average out the effect of the tidal oscillation, which requires considerable precision and care. The resulting residual or mean flow will be related to the river discharge, although the tidal response of the estuary can give additional contributions to the mean flow. The tidal excursion of a water particle at a point will be related to the tidal prism, the volume between high- and low-tide levels upstream of that point; and the instantaneous cross-sectional velocity at any time will be related to the rate of change of the tidal prism upstream of the section. In details, the velocities across the section can differ considerably. It has been found that in the Northern Hemisphere the seaward-flowing surface water keeps to the right bank of the estuary, looking downstream, and the landward-flowing salt intrusion is concentrated on the left-hand side (Fig. 2). This is caused by the Coriolis force, which deflects the moving water masses toward the right and is of increased importance for very wide estuaries. Of possibly greater importance, however, is the effect of topography, because the curves in the estuary outline tend to concentrate the flow toward the outside of the bends. Thus, in addition to a vertical circulation, there is a horizontal one, and the halocline slopes across the estuary. Because the estuary has a prismatic cross section, the saline water is concentrated in the deep channel and the fresher water is discharged in the shallower areas. Examples of partially mixed estuaries are the rivers of the Chesapeake Bay system.

Well-mixed estuaries. When the tidal range is very large, there is sufficient energy available in the turbulence to break down the vertical salinity stratification completely, so that the water column becomes vertically homogeneous. In this type of estuary there can be lateral variations in salinity and in velocity, with a well-developed horizontal circulation; or if the lateral mixing is also intense, the estuary can become sectionally homogeneous (also called a one-dimensional estuary). Because there is no landward residual flow in the sectionally homogeneous estuary, the upstream movement of salt is produced during the tidal cycle by salty water being trapped in bays and creeks and bleeding back into the main flow during the ebb. This mechanism spreads out the salt water, allowing salt to be maintained within the estuary, but it is probably an effective trapping mechanism for salt for only a small number of tidal excursions landward of the mouth. AccessScience from McGraw-Hill Education Page 5 of 12 www.accessscience.com

WIDTH:BFig. 2 Typical surface salinity distribution in Chesapeake Bay. Numbers indicate parts per thousand. (After H. E. Landsberg, ed., Advances in Geophysics, vol. 1, Academic Press, 1952)

Fiords. Because fiords are so deep and restricted at their mouths, tidal oscillation affects only their near-surface layer to any great extent. The amount of turbulence created by oscillation is small, and the mixing process is achieved by entrainment. Thus fiords can be considered as salt-wedge estuaries with an effectively infinitely deep lower layer. The salinity of the bottom layer will not vary significantly from mouth to head, and the surface fresh layer is typically no more than a few tens of meters deep. When the sill is deep enough not to restrict circulation, the inflow of water occurs just below the halocline, with an additional slow outflow near the bottom. When circulation is restricted, the replenishment of the deeper water occurs only occasionally, sometimes on an annual cycle and typically related to severe weather events. Between these replenishment episodes, the bottom layer can become anoxic, with very low dissolved oxygen (DO) levels.

The descriptive classification of estuaries outlined above depends on the relative intensities of the tidal and river flows and the effect that these flows have on stratification. A quantitative comparison between estuaries can be made using the diagram of Fig. 3, which is based on a stratification and a circulation parameter.

River plumes

The estuary represents the first stage of the blending of fresh river water into the sea. The water that is discharged from the estuary mouth typically has characteristics (such as salinity and temperature) that lie AccessScience from McGraw-Hill Education Page 6 of 12 www.accessscience.com

WIDTH:CFig. 3 Classification diagram for estuaries. An estuary appears as a line on the diagram; the upper reaches are less well mixed than the lower sections. Subscript letters refer to high (h) and low (l) river discharge; subscript numbers are distances from the mouth. J = James River; M = Mississippi; C = Columbia River; NM = Narrows of the Mersey; S = Silver Bay; JF = Strait of Juan de Fuca. (After D. V. Hansen and M. Rattray, Jr., New dimensions in estuary classification, Limnol. Oceanogr., 11:319–326, 1966)

between those of the fresh river water and seawater, due to mixing and entrainment processes within the estuary. The process of blending with the coastal ocean continues in a region known as a river plume. With regard to the mixing and blending processes, a river plume can be divided into two regions. Certain estuaries may not exhibit a well-defined near-field region; and in some estuaries with particularly low freshwater flow, there may be no evidence of a plume at all.

A near-field plume is often considered an extension of the estuary out into the coastal ocean. It can be broadly defined as the region where the velocity of the outflowing estuarine water (related to river velocity) is sufficient to dominate the physical dynamics of the system. In this region, the less dense estuarine waters flow in a lens near the surface, spreading laterally, as well as mixing vertically with the ocean water underneath. The typical extent of a near-field region is approximately 1–3 km (0.6–2 mi).

The far-field plume begins at the outer boundary of the near-field region, or at the mouth if velocities are not sufficient to generate a near-field region, and often extends several hundreds of kilometers down the coast. In this region, the estuarine water continues to be mixed into the surrounding ocean by wind- and wave-generated processes. Because of effects caused by the Earth’s rotation, a far-field plume will typically turn to the right outside of the river mouth and flow adjacent to the coast in the Northern Hemisphere or to the left in the Southern Hemisphere. AccessScience from McGraw-Hill Education Page 7 of 12 www.accessscience.com

The mixing in the near-field region can be much more intense than the wind-driven mixing in the far-field plume, but acts across a very small region, compared to the size of the far field. Thus, several recent studies have suggested that the total amount of mixing occurring in each of the two regions may be roughly comparable.

Flushing and pollution-dispersal prediction

Much research into estuarine characteristics is aimed at predicting the distribution of effluents discharged into estuaries. Near the mouth of a partially mixed estuary, the salinity of the estuarine water is very near that of the adjacent ocean water, implying that the incoming fresh river water has been diluted significantly by mixing with a much larger volume of ocean water. Consequently, estuaries are more effective than rivers in diluting and removing pollutants. It has been observed that increased river flow causes both a downstream movement of the saline intrusion and a more rapid discharge of water to the sea. The latter effect occurs because increased river discharge increases stratification; increased stratification diminishes vertical mixing and enhances the flow toward the sea in the surface layer. Thus, increased river discharge has the effect of increasing the volume of freshwater accumulated in the estuary, but to a lesser extent than the increase of the discharged volume. Obviously, it takes some time for the freshwater from the river to pass through the estuary. A rough estimate of the flushing time can be determined by dividing the total volume of freshwater accumulated in the estuary by the river flow. For most estuaries the flushing time is 5 to 10 days.

If a conservative, nondecaying pollutant is discharged at a constant rate into an estuary, the effluent concentration in the receiving water will vary with the tidal current velocity and will spread out by means of turbulent mixing. The concentrations will be increased during the next half cycle as the water passes the discharge point again. After several tidal cycles, a steady-state distribution will be achieved, with the highest concentration near the discharge point. Concentrations will decrease downstream but not as quickly as they do upstream. However, the details of the distribution will depend largely on whether the discharge is of dense or light fluid and whether the discharge is into the lower or upper layer. Since its movement will be modified by the estuarine circulation, the effluent will be more concentrated in the lower layer upstream of the discharge point, and it will be more concentrated in the upper layer downstream. To obtain maximum initial dilution, a light effluent would have to be discharged near the estuary bed so that it would mix rapidly as it rose.

For nonconservative pollutants, such as coliform sewage bacteria, prediction becomes more difficult. The population of bacteria dies progressively through the action of sunlight, and concentrations diminish with time as well as by dilution. The faster the mixing, the larger the populations at any distance from the point of introduction, since less decay occurs.

Because of the poor mixing of freshwater into a salt-wedge estuary, an effluent introduced in the surface layer will be flushed from the estuary before it contaminates the lower layer, provided that it is not too dense. AccessScience from McGraw-Hill Education Page 8 of 12 www.accessscience.com

Mathematical modeling

Increasingly, mathematical modeling is being used, with reasonable success in many instances, to predict effluent dispersal with a minimum amount of field data. Although the governing mathematical equations can be stated, they cannot be solved in their full form because there are too many unknowns. To reduce the number of unknowns, various assumptions are made, including some form of spatial averaging to reduce a three-dimensional problem to two dimensions or even one dimension. Mixing parameters, about which little is known, are assumed constant or are considered as a simple variable in space, and are altered so that the model fits the available prototype data.

The first step is usually to model the flow and salinity distribution. Because the density field is important in determining the flow characteristics, density and flow are interlinked problems. Then, for pollutant studies the pollutant is assumed to act in the same manner as fresh or salt water, or the flow parameters are used with appropriate mixing coefficients to predict the distribution. Simple models consider the mean flow to be entirely the result of river discharge, and tidal flow to be given by the tidal prism. Segmentation is based on simple mixing concepts and crude mixing ratios. Salinity and pollutant concentrations can then be calculated for cross-sectionally averaged and vertically homogeneous conditions by using the absolute minimum of field data. These models are known as tidal prism models. One-dimensional models are very similar, but use a finer grid system and need better data for validation. Two-dimensional models assume vertical homogeneity and allow lateral variations, or vice versa. There are difficulties in including the effects of tidally drying areas and junctions; the models become more costly and require extensive prototype data, but they are more realistic. The ideal situation of modeling the flow and salinity distribution accurately simply on the basis of knowledge about the topography, the river discharge, and the tidal range at a number of points is still a long way off.

Ecological environments

Estuarine ecological environments are complex and highly variable when compared with other marine environments. They are richly productive, however. Because of the variability, fewer species can exist as permanent residents in this environment than in some other marine environments, and many of these species are shellfish that can easily tolerate short periods of extreme conditions. Motile species can escape the extremes. A number of commercially important marine forms are indigenous to the estuary, and the environment serves as a spawning or nursery ground for many other species.

River inflow provides a primary source of nutrients such as nitrates and phosphates which are more concentrated than in the sea. These nutrients are utilized by plankton through the photosynthetic action of sunlight. Because of the energetic mixing, production is maintained throughout, in spite of the high levels of suspended sediment which restrict light penetration to a relatively thin surface layer. Plankton concentrations can be extremely high, and then, higher levels of the food web—filter-feeding shellfish and young fish—have an ample food source. The rich concentrations provide large quantities of organic detritus in the sediments which can be utilized by AccessScience from McGraw-Hill Education Page 9 of 12 www.accessscience.com

bottom-feeding organisms and which can be stirred up into the main body of the water by tidal action. For a more complete treatment of the ecology of estuarine environments from the biological viewpoint. See also: MARINE ECOLOGY.

The stratification present in estuaries tends to produce concentrated regions of detritus and microorganisms, which are attractive to other species as significant food sources. This can be particularly true where a region of strong stratification intersects the bottom of the estuary, a region known as a front. Higher populations of organisms, from juvenile fish to seals, can often be seen congregating near fronts.

There is a close relationship between the circulation pattern in estuaries and the faunal distributions. Several species of plankton peculiar to estuaries appear to confine their distribution to the estuary by using the water-circulation pattern; pelagic larvae of oysters are transported in a similar manner. The fingerling fish (Micropogon undulatus), spawned in the coastal waters off the eastern coast of the United States, are carried into the estuarine nursery areas by the landward residual bottom flow.

Sediments

The patterns of sediment distribution and movement depend on the type of estuary and on the estuarine topography. The type of sediment brought into the estuary by the rivers, by erosion of the banks, and from the sea is also important; and the relative importance of each of these sources may change along the estuary. Fine-grained material will move in suspension and will follow the residual water flow, although there may be deposition and reerosion during times of locally low velocities. The coarser-grained material will travel along the bed and will be affected most by high velocities and consequently, in estuarine areas, will normally tend to move in the direction of the maximum current.

Fine-grained material

Fine-grained clay material, about 2 micrometers in size, brought down the rivers in suspension can undergo alterations in its properties in the sea. Base (cation) exchange with the seawater can alter the chemical composition of some clay minerals; also, because the particles have surface ionic charges, they are attracted to one another and can flocculate. Flocculation depends on the salinity of the water and on the concentration of particles. It is normally complete in salinities in excess of 4 parts per thousand, and with suspended sediment concentrations above about 300 parts per million (1 mg∕liter), and has the effect of increasing the settling velocities of the particles. The flocs have diameters larger than 30 μm but effective densities of about 1.1 g∕cm,3 because of the water closely held within. If the material is carried back into regions of low salinity, the flocculation is reversed and the flocs can be disrupted by turbulence. In sufficiently high concentrations, the suspended sediment can suppress turbulence. The sediment then settles as layers which can reach concentrations as high as 300,000 parts per million and which are visible as a distinctive layer of “fluid mud” on echo-sounder recordings. At low concentrations, aggregation of particles occurs mainly by biological action. AccessScience from McGraw-Hill Education Page 10 of 12 www.accessscience.com

Turbidity maximum

A characteristic feature of partially mixed estuaries is the presence of a turbidity maximum. This is a zone in which the suspended sediment concentrations are higher than those in the river or farther down the estuary. This zone, positioned in the upper estuary around the head of the salt intrusion and associated with mud deposition in the so-called mud reaches, is often related to wide tidal mud flats and saltings. The position of the turbidity maximum changes according to changes in river discharge, and is explained in terms of estuarine circulation. Suspended sediment is introduced into the estuary by the residual downstream flow in the river. In the upper estuary, mixing causes an exchange of suspended sediment into the upper layer, where there is a seaward residual flow causing downstream transport. In the middle estuary, the sediment settles into the lower layer in areas of less vigorous mixing to join sediment entering from the sea on the landward residual flow. It then travels in the salt intrusion back to the head of the estuary. This recirculation is very effective for sorting the sediment, which is of exceedingly uniform mineralogy and settling velocity. Flocs with low settling velocities tend to be swept out into the coastal regions and onto the continental shelf. The heavier or larger flocs tend to be deposited.

The concentrations change with tidal range and during the tidal cycle, and fluid muds can occur within the area of the turbidity maximum if concentrations become sufficiently high. During the tidal cycle, as the current diminishes, individual flocs can settle and adhere to the bed, or fluid muds can form. The mud consolidates slightly during the slack water period, and as the current increases at the next stage of the tide, erosion may not be intense enough to remove all of the material deposited. A similar cycle of deposition and erosion occurs during the spring-neap tidal cycle. Generally, there is more sediment in suspension in the turbidity maximum than is required to complete a year’s sedimentation on the estuary bed.

Mud flats and tidal marshes

The area of the turbidity maximum is generally well protected from waves, and there are often wide areas of mud flats and tidal marshes (Fig. 4). These areas also exchange considerable volumes of fine sediment with sediment in suspension in the estuary. At high water the flats are covered by shallow water, and there is often a long stand of water level which gives the sediment time to settle and reach the bottom, where it adheres or is trapped by plants or by filter-feeding animals. The ebb flow is concentrated in the winding creeks and channels. At low water there is not enough time for the sediment to settle, and it is distributed over the tidal flats during the incoming tide. Thus, there is a progressive movement of the fine material onto the mud flats by a process that depends largely on the time delay between sediment that is beginning to settle and sediment that is actually reaching the bed. The tidal channels migrate widely, causing continual erosion. Thus, there is a constant exchange of material between one part of the marshes and another by means of the turbidity maximum. As the muds that are eroded are largely anaerobic, owing to their very low permeability, the turbidity maximum is an area with reduced amounts of dissolved oxygen in the water. AccessScience from McGraw-Hill Education Page 11 of 12 www.accessscience.com

WIDTH:BFig. 4 Aerial photograph of tidal flats showing the areas of pans, marshes, and vegetation between the channels, Scolt Head Island, England. (Photograph by J. K. St. Joseph, Crown copyright reserved)

Coarse-grained material

Coarser materials such as quartz sand grains that do not flocculate travel along the bed. Those coming down the river will stop at the tip of the salt intrusion, where the oscillating tidal velocities are of equal magnitude at both flood and ebb. Ideally, coarser material entering from the sea on the landward bottom flow will also stop at the tip of the salt intrusion, which becomes an area of shoaling, with consequent decrease of grain size inland. Normally the distribution of the tidal currents is too complex for this pattern to be clear. Especially in the lower part of the estuary, lateral variations in velocity can be large. The flood and ebb currents preferentially take separate channels, forming a circulation pattern that the sediment also tends to follow. The channels shift positions in an apparently consistent way, as do the banks between them. This sorts the sediment and restricts the penetration of bed-load material into the estuary.

Salt-wedge patterns

In salt-wedge estuaries, the river discharge of sediment is much larger, though generally markedly seasonal. Both suspended and bed-load materials are important. The bed-load sediment is deposited at the tip of the salt wedge, but because the position of the salt wedge is so dependent on river discharge, the sediments are spread over a wide area. At times of flood, the whole mass of accumulated sediment can be moved outward and deposited seaward of the mouth. Because of the high sedimentation rates, the offshore slopes are very low, and the sediment has a very low bearing strength. Under normal circumstances, the suspended sediment settles through the salt wedge, and there is a zonation of decreasing grain size with distance down the salt wedge, but changes in river flow seldom allow this process to occur. AccessScience from McGraw-Hill Education Page 12 of 12 www.accessscience.com

Fiord patterns

Sedimentation often occurs only at the heads of fiords, where river flow introduces coarse and badly sorted sediment. The sediment builds out into deltalike fans, and slumping on the slopes of the fan carries the sediment into deep water. Much of the rest of the fiord floor is bare rock or only thinly covered with fine sediment.

Bar-built estuary patterns

Bar-built estuaries are a very varied sedimentary environment. The high tidal currents in the inlets produce coarse lag deposits, and sandy tidal deltas are produced at either end of the inlets, where the currents rapidly diminish. In tropical areas, the muds that accumulate in the lagoons can be very rich in chemically precipitated calcium carbonate. Daniel G. MacDonald, K. R. Dyer

Bibliography

M. Barletta and A. R. A. Lima, Systematic review of fish ecology and anthropogenic impacts in South American estuaries: Setting priorities for ecosystem conservation, Front. Mar. Sci., 6:237, 2019 DOI: http://doi.org/10.3389/fmars.2019.00237

A. Paumier et al., Impacts of green tides on estuarine fish assemblages, Estuar. Coast. Shelf Sci., 213:176–184, 2018 DOI: http://doi.org/10.1016/j.ecss.2018.08.021

Additional Readings

P. Webb, Introduction to Physical Oceanography, Rebus Community, 2019