13.6: Estuaries

As freshwater flows into the oceans, it mixes with seawater. Regions where this mixing occurs are called “estuaries.” The seaward limit of an estuary is where the dilution of ocean water by freshwater is insignificant. The landward boundary, or head, of the estuary is the maximum landward limit of saltwater movement. Estuaries are present at mouths of major rivers and in many other semi-enclosed inlets or arms of the ocean into which streams and rivers flow. If freshwater discharge is very large, the seaward boundary of the estuary can be many kilometers offshore. The inland boundary can be tens or even hundreds of kilometers inland. The boundaries are not fixed, and can move seaward if freshwater flow rate increases or farther inland if it decreases.

The fact that most major cities are located on estuaries reflects the historical importance of estuaries as harbors and ports. Estuaries are also important to marine ecosystems because many species of fishes and invertebrates spend part of their life cycle in the oceans and part in an estuary or river, and many major fisheries and shellfisheries are in estuaries.

Estuaries differ in characteristics that include their length, width, depth, tidal range, freshwater flow rate, shape, and coastal character. These factors affect ocean and river water movements and mixing processes in the estuary. Consequently, there is no simple description of a typical estuary, and no single classification system can capture the many variations. Every estuary is different from all others and has its own unique behavior. However, estuaries are classified into general groups according to the geological processes that formed their embayment or, alternatively, according to their water circulation and mixing characteristics.

Geological Origin of Estuaries 

Almost all present-day estuaries were formed as sea level rose in the past 19,000 years from a low point approximately 130 m below current sea level (Fig. 6-18). As the most recent ice age ended and glacial ice melted, sea level rose. River and glacial valleys were flooded and coasts were modified by erosion, forming barrier islands in some locations (Chap. 11).

Estuaries are filled progressively with sediment supplied by the river. If sea level were to remain stable long enough, most estuaries would eventually become filled with sediment and deltas would form (Chap. 11). The inexorable process of sediment filling can be seen at locations such as the former Greek port of Ephesus, now in Turkey. This city, a bustling seaport less than 3000 years ago, is today many kilometers inland, behind a low coastal plain formed by the accumulation of sediment eroded from surrounding mountains (Fig. 13-17).

Figure 13-17. The ancient Greek city of Ephesus, now in Turkey. In ancient Greek times, Ephesus was a port city. The waterfront was just at the foot of the hill behind the temple in the center of this photograph. The ruins of the port are now several kilometers inland. In the background, you can see the wide flat coastal plain. This was once a large bay, but since ancient times it has filled with sediments.

In the past, during periods when sea level was falling, there were few estuaries. As sea level falls, a coast consists of newly exposed continental shelf. This new coastal plain is relatively featureless because the topographic lows in the shelf floor are filled with sediment transported by tides, waves, and currents. Estuaries will virtually disappear again when sea level falls in the future.

Rising sea level has created estuaries that differ according to the character of land that has been inundated. Four types are recognized: coastal-plain estuaries, bar-built estuaries, tectonic estuaries, and fjords. Coastal-plain estuaries were formed as sea level rose to flood river valleys (Fig. 13-18a). These estuaries, often called “drowned river valleys,” are especially abundant on passive margins, such as the east coast of the United States. Chesapeake Bay, Delaware Bay, and the New York Harbor are examples of coastal-plain estuaries. The Mississippi River delta is an example of a former coastal-plain estuary that has been filled with river-borne sediment.

Figure 13-18. The rise of sea level during the past 19,000 years has been a major factor in the creation of all present-day estuaries. Estuaries have a variety of geological settings and origins. (a) The most common estuary is a coastal-plain estuary (also sometimes called a drowned river valley) that forms as rising sea level floods the mouth of a river. (b) Some estuaries form where longshore drift builds a bar, spit, or barrier island that isolates a bay from free exchange with the ocean. (c) Tectonic estuaries are formed where a block of the Earth’s crust is lowered by tectonic processes at earthquake faults. (d) Fjords are created when rising sea level floods the steep-sided valleys that are cut by glaciers and exposed after the glaciers retreat.

Bar-built estuaries are formed when a sandbar is constructed parallel to the coastline by wave action and longshore drift (Chap. 11), and the bar separates the ocean from a shallow lagoon (Fig. 13-18b). Lagoons behind barrier islands are bar-built estuaries. Examples include Albemarle Sound and Pamlico Sound in North Carolina. Some estuaries have mixed characteristics. For example, the Hudson River estuary that passes through the New York Harbor and Raritan Bay is primarily a coastal-plain estuary, but the Sandy Hook spit gives Raritan Bay certain bar-built estuary characteristics (Fig. 13-19).

Figure 13-19. This satellite image of the Hudson River basin–New York Harbor–Raritan Bay estuary shows the complex character of this estuary. The Hudson River valley is a glacier-cut valley that was then shaped by a river and so can be considered a coastal-plain estuary. Parts of the river are steep-sided and deep enough that they may even be considered a fjord. Sandy Hook Bay and Jamaica Bay are bar-built. Raritan Bay and New York Harbor could be considered either coastal-plain estuary segments or bar-built, or both.

Tectonic estuaries are formed when a section of land drops or tilts below sea level as a result of vertical movement along a fault (Fig. 13-18c). Such estuaries are most often present on coasts along subduction zone or transform fault plate boundaries. The best-known tectonic estuary in North America is San Francisco Bay (Fig. 4-23), which is on a transform plate boundary.

Fjords are estuaries in valleys that were cut by glaciers when sea level was lower (Fig. 13-18d). These estuaries are generally steep-sided, both above and below sea level, and are often deep. Many fjords have a shallow sill near the mouth that was formed by sediment deposited at the lower end of the glacier when it flowed through the valley. Fjords are common on the Norwegian coast, the southwest coast of New Zealand, the Patagonia coast of Chile, and the Pacific coast of Canada and southern Alaska. Puget Sound in Washington State is also a fjord.

Estuarine Circulation 

In estuaries, less dense river water flows over seawater, causing vertical mixing between the two layers. The movements and mixing of freshwater and seawater are affected by many factors, including tidal currents and mixing, wind-driven wave mixing, shape and depth of the estuary, rate of freshwater discharge, friction between the moving freshwater and seawater layers and between the water and seafloor, and the Coriolis effect (CC12). Because some or all of these factors vary among estuaries, between seasons within individual estuaries, and from one part of an estuary to another, estuaries have a wide variety of circulation patterns.

Estuarine circulation is extremely important because many major cities are located on estuaries. These cities discharge large quantities of wastes, particularly sewage treatment plant effluents and storm water runoff, into the estuaries (Chap. 16). Estuarine circulation patterns determine the fate of these contaminants.

In many estuaries, circulation is altered by piers and other port structures, dredging of navigation channels, filling of wetlands, and construction of levees and other coastal structures (Chap. 11). These alterations can affect life cycles of marine species that inhabit or transit the estuary (Chap. 16).

Because circulation within each estuary is complex, varies temporally, and is unique to that estuary, detailed, multiyear physical oceanography studies of each estuary are necessary to understand its circulation. Such studies are difficult and expensive, particularly in large, complex estuaries such as Chesapeake Bay or San Francisco Bay.

Types of Estuarine Circulation

Estuaries can be classified according to the major characteristics of their circulation. The major types are salt wedge estuaries, partially mixed estuaries, well-mixed estuaries, fjord estuaries, and inverse estuaries

Salt Wedge Estuaries

In a salt wedge estuary, freshwater flows down the estuary as a surface layer separated by a steep density interface (halocline) from seawater flowing up the estuary as a lower layer that forms a wedge-shaped intrusion (Figs. 13-20a, 13-21a). Vertical mixing across the steep density gradient is slow in salt wedge estuaries, but the velocity difference between seaward-flowing river water and the underlying seawater creates friction between these layers. The friction causes turbulence and internal waves at the interface. The internal waves grow, and when they break, small quantities of high-salinity water are injected and mixed into the upper layer, which progressively increases in salinity toward the estuary mouth (Figs. 13-20a, 13-21a). Almost no freshwater is transferred into the lower seawater layer.

Figure 13-20. One way to classify estuaries is based on their circulation characteristics and the factors that influence their circulation. (a) Salt wedge estuaries are river-dominated and are strongly stratified with a sharp halocline. (b) Partially mixed estuaries have a weak halocline because the surface and bottom layers are partially mixed by turbulence induced by tidal currents. (c) In well-mixed estuaries, the water column is thoroughly mixed from surface to seafloor, and there is no halocline. However, salinity does increase progressively with distance from the head of the estuary to the sea and across the estuary as a result of the Coriolis effect.

Figure 13-21. Types of estuaries categorized by circulation pattern. (a) Salt wedge estuaries are characterized by a net (after tidal motions are averaged out) landward current in the lower layer and a net seaward current in the upper layer. There is little vertical mixing, but breaking internal waves on the halocline mix some seawater up into the surface layer. (b) Partially mixed estuaries are characterized by relatively strong net landward currents near the seafloor and relatively strong net seaward currents in the lower salinity (fresher) surface layer. (c) Well-mixed estuaries have no vertical salinity variations, but they do have variations in salinity both across and down the length of the estuary. The Coriolis effect causes net estuarine currents to flow landward at all depths on the right side of the estuary in the Northern Hemisphere (viewed from the ocean), and seaward flow at all depths on the left side. These currents are on the reverse sides in the Southern Hemisphere. (d) Estuaries in fjords have little current in the deep water below the depth of any sill. In the upper layers above the sill depth, there is generally a salt wedge or sometimes a partially mixed estuary circulation pattern.

Salt wedge estuaries are most likely to be present where freshwater input is relatively large and allows a thick, freshwater surface layer to develop. Estuaries that are narrow in relation to their depth also favor the development of a thick freshwater layer. Other conditions that favor salt wedge estuaries are small tidal range, and hence limited tidal currents and tidal mixing, and limited vertical mixing due to wind-induced motions (e.g., waves, Langmuir circulation, and Ekman transport). To understand how these factors favor the two-layer salt wedge configuration, compare the estuary with a blender filled with water and cooking oil. If the blender runs at high speed, the cooking oil is quickly mixed with and dispersed in the higher-density water. If the blender runs more slowly, the oil will tend to remain in a distinct upper layer, particularly if there is enough oil to create a thick oil layer. A thin oil layer will mix and disperse at a lower blender speed than a thick oil layer.

Partially Mixed Estuaries

Vertical mixing between freshwater and seawater is greater in partially mixed estuaries than in salt wedge estuaries, and therefore the density gradient between the two layers is much less pronounced (Fig. 13-20b). The most important process that causes vertical mixing across an estuarine halocline is the friction and resulting turbulence created by reversing tidal currents. Hence, partially mixed estuaries are most common where tidal currents are relatively fast, river flow rate is moderate, and river current speed does not greatly exceed tidal current speed. As in a blender running at moderate speed with roughly equal volumes of water and oil, some mixing occurs at the interface, but two distinct layers persist.

In partially mixed estuaries, freshwater and seawater layers are separated by a relatively weak halocline. Seawater moves landward up the estuary as a bottom layer and is diluted progressively with freshwater from above (Figs. 13-20b, 13-21b). Both layers move up and down the estuary with each tidal cycle. The distance that water moves in the estuary with each ebb and flood is the tidal excursion. The tidal excursion is usually much greater than the landward or seaward net movement that occurs as a result of the residual currents of the estuarine circulation (currents caused by the river flow and landward flow of seawater). Figure 13-21 shows only these residual currents. Residual currents are often difficult to measure because they are masked by the faster reversing tidal currents that must be averaged out to reveal the residual current.

As seawater moves landward and freshwater moves seaward in an estuary, the moving water is deflected by the Coriolis effect. Consequently, if we look at an estuary from the sea in the Northern Hemisphere, seawater moving up the estuary tends to be deflected to our right, and freshwater moving down the estuary tends to be deflected to our left. In salt wedge estuaries, this deflection causes the halocline to be tilted slightly upward toward the right bank (Fig. 13-21a). In partially mixed estuaries, the Coriolis deflection affects not only the mean flow, but also tidal currents. As tidal currents flow into the estuary, they are deflected to the right side (as viewed from the sea); as they ebb, they are deflected to the left side. Consequently, in a partially mixed estuary, movement of seawater landward in the lower layer is concentrated on the right side, and the lower-salinity estuarine outflow is concentrated on the left side. The halocline is strongly inclined upward to the right (as viewed from the sea) in many partially mixed estuaries. The incline is in the opposite direction in the Southern Hemisphere.

Well-Mixed Estuaries

In estuaries with swift tidal currents, mixing is very strong (like the blender at high speed), the water column is completely mixed, and no halocline is present (Fig. 13-20c). In these well-mixed estuaries, seawater moving landward is continuously mixed with freshwater moving seaward. Salinity decreases progressively from the ocean toward the head of the estuary. Although the salinity is uniform from surface to bottom, it varies during the tidal cycle, increasing as the tide floods and decreasing as it ebbs.

In large well-mixed estuaries, the Coriolis effect tends to separate the landward and seaward estuarine flows, so they are concentrated on opposite sides of the estuary. This separation is especially important in wide and shallow estuaries. Many such estuaries have a small residual landward movement of water along the right side (as viewed from the sea in the Northern Hemisphere), and a small residual seaward current on the left side. This is made possible by a residual current that flows from one side of the estuary to the other (Fig. 13-21c). This circulation causes salinity to be higher on the right side of the estuary and to gradually decrease across the estuary to the left side (Fig. 13-20c).

Well-mixed estuaries tend to be wide and relatively shallow. They usually have limited freshwater inputs and strong tidal currents. These factors and others that determine the circulation characteristics change with location in the estuary and sometimes with time. Consequently, an estuary’s circulation can vary from well mixed, to partially mixed, and to salt wedge at different times and at different locations within the estuary. San Francisco Bay is a good example. The central portion of the bay (closest to the ocean) has relatively strong tidal currents, is wide, and is generally partially mixed. The upper part of the estuary, which is closer to the Sacramento and San Joaquin rivers, is narrower and generally has salt wedge characteristics because tidal currents are somewhat reduced and freshwater inputs are large enough to form a thick, low-salinity surface layer. The southern part of San Francisco Bay is shallow, has very little freshwater input, and is generally well mixed. When river flow rates are low, seasonally or in drought years, the northern part of the estuary becomes partially mixed. During high spring river runoff, the southern and central parts of the bay can become partially mixed or even salt wedge estuaries.

Fjord Estuaries

Fjords are generally narrow and usually much deeper than most other estuaries. Most fjords are deep enough that vertical mixing does not reach the bottom waters, even if tidal currents are strong. Consequently, almost all fjords have a halocline separating high-salinity bottom water from lower-salinity surface water, and fjords are almost never well-mixed.

Circulation in many fjords is complicated by the presence of a shallow sill at the seaward end of the estuary that prevents the free exchange of deep water between ocean and fjord (Fig. 13-20d). Therefore, estuarine circulation is established only above the level of the sill. The fjord’s interior above the sill depth behaves as a salt wedge or partially mixed estuary. At the sill, vertical mixing is enhanced by turbulence caused by the flow of water over the sill (Fig. 13-21d). The deep water below the sill depth within the fjord is stagnant and is not involved in the estuarine circulation. In many fjords, the deep water becomes anoxic because of the decomposition of organic particles that have settled from above. Periodically this deep water may be displaced by high-salinity ocean water that floods over the sill in response to some unusual set of water movements. In such circumstances, hydrogen sulfide in the fjord bottom water is displaced into surface waters of the fjord and ocean, where it may cause extensive fish mortality.

Inverse Estuaries

Shallow estuaries in arid regions can sustain sufficiently high rates of evaporation that salinity is higher than that of ocean water outside the estuary. In this situation, estuarine circulation is inverted. High-salinity estuarine water flows seaward in a bottom layer, and ocean water flows landward as a relatively lower-salinity (and less dense) surface layer. Inverse estuaries generally occur at about 30°N or 30°S, in the region of atmospheric subtropical highs, where evaporation is strong and rainfall typically is low. Some well-known examples of inverse estuaries are the Red Sea, San Diego Bay in California, and Laguna Madre in Texas. In addition, the Mediterranean Sea behaves as though it were an exceptionally large inverse estuary.

Particle and Contaminant Transport in Estuaries

Most major cities have developed next to rivers and estuaries, partly because of their convenience for the disposal of sewage and other wastes. The common belief is that wastes discharged to a river or estuary are simply swept out to the ocean, where they are diluted in the vast volume of ocean water. However, a simple explanation of water movements and suspended-particle transport within estuaries reveals that this is not what happens.

Freshwater input at the head of the estuary creates the estuarine circulation. To sustain this circulation, the amount of seawater that flows through the estuary must greatly exceed the volume of freshwater discharged during the same period of time. To understand this process, we must first observe that freshwater entering the head of most estuaries has a salinity close to zero, whereas water discharged from the estuary mouth into the ocean has a salinity almost equal to that of the seawater. Freshwater mixes with seawater, so the salinity of the seaward-moving water progressively increases along the length of the estuary (Figs. 13-21, 13-22).

Figure 13-22. Seaward-moving surface water increases in salinity from nearly zero at the head of the estuary to 30 where it enters the ocean. Ocean water entering the estuary has a salinity of 33. Where seaward-flowing water reaches a salinity of 16.5 (half that of ocean water), it must consist of one-half river water and one-half ocean water. Seawater must flow landward to point A at a rate (volume per unit time) equal to the river flow rate, and seaward transport of low-salinity estuarine water must equal twice the river flow rate. Similarly, the landward flow rate of seawater at point B must equal 4 times the river flow rate, and the seaward flow rate of low salinity estuarine water must be 5 times the river flow rate.

Figure 13-22 depicts the relative residual flow rate (the volume of water that passes a particular point in the estuary per day) in various parts of an estuary. From the figure, we can see that the residual flow rate of seawater moving landward and the residual flow rate of estuarine water flowing seaward at the mouth of an estuary are both many times greater than the river flow rate. In addition, the seaward and landward flow rates are highest near the mouth of the estuary and decrease toward the head of the estuary. The residual flow rate is equal to the residual current speed multiplied by the cross-sectional areas of the estuary. Consequently, flow rate does not always translate directly into current speed, because the cross-sectional area of an estuary changes, sometimes in complex ways, along its length. However, the large differences in flow rate along an estuary generally cause both landward and seaward residual currents to be higher near the mouth of the estuary than near the head.

Rivers carry large quantities of suspended sediment particles, the largest of which are quickly deposited when the river reaches the flat coastal plain or, if there is no coastal plain, when the river enters the head of an estuary. The finest particles could remain in suspension as they are transported to the oceans, but the increased salinity in the estuary causes most of them to clump. Because the clumps are larger, they are then deposited.

Particles of intermediate size, which fall slowly through the water column, are usually deposited within the estuary. In well-mixed estuaries, these particles are deposited primarily on the left side (as viewed from the sea in the Northern Hemisphere). In other estuaries, seaward-moving particles fall through the water column into the landward-moving bottom layer, where they are transported back toward the head of the estuary. These particles often are deposited near the estuary’s head where tidal and residual currents are low, often accumulating as a shoal (Fig. 13-21a,b). However, when elevated river flows extend the head of the estuary seaward, the particles may be resuspended by the swifter river currents. In many estuaries, these recycled particles are eventually deposited in wetlands (Chap. 11).

Sediments also can be transported from the ocean into estuaries, particularly in estuaries where tidal currents are strong enough to resuspend sediments that accumulate on the nearshore continental shelf.

Substantial quantities of suspended sediment can be transported from the ocean into well-mixed estuaries, where they accumulate along the right side (as viewed from the sea in the Northern Hemisphere). Smaller quantities of suspended sediment are carried from the ocean into partially mixed and salt wedge estuaries. Even particles carried into the ocean by the seaward-moving surface layer may return to the estuary if they sink and are transported landward in the seawater layer.

The fate of particles in an estuary depends on the particle size and where the particles are introduced to the estuary. Floating and extremely small particles introduced to the surface layer (or to the left side of a well-mixed estuary) are transported to the oceans, although many will clump at higher salinities. Small or low-density particles in bottom waters are transported landward until the current slows and they are deposited. Such particles may accumulate in the estuarine sediments or in the sediments of adjacent wetlands, and they may be resuspended and carried seaward when river flow rates are high. Particles of intermediate size in the surface layers of salt wedge or partially stratified estuaries are carried seaward until they sink below the halocline, then are transported landward until they settle out in the upper parts of the estuary where current speeds are lower. Large particles tend to accumulate near the point of introduction.

The important point of the preceding discussion is that suspended particles tend to be retained within the estuary by the estuarine circulation and are not simply flushed into the open ocean. Because most toxic metals and organic compounds attach to particles, particularly the organic particles in treated sewage effluents, they tend to accumulate in the estuary and are not flushed out to the ocean.

A lack of understanding of estuarine circulation and its effect on particles and the fates of toxic metals and organic compounds is one reason that many estuarine ecosystems have been and continue to be severely impacted by human activities. One of many examples of the misuse of estuaries for waste disposal is in San Francisco Bay, where navigation channels, marinas, and ports must be dredged to maintain the desired depths. Most of the dredged sediments were, for decades, and some still are, dumped near the bay’s mouth at a site between Alcatraz Island and San Francisco (Fig. 13-23). The swift tidal currents were expected to flush the material out to the ocean, where it would be dispersed. However, the dump site is near the right bank (viewed from the sea) of a partially mixed estuary, and the dumped dredged sediments instead fall to the floor of the bay and disperse in the lower part of the water column.

Figure 13-23. This image of San Francisco Bay, taken by the Aqua/MODIS satellite, shows the high turbidity in the upper estuary, where there is a salt wedge circulation, and the location of the Alcatraz dredged-material dump site near the estuary mouth, where the circulation is partially mixed.

With an understanding of estuarine circulation, we realize that much of the dredged material dumped at the Alcatraz dump site, with its toxic load, is not transported to the ocean. It is instead transported back within San Francisco Bay by estuarine circulation and eventually is deposited in low-energy areas, including the same channels and harbors from which it was originally dredged. Perhaps more importantly, continual dredging and disposal disturbs the particle-associated toxic metals and organic compounds, which were relatively safe when buried in sediments, and resuspends them in the estuarine circulation, where they are more directly in contact with the biota (Chap. 16). Fortunately, the most contaminated dredged material is no longer dumped at this dump site.

Estuarine Biology

Estuaries are difficult places for organisms to inhabit because they are subject to rapid and irregular changes in environmental factors such as salinity, temperature, and current speed. The stress induced by salinity changes is particularly challenging because organisms must be able to cope with widely varying osmotic pressures (Chap. 14).

Because estuaries are high-stress environments, they support fewer species than the adjacent ocean or freshwater environments support (Fig. 13-24). However, there is an abundant supply of nutrients and sunlight in most estuaries. As a result, estuarine biomass is typically much greater than biomass in freshwater and ocean environments, with the exception of the most productive coastal upwelling areas.

Figure 13-24. The number of species of marine, estuarine, and freshwater origin varies within the salinity gradient of a typical estuary. The fact that the total number of species is much lower in the brackish-water zone than in the freshwater or the marine zones reflects the stressful environmental conditions, particularly variable salinity, in the brackish-water region.

Although estuaries typically have relatively high turbidity, they are also generally shallow, and in many, light penetrates to the bottom. In such estuaries, primary production is dominated by benthic microalgae, particularly diatoms, that form mats that carpet the sediments. Macroalgae can grow on the estuary floor in areas where current speeds are low. In addition, rooted aquatic plants are abundant in the tidal salt marshes and mangrove forests along the edges of many estuaries. In contrast, because phytoplankton are continually being swept seaward with the estuarine surface layer flow, their populations are relatively limited in estuaries that have short residence times.

Nutrients are generally abundant in estuaries because they are supplied in substantial quantities in river water that has leached them from soils and rocks of the drainage basin. In addition, once in the estuary, nutrients tend to be retained and recycled within the estuarine circulation. Nutrient-containing detritus, which flows seaward in the surface layer, sinks below the halocline and then returns with the landward-flowing ocean water. This circulation tends to concentrate detritus in the region near the landward limit of seawater intrusion (Fig. 13-25), which has low residual currents. This region has high turbidity as a result of the accumulated detritus and other particles. The nutrient-rich detritus supports a varied community of fish and invertebrate larvae and juveniles.

Figure 13-25. In many estuaries, an accumulation of particulate matter creates high turbidity near the landward extent of seawater intrusion. Particles transported seaward in the upper layer may sink to the lower layer before leaving the estuary and then return to the high-turbidity area in the landward-flowing lower layer. Many planktonic estuarine organisms use this pathway to stay within the estuary or to reach an appropriate point in the estuary at the right time in their life cycle. The maximum primary productivity in an estuary is usually just seaward of the high-turbidity zone. In this region, more light can penetrate. In addition, nutrients are available because they are regenerated by the decomposition of suspended detritus particles transported up the estuary in the lower layer and are then mixed into the surface water at the turbidity maximum. Maximum zooplankton populations and growth occur downstream of this area to take advantage of the phytoplankton drifting seaward in the upper layer.

In many estuaries, much of the detritus that accumulates in the upper part is derived from algal mats from the adjacent wetlands (Chap. 11). Where residual currents are slow, or the estuary is long, a food chain from phytoplankton to zooplankton to estuarine fishes is established that is analogous to the food chain in the coastal upwelling ecosystem described previously in this chapter. Zones of high phytoplankton abundance, high zooplankton abundance, and high fish abundance form a seaward progression from the head of the estuary (Fig. 13-25). Some planktonic organisms and larval fishes that are being transported seaward in the estuary’s upper layer and away from the region of abundant food supply sink into the seawater layer and are transported back into the food-rich region.

Because they provide abundant food and substantial protection, estuaries are ideal habitats for the larvae and juvenile stages of marine animals. Particularly in wetlands, there are many shallow-water hiding places that are accessible to small swimming organisms but not to their larger predators. In addition, predators are less common in estuaries than in less stressful freshwater or ocean environments. Many marine fishes and invertebrates use estuaries as nursery grounds. Some species briefly visit the estuary to spawn, and others release vast numbers of eggs in the adjacent ocean so that they are transported into the estuaries by the estuarine circulation. Along the southeast coast of the U.S., more than half of the commercially important marine fish species are known to use estuarine wetlands as nursery areas or for breeding. Because of the abundant supply of detritus, estuaries also support huge populations of commercially important shellfish, including clams, oysters, mussels, and crabs.

Anadromous and Catadromous Species

Estuaries are important to two categories of migratory fishes that live most of their lives either in the oceans or in freshwater. Anadromous fishes, such as salmon and striped bass, live most of their lives in the ocean but return to freshwater to spawn. The stress involved in the return to freshwater and the energy lost in spawning are such that the adults die soon after they spawn and do not return to the ocean. Their offspring develop in freshwater and migrate to the estuary, which provides abundant food and relative safety from predators. Eventually, they migrate out to sea, where they live several years before returning to the rivers to spawn.

Catadromous fishes use a strategy opposite that of anadromous fishes. Catadromous species live most of their lives in freshwater and then journey to the oceans to spawn and die. Freshwater eels are the best-known and most abundant catadromous species. Sexually mature eels in rivers on both sides of the North Atlantic Ocean make a one-way journey to spawn in the southwest part of the Sargasso Sea. Their offspring return to the rivers, riding part of the way on the Gulf Stream. North Pacific eels make a similar journey to the equivalent southwest area of the North Pacific Gyre, and their offspring use the Kuroshio (Japan) Current for the ride home.

Anadromous and catadromous species do not generally spend much time in the estuary during their reproductive migration, but transiting the estuary is especially stressful because they must make the osmotic-pressure adjustment from seawater to freshwater, or vice versa, very quickly. Striped bass, for example, rest in the outer part of the estuary to adjust to lower salinity before moving farther upstream. Many estuaries also have relatively high concentrations of toxic metals and organic compounds that are likely to cause additional stress on anadromous and catadromous species as they transit the estuary. Such contamination by toxins may account, in part, for the decline of these species in some areas.