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They can have many different names, such as bays , harbors , lagoons , inlets , or sounds , although some of these water bodies do not strictly meet the above definition of an estuary and may be fully saline.

The Estuarine Ecosystem. ecology, threats and management

Many estuaries suffer degradation from a variety of factors including: sedimentation from soil erosion from deforestation , overgrazing , and other poor farming practices ; overfishing ; drainage and filling of wetlands; eutrophication due to excessive nutrients from sewage and animal wastes; pollutants including heavy metals , polychlorinated biphenyls , radionuclides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.

The word "estuary" is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus , meaning tide. There have been many definitions proposed to describe an estuary. The most widely accepted definition is: "a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with freshwater derived from land drainage".

A more comprehensive definition of an estuary is "a semi-enclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff; however the freshwater inflow may not be perennial, the connection to the sea may be closed for part of the year and tidal influence may be negligible". An estuary is a dynamic ecosystem having a connection to the open sea through which the sea water enters with the rhythm of the tides.

The sea water entering the estuary is diluted by the fresh water flowing from rivers and streams. The pattern of dilution varies between different estuaries and depends on the volume of fresh water, the tidal range, and the extent of evaporation of the water in the estuary. Drowned river valleys are also known as coastal plain estuaries. In places where the sea level is rising relative to the land, seawater progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley.

This is the most common type of estuary in temperate climates. The width-to-depth ratio of these estuaries is typically large, appearing wedge-shaped in cross-section in the inner part and broadening and deepening seaward. Examples of this type of estuary in the U.


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Bar-built estuaries are found in place where the deposition of sediment has kept pace with rising sea level so that the estuaries are shallow and separated from the sea by sand spits or barrier islands. They are relatively common in tropical and subtropical locations. These estuaries are semi-isolated from ocean waters by barrier beaches barrier islands and barrier spits. Formation of barrier beaches partially encloses the estuary, with only narrow inlets allowing contact with the ocean waters.

Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U. The barrier beaches that enclose bar-built estuaries have been developed in several ways:. Fjords were formed where pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are typically rocks, bars or sills of glacial deposits , which have the effects of modifying the estuarine circulation.

Fjord -type estuaries are formed in deeply eroded valleys formed by glaciers. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial moraines or rock bars form sills that restrict water flow.

The width-to-depth ratio is generally small. In estuaries with very shallow sills, tidal oscillations only affect the water down to the depth of the sill, and the waters deeper than that may remain stagnant for a very long time, so there is only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted, and there is a slow but steady exchange of water between the estuary and the ocean.

These estuaries are formed by subsidence or land cut off from the ocean by land movement associated with faulting , volcanoes , and landslides.

Ecosystems

Inundation from eustatic sea level rise during the Holocene Epoch has also contributed to the formation of these estuaries. There are only a small number of tectonically produced estuaries; one example is the San Francisco Bay , which was formed by the crustal movements of the San Andreas fault system causing the inundation of the lower reaches of the Sacramento and San Joaquin rivers. In this type of estuary, river output greatly exceeds marine input and tidal effects have a minor importance. Freshwater floats on top of the seawater in a layer that gradually thins as it moves seaward. The denser seawater moves landward along the bottom of the estuary, forming a wedge-shaped layer that is thinner as it approaches land.

As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example of a salt wedge estuary is the Mississippi River. As tidal forcing increases, river output becomes less than the marine input. Here, current induced turbulence causes mixing of the whole water column such that salinity varies more longitudinally rather than vertically, leading to a moderately stratified condition.

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Examples include the Chesapeake Bay and Narragansett Bay. Tidal mixing forces exceed river output, resulting in a well-mixed water column and the disappearance of the vertical salinity gradient. The freshwater-seawater boundary is eliminated due to the intense turbulent mixing and eddy effects. The lower reaches of Delaware Bay and the Raritan River in New Jersey are examples of vertically homogenous estuaries. Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of fresh water.

A salinity maximum zone is formed, and both riverine and oceanic water flow close to the surface towards this zone. Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine embayment to any of the other estuary types. The most important variable characteristics of estuary water are the concentration of dissolved oxygen, salinity and sediment load. There is extreme spatial variability in salinity, with a range of near zero at the tidal limit of tributary rivers to 3.

At any one point the salinity will vary considerably over time and seasons, making it a harsh environment for organisms. Sediment often settles in intertidal mudflats which are extremely difficult to colonize. No points of attachment exist for algae , so vegetation based habitat is not established. The problems ofestuarine pollution and management are thosedealing with a sheltered environment that actsas a trap for sediment and contaminants, andas areas subject to intense pressure frommankinds activities such as land-claim orreclamation.

Estuaries are the most protected habitat inthe United Kingdom and elsewhere meas-ured as the percentage of the total habitat thatis subject to protection orders , and for moststudents on aquatic biology are their mostaccessible marine habitat. While few UK,European, or American Universities haveships for access to the sea, many are located onthe shores of estuaries. Thus for manyUniversity courses, their local estuary is themarine habitat, which can be studied, whilethe open seas have to be studied only infilms or video.

Throughout the book exam-ples are drawn from Britain, Europe, andAmerica, as well as other areas, which havebeen well studied such as South Africa andAustralia. May we thank the following authors andpublishers for allowing us to use copies oftheir figures: Kai Olsen Ophelia; 3. The authors prepared other figures. Sources of information that are not mentionedin the text are listed in the reading list, whichhas been fully updated for this edition.

Estuaries have for long been important tomankind, either as places of navigation, or aslocations on their banks for towns and cities. Nowadays they are under pressure, either asrepositories for the effluent of industrialprocesses and domestic waste, or as primesites for land-claim to create sites for industryor urban development. Against this back-ground the biologist has been attracted toother functions of estuaries: Vital feedingareas for many species of birds, especiallywaders and wildfowl, like the locations ofcoastal fisheries or as fascinating areas presentchallenges to our understanding of how ani-mals and plants adapt to their environment.

The estuarine environment is characterized byhaving a constantly changing mixture of saltand freshwater, and by being dominated byfine sedimentary material carried into theestuary from the sea and from rivers, whichaccumulates in the estuary to form mudflats. The mixtures of salt and freshwater presentchallenges to the physiology of the animals,which few are able to adapt to.

In this book we shall examine estuaries bytrying to understand the interactions and feed-ing relationships, which make up the estuarinefood web.

Estuarine, Coastal and Shelf Science

Estuaries have been claimed to beamongst the most productive natural habitatsin the world, and we shall attempt to explainwhy they are so productive, and how theenergy produced is utilized by succeedingtrophic levels. Estuaries are transition zonesbetween rivers and the sea, which differfrom both in abiotic and biotic conditions. Temperature, salinity, and turbidity fluctuate. From a biotic viewpoint, estuaries arehighly productive ecosystems ranking at thesame level as coral reefs and mangroveswamps. An elevated productivity is main-tained because of high nutrient levels in bothsediment and water column.

Before we can examine the life of an estuarywe must first examine the physical and chem-ical features, which mould the estuarine envi-ronment. An estuary is a partially enclosedbody of water formed where freshwater fromrivers and streams flows into the oceans, mix-ing with the seawater. Estuaries and the landssurrounding them are places of transition fromland to sea, and from fresh to saltwater. Donald S.


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  • Print Save Cite Email Share. Show Summary Details. Subscriber Login Email Address. Library Card. View: no detail some detail full detail. Front Matter Title Pages Preface. Chapter 1 The estuarine environment. Chapter 2 Life in estuaries. Chapter 3 Primary producers: plant production and its availability. Chapter 4 Primary consumers: herbivores and detritivores. Chapter 5 The secondary consumers: carnivores. Chapter 6 Estuarine uses and users.