Estuary
Definition of estuary:
A semi-enclosed embayment of the coast in which fresh run-off water mixes with saline water entering from the ocean.
This is the common definition for estuary, other definitions can be discussed in the article
|
This is the definition given by Cameron and Pritchard (1963) [1]. However, it excludes the tidal river upstream of the seawater intrusion limit, where tidal motion can still have important consequences for ecosystem, water quality and morphology – for example: import of marine sediments and organisms, generation of a high turbidity zone, sedimentation/erosion of intertidal wetlands.
Estuaries are characterized by the presence of tides and a prominent system of channels and shoals (open bays and enclosed coastal seas such as the Bohai Sea or North Sea therefore do not qualify as estuaries). According to this characteristic, estuaries can also be defined as
Definition of estuary:
Semi-enclosed inlet system where the tide propagates along channels.
The part of the estuary upstream of the seawater intrusion limit, but subject to tidal motion, is usually referred to as 'tidal river'. This is the common definition for estuary, other definitions can be discussed in the article
|
According to the first, more common definition, the tidal river is excluded from the estuary, but included according to the second definition. The second definition, which is closer to the etymological origin of the word estuary, includes tidal inlet systems without significant fresh water inflow, for example:
- drowned coastal valleys;
- tidal lagoons – drowned coastal plains semi-closed by a barrier system;
- inverse estuaries – estuaries where evaporation exceeds fresh water inflow.
From a morphological or ecological viewpoint, such inlet systems can be very similar to estuaries with small fresh water inflow according to the first definition.
Contents
[hide]Hydro-sedimentary dynamics
Estuaries can greatly differ in size, from tiny water bodies of order 1 km2 to basins of over 10,000 km2 (e.g. St. Lawrence estuary, Chesapeake Bay). Most of the small estuarine basins are more properly called coastal lagoons, because they are seasonally or infrequently open to the sea and can be hypersaline. Estuaries are widely diverse and can be classified according to different criteria:
tidal range
- microtidal: tidal range < 2 m;
- mesotidal: tidal range 2-4 m;
- macrotidal: tidal range > 4 m.
salinity [math]\delta = \Delta s / s[/math] (ratio of bottom-surface salinity difference and sea salinity)
- well-mixed: [math]\delta \lt 0.05[/math] (see Seawater intrusion and mixing in estuaries)
- partially mixed: [math]0.05 \lt \delta \lt 0.25[/math] (see Estuarine circulation)
- stratified: [math]\delta \gt 0.25[/math] (see Salt wedge estuaries)
morphology (see Morphology of estuaries)
- river-dominated: delta-shoal system;
- tide-dominated: funnel-shape channel system;
- wave-dominated: coastal barrier system.
The main drivers of estuarine hydro-sedimentary dynamics are tide, river and waves. The role of waves is strongest around the inlet zone, where wave-driven alongshore sediment transport can generate an inlet barrier (see Sand spit). However, also inside the estuary, locally generated waves influence morphologic evolution by stirring up tidal flat sediments. In meso-or macrotidal estuaries (and often even in microtidal estuaries) tidal currents are primary agents controlling water quality, salinity intrusion and dispersal of dissolved constituents, see Dispersion processes in estuaries. Tidal currents are also main drivers for sediment transport and play a major role in shaping estuarine morphology, see Morphology of estuaries and Tidal asymmetry and tidal inlet morphodynamics. River discharge can be as important as the tide for water quality and salinity intrusion by generating stratification and density driven residual currents, see Estuarine circulation. These circulations act together with tidal asymmetry in trapping sediments in estuaries, see Estuarine turbidity maximum. All of these processes create a very special and highly diverse habitat that provides the conditions for spawning, nursery and food supply that many marine animals depend on.
Ecology
As transition zones between rivers and seas, estuaries comprise a great diversity of habitat types ranging from terrestrial with an aquatic influence (salt and freshwater marshes) to tidally dominated pelagic and benthic habitats, consisting of subtidal and intertidal areas with different bed structures, different sediment composition and different salinity regimes. Receiving high inputs of energy and nutrients from ocean, land and atmosphere, estuaries play a significant role in biogeochemical cycles. Important nutrient cycling processes in estuaries include nitrogen fixation, denitrification, uptake, remineralization, and sequestration in plants and sediments, see Nutrient cycling. Although species diversity is typically low, estuaries are very productive ecosystems, characterized by high abundances of species adapted to the specific estuarine conditions of strongly varying salinity, turbidity (light), temperature and currents and alternating submergence-emergence in intertidal zones, see Estuarine ecosystems.
Typical estuarine habitats are discussed in several Coastal Wiki articles:
- Intertidal mudflats and salt marshes in temperate climate zones
- Seagrass meadows in shallow waters of temperate and (sub)tropical coastal areas
- Mangrove forests in tropical and subtropical regions
Human interventions
Most of the world’s ports and many coastal cities are on estuaries, which have been heavily modified to support human activities. Shorelines have been altered by the installation of docks, jetties, and hard barriers such as levees, seawalls, and tidal gates. Marshes and shoals have been embanked and converted to farmland or filled and converted to hard structures such as airports, roads, and residential and industrial areas. This has resulted in extensive losses of estuarine habitat as well as increasing risk of shoreline erosion. Land use in the watershed can have substantial impacts on an estuary. Extensive agriculture and urban development demand freshwater, cause erosion, and increase peak flow during storms. Diversion of freshwater from the watershed and impoundment of water behind dams reduce the rate and dampen the seasonal pattern of freshwater flow into estuaries. In addition, rivers and estuaries can be starved of their sediment supply by trapping behind dams. See Threats to the coastal zone.
Although still rather sparse, there are also initiatives to restore parts of estuaries to their original state, see Restoration of estuarine and coastal ecosystems, Principles of conservation, rehabilitation and restoration of estuarine and coastal habitats and US National Estuary Program.
Articles on estuaries
- Estuarine ecosystems
- Seawater intrusion and mixing in estuaries
- Estuarine circulation
- Salt wedge estuaries
- Morphology of estuaries
- Tidal asymmetry and tidal inlet morphodynamics
- Estuarine turbidity maximum
- Estuarine morphological modelling
- Physical processes and morphology of synchronous estuaries
- Tidal rivers
- US National Estuary Program
Reference
- Jump up ↑ Cameron, W. M. and D. W. Pritchard (1963) Estuaries. In M. N. Hill (editor), The Sea, Vol. 2. John Wiley & Sons, New York, pp. 306–324
Please note that others may also have edited the contents of this article.
|