Pei-Yuan QianChapter 8: 328-369

Outline of Lecture

Definition Classification of estuaries Physical characteristics of estuary

environment Animal adaptation Food-web of estuaries

Definition

Estuary is a semienclosed coastal body of water that has a free connection with the open sea and within which seawater is measurably diluted with freshwater. The decrease in salinity in an estuary precludes those species incapable of volume regulation. A critical salinity of 5 to 8‰ precludes species incapable of extensive regulation of specific inorganic ion concentrations.

Estuary is coastal bays in which freshwater and salt water mix, thus permanent impoundments

Pearl River Estuary

Rivers Discharge (m3 s-1)

Amazon 175,000Congo 39,640Orinoco 33,950Yangtze Kiang 22,000Brahmaputra 19,200Mississippi 17,800Yenissei 17,800Lena 16,300Parana 14,900Mekong 14,900St. Lawrence 14,160Irrawady 13,560Pearl River 12,500Ob 12,200Ganges 11,600

Rivers Discharge (m3 s-1)

Tocantins 11,000Aur 10,300MacKenzie 9,710Columbia 7,960Indus 7,550Magdalena 7,500Zambezi 7,080Danube 6,530Yukon 6,220Niger 6,090Uruguay 5,500Ogoove 4,710Huang Ho 3,900Sepik 3,800Frazer 3,540

Biologically productive, together with salt marshes and mud flat

Nursery ground for fisheries

Migration pathways for many important commercial fisheries

A link for global geochemical cycling of elements between terrestrial water and oceans

Gateways of inputs of both natural and anthropogenic substance (nutrients, metals, and contaminants)

Entrance for riverine sediments from land to the sea

Estuarine delta regions often become most populated cities in the world, Shanghai, San Francisco, New York

Significance of estuaries

Classification of estuary Based on the origination

1. Coastal plain estuary

2. Tectonic estuary

3. Semienclosed bay or lagoon

4. Fjord

Classification of estuary

Delaware Bay

Chesapeake Bay

Coastal plain estuaries on US east coast

Coastal plain estuary -- formed at the end of the last ice age when the rising sea level invaded low-lying coastal river valleys. Most common estuaries, broad & shallow, found along the northern and central Atlantic coast, e.g. Chesapeake Bay, Pearl River estuary in China.

Tectonic EstuaryTectonic estuary

-- the subsidence of the land in response to crustal movements allows the sea water reinvading, e. g. the San Francisco Bay.

Semienclosed bay or lagoon – Sand bars build up parallel to the coastline and partially cut off the waters behind them from the sea. Common in USA, such as Matagorda and San Antonio Bays

Semienclosed bay or Lagoon

Fjord -- Valleys have been deepened by glacial action and are then invaded by the sea. Very common in British Columbia.

Fjord

Freshwater from river

Seawater from ocean

The general pattern of fw and sw mixing in an

estuary

Well-mixed estuary

Stratified estuary

Typical estuary

Salinity changes in estuary

High Tide River Flow Decrease

515202530 10

5101520 0

River Flow IncreaseLow Tide

or

or

Estuarine Front

Estuarine Plume Riverine Plume

Riverine Front

Ebb-LLW

River

Salt Wedge

Estuarine Plume

Flood-HHW

Oceanic Water

Entrainment

Nutrient-rich water

Classification of estuaryBased on physiograph Positive (highly stratified) estuary -- freshwater flows

downstream over a deeper layer of higher salinity ocean water. Highly stratified estuaries only exist where river flow strongly dominates over tidal motion.

Negative estuary -- exist where tidal flow strongly dominates over river flow.

Neutral estuary -- vertically homogeneous estuary--At low tide, the salinity is dominated by downstream river flow whereas at high tide the inrush of SW determines the salinity. Only estuaries of very small extent permit such domination by tidal motion.

Seasonal estuary -- Seasonal variation in rainfall (or tropical storms and hurricanes) increase freshwater drainage and shift downstream lines demarking constant surface salinity (isohalines). During dry seasons isohalines shift upstream.

Classification of estuary

FW input>E

FW input<E

Entrainment: a process of vertical mixing between two layers of water which flow at different velocity in the water column.

Riverine Plume: a surface layer of water which flows out of the river channel into the estuary, usually distinct in color (brown), and bounded by the riverine front.

Front – an interface or zone between two different water bodies which move toward each other.

Riverine Front: a front where the river outflow meets with saline water, usually distinct in color, debris gathering line

Salt Wedge – A body of saline water invading the river channel underneath the freshwater flow

Estuarine Plume – a surface layer of water which flows out of the estuary into the open region of the coastal ocean.

Estuarine Front: a front where the estuarine plume meets with oceanic water. This front is much wider than the riverine front

Physical processes in an estuary

This is the riverine front (color front) between the riverine plume and seawater.

Riverine plume

Salt water

Riverine front

Physical characteristics

The freshwater derives from land drainage and tends to float as a low-density surface layer over denser seawater tidal mixing can reduce or obliterate this stratification.

Large amount of nutrient input from land Salinity decreases toward land Restricted exchange allows rapid changes in salinity,

temperature, nutrients, and sediment load Muddy substrata Depleted oxygen in substrata Limited wave action, highest current velocities

occur in the middle of channels

Effects of River outflow on coastal oceans

Stabilization of the water column: mixing increase the stability of water column(stratification & mixing)

Sedimentation: turbidity due to suspended solids reduces light sedimentation affects coastal benthic communities

Nutrients & primary productivity: high nutrient input leads to high productivity

River delta formation or erosion

Sedimentation: sediment particles sinking to the bottom of the estuarine bed or coastal ocean bottom

Hypoxia of near bottom water Muddy substrata High current velocities, different water masses (including

different layers) flow in different directions or changing directions due to tidal cycles, river discharge and winds

Pollutants

Physical characteristics

Variability in salinity due to: seawater heavier than

freshwater; periodical tide events; the Coriolis effect

that causes a moving body to the deflected from a straight

path.

River Outflow Estuarine Plume

Salt Wedge Oceanic Water

Nutrients (N, Si, DOM)

TurbidityLight penetration

Chl aSalinity

A summary of physical-biological coupling processes in an estuary

0 5 10 15 20 25 30 35

Freshwaterspecies

Stenohaline species

Marine species

Euryhaline marine species

Brackish water

species

Transitional

Salinity

Nu

mb

er o

f sp

ecie

s

Features of faunal composition in estuary

Stenohaline marine animals, usually restricted to the mouth of estuaries, salinity > 25, as they are unable or barely able to tolerate salinity changes.

Euryhaline marine animals, more tolerable to salinity change between 10-30, few down to 5, and so can be found throughout the range of estuarine salinities

Brackish estuarine species: true estuarine animals found in salinity between 5-18, not found in freshwater and not in full seawater.

Freshwater species: can not tolerate salinities > 5, are restricted to upper reaches of estuaries

Transitional species: migratory fishes that pass through the estuary

Features of faunal composition

Faunal composition Marine – the largest in terms of numbers of

species, restricted to the mouths in salinity >25

Freshwater – restricted to the upper reaches in salinity <5

Brackish water – the middle reaches of the estuary in salinity between 5-18

Limited in the number of large plants, seagrasses, macroalgae and diatoms can be common

Diatoms are dominant phytoplanktons but composition and abundance varied with tubulence, turbidity, and flushing rate

Terms Osmoregulators: the organisms have

physiological mechanisms to control the salt content of internal fluids (ture esturine species) Move water move ions Adjust internal water-ion balance

Osmoconformers: the organisms unable to regulate its internal fluid and salt balance, therefore ones with a varying internal salt concentration – tolerate fluctuations in salinity without tissue damage (ture marine species)

Biotic responses to the estuarine gradient

Two transitions in estuaries: The critical salinity: This region encompasses an

approximate salinity range of 5-8‰ and marks a pronounced minimum of benthic invertebrate species richness. A relatively rich fauna of bivalve mollusks and other invertebrates reside in freshwater. Freshwater species decrease, however, in numbers at a maximum salinity of

The mouth and lower reaches of the estuary: organisms must adjust physiologically to lowered salinity. Salinity can change from fresh to completely marine in these area. If salinity is tidally regulated, benthic organisms may experience fresh and saltwater in a single tidal cycle--more of a physiological challenge because of the time require for acclimation. Seasonal estuaries show seasonal shifts up- and down-estuary, but the rate of salinity change at any point is slow--permitting acclimation.

Adaptation in distribution pattern

Species richness generally diminishes steadily up-estuary and reaches a minimum at salinity of 5-8‰. Species richness then increases again in freshwater.--Estuarine marine bivalves are also rare at this salinity but increase steadily in species richness with increasing salinity. The steady decrease in the estuary must be related to

the steady reduction of species capable of extensive cell volume regulation.

Changes in physiological adaptations Changes in body size and genetically determined

morphological features. In many cases, the maximum size of bivalve mollusks decreases with decreasing salinity--may simply be related to the negative role of decreased salinity in growth.

Adaptation in distribution pattern(con’t) An increase in niche breadth as competitors disappear in the

decreased salinity.The diminution of species richness in brackish water is

accompanied by niche expansion of those species capable of invading the estuary or surviving in a brackish sea.

A related phenomenon to niche expansion is the presence of enormous populations of a relatively few species--may be due to lack of competitors and the nutrient enrichment of estuaries. Large fluctuating populations of phytoplankton, invertebrates, and fishes dominate the estuary.--rich invertebrate fisheries

Adaptations to avoid transport in the surface layer to the open sea. Estuarine flow results in a net transport of surface water to the open sea--dilute the larval populations of estuarine species. In many species, larvae are adapted to stay near the bottom during ebb tide and rise up into the water during the flood--counteract the seaward dilution effect.

Adaptation in distribution pattern(con’t)

Genetic divergence from open marine conspecifics. Sharp differentiation (changes in gene frequencies) has been found over a short geographic space (from the mouths of estuaries to brackish water seas). Selection may act on variation at individual loci, but it is probable that estuarine populations have diverged broadly into genetically distinct races relative to their open marine conspecifics. Consequently, fine-scale adaptation to local estuarine conditions has resulted.

Adaptation in distribution pattern(con’t)Many fish species spawn offshore but spend some

period feeding in estuaries (estuaries are therefore crucial to many species as nurseries. The abundance and fluctuating nature of the food supply have favored the following characteristics of the feeding ecology of juveniles of common marine fishes: flexibility of feeding habits in time and spaceomnivoresharing a common pool of food resources among speciesexploitation of food chains at different levels by the same

speciesontogenetic changes in diet with rapid growthshort food chains based on detritus-algal feeders

Animal’s adaptation

Nutrient inputs River input--freshwater drainage delivers large

amounts of nutrients in dissolved and particulate form Oceanic import --Dissolved nutrients arrive with deep-

water flow into estuaries. Vertical mixture with the surface water permits the phytoplankton to use this nutrient source; benthic algae and sea grasses may also benefit.

Regeneration from the bottom -- Bottom sediments consist of an active microbial community whose ability to decompose particulate organic matter permits the continuous recycling of nutrients between the bottom and the overlying water.

Food-web of estuary

Food-web of a typical estuary

Characteristics of food webs in estuaries

Highly dynamics--The extensive variation of size, nutrient input, and tidal exchange among estuaries suggests a large variation in the pattern of connections between primary and secondary production.

Detritus is a major factor in the webs, and detritus pathways dominate webs.

Detritivorous are the major consumers in many webs.

Phytoplankton and seaweeds may still play important roles in the webs.

Relative short food-chains.