This has several results. The water level in the estuary fluctuates regularly unlike that of river. For this reason the habitat that is covered at high tide and uncovered at low tide is a prominent one in the estuary, and has no analogue in any purely freshwater ecosystem.
Further, the salinity is exceedingly variable and may change by a factor of ten over the course of a day at any location. At low tide, most of water passing through the estuary in fresh-water, and the salinity is correspondingly low.
At high tide, most of the water may be of marine origin, and the salinity correspondingly high. Flow may be somewhat stratified, or mixing may be incomplete, and the salinity at any time may vary considerably from one location to another.
The intensity of current, and hence of the degree of mixing, is a function of the intensity of the tides and of the-rivers flow rate. In other words, an estuary is an exceedingly variable environment.
Further, salinity of estuaries remains highest during the summer and during periods of ought, when less fresh-water flows into the estuary. It is lowest during the winter and spring, when rivers and streams are discharging their peak loads.
No stenohaline organism (such as Echinodermata, Cephalopodan, and other mollusks) could hope to survive in the estuaries. Likewise, no organism that could not tolerate strong currents and the turbid water that results from strong currents could live there.
The temperatures in estuaries fluctuate considerably diurnally and seasonally. Waters are heated by solar radiation and inflowing and tidal currents. High tide on the mud flats may heat or cool the water, depending on the season. The upper layer of estuarine water may be cooler in winter and warmer in summer than the bottom, a condition that, as in a lake, will result in a spring and autumn overturn.
In Cochin backwater, which is a tropical estuary, Gopinathan et al., (1974) have found that its hydrological parameters fluctuate three times in the year. During the pre-monsoon season (February- May) a stable condition exists in the estuary without any vertical gradients (stratifications) in salinity or temperature.
However, during monsoon season (June-September) there is considerable influx of fresh-water from precipitation and land run-off and during post- monsoon season (October-January), there is reduction in the discharge of fresh-water and the earlier brackish condition is restored by the incursion of sea water.
The tidal cycles, upwelling, monsoon piling and sinking in the Arabian Sea, also influence the seasonal pattern of the hydrological conditions of the back water. The seasonal and tidal cycles cause changes in nutrient concentration in the estuary which are found to be related with seasonal and spatial variation of phytoplanktons.
Some common planktons of this estuary are diatoms (Skeletonema costatum), dinoflagellates (Prorocent- rum micans and Ceratium furca), silicoflagellates (Diclyocha fibula and Destephanus speculum), chlorophvceans (Spirogyra), desmids (Euastru n, Cosmarium, Closterium and Micrasterias) and blue- green algae (Trichodesmium theibautii, Oscillatoria sp., and Meres- mopedia), all of which exhibit seasonal fluctuations.
Anyhow, all estuaries have high productivity. As estuary’s high productivity is exceeded among aquatic communities only by coral reefs. The reason is that although an estuary may be a harsh environment in some ways, it is not in others.
Because the typical estuary is shallow and turbulent, the amount of dissolved oxygen tends to be fairly high although it may be low in bottom layers of those estuaries where water does not mix from top to bottom.
More important, the tidal action acts to accumulate (concentrate) the nutrient and energy materials that wash in from upstream or, in some cases that enter from the nutrient rich bottom waters of the sea.
The mechanism of nutrient concentration is very simple-particulate nutrient material enters the estuary at its upper end, is carried seaward by the falling tide and is brought back through the estuary the rising tide, and so on for several cycles. The length of time takes from a nutrient particle to traverse the estuary is substantially greater than it would take for it to traverse a similar length of even the most slowly flowing river.
Thus, the estuary acts as a nutrient trap, with an average nutrient level significantly high then the river or the sea that it connects. Likewise, the concentration of energy-rich organic materials remains high in estuaries.
The primary result of the concentration of nutrients and fixed carbon is a very high level of production within the detritus food chain. This has two aspects. First the nutrient material is broken down bacterially at a very high rate, and recycled back into soluble form.
This allows a very high rate of gross primary production for estuarine plants. More important, perhaps, the amount of nutrient- rich organic detritus allows a level of productivity for detritus-eating animals much higher than could be maintained on the basis of the primary production by estuarine plants.
Biotic Communities of Estuaries:
Carikker (1967) has classified the regions of estuaries into the head, where fresh-water enters the estuary, upper, middle and lower reaches with increasing range of salinities and the mouth with salinity nearly equal to the sea. He has also classified the animals inhabiting the estuarine region into oligohaline (0.5 to 5 per cent), mesohaline (5 to 18 per cent) in the upper reaches, polyhaline (18 to 25 per cent) in the lower reaches.
25 to 30 per cent in the middle and euhaline (30 to 50 percent) in the mouth of the estuary. Krishnamoorthy (1963) in fact reported that the extent of penetration of the polychaets, Marphysa gravelyi, Diopatra variables, Cymene insecta, Loimia medusa, Glycera embranchiata, Onuphis ercmita into the Adyar estuary in Madras from the Bay of Bengal vary with varying salinities at constant temperatures.
Furthermore an estuary being a transitional zone between the fresh-waters and seas is an ecotone and, therefore, typical estuarine forms are unique in their habitat. Thus, the estuarine community is a mixture of three components: the marine, the fresh-water and the brackish water.
However, the diversity of both fresh-water and marine components reaches a maximum. Total diversity of the estuarine community is lower than that of either of the more normal environments. The number of interaction between species is not high and some estuarine populations may fluctuate greatly in size.
The plants of the estuary are of four basic sorts: phytoplankton marginal and marsh vegetation, mud-flat algae, and epiphytic plants-growing on the marginal marsh vegetation. Because of the turbid water found in estuaries, phytoplankton are normally uncommon.
However, in Hooghly-Matla estuarine system in West Bengal, Gopalakri; hymen (1971) has reported an abundance of phy- toplanktonic forms—several species of diatoms, Synedra, Navicula, Rhizostoma, Fmgilaria, Asteriobella, Biddulphia, PlanktonieIla, Hcrrn- discuss Chactoceros Cyclotella, Stephanodiscus, Triceratium; several species of green algae like Pediastrum, Spirogyra, Eudorina, Tribo- rtma, Closterium, Zygonema, Pandorina, Volvox, Clilorclla ; and blue-green algae like Microcystis, Oscillatoria, Anabaena, and Tri- chodesmus. Most estuarine algae are of marine origin. Further, the most significant estuarine plants are marsh grasses such as Sparirm.
Salicornia, and Scirpus, as well as some submerged filamentous algae such as Cladophora, Chara and Enteromorpha. Very few animals feed on these plants directly, but a very large amount is consumed as detritus.
The estuarine animal communities include zooplanktons and other animals. For example, the planktonic animal forms of Hoogly-Matla estuary of India include flagellate protozoans such as Euglena, Ceratium, Peridinium, Noctiluca; other protozoans such as Difflugia, Arcella, Vorticella , rotifer species such as Brachionus, Keratella, Asplanchna, etc., copepods like Diaptomus, Pseudodiap- tomus, Cyclops and Paracalanus; cladocerans of the genus Bosmina, Bosminopsis, Ceriodaphnia, Moina, besides a number of isopods (Gopalakrishnan, 1971).
All these forms remain confined to water and lead a pelagic existence. The best known estuarine animals are detritus feeders like oysters (Ostrea sp. of Lake Chilka), clams, lobsters, and crabs. Several insect larvae, annelid worms, mollusks, enter the estuary from the fresh-water, while, most marine phyto- plankton, Crustacea, annelid worms, anemones and Bxyozoa enter a estuary from marine ecosystems.
For example, out of 130 species of fishes and 30 species of prawns of Hoogly-Matla estuary, several fishes such as Hilsa, Harpodon, Mugil, Trachvurus, etc., migrate from sea in to the mouth of the estuary, to form important fish catches, while some like palaemonid prawns are fresh-water inhabitants and come to live in estuaries.
Adyar estuary of Madras coast is found rich in invertebrate and vertebrate fauna (Aiyar and Panikkar, 1937). The common invertebrates of this estuary are sea anemones (e.g., Phytocoetes gangeticus, Phytocoeteopsis ramunnii.
Stephensonaetis ornata, Pelo- coetes exul, Boloceractis gopalai, etc.); hermaphrodite nereids (e.g., Lycastis indica) ; the tubicolous polychaet Diopatra variables ; the burrowing polychaet Marphysa gravelyi, the oligochaet Pontodrilus bermudensis ; the mysids Rhopalophthalmus egregius and Mesopo- dopsis orientalis ; the crabs Uca annulipes ; Uca triangularis, Neptu- nus pelagieus, Scylla serrata, Veruna litterata, Sesarma tetragonum ; the hermit crabs Clibanarius olivaceus and Clibanarius padavensis, and several molluscs (e.g., Ostraea arkanensis.
Modiolus undulatus, Mzretrix casta, Cuspidaria cochinensis, Stenothyria blanfordiana, Amnicola stenothyroides, Potamides cingulatus, Styliger gopalai and Bursatella leachii).
The vertebrate fauna of Adyar estuary includes the fishes like Etroplus maculatus, Etroplus suratensis, Acentrogobius ridipunctatu3, Acentrogobius neilli, Panchax parvus, Aplocheilus miastigma, and mud skipper Periophthalmus koelreutcri and snakes such as Matrix piscator and Cerberus rhyncops.
In short, the estuarine ecosystem is a complex and interesting one. It is also very vulnerable environment, because it has served as conduits for shipping and as sites for cities throughout the history man. The organisms that inhabit an estuary are adapted to a vastly changing environment, but because estuaries are strategically located, man-made alterations to their ecosystems are substantially greater than in almost any other ecosystem.