Current and salinity of estuaries are complex. The current results from the interaction of one direction flow. The flow varies with the season and the rainfall. The salinity of estuaries varies horizontally and vertically. As a result of low salinity fresh water has a lower density than sea water. The water level in the estuaries fluctuates regularly as a result of tidal action.

The salinity is exceedingly variable and may change by a factor often over the course of a day at any location. At low tide the salinity is correspondingly low but at high tide the salinity is high. But the salinity of estuaries remains higher during summer and during the periods of drought. The temperature fluctuates diurnally and seasonally. The main subsystems of estuaries are as follows:

(I) Shallow Water Production Zone:

The producers are sea-weed or sea-grass beds, algae mats and small marshes. As they are rich in diverse types of producers, these remain programmed for year around photosynthesis. These are three types of producers such as:

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(a) the macrophytes (sea-weeds, sea-grass and marsh grasses) (b) benthic microphytes and (c) phytoplanktons.

(II) Sedimentary Subsystem:

These are present in deeper channels and lagoons in rich respiration exceed production. The nutrients are regenerated, recycled, stored and vitamins and growth regulators are manufactured. The consumers are often variable in their feeding habits.

(III) Plankton and Nekton:

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These keep on producing, converting and transporting nutrients and energy while responding to diurnal, tidal and seasonal. The halo plankton comprises relatively few species.

Estuaries are highly productive as compared to adjacent rivers or sea because of the following reasons.

(a) Less salinity than the ocean.

(b) Rooted plants supported in shallow water.

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(c) Easy access to deep sea.

(d) A high concentration of nutrients origi­nating from the seas as well as land.