Marine Biology

Wave action

Estuaries are surrounded by land on there sides. This means the water distance over which a wind can blow to create waves is minimal, atleast with respect to the oceans. Since the height of waves depends on the fetch or the open water distance over which the wind can blow, a small area of water can generate only small waves. The shallow depth of the water in most estuaries also precludes development of large waves. The mamowness of the mouth of the estuary, coupled with the shallow bottom, rapidly dissipate the effects of any waves enter in the estuary from the sea. As a result of these processes, estuaries are generally places of calm water. Reduced wave action also promotes the deposition fine sediments, allowing the development rooted plants and further stabilizing these sediments.

Currents in estuaries are caused primarily by tidal action and river flow. Currents are generally confined to channels, and within these channels, velocities upto several knots can occur. The highest velocities occur in the middle of channels, where the frictional resistance from the bottom and side banks is lowest. Although the estuary is an overall place of sediment deposition, the channels where currents are concentrated are often areas of erosion. Whenever currents change position new channels are quickly eroded and old channels filled. This is particularly true in the intertidal areas where a natural cycle of erosion and deposition occurs. However, in most estuaries, deposition exceeds erosion, so there is a net accumulation of silt. In intermittent estuaries, where the mouth is often closed off from the sea during the dry part of the year, water movement is severely reduced at that time, leading to stagnation, reduced oxygen content, algal blooms and fish kills. For most estuaries, there is a continual input of freshwater at the head. A given amount of this freshwater moves down the estuary, mixing to a greater (or) lesser degree with seawater. A volume of this water is eventually discharged from the estuary or evaporated to compensate for the next volume introduced at the head. The time required for a given mass of freshwater to be discharged from the estuary is the This time interval can be measure of the stability of the estuarine system. Long flushing times are important to the maintenance of estuarine plankton communities.