In the average estuary into which a stream debouches, there are definite relationships between the freshwater microbes and those derived from the ocean. The fresh water from the stream flows over the salt water from outside which forms what is known as a ‘salt wedge’ as it tapers upstream along the bottom (Fig. 17). In floods, the fresh water pushes the salt water out to sea, and steepens the gradient of the salt wedge. When there are strong offshore winds, the salt water moves into the stream and may even push up several miles. During freshets, the freshwater microbes, especially the phytoplankton, move downstream and tend to take over the estuary. With strong onshore water movement, the marine flora moves upstream, especially in the lower waters of the salt wedge, and may for a time establish itself there. Where there is a sill between the sea and the estuary, the ocean water with its flora will cascade into the estuary, and may establish itself in the hypolimnion or lower stratum of water and remain there indefinitely, blooming when conditions are favourable, e.g. when mixing occurs. In lagoons with a small run-off and no strong fresh streams, évaporation from the surface may make the top layer of water heavier than the bottom layer and the water mixes from top to bottom of the lagoon. This also mixes the flora and the nutrients of both layers, and we often get a phytoplankton bloom.
KeywordsStream Flow Continental Shelf Salt Water Estuarine Environment Organic Detritus
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