Substrate

Substrate

Most estuaries have soft, muddy substrates. These are derived from sediments carried into the estuary by both seawater and freshwater. Wind (Aeolian) transport of larger sand particles into the estuary is often significant in certain areas, particularly coastal lagoons behind barrier beaches. In the case of freshwater, rivers and streams carry silt particles in suspension. When these suspended particles reach and mix with seawater in the estuary, the presence of the various ions in the seawater causes the silt particles. These particles settle out, forming the characteristic mud bottom. Seawater also carries much suspended material. When it enters an estuary, the sheltered conditions reduce the water motion that has kept the particle in suspension. As a result, the particles get settled out and contribute to the forming of the mud or sand substrate. The relative importance of freshwater-borne or marine borne particles to the development of the muddy substrate varies from estuary to estuary and also geographically. In North America and Europe, the main sedimentary source is the sea.

The deposition of particles is also controlled by currents and the size of the particle. Large particle settle out faster than small particles in suspension. When strong currents prevail, the substrate will be course (sand or gravel), as only large particles can settle out;  where waters are calm and currents weak, fine silt will settle out. Thus, both seawater and freshwater drop their coarse sediments first, the former at the mouth of the estuary and the latter in the upper reaches of the river itself, so the area of mixing is dominated by fine silt.(mud), resulting from decreased water movement and flocculation from the intermixing of the water masses.

Catastrophic events may also play a significant role. Massive storms and their accompanying floods may produce large deposition or removal of sediment in estuaries, causing severe mortality of organisms.

The particles that settle out in the estuary are terrestrial and marine organic matters. As a result, the substrate that accumulates is very rich in organic material. This material may serve as a large food reservoir for estuarine organisms. The large surface area relative to volume of the very small particles means that there is a very large area for bacteria to inhabit. 

 
Last modified: Wednesday, 4 April 2012, 7:05 AM