Substrate

Substrate is an element determining the sort of marine organisms encountered in an area. In general there are two types of substrate.

Hard substrates

Hard substrates (rocks, boulders, pebbles, hard surfaces) which consists of rocky surfaces of the coast and boulders and pebbles in the coastal fringe. Man made structures like harbour constructions or off - shore constructions are called secondary hard substrates, they also include ship wrecks and floating objects (ships, boats, buoys etc). In some cases hard substrate can be created by plants or animals with the capacity to accumulate Calcium carbonate (CaCO₃). This kind of substrate is called biogenic and is a very good example of the way that organisms can affect the environment by changing some of its its major features.

Soft substrates

Soft substrates , sediments consisting of grains of different size and mainly categorised as sand,silt or clay, according to size.



Sediments are classified according to the size and size distribution of their grains in the categories described in Figure 9 after the Wentworth classification Scheme.



Most sediments do not fall into a single category, but are characterized by a mixture of grains of different size. In this case they are represented either descriptively, using the names of the dominant parts (silty sandy etc.) or by means of mathematical indices such as mean grain size. Another way to present grain size mixtures is the use of triangular diagrams.


Sediment composition is closely related to water movement. In general the higher the water velocities the larger the grain diameter it can remove. The underlying processes are called erosion and deposition. That is why in exposed beaches it is mainly sand and pebbles that are found, while in more protected, less exposed beaches there are varying amounts of mud as well. Below the zone affected by waves there are usually high percentages of silt and clay, unless there are strong water currents.

Redox potential

Another important parameter characterizing marine sediments is the redox regime; this is related to the concentration of oxygen dissolved in the water occupying the space between the grains (interstitial water). This regime is described by the redox potential (Eh) measured in mV. In general oxidizing conditions (high Eh values up to +400 mV) mean high oxygen concentration while reduced conditions (low Eh down to -200 mV) reveal a lack of oxygen. Since most animal species need oxygen, it follows that low Eh sediments are poor in fauna.

Redox potential is affected by the following parameters.

• sediment grain size The larger their diameter the better the water circulation in the sediment and consequently the easier the oxygenation. Thus fine material (silt, clay) should be expected to show lower Eh values than sandy sediments.
• organic content Decomposition of organic content by bacteria leads to fast oxygen consumption. As a result sediments with high organic content usually have extremely low Eh values.
• oxygen concentration in sea water above the sediment If the oxygen content of the sea water is low then its ability to supply oxygen to the interstitial water is limited, hence the Eh values cannot be other than low.

Figure 11 shows the change of redox potential in relation to depth from the sediment surface in two sediment types. In both cases there is a brown layer where oxidising conditions prevail, deeper down there is a black layer where reduced conditions prevail and in-between the two there is a thin grey layer where the redox potential changes rapidly in a rather small distance. This grey layer is called the "Redox Potential Discontinuity" layer (RPD). Here it is easy to see that in fine sand sediment the RPD is closer to the surface than in coarse sand, because the penetration of oxygenated water is obviously more difficult in fine grain sediments.