Rationale behind main divisions

Table 4  Rationale behind the main divisions adopted in the primary habitat matrix (EUNIS levels 2 and 3)
Rock, Sediment
A primary distinction is made between communities which develop on hard substrata (epibiota) and those which can develop in soft sediments (infauna). Sediments can support distinctive epibiota as well as infauna. The term rock is used in a broad sense to indicate hard substrata such as bedrock, boulders, stable cobbles, artificial substrata and biogenic substrata. Sediments also include pebbles and cobbles which are essentially mobile (shingle) or may have a small proportion of stones and shells on the surface, supporting epibiota. Where biogenic substrata develop on substantially sediment substrata, they are included in the sediment section of the classification.
Littoral, Sublittoral (Infralittoral, Circalittoral)
These represent the major divisions in a vertical gradient from the terrestrial environment to the edge of the continental shelf (about 200 m depth). The main factors which control the zonation are immersion, thermal stability, light, wave action and salinity. They interact in a complex manner to produce a general zonation pattern, applicable to both rock and sediment habitats throughout Europe and beyond. Table 5 illustrates the inter-relationship of the factors for each zone, and Figure 1 provides a typical schematic profile of this zonation pattern.
High energy rock, Moderate energy rock, Low energy rock
These are defined on an energy gradient, reflecting exposure to wave action or tidal currents, or a combination of both (note, this energy gradient was reflected in the 1997 classification, but expressed as 'exposure'; the resulting confusion with wave exposure has now been removed). This energy gradient is broadly paralleled in sediment habitats, where coarse clean sediments occur in high energy conditions and fine muds occur in low energy conditions. Although the effects of wave action and tidal currents can be significantly different, there are many instances where the increase in tidal current strength in wave-sheltered habitats gives rise to communities similar to those found on more wave-exposed coasts but in reduced tidal currents. For example, increased currents in the infralittoral zone change the kelp Laminaria saccharina communities of very wave-sheltered sites to Laminaria hyperborea communities similar to those on open, more wave-exposed coasts. Very strong tidal currents in the circalittoral appear to override the effect of wave action to a large extent, giving rise to a suite of associated communities of barnacles, cushion sponges and the hydroid Tubularia indivisa which are less obviously affected by wave action. These communities are similar in character to those of surge gullies which are subject to extreme wave action.
Coarse sediments, Sands, Muds, Mixed sediments, Macrophyte communities on sediments, Biogenic Reefs
The particular sediment grade, typically derived from the hydrodynamic conditions of the site, strongly influences community structure. The four main divisions adopted here reflect major changes in species character, particularly related to the amount of silt or clay in the sediment. In addition, some sediments support communities of macrophytes (angiosperms and seaweeds) which attach to small stones and shells on the sediment surface, whilst on others biogenic reefs develop in which a particular species aggregates to form a stable surface upon which other species can live. With both macrophyte and biogenic reef communities the underlying sediment may support infaunal communities according to the particular sediment type; however the prominent character of the epibiota communities has led to a preference to group such biotopes under these separate major categories.