The coast (littoral shores) is defined as marine habitats where saline (>30 S) or brackish (0,5–30 S) water washes over the ground at high tide or in the surf. On sheltered shorelines, littoral shores are mainly delineated by tidemarks. Along exposed shorelines, where ocean spray or splash reach varying distances in windy weather, the upper limit of the littoral shore can be much farther inland than the high-tide mark (supralittoral zone), and the lower limit may likewise be somewhat below the low-tide mark. The coast also includes saline and brackish pools above the spring high tide limit, together with the littoral fringe, where sea tar lichen (Verrucaria maura) grows. The width or extent of the littoral shore at any given location is determined by the difference in height between high and low tides, in addition to exposure and the littoral slope. The coast in Iceland has an estimated cover of 1,008 km2, including islands, skerries, and beach ridges. Beach ridges are typically narrow ridges of land above the high-tide mark that are shaped and strongly affected by the adjacent ocean.
How often and how long the various littoral zones are submerged or exposed to air are factors that shape the biota. The lower littoral zone is more frequently submerged than the uppermost littoral zone, and for longer periods of time. The mid littoral zone is a strip of shoreline that is consistently submerged and exposed by the changing tide. Exposed shores tend to remain damp longer than sheltered ones. Fine sand retains moisture better than coarse gravel, and ocean waves recede more quickly from more steeply inclined littoral slopes. Seawater may remain in depressions, hollows, and coastal gullies. Moisture is also retained longer on seaweed-covered shores than on bare rocky shores. On very exposed shores, most organisms are firmly anchored to the substrate, although motile animals may thrive in the shelter of the seaweed. More sheltered shores are home to larger numbers of motile animals. Other physical variables affecting coastal and littoral communities include salinity, temperature, shelter from drought, and sun exposure.
Classification and mapping of coastal habitat types is based on the EUNIS system, where applicable, and on data and classification from Dr. Agnar Ingólfsson. The uppermost levels of the EUNIS system take into account the littoral substrata, and the physical and chemical environment, such as exposure, ocean temperature, salinity, and climate. Classification at lower levels is based on the species composition of the biota, i.e., dominant vegetation and animal life. Icelandic coastal and littoral habitats are classified into 24 habitat types, organised hierarchically into four levels. The broadest classification (Level 1) is that of littoral shores. Level 2 divides these habitats into rocky shores or sandy and muddy shores. These, in turn, are classified into more specific habitat types at Levels 3 and 4 and in few cases down to Level 5.
Fact sheets on littoral habitat types provide concise but detailed descriptions of each habitat type, a list of characteristic species, and two photographs. The rough distribution of habitat types is shown on a basic 10x10 km grid square map. A more detailed map can be accessed in the map viewer.