The coastal-marine area is divided into zones where environment conditions (light, wetness, salinity, hydro-dynamism, nutrients and typology of substratum) are relatively homogenous in one zone but different from adjacent zones by environmental discontinuities.
The terrestrial part is called the adlittoral region, followed by four other zones:
- Supralittoral
- Mediolittoral
- Infralittoral
- Circalittoral
A zone is distinguished from adjacent zones by distinctive features, namely the range of depths in which different organisms survive, with each zone hence being characterised by specific assemblages of living organisms. The four zones can then be subdivided as follows:
1. Supralittoral
a) Rocky shore
b) Sandy shore
c) Posidonia banquettes
2. Mediolittoral
a) Upper mediolittoral zone of rock shores
b) Middle and lower mediolittoral zone of rock shores
c) Vermettid / coralline algal ‘trottoir’ (or ‘rim’)
d) Soft substratum shores
3. Infralittoral
a) Hard bottom assemblages
b) Cystoseira ssp. communities
c) Soft bottom assemblages
d) Seagrass meadows
e) Posidonia ‘barrier reefs’
4. Circalittoral
a) Coralline communities
b) Mäerl communities
1. Supralittoral
This zone is subdivided into hard and soft substrata. The soft substratum is either of the slow-drying form, with the most common type being Posidonia banquettes, or the rapidly drying form, consisting of sand and burrowing animals. The supralittoral zone is characterised by organisms that require some wetting with seawater but not immersion.
a) Rocky shore
Rocky shores are formed because of the erosion of bedrock, which is brought about by the pounding of sea waves. Harsh conditions of wave action combined with weather conditions make this a hostile environment for the survival of most living organisms. Rocks in this zone have a characteristic blackish-brown hue, caused by microscopic algae or cyanobacteria (primitive microorganisms), which live on or in the rock. This area may also be covered by maritime lichens, which are mostly Verrucaria spp. Hollows in the rock collect seawater. These seawater rock pools are unstable, unlike the freshwater rock pools, because of the waves pounding on the rocks, which replace the water in the pool and species living within, especially when considering the most seaward rock pools. On the contrary, rock pools that are further inland and less subject to wave action are able to sustain a number of species, such as periwinkles, the most abundant usually being the black periwinkle (MT: żibġet il-blat sewda; SN: Littorina [=Melarhaphe] neritoides) and the isopod commonly known as the sea-slater (MT: id-dudu tas-sajd; SN: Ligia italica). The lower limit of this zone is characterised by more resilient species such as the flattened barnacle (MT: il-koċċla ċatta; SN: Chtalamus depressus) and limpets (MT: l-imħar); SN: Patella spp.), which attach to rocks at the seawater’s edge.
b) Sandy shore
Sandy shore habitats form by way of the deposition of sand and fine sediments as a result of erosion of bedrock. This habitat type is mainly dominated by burrowing animals such as sandhoppers (MT: il-bergħud tar-ramel; SN: Talitrus saltator), amphipods, beetles and woodlice. The highest point reached by waves, forming a visible sub-zone on most sandy shores, collects stranded material, which provides a cool moist environment for small invertebrates, both terrestrial and marine (flies, beetles, centipedes, spiders, isopods, amphipods and gastropods).
c) Posidonia banquettes
In the Mediterranean, as well as on the sandy shores in the Maltese islands, banks of seagrass leaves (MT: l-alka; SN: Posidonia oceanica) and seaweed debris are deposited by wave action. These Posidonia banquettes support an interesting community of terrestrial and marine species including amphipods, snails, spiders, beetles flies and others.
2. Mediolittoral
This zone is colonised by organisms that tolerate regular immersion in seawater but not continuous submersions. The mediolittoral may have an extent of between 10-150 cm (very rarely 200 cm), depending on the degree of exposure. Organisms colonising this zone are characterised by vertical zonation (i.e. the occurrence of dominant species in distinct horizontal bands) allowing the distinction between the upper, middle and lower zones.
Organisms in the upper mediolittoral, which is exposed for far longer periods than the lower mediolittoral, must have a greater tolerance to desiccation because of the continual alteration between a marine and terrestrial habitat. This causes problems of heat stress, desiccation, oxygen shortage and reduced feeding opportunities. Activity and behaviour of several organisms is related to fluctuations in water level (e.g. some organisms, which are mobile when prone to desiccation or heat stress, migrate to deeper water during sea-level lows; others, especially sessile ones, become active only when covered with water). On the other hand, the pounding of waves generally causes a wave shock, especially during rough weather, causing physical damage to organisms due to abrasion, hydrostatic pressure and pressure drag. Most of the animals found in this zone counteract this by adopting various shapes to break the wave. For instance, limpets occurring in rock depressions, have a characteristic cone-shaped shell, which coupled with the powerful suction of its foot, allows the animal to withstand the pounding waves of the lower littoral zone.
Local communities may be prone to seasonal influxes of predators (e.g. migratory shore birds) that can have devastating effects (an example of biological disturbance); hence, the development of behavioural and structural defences by many mediolittoral species.
a) Upper mediolittoral zone of rock shores
On rocky shores, the upper reach of this zone is characterised by: species of barnacles such as the star barnacle (MT: il-koċċla komuni; SN: Chtamalus stellatus); several microscopic blue-green algae also occur in this sub-zone; limpets (species of Patella; different species occur at slightly different levels) occur at the lower limit of the barnacle zone; the chiton Lepidochitona corugata occurs within the limpet zone.
b) Middle & lower mediolittoral zone of rock shores
Between the middle and the lower mediolittoral the shore is generally dominated by the attached snail Dendropoma petreaum in exposed parts, while in sheltered microhabitats one finds the species of snail known as Vermetus triqueter. In polluted waters, the species of algae forming algal belts on the lower mediolittoral rock become replaced by species of Enteromorpha. Several other seaweeds grow on this platform or on rocks at equivalent levels where this is lacking.
In the mid-mediolittoral, macroalgae (large algae) become evident. Different shores have different assemblages of species depending on geographical location, exposure, nature of the substratum and other factors. Characteristic species include the red algae Laurencia sp. (MT: il-lawrenzja; SN: Laurencia papillosa), Polysiphonia sertularoides (which often grows on the shells of limpets), the coralline Phymatolithon lenormandii, species of the brown alga Dictyopteris sp. and, particularly in sheltered conditions, species of green algae are found.
Vagile top-shells, such as the toothed top-shell (MT: il-bebbuxu tal-mazza; SN: Monodonta turbinata), occur throughout the mediolittoral during calm weather, but seek the shelter of pits and grooves in the lower reaches of the shore during rough seas. The lower mediolittoral often includes a belt with the coral weed (MT: il-korallina; SN: Corallina elongata) often accompanied by other corallines.
The fringe between the lower mediolittoral and upper infralittoral is marked, in exposed shores, by a belt of Cystoseira species, particularly C. amentacea (protected species) and C. compressa (in calmer and sometimes polluted conditions).
c) Vermettid / coralline algal ‘trottoir’ (or ‘rim’)
On some shores, the shells of the vermettid gastropod Dendropoma petraeum are embedded in encrustations of the calcareous coralline red alga Neogoniolithon notarsii to form characteristic platforms known as ‘trottoirs’ or vermettid rims.
A second type of threatened trottoir, which may form in the Western Mediterranean, is that formed by the coralline alga Lithophyllum lichenoides. In the Maltese Islands, this forms small hard cushions and is restricted to two or three areas.
d) Soft substratum shores
This type of mediolittoral exhibits harsher conditions than the hard type, since, in addition to those attributes mentioned above, the sand is mobile. Only fauna, usually bacteria, worms of various types, amphipods, diatoms and dinoflagellates, are present in this zone. All these generally live within the sand (endobenthos) or else these animals burrow within the sand.
3. Infralittoral
The infralittoral zone extends from the lower limit of the mediolittoral, which is never or seldom exposed down to the lowest limit, where seagrasses and photophilic (light-loving) algae can live. The lower limit of this zone on average lies at circa 50 m depth in clear seas.
a) Hard bottom assemblages
On hard bottoms, the infralittoral zone is vegetated with photophilic (light-loving) macroalgae in the upper regions and sciaphilic (shadow-loving) communities in shady areas. The most notable and dominant communities in this zone are ‘forests’ dominated by species of brown algae, namely Cystoseira spp., and by Dictyopteris (MT: ħabaq il-baħar; SN: Dictyopteris membranacea [= D. polipodioides]). The latter grows mainly on shady and steeply sloping rocky bottoms.
b) Cystoseira ssp. communities
Cystoseira is a very large genus of tough brown seaweeds adapted to withstand considerable hydrodynamism. Different species of Cystoseira occur in different parts of the Mediterranean. Moreover, different species occur in different environmental conditions and in different sub-zones in the same geographical locality. In the central Mediterranean, rainbow bladder-weed (MT: iċ-ċistosejra kaħla; SN: Cystoseira amentacea) is common on exposed, unpolluted shores, while flat bladder-weed (MT: iċ-ċistosejra tal-friegħi ċatti; SN: Cystoseira compressa) occurs in moderately polluted conditions.
The upper infralittoral on exposed shores is also characterised by species of Cystoseira schiffneri v. tenuiramosa and common sargasso-weed (MT: is-sargassu komuni; SN: Sargassum vulgare). The latter can grow where nitrate / sulphate levels are rather high.
Cystoseira forests often exhibit a four-storeyed structure. This consists of:
- a basal storey of encrusting species (e.g. Peyssonnelia spp.);
- a second storey with low growing erect species and shrubby forms such as Udotea (MT: l-imrewħa tal-baħar; SN: Udotea petiolata [Flabellia petiolata]) and the green alga Dascycladus vermicularis, and;
- a third storey with tall forms of larger plants (e.g. Dictyopteris), and;
- the large Cystoseira forming the last layer.
A multitude of microhabitats occurs within these algal beds, supporting a very rich flora and fauna. Algal beds provide habitats for a large number of invertebrates and fishes. Such biotas are distributed among the three different regions of the forests: the surface canopies, the sub-canopy and the substratum.
Some organisms actually burrow into the rocks surrounding the plants (e.g. some sponges and bivalves). The holdfasts of the larger algae and the actual surface of the substratum again provide another type of microhabitat and in many sessile animals are found here. Moreover the algae themselves provide a surface for attachment of other algae (epiphytes) or sessile fauna (such as bryozoans). Additionally some swimming animals feed shelter or lay their eggs in the algal beds (e.g. many fish and copepods).
c) Soft bottom assemblages
Soft bottom assemblages are mainly dominated by sea grasses particularly the Neptune seagrass (MT: l-alka; SN: Posidonia oceanica) and the lesser Neptune seagrass (MT: l-alka rqiqa; SN: Cymodocea nodosa). Soft bottom assemblages differ from hard bottom ones in a number of ways: organisms mostly live in the substratum; except for seagrasses there is little or no fixed vegetation; the environment is more structurally and spatially uniform; the granulometry (particle size) has a large effect on organisms; organisms can be subdivided in two feeding groups: suspension feeders and deposit feeders.
Wave action produces a coarse and well-sorted (i.e. low variation in particle size) sediment. Large waves lift these surface sediments into a granular suspension tossed shore ward and then seaward by the passing waves.
The physical stability of the bottom increases with increasing water depth as wave-generated bottom currents decrease. As a result, bottom sediments grade from coarse to fine sand with increasing water depth and decreasing wave disturbance. However, seagrass cover stabilises the bottom sediment by damping the direct effect of waves and currents.
d) Seagrass meadows
Seagrass meadows are perhaps the most important natural marine habitat type in the Maltese Islands in terms of productivity and with respect to providing shelter and a place for breeding and feeding for a number of marine organisms.
Soft bottoms occurring in shallow and sheltered waters (5-10m) mainly consist of Cymodocea nodosa meadows, usually in the form of sparse stands with patches of sand in between, and in certain localities the red seagrass Halophila stipulacea (a red sea migrant that is locally restricted to two localities).
In deeper water, meadows consisting of the Neptune seagrass (MT: l-alka; SN: Posidonia oceanica) – species is endemic to the Mediterranean – take over. These are amongst the most important marine ecosystems in the Mediterranean in view of their productivity, high species-richness and their functional role in stabilising sediments, nutrient cycling and as refuges, breeding, and nursery grounds for a number of marine species. In the Maltese Islands, they are found in a reticulate or continuous meadow type.
e) Posidonia ‘barrier reefs’
This is the rarest type of the Posidonia meadow locally, where a mass of Posidonia rhizomes together with high level of sediment form a thick bank or ‘reef’ several metres above the level of the bottom; its distribution is very restricted in the Maltese Islands.
4. Circalittoral
The circalittoral zone extends from the lower limit of the infralittoral down to the maximum depth where multicellular photosynthetic forms can exist; in practice this is about 200 m and light intensity, at such depths, is very low.
On hard substrata, the circalittoral is dominated by attached forms, such as encrusting algae, tubeworms, bryozoans, sponges and corals. This same assemblage of species is also found in shallower water, where environmental conditions mimic those of the circalittoral zone, for example in deep submarine caves. On the other hand, soft circalittoral substrata are characterised by burrowing animals (e.g. heart urchins) or those that live on (e.g. brittle stars and sea cucumbers) or partly embedded in the sediment (e.g. sea pens and soft corals).
a) Coralline communities
The most characteristic of Mediterranean circalittoral communities is the coralgal (coralligene, coralligenous). This consists of more or less massive bioconstructions formed by coralline algae, such as Mesophyllum lichenoides, Neogoniolithon mamillosum and Peyssonnelia rosa-marina. This displays a complex architecture with numerous hollows, caverns and crevices and becomes settled by a rich zoobenthos dominated by sponges, hydroids and bryozoans; this may either form massive reefs composed of accretions of encrusting calcareous algae or maerl composed of nodules of free-living calcareous algae.
b) Mäerl communities
Mäerl beds consist of loose lying, living or dead, coralline red algae, usually aggregated into masses on shell gravel mixed with coarse sand. The most common mäerl-forming species are the mäerl coralline algae (MT: il-korallina tar-ramel ħaj), such as Lithothamnion corallinoides, L. minervae and Phymatholithon calcaraem. These are slow-growing and, when dead, the accumulation of their calcareous skeleton forms mäerl deposits. These provide attachment to a variety of algae, particularly the green algae Udotea ssp. (MT: l-imrewħa tal-baħar; SN: Flabellia petiolata;), and the red algae Vidalia ssp. (MT: il-vidalja; SN: Osmundaria volubilis;). Very productive and large mäerl grounds exist towards the Northeast of Malta.
