Littoral Zone Ecosystems

Overview

Littoral ecosystems are considered to be one of the most productive marine zones. The high levels of light intensity, the relatively high temperatures, and the presence of both macro and micro algae; attract a large number of species of fish and invertebrates to the coastal zones, where they find rich feeding and, frequently, breeding grounds.
The Mediterranean Sea is characterised as a biodiversity hotspot, supporting over 8500 species of macroscopic marine organisms, corresponding to between 4% and 18% of the world marine species. This is a very high percentage, keeping into account that the Mediterranean covers only 0.82% in surface area and 0.32% in volume of the world’s oceans. The degree of endemism (species living solely in the Mediterranean) is high, as over one quarter of the total number of species of flora and fauna in the Mediterranean are endemic. The paleogeographic, hydrological and ecological characteristics (its variety of climatic and hydrologic situations within a single basin) are responsible for these levels of high biodiversity, of which probably have no equals in the world.
The extensive coastline of Greece, exceeding 18.000km, is the longest/largest in Europe and is comprised of a vast region for species’ settlement. This, in combination with the relatively low levels of coastal development (in comparison to Western Mediterranean), makes the littoral zone ecosystems of the Greek seas and North-Eastern Mediterranean one of the most biodiverse marine zones of the entire Mediterranean.

Species

1. Algae
Over 550 species of algae have been recorded in the Greek seas. Algae (photosynthetic organisms) are at the centre of most food webs in the littoral zone, meaning that they are the basis of most of the life within the marine environment.
Algae can only grow in areas where there is enough light penetration for photosynthesis to occur. Typically, algae reach depths of 40m, but this can be deeper where the water is clear and shallower when the water is more turbid.

Algae can be used as bio-indicators for the quality of water in an area. For example, green algae are usually dominant near sewage outflows and generally in areas with poor water quality, whereas red and brown algae are most commonly found in areas with good water quality.

Sea grass
Seagrasses are higher marine plants, in relation to many plants found on land. There are differences between seagrasses and algae; seagrasses have a root system and produce flowers, whereas algae are attached to substrate by a holdfast and take up nutrients directly from the surrounding water rather than from the soil.
In the Eastern Mediterranean, four species of seagrass are found; Posidonia oceanica, Cymodosea nodosa, Zostera noltii and Halophila stipulacea. Seagrass beds are very important habitats for marine ecosystem systems. They provide food and shelter for a large number of marine species; they also prevent beach erosion by stabilizing the sediments through their rhizomes. Posidonia seagrass beds are of exceptional importance in the eastern Mediterranean, and are one of Archipelagos’ research and conservation priorities. A specific section on Posidonia oceanica – the forests of the sea can be found here.

2. Invertebrates
Invertebrates are defined as being organisms without a spine; however despite this common factor, they are the most diverse taxa group on the planet. Over 6500 species of marine invertebrates are found in the Mediterranean alone, including amongst others; sponges, jellyfish, sea urchins, starfish and squids.

3. Fish
There are over 600 species of fish in the Mediterranean, 86 of which are endemic to the area. Their different body shapes, fin morphologies and colouration are important in the process of distinguishing differences between similar looking species. Many fish change colour during the breeding season; and often males, females and juveniles all have different colouration and patterns. Their colouration may be extremely well suited to camouflage such as that of Wide-eyed flounder (Bothus podas podas), which is virtually the same colour as the sediment. Fish can be found around rocks and algae, in seagrass beds (which provide an ideal nursery ground for juvenile fish and other marine species), or swimming in open water. Their behaviour can vary greatly, from schooling to completely solitary, from actively swimming to remaining motionless on the sea floor, and from predatory or planktivorous to herbivorous.

Threats

Due to their close proximity to cities and villages, littoral ecosystems often are affected by anthropogenic activities more than any other marine habitats.

1. The primary threat faced by littoral ecosystems, is that of physical disturbances which are caused by trawling (purse and boat seiners which is illegally practiced by trawlers), as well as by anchoring (mainly of recreational boats but also by small scale fishing vessels). These physical disturbances contribute to the loss of important habitats such as P. oceanica meadows.

2. Eutrophication and pollution from untreated sewage outlets and fish-farm effluents can smother benthic species and reduce water visibility, which in turn decrease the depth to which photosynthetic organisms can survive. Meadows of Posidonia seagrass or Cystoseira algae in particular, are very sensitive to water and sediment enrichment. In bays with low water exchange, even small amounts of nutrient or organic input from anthropogenic sources can induce habitat decline.

3. The accumulation of toxic chemicals in the water column can inhibit growth rates and bioaccumulate in the food chain. Mercury is a heavy metal commonly found in a number of commercial fish species, such as swordfish and tuna. These chemicals bioaccumulate throughout the food chain and can be particularly dangerous to wildlife and humans, especially to fetuses, infants, and nursing mothers.

4. The ever increasing ship traffic and the opening of the Suez Canal has lead to an increase in invasive species in the Mediterranean basin. For example a total of 116 invasive fish species and 110 algae species has been recorded to date in the Mediterranean, which out-compete native species and homogenize the once diverse environment. Of particular concern is the extensive spread of the invasive green algae, Caulerpa racemosa.

5. Climate change alters seawater temperature and deep-water currents; this affects the spread of some species, with more tolerant species able to survive while fragile, slow growing species are wiped out. It can also lead to a rise in sea levels, resulting in the migration of both invertebrate and algae species further up the littoral zone. The effects of climate change on the Mediterranean Sea cannot be fully observed and investigated yet, as they are in the early stages of influencing change.