In many coastal zones, population increase and changes in economic activities are leading to alterations of coral reefs, the seafloor, beachfronts and shores. Urban expansion can result in destruction of important coastal habitats, particularly wetlands. As individual species respond to new conditions, the composition and geographic distribution of ecosystems will change and biological diversity may be threatened as species become locally extinct.

The loss of biological diversity may take many forms but the most dramatic aspect is the extinction of species. The extinction rate depends mainly on environmental changes and the possibility of species to adapt to these changes. In the recent years, species extinction caused directly or indirectly by humans occurs at much higher rate than the natural extinction.

Four factors in the coastal zone are of special interest considering the loss of biodiversity:

1. Eutrophication
2. Loss of habitats
3. Overexploitation of species, objects of fishery
4. Introduction of alien species

Example Posidonia oceanica

The beds of Posidonia oceanica, an endemic species of the Mediterranean Sea, occur in linear fringes in most of the infralittoral, between 0.2 and 40 m depth. It is a priority habitat in the EU Habitats Directive. Posidonia beds are one of the most important ecosystem of the Mediterranean Sea for a number of reasons:

• they are the nurseries of the sea (high primary productivity and supply of oxygen)
• support 25% of the region's flora and fauna and provide essential feeding grounds for sea turtles, waterfowl, cephalopodes, crustaceans, shellfish and finfish
• they are of great economic importance for fisheries and tourism
• they protect against coastal erosion; a loss of 1 m of Posidonia bed may cause a shoreline regression of nearly 20 m.

Posidonia beds are not rare (France alone has 115 000 hectares), but they have suffered a progressive and irreversible regression throughout the Mediterranean due to:

• Sand extraction and development of infrastructure, harbours and artificial beaches, enhancing turbidity and covering the beds with sand;
• Damming of rivers. Changes in sedimentation in the littoral zone have led to either exposing or burying of habitat;
• Trawling and anchoring are especiallly destructive to exposed rhizomes;
• Eutrophication, augmenting algal blooming. Sewage and industrial waste discharge cause a complete loss of the habitat locally.
• Caulerpa taxifolia (a tropical alga introduced in the French Mediterranean in 1984) that is progressively overwhelming Posidonia beds.

The situation in the Western Mediterranean is most serious. Shoot density is rapidly decreasing, up to 50% over a few decades. Besides, increased turbidity and pollution have resulted into a squeeze of the beds; in various places living beds have withdrawn between 10 and 20 m depth. Dead beds occur abundantly, even in waters which have already been protected for 35 years. For the French mainland coast habitat loss is estimated 10-15%; but taking into account the decrease of shoot density the overall decline of the resource will be between 30 and 40%. This is probably a good estimate for most Western Mediterranean coastlines, although the situation around the islands and in the Eastern Mediterranean is better (www.coastalguide.org/eco/index.html)

Example eutrophication; Black Sea coast, Russia

High content of organic and blooming of unicellular algae based on the rich content of biogenic matters increase the water turbidity. This means that the photic layer becomes more shallow. The macroalgae follow the lower border of the photic zone, along the slope, because they need sunlight for photosynthesis.

The following data are known from the Russian coast of the Black Sea. During the last two decades, the belt of macro-algae in the area of Novorossisik (Gelengik) moved up on about 20 meters due to high turbidity of water (and in some area, for example sub tidal slope near Odessa Cystozira disappeared completely, Zaitsev, 1991). Mussel beds, that in previous time were hidden by the brushwood of Cystozira, became available for carnivorous gastropod Rapana and practically disappeared (were eaten). See Vinogradov et al, 1991. (www.ocean.ru/index.htm)