Box 9S Marine seagrass

Mediterranean marine seagrass (Posidonia oceanica).

Source: E G Verriopoulos


Rich and highly productive communities of attached macrophytes are found in many of Europe's coastal waters. Due to its high productivity, seagrass contributes to the process of water oxygenation: 1 m2 of Posidonia bed produces between 4 and 20 litres of oxygen per day. More than 400 algae and 1000 animal species live in Posidonia beds, also providing shelter and nursery habitat for many crustaceans and fish.

As the only flowering group of marine plants, seagrasses form meadows which play an important role for stabilising sedimentary deposits and trapping nutrients. In fact, waves and currents are damped by the matte and the transient sediment is accumulated by the filtering mechanisms of their leaves and the rhizomes. The effectiveness of seagrass for coastal protection becomes apparent in areas where substantial changes have followed seagrass regression.

Several factors can contribute in synergistic ways to disrupt the equilibrium of seagrass beds: marine coastal construction, resulting alterations of the hydrological patterns, eutrophication and pollution of coastal waters, increased water turbidity, fishing activities and anchorages are among the possible factors responsible for the destruction and decline of seagrass beds.

Though often associated with tropical waters, there are a number of temperate seagrass species such as Zostera marina, and important meadows are found as far north as Breidafjördur in Iceland. Many Atlantic areas of Zostera have yet to recover from the deteriorating disease in the 1930s which was associated with raised water temperatures.

Of particular importance for the Mediterranean sea is Posidonia oceanica (see illustration) with an average biomass growth of 38 tonnes/year/hectare of dry weight. In the case of the Hyères archipelago in southern France, studies have shown a decline of Posidonia meadows from 30 000 hectares in 1900 to 22 500 hectares ­ a regression that is only expected to level off by the year 2015 if coastal development and pollution are reduced.