| Three of the four species of European seagrasses, Zostera marina, Zostera noltii and Cymodocea nodosa, cover shallow coastal sediments when water movement is not excessive and light penetrates sufficiently deep. The fourth, Posidonia oceanica, largely occurs in deeper Mediterranean waters, where this slow-growing long-lived species forms reefs of the decaying remnants of rhizome networks and covers wide expanses of the Mediterranean sea floor. All four species have been subject of substantial research (for example: a search on the Web of Science produced 502 papers for Posidonia oceanica, 140 for Zostera noltii and 208 Cymodocea nodosa). All seagrasses form ‘lawns’, meadows or beds that are partial to exclusive habitat to numerous organisms not present on sediments without such plants. Furthermore, these seagrass beds act as nutrient sinks, sediment traps and wave breakers through a number of mechanisms (e.g. Hemminga & Duarte, 2000). Areal extent and density of these seagrasses thus are important parameters for coastal management. Amongst others eutrophication has led to increased turbidity and hence reduced light availability to and depth penetration of seagrasses in many European waters but also elsewhere (cf Hemminga & Duarte, 2000). This has led to substantial reductions in areal extent, increased fragmentation as well as complete loss.
Maintenance of stands of the three shorter-lived species over longer time scales (in the order of > 10 years) depends on the successful establishment of new individual plants from seeds. Establishing seedlings, however, often fail to do so successfully outside the shelter of existing beds. Also, fragmented stands, expand more slowly and produce less seeds (Vermaat et al., 1987). All taken together, this may lead to a self-accelerating decline, probably starting when fragmentation has led to seagrass canopy covering less than 50% of the area. Frederiksen et al. (2004a, 2004b) showed that apparently stable seagrass stands are subject to considerable interannual changes of local disappearance and recolonisation (40-60% in 7 years), particularly in sites exposed to strong water movement with a conspicuous sand bar geomorphology. In a fragmented meadow the majority of seagrass patches is often small (< 10 m2), originating from a successful seedling) and has a high risk of extinction. Thus, we must conclude that such seagrass meadows depend on seedling establishment and early patch expansion as a critically sensitive phase. This phase can easily be disrupted by stochastic and predictable environmental adversity, such as ice scour, storm disturbance and high turbidity, and makes these stands susceptible to major declines. It took eelgrass (Zostera marina) more than 10 years to recolonise areas in Denmark that had been devoided after the wasting disease in the 1930s. Initial, slow recolonisation from a few surviving stands occurred probably primarily from seeds.
The long-lived Mediterranean deep-water Posidonia oceanica is estimated to cover some 8% of the Mediterranean sea floor (Gazeau et al., 2003) and is observed to be declining widely (Marba & Duarte, 1997). Because the decline occurred simultaneously at 15 sites studied along a 1000 km stretch of the Spanish coast, this decline was attributed to a global factor such as climate change and not to locally variable human interference. Superimposed on this overall decline, the authors identified human-induced decline in two sites.
Seagrass meadows have become recognised as habitats of importance. Coastal management thus needs to recognise the importance of water quality (notably light availability), hydrodynamics and sea-floor topography in their interaction with the dynamics and seasonal timing of recruitment of seedlings and establishment of new patches. Spatial extent and pattern dynamics are highly useful for a rational management-oriented monitoring. Increased patchiness or fragmentation of previously dense and homogeneous seagrass meadows should be considered as a serious indication of reduced vigour, just as decreased depth penetration. In open coastal waters where water and sediment movement is considerable, seagrass beds often are inherently heterogeneous, forming elongated bands shaped by moving sandbars. | 
