Photograph by Yiannis Issaris

Direct erosion by anchoring and fishing

Trawling is one of the most important causes of large-scale degradation of P. oceanica meadows, particularly in deep meadows (e.g. Ardizzone and Pelusi 1984, Erftemeijer and Robin Lewis 2006). The repeated use of trawl gear over the seabed pulls up P. oceanica leaves and rhizomes (100,000 to 360,000 shoots hour -1 , Martín et al. 1997), largely reducing plant density and cover. As the trawl passes over the seabed, it also re-suspends the sediment and alters the substrate structure, increasing turbidity and nutrient concentrations in the water column. Reduced plant cover and the altered sediment interact in a negative way to maintain silted conditions. The slow regrowth of seagrass further prolongs the impact of trawling which can sometimes run into decades (González-Correa et al. 2005).

In sites frequently visited by pleasure boats, there is significant removal of seagrasses by boat anchors. Also moorings consisting of a dead weight lowered to the seabed, attached to a partially crawling chain, form characteristic bare circles in P. oceanica meadows. These clearings persist for many years. If the anchoring density and frequency are too high, the subsequent erosion may be accelerated by enhanced hydrodynamics.

This part is more detailed in the boat section.

Posidonia - Pressures
Posidonia - Pressures

Water and Sediment eutrophication

P. oceanica meadows are very sensitive to water and sediment enrichment with organic matter and nutrients (Phosphorus). This occurs through a series of cascade effects. When dissolved nutrients are high, epiphytic algae grow much faster and shadow the seagrass leaves, reducing seagrass light harvest and enhancing leaf grazing (Ruiz et al. 2001). Organic matter increases sediment microbial activity, producing anoxia and increasing sulphate-reduction rates in the sediment. The excess hydrogen sulphide rapidly reacts with oxygen pumped through the seagrass roots, and may even penetrate the plant tissues, enhancing P. oceanica mortality (Frederiksen et al. 2007). Therefore, untreated sewage outlets, fish-farm effluents or runoff from fertilized agricultural areas are serious threats to neighbouring P. oceanica meadows. In bays with low water exchange, even small amounts of nutrient and organic input from houses or boats may induce seagrass decline (Marbà et al. 2002).

Coastline transformation effects

P. oceanica meadows are very sensitive to erosion. Coastline transformation, with the proliferation of roads and houses and the regulation of continental river-flow, sharply reduces sediment inputs to the submersed coastal habitats, thereby promoting meadow erosion in their area of influence. Piers and other coastal constructions destroy the underlying communities and may alter the pattern of coastal currents thus passing on the effects of siltation or erosion to other meadows. Dredging and sand reclamation activities close to meadows have a high risk of direct meadow removal and may produce bed siltation or erosion. Finally, beach re-filling (Medina et al. 2001) may change sediment conditions and produce long-term siltation of the adjacent underwater meadow, slowing seagrass recovery (González-Correa et al. 2007b).

Barcos atracados en un puerto - Posidonia
Photograph by Elias Papoun​
Posidonia - Pressures
Photograph by Ivan Bandur​

Salinity increase in the vicinity of water desalination facilities

P. oceanica is especially sensitive to increases in salinity levels (Fernández-Torquemada and Sánchez-Lizaso 2005). Salt concentrations above 39 p.s.u. induce rapid plant death (Sabah et al. 2003). Thus, the brine (40-80 p.s.u.) from water desalination facilities, poured directly onto P. oceanica meadows, can produce diebacks across large areas. Moreover, pipelines constructed to divert the brine to offshore areas destroy considerable meadow surfaces. The present and projected increase in coastal desalination facilities is therefore an emergent threat to P. oceanica meadows.

Proliferation of invasive algal species​

The sustained increase in global marine transport favours the proliferation of exotic species which harm existing communities (Galil 2007). In the Mediterranean Sea, around 100 exotic macrophytes have been introduced in the last decades, of which at least 10 have an invasive behaviour (Ballesteros 2007). Those that most affect P. oceanica meadows are the green algae Caulerpa taxifolia and C. racemosa. Although these species do not apparently penetrate into dense healthy meadows, they may, when associated with other perturbations (e.g. eutrophication, bottom trawling), enhance meadow decline, since they compete for space and light and increase the contents of labile organic matter in the sediment.

Posidonia - Pressures
Posidonia - Pressures
Photograph Daisee Aguilera​

Climate change effects

High temperatures and prolonged heatwaves reduce P. oceanica shoot growth (Mayot et al. 2005) and increase shoot mortality (Díaz-Almela et al. 2007). Sexual recruitment may be enhanced by temperature, but the balance is still negative.

The observed trend in Mediterranean Sea warming and the expected increase in the number of heatwave episodes (Cubash et al. 2001), as well as other observed trends in Mediterranean Sea climate (e.g. a general reduction in water transparency and the greater frequency of severe storms, Duarte et al. 1999) suggest that P. oceanica meadows will have to cope with enhanced climatic stress in the coming decades.