Submerged vegetation

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Eelgrass (Zostera marina)	Bladderwrack (Focus vesicolosus)
Macroalgal community	Gut weed (Enteromorpha sp.)

Ecology of submerged vegetation in coastal areas

Submerged vegetation (macrophytes) in coastal areas comprises:

Submerged vegetation is confined to the shallow coastal zone where enough light penetrates the water column to allow plant growth on the sea bottom. The marine flowering plants are rooted in the sediment and, therefore, require a sandy or soft substrate for growth. Macro algae, on the other hand, generally grow attached to firm substrates such as stones, though some macro algal species (e.g. sea lettuce, Ulva sp.) float freely and a few species grow in sand (e.g. Characeae).

Submerged vegetation plays a key role in influencing the structure and ecology of coastal ecosystems. Benthic plants can be very productive (Westlake 1963, Mann & Chapman 1975, Sand-Jensen & Krause-Jensen 1997, Krause-Jensen et al., 1998). On the continental shelf, the contributions of benthic and planktonic algae are nearly equal, and benthic algae contribute some 10% of the total marine primary production (Charpy-Roubaud & Sournia 1990). Submerged vegetation, as for example sea grass beds, also constitute recruitment areas and shelter for benthic fauna, zooplankton and fish fry (Rasmussen 1973), and provide important food sources for waterfowl (Noer et al. 1996). In addition, sea grass beds improve sediment stability, in part because of their network of underground stems and roots, and in part because vegetation cover reduces water movement at the sediment surface (Ward et al. 1984).

The various types of  submerged vegetation have different growth strategies. The kelps and most sea grasses are perennial and relatively stable plant communities characteristic of areas with low nutrient loading, whereas annual fast-growing macro algae such as sea-lettuce are unstable communities characteristic of areas with high nutrient loading (e.g. Pedersen & Borum, 1996).

Along with the increased nutrient loading of coastal ecosystems during the last decades, the depth penetration and area distribution of especially flowering plants and perennial macro algae has been severely reduced in many areas (e.g. Orth & More 1983, Nienhuis 1983, Cambridge & McComb 1984). The main cause is shading from extensive phytoplankton blooms, shading from epiphytes, i.e. algae growing on the submerged vegetation, and shading from annual free-floating macro algae (Sand-Jensen & Borum 1991). When sea grasses disappear, the sediments become exposed to resuspension, and the resuspended material further reduces light penetration to the benthic communities. Changes from communities of perennial sea grass beds to communities of free-floating macro algae have severe impacts on oxygen conditions and carbon- and nitrogen flow in coastal ecosystems (Duarte 1995, Krause-Jensen et al. 1996, Valiela et al. 1997).