Throughout geological history, plants have taken over virtually the entire world, but different groups dominate in different biomes. On land, it is mainly taller plants (now we are not referring to their actual height, but to their evolutionary “advancedness”) that are dominant, while in the waters, the species diversity is made up of lower plants, mostly algae. But it’s not the same in different waters: while in freshwater environments there are quite a few taller, i.e. vascular plants, the sea is an environment that plants with the modern Mesozoic invention of vascular bundles somehow didn’t want to go to. But the exceptions prove the rule!
If we look at the total species diversity of vascular plants, we arrive at a figure of just over 300 000 species. Of this number, we can count a full 65 species among marine plants, barely a quarter of a percent!
The marine flora is far dominated by lower plants, i.e. algae, which can form quite large and massive forms in this environment, especially the brown algae, the seaweeds, of which Macrocystis pyrifera, for example, can form stalks tens of metres long. Large stands of Sargassum are also well known.
Evolution has given plants one huge advantage – under normal conditions, they don’t need any energy to pump water. They work according to a simple physical law – water moves from a less concentrated environment (which is the soil) to an environment with a higher concentration of solutes, which is the cell juices. This concentration gradient requires no help in the form of additional energy.
Vascular plants in the sea must be able to cope with one major handicap, which is the salinity of the water. In a saline environment, where the concentration gradient is reversed, vascular plants have a hard time. Survival in such an environment is usually not possible without additional mechanisms – for example, the communities on coastal and inland salt marshes, which are essentially composed of only species that “know how” to do it, also function on this basis. However, the ability to cope with an environment with a higher concentration of dissolved minerals also varies according to this concentration.
Some freshwater species can tolerate moderate salinity and are therefore able to grow in brackish waters. Of our native aquatic plants, this is true for example of the red-headed sturgeon (Stuckenia pectinata), the mudwort (Zannichellia palustris) and the sea-rush (Najas marina), which grow even in the very low-salinity water of the Baltic Sea bays (I tasted this water myself on the Estonian coast and it was far from salty). However, even these species cannot grow in a really salty sea.
It is also remarkable that all vascular plants that have managed to overcome the problem of salty sea water come from a single evolutionary branch. They are all monocotyledons of the order Alismatales, which belongs to four families: Cymodoceaceae, Hydrocharitaceae, Posidoniaceae and Zosteraceae. Three of them are exclusively marine, only among the vascular plants are mixed also species from fresh waters – among them are also our representatives, e.g. the frog’s frogweed (Hydrocharis morsus-ranae) or the popular among aquarists plants of the genus Vallisneria.
Grasses on the seabed are found mostly in shallow waters; it is reported that the deepest Halophila decipiens was found at a depth of 58 m. In contrast, they do not like to be exposed during high tide. They particularly like slowly sloping sandy bottoms. In the tropics, they are found mainly on islands with a coral barrier; their communities are rather species-poor (14 species were found in the richest one), but usually composed of several species from both the Cymodoceaceae and Hydrocharitaceae families.
In colder waters, their diversity is lower: in the Mediterranean Sea, only Posidonia oceanica, a family with a single genus to which 8 other species belong, forms such stands. In the seas of temperate and cold climates, the species of the family Zosteraceae are mainly found: species of the genus Zostera (15 species in total) are found in both the northern and southern hemispheres, e.g. in the North Sea around the European mainland it is mainly Zostera marina; in the Pacific around the coasts of the United States and Canada and in East Asia there are also species of the genus Phyllospadix.
Submerged marine plants have specific adaptations: these include predominant vegetative reproduction and – especially among larger species – the ability to form very long-lived clones. One of the clones of Posidonia oceanica carries the palm of victory in the competition for the oldest living organism: it has been shown to be about 200,000 years old. Sexual reproduction, on the other hand, is associated with various peculiarities: these plants have large, pentlike pollen grains, and in species of the genus Phyllospadix, a very unusual dispersal of seeds has been found by the droppings of fish and turtles that graze the vegetation of these plants.
The importance of seagrass beds is considerable. The various species have long creeping rhizomes and make a significant contribution to stabilising the seabed, in other words: protecting it against erosion. They contribute to assimilation, consume carbon dioxide and produce oxygen. They provide important shelter and food for many animal inhabitants, such as sea turtles and ochechule (manatees and dugongs).
From a conservation point of view, they are therefore of considerable importance. In the Mediterranean, however, the vegetation is threatened by the highly competitive introduced American green alga Caulerpa taxifolia, which escaped from the famous Monaco marine aquarium after 1980 and has since spread avalanche-like, displacing native vegetation including Posidonia oceanica.