Waters that have become turbid as a result of eutrophication have reduced the available habitat of aquatic vascular plants. The majority of endangered species in the Baltic Sea live in shallow vascular plant communities.
Vascular plants usually grow in depths less than four metres
As on land, the life of marine vascular plants begins from the bottom. The depth of the vegetation will depend on the turbidity of the water: in the clear waters of the outer archipelago, certain vascular plants, such as seagrass, can be found at depths of six or seven metres. However, the majority of vascular plant communities are found in shallow, sheltered bays with mud bottoms, at depths of less than four metres.
The flower cluster of the grey club-rush rises high above the water surface.
Plants collect sunlight by many different methods
As a fluid and denser element than air, water better supports the structure of aquatic plants, which are more flexible than their terrestrial relatives.
In certain species, particularly those familiar to anglers, such as pondweeds and water milfoils, long and narrow stems and leaves grow rapidly from the bottom towards the light and bend according to the waves, as well as changes in sea level. Another efficient method of gathering sunlight is favoured by the quillwort (Isoëtes spp.), where a sturdy leafy rosette begins to grow in spring, supported a nutrient-storing root or rhizome.
In some plants, most of the area available for photosynthesis is via the leaves floating on the water's surface, which means that the stems and roots are more fragile, and growth towards the surface is rapid. This strategy is used by the photogenic species of European white- and yellow water lilies.
Also, drifting vascular plants have found their way from fresh- and stagnant waters to protected brackish water bays. Such plants either float freely on the surface or within the water column itself. Of these, the most visible are the duckweeds, as well as the hornworts, which thrive in eutrophicated waters.
A “meadow” of water naiads, i.e.
Najas marina, in shallow water.
Reproduction occurs by various means
Plants can also grow vegetative runners along the bottom from which new shoots can be raised. Similarly, rhizomes can creep along the bottom to increasingly larger areas, as long as the vegetation is firmly anchored to the first growing habitat.
Sexual reproduction is riskier. If a flower is at the surface, pollination either by flying insects or wind ensures a mixing of sex cells or gametes. However, success cannot always be guaranteed by this method.
If plants do not have access to the surface, some species may keep their inflorescences (flower heads) tightly sealed and self-pollinate instead. One of the most ingenious solutions includes forming a gas bubble around the inflorescence for self-pollination. Pollinated seeds or spores are released into the water and the trek towards new habitats using water currents continues.
Vascular plant species:
- Seagrass (Zostera marina)
- Pondweeds (Stuckenia spp., Potamogeton spp.)
- Water milfoils (Myriophyllum spp.)
- Quillworts (Isoëtes spp.)
- European white water-lily (Nymphaea spp.)
- Yellow water-lily (Nuphar lutea)
- Duckweeds (Lemna spp.)
- Hornwort (Ceratophyllum demersum)
- Bladderworts (Utricularia spp.)
- Spiny naiad (Najas spp.)