Seaweeds
Even though seaweed looks like a plant, it is not categorized as one because its internal structure is much simpler. Seaweed has no real organs such as leaves, stems, roots and flowers. That is why one speaks of 'thallus' instead of 'plant'. Seaweeds form vegetations in shallow coastal waters, usually on rocky coasts and dikes. The thallus is tough and bendable, so it is able to withstand surf.
Reproduction
The most important difference between seaweed and other plants is the different manner of reproduction. The vegetative phase in seaweed is much more prominent than in 'higher plants'. The life cycle is the most simple in flowering plants. The plant is the sexual phase; the asexual phase only has a short life in a flower. Seaweeds have much more variation in the life cycle. The asexual phase is almost always a separate thallus; sometimes, the sexual phase is practically lacking. The life cycle of red seaweed often consists of two vegetative and one sexually free-living phase.
In general, seaweeds multiply through microscopically small spores. These spores join the phytoplankton for a short period of time to spread out through the water. After awhile, a spore attaches itself to a place with sufficient light and grows into an adult seaweed. Most seaweed live only one year, some species can live for several years.
Categories
Seaweed can be divided into a number of large groups: green seaweed, brown seaweed and red seaweed. 'Blue-green' algae is not a seawee but a bacteria. The division by color has to do with the color of the material for photosynthesis that is present in the highest concentration.
Seaweed comes in all kinds of shapes: stringy species such as green hair algae and hair pen, branched species such as banded weed, leafy seaweeds such as ulva and purple laver, and more complicated shapes such as toothed wrack and sea belt. Some seaweeds such as knotted wrack and bladder wrack have floating bladders, allowing them to float up straight. Seaweeds produce slime to prevent dehydration and protect them from sun and wind during low tide.
The increase of nitrogen in the environment allow green seaweeds to expand so much that they form blankets on the tidal flats. Marine animals suffocate under these blankets.
Where is it found?
Rich seaweed flora grows along the rocky coasts of England, Scotland and Norway. In the Netherlands, there are only artificial rocky coasts, dikes, breakwaters and harbour heads.
A rich seaweed flora is present especially in the delta region. It is necessary that the dikes are constructed from rough rocks. It is difficult for plants to root themselves on poured asphalt and basalt. Clear zones can often be seen on the dike slopes, each having their own seaweed growth. In the zone under the low-tide mark (the infra littoral zone), seaweed such as sugar kelp can be found. The zone between the low-tide and high-tide mark is called the littoral zone. In the low littoral zone, one finds toothed wrack, Irish moss and bryozoan. Spiral wrack grows in the central littoral zone (just under the flood-mark), and small gutweed is found in the splash or supra littoral zone. The zone for lichen is found higher up. This zoning has developed as such because each species has its own degree of resistance to dehydration.
Seaweed is found on the pontoons in harbours, on sheet-piling from salty canals and in saltwater regions where no tides exist, such as the Grevelingen. One usually finds the same species as is tidal regions, however in different proportions. Sandy coasts are not suitable for seaweed to establish itself. Sand moves about too much, offering no foothold. Only on tidal flats with little current are types such as sea lettuce and gutweed present. Seaweed species found on salt marshes are specially bonded to this environment, for example scorpion wrack (Bostrychia scorpioides).
Not all seaweed are indigenous. Japweed, also known as Japanese sargassum, is an example of a sort which has travelled from afar, yet survives very well. Sea thong washes ashore regularly, however is not known to grow in Dutch coastal waters. Many seaweed species lying on the beach have had a long sea voyage lasting many months, and originate from the coasts along the English Channel.
Source: de Vleet, Ecomare
