If you look at a rocky shore when the tide is out you immediately notice that the plants at the top of the shore are different from those at the water's edge. Those in the middle are different again. The seaweeds are arranged in zones according to how high or low on the shore they are. Each plant species is specially suited to the zone it lives in.
Link to- Why aren't all seaweeds green?
Northton Zonation (Photo: Paul Tyler)
A sheltered rocky shore at Northton. The different zones are plain to see, with different plants growing at different levels.
The Splash Zone
Plants living in the Splash Zone don't normally get covered by the tide but regularly get splashed with seawater and occasionally submerged by a very high tide.
This zone is dominated by Lichens.
Yellow & Black Lichens on Rock (Photo: Paul Tyler)
These lichens are very tolerant of salt water, and are found all round the island above the high water mark
There are 2 kinds of lichen in this photo - the yellow one which is commonly found in this zone, and the stuff that looks like black paint is another species of lichen. Very often what appears to be black rocks on the shore is actually coated with this lichen.
Yellow Lichen on Shiant Island (Photo: Paul Tyler)
On the Shiant islands the yellow lichen extends a considerable distance above the sea.
Here are some more details on Lichens
During periods of neap tides, which occur once a fortnight, the tides do not extend so high up the shore line and these plants may not be submerged for several days. The Channelled Wrack and Spiral Wrack occupy this niche on Harris' Rocky Shores
This, the widest part of the shore, is dominated by 2 species of weed Bladder Wrack and Knotted Wrack . They can both occupy the same zone and probably compete with each other for space. They can withstand a certain amount of drying out but need to be covered by the tide every day to survive.
At the lowest edge of the tide you will find another wrack- Serrated Wrack and on a very low tide you will see much bigger plants growing at the water's edge. This is kelp, a plant with a large blade-like leaf and a strong stem to support it. There are several different kinds of kelp which, although similar, are easy to tell apart see- Oarweed, Forest Kelp and Sugar Kelp. These kelps are truly Subtidal Plants.
Along with Knotted Wrack (see above), Kelp plants have also been important in the seaweed industry. Usually the plants washed up by storms were used, although in some countries today it is harvested by special cutting machines. The word kelp originally referred to the melted ashes left behind after burning seaweed, which was the raw material for the seaweed processing factories. Nowadays the weed is dried and processed chemically, burning it wastes too much of the desired ingredients and contaminates it with sand and dirt.
By Paul TylerNext Section