Coast & Sea Areas

Wild Harris

Subtidal Plants

Like all plants, seaweeds need light to grow. Unfortunately for them, seawater absorbs light - the deeper you go, the darker it gets. This means that plants are limited to growing close to the sur face; below a certain depth, there is insufficient light for seaweeds to survive.

sugarkelp 2.jpgSugar Kelp    (Photo: Sue Scott)

The depth at which this limit is reached depends entirely on how clear the water is. In a sheltered sealoch, where the water is often clouded by silt, kelp plants may stop growing at depths below 5m. By contrast, in the clear waters around St Kilda, far from any stirred up mud or river silt, the same kelp plants can be found growing at 30m or more.

Some plants are better than growing in low light conditions than others. In general, red seaweeds survive better in dimmer light, so these are often found deeper than other species.


Alaria, also known as Dabberlocks- See rocky shore plants, Forest kelp, Sugar kelp, Furbellows

 Bootlace weed


Red Weeds

Lithothamnion (see rocky shore),  There are many species of leafy red weed found below the kelp forest. Many of these can live with less light than brown weeds, so are found in deeper water.

Two of these are shown here. Red serrated weed and Red Weed

flat red weed.jpgRed Weed    (Photo: Sue Scott)

Red weed with smooth flat leaves

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