Kelp Forests - Scottish Natural Heritage

Sugar kelp
Individual, scattered kelp plants can survive down to the
depth at which average light values fall to 1% of that at the
surface. Some smaller red seaweeds require even less light
and therefore have a greater critical depth. In clean, clear,
west coast sea water kelp is found down to depths of 30m,
and in the crystal clear waters around St Kilda it has been
recorded at 47m. In the more turbid waters of north-east
Scotland, however, it grows only down to between 18m and
20m and in many places much shallower than this such as
in the peaty waters of many of the sea lochs.
On the open coast the density of the kelp declines with
depth as the light intensity decreases. The term ‘forest’ is
used to refer to the dense stands of kelp that are found in
shallower water, subject to higher ambient light intensity. In
this region only a limited amount of light can penetrate
through the massed kelp fronds (or ‘canopy’) to the sea
bed. The kelp ‘park’ occurs at greater depths; plants are
more widely scattered than in the ‘forest’, allowing more of
the ambient light to penetrate to the sea bed. The boundary
between the ‘forest’ and the ‘park’ can be as a shallow as
4m in turbid areas, and in clear water at anything from 14m
to 30m.
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