Kelp Forest

Scotlands Living Landscapes

• Foreword
• Introduction
• What is kelp and where is it found?
• Why kelp forests are important in Scottish coastal waters
• Scottish kelp plants
• Light – the critical factor
• Oarweed forests
• Cuvie forests
• Kelp parks
• Beyond the kelp zone
• Kelp productivity
• Uses of Kelp
• Kelp harvesting
• Afterword

Light – the critical factor

Light is absorbed by seawater; the deeper you are in the sea, the darker it becomes. Kelp and all other seaweeds only occur in relatively shallow areas of the sea, where light intensity is high enough to permit photosynthesis. This layer is known as the photic zone. Within this zone there is a critical depth, different for each species of seaweed, above which the particular species can obtain sufficient light energy for growth and reproduction, but below which this is not possible.

However, the densest kelp growth does not take place at light levels approaching those of critical depth, but in shallower water where light intensity is higher. Cuvie forests typically extend to a depth of about 20m off the west coast such as the Outer Hebrides, and to only about 10m off north-east Scotland.

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.