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Biodiversity & Conservation

Water clarity / Turbidity

The turbidity (clarity or opacity) of sea water is dependant on the concentration of substances that absorb or scatter light, including inorganic and organic particulates and dissolved coloured substances. The following scale refers to the effect of changes in light penetration, essential for photoautotrophs, because of changes in turbidity. The scale refers to the depth at which the incident surface illumination is reduced, approximating to the lower limit of growth in photophilic algae, to 1% of surface intensity in kelps (laminarians) or 0.05% of surface intensity in foliose algae. It should be noted that turbidity may vary with season and coastal waters are likely to have a higher turbidity at times as a result of winter storms and riverine runoff.

Clarity / Turbidity Foliose algae penetration Description
Extreme turbidity / Poor clarity 0 m Typical of turbid coastal waters, turbidity maxima of estuaries or estuaries with high sediment loads e.g. Severn estuary. Insufficient light penetrates to support algal growth in the sublittoral.
High turbidity / Low clarity Kelp 0 - 5m
Foliose algae <10m
Approximates to turbid coastal waters and upwelling zones (coastal 9; Jerlov, 1971). Sufficient light for sublittoral algae including kelps to about 2-5m e.g. Bristol Channel and southern North Sea.
Medium turbidity / Medium clarity Kelp 5 - 10m
Foliose algae <20m
Approximates to coastal 7-9 (Jerlov, 1971) e.g. Helgoland, German Bight, and Lundy.
Low turbidity / High clarity Kelp 10 - 20m
Foliose algae <10m
Approximates to relatively clear waters of coastal 3-7 (Jerlov, 1971) e.g. off Aran Island; Roscoff, France and off Plymouth.
Very low turbidity / Very high clarity Kelp 20 -30m
Foliose algae <30m
Approximates to coastal 1-3 (Jerlov, 1971), e.g. St Kilda.
Oceanic water Kelp >30m
Foliose algae >50m
Approximates to oceanic III (Jerlov, 1971), e.g. Rockall