Light, depth and water clarity

The light quantity and quality that is available to a kelp plant is dependent on the depth of water above the plant (and tidal changes in that depth) and also on the type of water present. Sea water optical types were classified by Jerlov (1951; 1976) into:

  • oceanic types (I, II, III), which are relatively clear because of low concentrations of particulate matter (biotic or abiotic); light at the bottom of the photic zone is blue in colour.
  • coastal types (1 - 9), which contain higher concentrations of particulate matter (biotic and abiotic); at the bottom of the photic zone light is green in colour.

A detailed explanation of the interactions between light, depth and water clarity and the consequences for algal photosynthesis and growth can be found in Lüning, 1990, pp. 277-320.

The total irradiance that penetrates to different depths will change in different optical water types (summarised in linked figure). In addition, the quality of that light (i.e. its spectral composition) will change depending on the coloration and particulate loading of the water.

Figure - Change in irradiance with depth in different water types

The irradiance requirements of several species of kelp have been determined experimentally and are known to be different for the different phases of the life cycle (sporophyte and gametophyte). In the field, the light requirements of the different kelp species determine the depths at which they may be found within an area with water of any given quality. In areas where the water is clear, light can penetrate to, and kelp plants can grow at, much greater depths (see table below) than where water is turbid or loaded with DOM (dissolved organic matter). For example, kelps may be found below 100 m in the Mediterranean, with its clear water which is classed as Oceanic III (Fig. 1), but are generally restricted to a maximum depth of 35 m in Europe (offshore from Ireland, the Scilly Isles, Rockall with water clarity of Coastal 3, Fig. 1., or 47+ m on St. Kilda, D. Connor, pers. comm.), and to as little as 6-7 m depth around Helgoland, which is surrounded by the silt-laden waters of the German Bight recorded as water of type Coastal 7 (see linked figure). In very turbid waters (e.g. some sites on the east coast of England or in the Bristol Channel), the depth limit for kelp growth may be reduced to about 2 m, and the absence of kelp from even more turbid sites (e.g. south east Kent, inner Bristol Channel) may be attributed to the lack of sufficient light in subtidal habitats (Dring, 1987).

 

Depth distribution of established kelp species in European waters

kelp species

usual habitat zone & depth ranges (from MLWS) at example locations

Alaria esculenta Upper sublittoral: Strangford Lough, 0-5 m (pers. obs.); Aran Islands, 15 m; Rockall, to below 35 m (T. Hill, pers.comm.)
Laminaria digitata Upper sublittoral: Helgoland, 0-1.5 m; Rockall, to 20 m (Lüning, 1990); Brittany, 0-10 m (Gayral & Cosson, 1973); east Kent, 0-2 m (I. Titley, pers.comm.)
Laminaria hyperborea Mid and low sublittoral: Menai Straits, 0-2.5 m; Norway 0-34 m; southwest England, 0-36 m (Kain, 1971); Hebrides, 0-20 m (Norton & Powell, 1979); Helgoland, 1.5-7 m (Lüning, 1990); St. Kilda to 47 m (MNCR survey)
Laminaria saccharina Upper sublittoral: Helgoland, 0-1.5 m; Norway, 0-8 m (Lüning, 1990); as L. faroensis & L longicruris, Shetland, to 25-30 m (I. Titley, pers. comm.)
Saccorhiza polyschides Carsaig, Argyll, 0-15 m; Isle of Man, 0-24 m; Cornwall, 0-35 m (Norton, 1970)

There is a considerable extension in the depth ranges at which different kelp species may be found when they occur in single-species stands. For example, in the absence of the intense shade of the L. hyperborea canopy (due to harvesting or to severe wave action for example), L. digitata and A. esculenta may be found at much greater depths than normal (as on Rockall).

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