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;
- 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
- 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,
|Depth distribution of established kelp species in European
usual habitat zone & depth ranges (from
MLWS) at example locations
||Upper sublittoral: Strangford Lough, 0-5 m (pers. obs.); Aran
Islands, 15 m; Rockall, to below 35 m (T. Hill, pers.comm.)
||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.
||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)
||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.)
||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