Light and the compensation depth

In the process of photosynthesis the energy of solar radiation becomes fixed as chemical energy in organic compounds. The efficiency of the ocean surface in this energy transformation must vary with locality and conditions, but is probably on average about 0.1-0.5 per cent overall, an efficiency a little lower than that of the land surface.

The ability of plants to absorb and utilize light in photochemical reactions is due to their possession of the green pigment chlorophyll and certain other accessory photosynthetic pigments. These are contained in organelles known as chloroplasts, except in blue-green algae and photosynthetic bacteria where they are diffused in the protoplasm. Chlorophyll occurs in several forms, but only chlorophyll a is common to all photosynthetic organisms. Diatoms and dinoflagellates also contain chlorophyll c together with various xanthophyll and carotenoid pigments which give a golden to brownish appearance to their chloroplasts.

It appears that chlorophyll a is the essential form for conversion of light energy to chemical energy. This pigment is green in colour because it absorbs light in the blue and red parts of the spectrum. Red light is rapidly absorbed by water; consequently in aquatic plants the absorption of radiant energy by chlorophyll a must be mainly limited to the blue wavelengths. The possession of accessory red and yellowish pigments is important in extending the range of wavelengths which can be absorbed, presumably for energy transfer to chlorophyll a. The red and brown colours of many seaweeds are due to these additional pigments and seaweed depth distributions are related to the possession of these pigments.

Photosynthesis is confined to the illuminated surface zone of the sea, and a useful measure of the extent of this productive layer is the compensation depth, i.e. the depth at which the rate of production of organic material by photosynthesis exactly balances the rate of breakdown of organic material by plant respiration. Below the compensation depth there is no net production. The compensation depth obviously varies continually with changes of illumination, and must be defined with respect to time and place. In clear water in the tropics, the noon compensation depth may be well below 100 m throughout the year. In high latitudes in summer, the noon compensation depth commonly lies somewhere between 10 and 60 m, reducing to zero during the winter months when virtually no production occurs.

Photosynthesis varies in proportion to the light intensity up to a limit at which plants become light-saturated, and further increase of illumination produces no further increase of photosynthesis. Exposure to strong light is harmful and depresses photosynthesis, the violet and ultraviolet end of the spectrum having the most unfavourable effects. In bright daylight the illumination at the sea surface seems often to be at or above the saturation level for most of the phytoplankton, and measurements of photosynthesis in these conditions show that maximum production occurs some distance below the surface, usually somewhere between 5 and 20 m depending upon light intensity, and falls off sharply above this level (Figure 5.5).

Arbitrary units of gross primary production or respiration ._1 2 3 4

Arbitrary units of gross primary production or respiration ._1 2 3 4

c

o

D

Q.

a:

r

-

y

' Compensation depth

1 2 3

Net primary production

Figure 5.5 Generalized diagram relating primary production rate to depth in middle latitudes during bright sunshine. Below the compensation depth there is no net production.

Figure 5.5 Generalized diagram relating primary production rate to depth in middle latitudes during bright sunshine. Below the compensation depth there is no net production.

Correspondingly, the maximum quantity of phytoplankton is seldom found very close to the surface, and except for a few species that seem to thrive in the uppermost few centimetres the greater part of the phytoplankton can be regarded as 'shade plants'. By absorbing light the plants themselves reduce light penetration through the water, and as the population increases the compensation depth tends to decrease.

Above the compensation depth the rate of photosynthesis exceeds the rate of respiration and there is a net gain of plant material; below it there is a net loss. At a particular level the total loss by algal respiration in the water column above may exactly equal the total gain by photosynthesis. This level is termed the critical depth. The distance between compensation depth and critical depth depends upon the proportions of the phytoplankton stock above and below the compensation depth. This is determined mainly by vertical water movements.

For the standing stock of phytoplankton to increase, its total photosynthesis must exceed its total respiration. This is possible in a stratified water column when the depth of surface wind-mixing is limited by a thermocline and there is very little transport of phytoplankton below the compensation depth. Around the British Isles these are the conditions of spring and summer when the water column is stabilized by thermal stratification. There is then an overall gain of organic material within the water column and the critical depth must lie below the sea-bed. But in autumn, once vertical mixing begins to distribute much of the phytoplankton to levels well below the compensation depth, a stage is soon reached where total losses by plant respiration are greater than total gains by photosynthesis, the critical depth rises and the standing stock is sharply reduced (Figure 5.6).

Survival of phytoplankton over the winter, when little light energy is available, is effected in several ways. During productive periods the plants build up food reserves, notably as oil droplets, on which they can draw when there is insufficient light for net production. Some species develop resting spores which pass unfavourable periods in a state of dormancy, germinating when conditions become propitious. The dissolved organic matter in seawater provides an energy source which some phytoplankton can utilize if light is inadequate for their needs.

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Responses

  • vilma
    Is there organic production above the light compensation level?
    5 years ago
  • kimmo rehn
    How does compensation depth relate to primary productivity?
    5 years ago
  • Roope
    When is compensation depth maximum?
    3 years ago
  • Rocco
    How is compensation depth determined?
    3 years ago
  • olli savolainen
    What is the compensation depth for photosynthesis and how is it measured?
    3 years ago
  • Hiewan
    What is compensation depth of an algae?
    5 months ago

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