from R.H. Whittaker, 1970. Communities and Ecosystems, Macmillan Co., New York, NY158pp.
for information and development purposes only. These are comments on marine communities. Compare as needed with freshwater phenomena.
p. 88-89
Only that uppermost part of the column that is illuminated by sun-light counts as an environment of primary productivity. Ocean waters are often highly transparent; visible amounts of sunlight penetrate well below 100 m depth in some seas, especially in the Tropics. The depth of the compensation point-the light intensity at which plant respiration equals photosynthesis-may be used as the lower limit of the productive, lighted, euphotic layer. The light intensity at the compensation point varies with species and physiological state of plants, environmental factors other than light, and span of time considered, but 1 per cent of full sunlight is a conventional approximation. Depth of the productive layer is on this basis between 30 and 120 m in most open ocean waters. A 1 per cent limit of light intensity corresponds to that on the floor of many forests, but terrestrial communities, even forests, are shallower photosynthetic systems than the marine plankton. Depth of light penetration in the open ocean is in large part determined by absorption by the plankton itself; more correctly it is determined by absorption by the seston-plankton plus dead organic particles in the water-as well as by the water itself. Depth of light penetration is consequently inversely related to productivity. Depth of light penetration decreases from tropical open oceans to temperate open oceans and from these to inshore waters in which both high plankton production and particles derived from other sources limit light penetration.
(The depth may be estimated from the relation of the extinction coefficient, k, to depth in meters, L, of a limiting light intensity, Ld. below a surface sunlight intensity, I0, through water with a chlorophyll content, Ch, of the seston in micrograms/ liter of water. The extinction equation and some observed relationships of k, Ch, and L are
Id = I0e -kL,
k = 0.04 + 0.0088Ch ± 0.054Ch2/3 ,
hence
if Id = 0.01 x I0,
L = 4.605/k.)
Neither on land nor in the sea is photosynthesis simply proportional to light intensity. At lower light intensities, however, photosynthesis approaches a direct, linear relation to light intensity. Above these low light intensities a point of light saturation occurs beyond which photo-synthesis does not increase with increased light intensity, and at still higher light intensities, such as those close to the ocean surface at mid-day, photosynthesis is inhibited and occurs at lower rates than at inter-mediate light intensities. For this discussion of the broad relationships of marine plankton production, the complex interrelations of light, chlorophyll, and photosynthesis with depth may be shortcut. We may consider that summer sunlight intensities are of similar magnitudes in temperate and tropical waters, that over-all efficiencies of light use by plankton at all depths are similar in temperate and tropical waters of similar productivities, and that productivity is controlled by factors other than light intensity. The effective factors are, primarily, nutrients.
As described above, the plankton has a problem with sinking. Despite all adaptations, a certain fraction of the plankton organisms and their dead remains must sink below the lighted zone, carrying with them nutrients incorporated in protoplasm and skeletons. The loss of nutrients from the lighted zone is intensified by another phenomenon. The warm waters of the open ocean in the tropics and in the temperate-zone summer are (like those of many lakes in summer) stratified:
a layer of warmer and less dense water floats on top of the colder and denser water of the depths, separated by a zone of relatively rapid temperature change per unit depth, the thermocline. Because water density decreases upward in the thermocline, its waters are stable. Vertical movements of water by waves and other forces affect primarily the warm waters above the thermocline. The thermocline is consequently a relative barrier to the return of nutrients from the lower levels to the upper warm and lighted waters. Depth of the warm water above the thermocline, and of the lighted water above the 1 per cent level may correspond roughly, though they will not necessarily do so. But the sinking of plankton implies the depletion to low concentrations of the nutrients in the warm and lighted, productive surface waters, and consequently the low productivity of the open oceans.