Biotic Regulation versus Nutrient Limitation: Additional Considerations No. 3
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Biotic regulation and the concepts
of genetic adaptation and nutrient limitation

Additional considerations No. 3

As we have already noted, the marine biota is currently excluded from the global carbon budget. Its productivity is believed to be limited by nutrients other than carbon. The primary productivity in the ocean is concentrated near the surface, in the upper layer penetrable by sunlight. The concentrations of nitrogen and phosphorus in the surface waters are lower than demanded by the stoichiometric proportions, if one calculate the needed N and P amounts from the available amount of carbon. This allows one to speak of nitrogen or phosphorus limitation of primary productivity in the ocean.

The surface concentrations of phosphorus and nitrogen are considered to be something external with respect to the marine biota. In reality, however, it can be easily shown that these concentrations are dictated by the biota's organisation and functioning and that they can be changed by the biota within wide margins.

Indeed, a considerable part of the organic matter synthesised by phytoplankton (so-called new production) is decomposed at depths, i.e. at a considerable distance from the surface. The inorganic nutrients forming in the course of biological decomposition propagate upwards by means of physical diffusion. (This phenomenon is known as the biotic pump). As a result, the concentrations of the inorganic nutrients appear to be larger at depths than at the surface, the difference between them being a function of the average depth of decomposition. The latter varies greatly among different regions of the world ocean, causing different values of primary productivity, surface temperature and other characteristics.

The average depth of decomposition is fully determined by the structure of marine community (in simple words, on at what depth the major decomposers (heterotrophs) live). Thus, the marine biota itself determines the value of the surface concentration of inorganic nutrients, that may be in principle be changed by the ecological community within the interval from the depth values (which differ only slightly among different oceanic regions) to the very low, almost zero surface values. This might cause significant changes of biological productivity.

The so-called oligothrophic tropical waters of the world ocean are characterised by a normal depth concentration of P and N and a large average depth of decomposition of the new production. If the marine ecological communities in the tropics were structured in a way that major heterotrophs lived at lower depths closer to the surface, the surface concentrations of inorganic nutrients could be dramatically higher. Unfortunately, marine ecology does not even pose the question of why the equatorial ecological communities are organised as they are.

In local areas where physical fluxes of organic substances are more powerful than biological ones, the concentrations of nutrients are dictated by physical factors. In such areas it is possible to observe an increased productivity of the biota. The increased productivity in these areas is likely to constitute a biotic reaction to the continuously perturbed environmental conditions. However, in such areas with too powerful physical fluxes the biota is just too weak to form and maintain the environment in an optimal state for a long term. In this sense the biotic reaction in such areas does not correspond to biotic regulation. Therefore it may be analysed on the basis of physico-chemical regularities alone and be interpreted as a manifestation of the limiting principle in action.

In the upwelling regions the mixing of deep and surface waters is enhanced due to physical reasons, which creates elevated concentrations of nutrients at the surface. Physical fluctuations in upwelling intensities lead to effects exemplified by the Peruvian El Ninios. Marine biota living within the upwelling regions must be adapted to them (as well as the whole marine biota is adapted to seawater, the terrestrial biota - to land, the biota of mountainous regions - to mountains, etc.). As noted above, if the abiotic fluxes of nutrients in the upwelling regions by far exceed the biotic ones, the biota of these regions is unable to maintain an optimal environment, which is thus formed by physical factors. In such a case the biota continuously exists in a strongly perturbed environment, where the limiting principle may act (and does act!) as well as it acts in agricultural systems continuously perturbed by humans.

Upwellings, powerful oceanic streams, many rapid rivers - all they are characterised by a biota which is unable to control its local environment and in all of them the limiting principle may be valid. Within these areas the biogeochemical cycles are not closed. Therefore, a stationary stable state of environment in these areas is maintained due to contacts with the rest of the biosphere, where the biotic regulation of the environment is active. Due to the fact that the above areas with no biotic regulation of nutrients occupy but a negligible part of the Earth's surface, on a global scale the Earth's environment appears to be under biotic control. Anthropogenic activities extend such 'uncontrolled' areas to a dangerously large scale, which threats to destroy the global biotic regulation.

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Main page in English   6 October 2001.