MAP Technical Reports Series No. 106 UNEP

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specific algal blooms. Also species reproductive strategies, the interactions within the biotic
network and along the food-chain (Mann, 1969; Fenchel, 1988), the presence and dynamics of
complexing agencies, the role of nutrient turnover rates and microbiological interactions (Seki
and Iwami, 1984), etc., determine the dynamics at the primary production level.
3.3.3 The limiting factor concept, and the concept of target factors
The complexity portrayed in previous paragraphs is of but little use for practical purposes,
unless restated. As already said, the manifest production is governed at any time by numerous
factors simultaneously (acute limiting conditions or factors); yet, the overall productivity of waters
under otherwise comparable conditions is largely determined by those factors that limit
production over a substantial length of time during the main growth period (chronic limiting
conditions or factors). Though not in all circumstances, chronic limitation is most often due to
limited availability (systems internal) and supply (from outside the system) of key nutrients. Both,
acute and chronic limitation, are governed by some generalized form of the Michaelis-Menthen
relationship (cf. above).
To evaluate which element, or group of elements, are most likely to act as limiting factor,
the amounts and proportions of all critical elements present in biomass must be ranked against
their concentration and proportions in the water environment. With this, one can eliminate
immediately hydrogen and oxygen, and the elements of group 2 (section 3.2.1) as potentially
limiting. This leaves carbon, nitrogen, phosphorus and sulphur (group 1). Carbon in the form of
free CO 2, bicarbonates and carbonates, and sulphur in the form of sulphate are normally in
excess relative to nitrogen and phosphorus in both, fresh and marine waters, while trace
elements may or may not be. This makes nitrogen and phosphorus primary candidates for
chronic nutrient limitation.
Nitrogen and phosphorus are singled out as target substances not only because of their
overriding role in regulating aquatic productivity, but also because these factors are the only ones
that are amenable to control at source. Because control options for either nitrogen or
phosphorus reduction at source differ in regard to technologies and respective strategies that
are of dissimilar administrative and legislative economic consequences, it is further important
to know whether nitrogen or phosphorus is the prevailing limiting factor in any given situation.
On a worldwide oceanic scale the prevailing limiting conditions vary considerably. For
open oceanographic conditions, nitrogen rather than phosphorus is generally assumed to be the
production limiting factor. Nitrogen can also be limiting in coastal waters as evidenced by
experimental studies near a South California sewage outfall (Eppley, 1971), along the North
American east coast (Yentch and Vaccaro, 1958; Ryther and Dunstan, 1971), and along the
Swedish West coast (Rydberg, 1982). Still, in other circumstances nitrogen and phosphorus
availability may be fairly balanced (Skagshaug and Olsen, 1986). In contrast to oceans,
phosphorus limitation seems to be the norm in fresh waters (OECD, 1982), though exceptions
to this rule are known.
3.3.4 Oceanic versus inshore processes
While eutrophication has been found to potentially affect fresh water lakes of practically
any size and depth, it is hardly justifiable to consider oceans as a whole to become eutrophic
within a few years or even decades. Nevertheless, there are signs and actual trends of
increasing productivity of larger marine areas, particularly of enclosed seas (e.g. the
Mediterranean, the Baltic, the Black Sea, the Seto Inland Sea) and relatively shallow