Department of Biology,

Subfaculty of Zoology of Vertebrates and General Ecology,

Moscow State University,

Vorobyovy Gory, Moscow 119899, Russia

The undesirable consequences of phosphorus enrichment in reservoirs are primarily connected with the emerging abundance of cyanobacteria which are not utilized by the consumers and form, as a rule, a trophic deadlock in the majority of nutrition chains. Redundant blooming of the Protococcales does not lead to negative effects since the Protococcales are actively consumed by higher trophic levels of water ecosystems.

The N:P ratio turns out to be the factor which regulates the domination in algocoenoses, in particular, that of blue-green or green microorganisms.

In order to replace the blue-green blooming of eutrophic reservoirs by the green one, it is suggested to apply, instead of the traditional phosphorus load decrease, addition of nitrogen compounds, thus varying the N:P ratio (nutrient manipulation). Utilization of the redundant green algal biomass may occur in a natural way or be conducted with the aid of biomanipulation, i.e., by introduction of planktivorous fish into the reservoir.

If the N:P ratio is decreased due to addition of phosphorus compounds, that leads to cyanobacterial blooming of a reservoir. This can turn out to be useful for solving the problems of algological purification of sewage, its disinfection and further utilization in irrigation systems.

The anthropogenic enrichment of natural waters by mineral nutrition components for algae leads to water quality changes which are undesirable from the standpoint of many aspects of water management: for everyday life, recreation, fish breeding, power engineering, etc. A typical cause of reservoir eutrophication is the increased concentration of phosphorus compounds (Vollenweider, 1971; Sirenko & Gavrilenko, 1978).

A typical consequence of reservoir eutrophication is the explosive blooming of blue-green algae (cyanobacteria) with subsequent dying-off of their redundant biomass, toxin production, oxygen regime violation, organoleptic indications of rotting, etc.

From the ecologo-trophological viewpoint, accumulation of redundant blue-green biomass in the course of reservoir eutrophication is connected with the fact that the majority of taxons of cyanobacteria form a trophic deadlock in the trophic chains of hydrobionts. Indeed, the representatives of peaceful zooplankton avoid nourishing large colonial and filiform blue-green species (Gusynskaya, 1978; Hanazato & Yasuno, 1988). Having a high biomass, the cyanobacteria may suppress the life of plankton crustaceous by toxin production (Haney, 1987; Lampert, 1987).

The same tendency is observed in the nutrition of herbivorous fish. The blue-greens are as a rule avoided by silver and motley carp due to low nutrition value and even toxicity of representatives of this division. It is believed (Maliarevskaya, 1973; Topachevsky et al., 1975) that even if a white planktivorous fish capture blue-greens, they are weakly destructed in its bowels while the destructed ones are poorly assimilated. Whereas small species of cyanobacteria still can be consumed by fish, the thread-shaped species (Phormidium, Oscillatoria, Aphanizomenon) are unsuitable for nutrition (Danchenko, 1974).

A standard way of overcoming the eutrophication is to lower the phosphorus load on the reservoirs. However, this way is, as a rule, hardly practicable since the sources of phosphorus compounds are unlocalized, numerous, diverse and connected with such features of human activities whose limitation would require that millions of people change their way of life. Standard methods of overcoming the consequences of eutrophication are the chemical methods of phosphorus compounds sedimentation (Safferman, 1963) and the physico-chemical methods of struggle agaimst the blue-greens: mechanical biomass removal, aeration of huge territories, application of algicidal preparations, coagulating agents and ultrasonics (Komarenko & Vasilyeva, 1972; Schmidt, 1973; Bumbu et al., 1974, 1978). All these methods are expensive and inefficient, while algicidal preparations, if applied, unfavourably affect the life of other hydrobionts.

A complex of ecological methods of ecosystem management is suggested in order to overcome the undesirable consequences of increased phosphorus concentration in reservoirs. We would like to note two possible mechanisms leading to blue-green hyperblooming due to phosphorus abundance.

The first one is connected with the ability of some taxons of cyanobacteria (and only cyanobacteria) to fix the nitrogen dissolved in water (Carpenter & Price, 1976; Bryceson & Fay, 1981; Bothe, 1982). When phosphorus salts are present in excess, the development of algae is restricted by nitrogen compounds and an absolute advantage is gained by the taxons of blue-greens capable of nitrogen fixing. In other words, non-nitrogen-fixing organisms (greens, diatoms, euglenic, some species of blue-greens and others) grow proportionally to the available (low) nitrogen resources, while the nitrogen-fixing cyanobacteria proportionally to the high phosphorus supply.

The second mechanism of domination of the blue-greens, more precisely, of their non-nitrogen-fixing species, can be connected with the low nitrogen to phosphorus ratio optimal for the growth of the blue-greens. The nitrogen to phosphorus ratio in mineral nutrition components is a regulating factor for species domination in algocoenoses.

The latter is indicated, in particular, by an analysis of models of mathematical ecology (Levich & Lebed', 1987; Levich, 1989; Levich & Lichman, 1992; Levich et al., 1993). As follows from the analysis, the nutrient concentration ratios which are optimal for different microalgal groups are close to the ratios of cellular requirements in these substances for each of the groups considered.

The unambiguous effect of the nitrogen to phosphorus ratio on different microalgal taxons has been proved in a number of empirical studies of natural and laboratory phytoplankton.

The results of our own experiments on accumulative pond phytoplankton cultivation in vitro (Levich et al., 1992) have shown that a change of the initial nitrogen to phosphorus concentration ratio causes a change in the algocoenosis composition. Figure 1 depicts the dependence of the biomass of algal divisions at the stationary growth stage (i.e., when cell division has stopped) on the initial environmental nitrogen to phosphorus ratio. Ratios greater than five appreciably transform the community structure in the direction of absolute domination of the greens. The dependence curve of algal biomass vs. nitrogen to phosphorus ratio for the greens has a single peak at the ratio equal to 20, which corresponds to the most rapid growth. For the diatoms and blue-green algae a maximum biomass is achieved at low ratios (from two to five). Herewith among the cyanobacteria the non-nitrogen-fixing ones were predominant. This leads to the conclusion that it was not the absolute amount of nitrogen that affected the improvement or worsening of their growth conditions, but the environmental nitrogen to phosphorus ratio. Other authors' data also indicate that the blooming of the blue-greens and other divisions of phytoplankton from natural reservoirs can be successfully regulated by varying the ratios of mineral nutrition components (Suttle & Harrison, 1988; Wilcox & De Costa, 1990).

The experiments on insertion of mineral nitrogen and phosphorus compounds to ponds in different quantitative combinations enable us to speak of the possibility of directed regulation of blooming types in natural conditions as well (Levich & Bulgakov, 1992). Figures 2 and 3 show the seasonal dynamics of biomass of the Protococcales and Cyanophyta in experimental and control ponds. The inserted nitrogen to phosphorus ratio was (at average throughout the season) equal to 4 in the control ponds and 25 in the experimental ones. It turned out that, as the nitrogen to phosphorus ratio got higher, the biomass of the Protococcales increased while that of the cyanobacteria decreased. Just as in the experiments in vitro, stumulation of the blue-greens at low ratios takes place at the expense of the species uncapable of nitrogen fixing.

Confrontation of numerous data from the lakes of the world (Schindler, 1977; Smith, 1983; Varis, 1991) confirms the conclusion that the blue-greens are most viable at lower nitrogen to phosphorus ratios than are, for instance, the green algae (Fig. 4). This enabled Tilman (1982) to call the discovered law a dramatic influence of such a factor as nitrogen to phosphorus ratio on the taxonomic composition of lake algocoenoses. See also the works of Findley & Kasian (1987), McQueen & Laan (1987), Stockner & Shortreed (1988), Cho et al. (1990) and Haarhoff et al. (1992).

Regulation of species composition of algocenoses with N:P ratio is observed also in laboratory experiments using artificial communities (Suttle & Harrison, 1988; Levich & Bulgakov, 1993).

The suggested ecological way of getting rid of the redundant production of trophically useless cyanobacteria resulting from phosphorus concentration consists of two methods of ecosystem control.

The first stage, nutrient manipulation, consists in increasing the nitrogen to phosphorus ratio in the water of the eutrophicated reservoir. The uncommonness of this method is that the necessary increase is achieved not at the expense of phosphorus extraction but at the expense of addition of nitrogen compounds to the eutrophicated reservoir. As shown by our model and experimental studies, at certain values of the nitrogen to phosphorus ratio nutrient manipulation suppresses the cyanobacterial blooming and leads to the domination of the Protococcales.

The second stage of the control, biomanipulation, consists in introducing planktivorous fish to the reservoir, to transform the redundant primary production of the actively consumed Protococcales to the secondary production of fish. For this purpose it is reasonable to put into operation the rational fish-breeding reservoir fertilizing system, according to which silver and motley carp are introduced in the ponds (Levich et al., 1995). An approbation of this system in fry and marketable fish breeding ponds has demonstrated the high efficiency of the above measures.

The planktivorous fish productivity of the experimental ponds, being compared with the ponds fertilized according to the normative technology, turned out to be 19 percent higher, while the total fish production, including carp, silver carp and monley carp was 16 percent higher. Herewith the excess of nitrogen inserted to the water did not lead to an excess of nitrogen compounds (nitrates, nitrites, etc.) in fish tissues. The salt contents were equal in the fish from both the experimental and control ponds and did not exceed the upper bound prescribed by the state sanitary standard.

Introduction of herbivorous fish is not the only way of utilizing the redundant (due to eutrophication) green algal biomass. The green cells are actively consumed by peaceful zooplankton, which in turn serves as food for predatory hydrobionts, so that the primary production, in proportions common to trophic pyramids, is transformed to the final levels of the pasture and detrital nutrition chains of the reservoir. That is why there are no catastrophic consequences of stormy blooming of reservoirs with green, diatomic and other actively consumed algae: the algal blooming peak is followed by zooplankton abundance peaks, etc. Thus, unlike the cyanobacteria, the biomass of consumed algae is not accumulated and does not decay. This is the reason why the paradoxical addition of nitrogenic forms of nutrients to a reservoir rich in phosphorus does not aggravate the undesirable consequences of eutrophication (reservoir poisoning, oxygen deficiency).

The suggested blooming type regulation methods (changing of the domination of blue-greens by that of green algae) are applicable for recreation reservoirs, cooling reservoirs and those of drinking and complex usage. Nutrient manipulations may play a specific role as elements of a rational ecology-based fertilizing system (Levich et al., 1995) for fish-breeding reservoirs, especially with fish polyculture including herbivorous fish.

There are situations when the opposite direction of blooming regulation is desirable. The case in point is that of accumulating reservoirs for unpurified domestic and zoological sewage which could be used for agricultural crops watering after biological purification. Low values of the nitrogen to phosphorus ratio (due to added phosphorus compounds) lead in such reservoirs to cyanobacterial blooming. The cyanobacteria, due to their biocidal (bactericidal, fungicidal, etc.) properties (Jakob, 1957; Ramamurthy & Seshadri, 1986; Kozhevnikov et al., 1972; Mason et al., 1982; Gleason & Paulson, 1984; all citations with Sirenko & Kozitskaya, 1988) are sufficiently capable of conducting disinfection of sewage from poultry plants, animal farms, some kinds of animal husbandry complexes and communal services. The toxins of the majority of blue-green algae exhibit a wide spectrum of antimicrobial action with respect to representatives of saprophyte and pathogenic microorganisms. At high toxin concentrations the saprophyte bacteria are suppressed, as well as pathogenic microorganisms (salmonella, staphilococci) and helminthal eggs (Kirpenko et al., 1977).

An excess of cyanobacteria in irrigation water can positively affect the development of agricultural crops, in particular, cotton. Irrigation of soils with water rich in phytoplankton improves their biological state (oxygen supply, fixation of nitrogen and humic substances in the soil, its enrichment with proteins, vitamins, auxins, microelements, indispensable aminoacids, mineral salts); the struggle against root putrescence is more successful, the cotton morbidity is lowered (Muzafarov et al., 1984). Some species of the cyanobacteria are able to produce metabolites which stimulate the germinating capacity of cotton, wheat and rice seeds and the growth of adult individuals (Kasymova et al., 1984; Krasnianskaya et al., 1984).

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