The nitrogen:phosphorus ratio

as a factor regulating phytoplankton community structure*

N.G.BULGAKOV and A.P.LEVICH

Department of Biology, Moscow State University

Subfaculty of Zoology of Vertebrates and General Ecology,

Department of Biology, Moscow State University

Vorobyovy gory, 119899 Moscow, Russia

 

Abstract: The aim of this review was to rationalize the standpoint according to which: 1) relative biomass of species in a community are determined by ratios of growth-limiting resources; 2) absolute concentrations of resources determine only the total biomass of a community; 3) optimal resource ratios for species relative biomass are determined by specific physiological characteristics of a species. Shifts in phytoplankton species composition following changes in N:P ratio have been observed in artificial laboratory microcosms and natural phytoplankton communities in vitro and in situ. The experiments reported and reviewed here have shown that high N:P weight ratios (20-50:1) can favor the development of Chlorococcales, while N:P ratio reduction to values of 5 to 10 frequently leads to a community dominated by Cyanophyta. As shown by calculus of variations, relative abundance of different phytoplankton species should depend only on the relative amounts of N and P in the environment, so that the optimal N:P ratio for a given species is equal to the ratio of its minimum cell requirements for these elements. An empirical test of this law reveals itself in the fact that for several species of Chlorococcales and Cyanophyta the ratios of their cellular requirements for N and P calculated in the experiment were close to their optimal (for growth) environmental concentration ratios. For instance, an experimental increase in the N:P ratio from a value of 4:1 to 25-50:1 by mass in the water of fish-breeding ponds led to an increased abundance of Chlorococcales; this species shift was due mainly to Scenedesmus quadricauda, which has a high optimal N:P ratio for growth.

Introduction

The primary macronutrients limiting the development of phytoplankton biomass in nature are phosphorus (P), nitrogen (N) and silicon (Si). The latter is contained in the composition of diatoms because it is essential in the development of their silicaceous frustules. Each of these nutrients, taken separately, may be a limiting factor, as has been repeatedly shown in the literature. However, there are a large number of publications which suggest a significant regulating role by nutrient supply ratios (N:P, Si:N, and Si:P) in aquatic environments (Pearsall 1930, 1932; Rhee 1978; Smith 1983, 1986; Pick & Lean 1987; Stockner & Shortreed 1988; Suttle et al. 1991; etc.). These works have demonstrated the algocoenosis restructuring which results from changes in nutrient supply ratios both in natural ecosystems and in laboratory experiments with microalgal communities. On the basis of a literature review and results from their own studies, the authors discuss below which of the factors determines the composition of an algocoenosis to a greater extent — the absolute element concentrations or their relative quantities.

 

Evidence from laboratory microcosms

Data from previous studies

A significant number of papers present the results of laboratory culture experiments. Those experiments had the one common scheme: a varied sequence of nutrients supply ratios was created in the algal growth medium and then the optimal (leading to dominating position in the community) ratios for the community members were found.

Tilman (1977) studied mixed semicontinuous cultures of the two diatoms A. formosa and Cyclotella meneghiniana Kutz. under a wide range of Si:P supply ratios. When the molar ratio of these elements was smaller than 6:1 by moles, A. formosa dominated, while C. meneghiniana dominated when the ratio exceeded 90:1 by moles.

Rhee (1978) concluded on the basis of experiments with monocultures of Scenedesmus sp. that for this alga an N:P supply ratio equal to 30:1 by moles is the optimal ratio for growth: at higher and lower ratio values the culture growth is limited by N and P, respectively. The optimal N:P molar ratio found for the cyanobacterium Anacystis nidulans P. Richt. is also 30 (Sirenko 1972).

Holm & Armstrong (1981) cultured a 2-species culture of the diatom Asterionella formosa Hass. and the cyanobacterium Microcystis aeruginosa Kutz. emend Elenk. in a semicontinuous cultivation regime. The atomic Si:P supply ratio was varied from 2 to 200 between treatments. As the Si:P supply ratio was increased, the biomass ratio Asterionella:Microcystis changed from 1:99 to 96:4. It turned out that A. formosa is a species with a greater demand for Si but gains an advantage over its rival under condition of P limitation. It is noteworthy that similar Asterionella:Microcystis biomass ratios were formed at different absolute concentrations of Si and P, but only when the Si:P supply ratios in the medium were similar. For instance, the alga M. aeruginosa could dominate both at a ratio of 25 (25 mM Si and 1 mM P, 98 % of the total biomass) and at a ratio of 10 (100 mM Si and 10 mM P, 97 %), while A. formosa dominated both at 150 (300 mM Si and 2 mM P, 93 %) and 200 (100 mM Si and 0.5 mM P, 96 %) ratios.

Several species resource requirements under limiting nutrient conditions has been established using batch monocultures as well. When the productivity of two species of green algae, Scenedesmus quadricauda and Stigeoclonium tenue (Ag.) Kutz., were compared, the critical N:P supply ratios by weight turned out to be 22:1 for the former and 17:1 for the latter (Vries & Klapwijk 1987). In this study the critical value was the lowest N:P supply ratio value which, being exceeded, led to P growth limitation.

The growth of the cyanobacterium Synechococcus Nag. grown with the green alga Scenedesmus quadricauda (Turp.) Breb. was studied under conditions of P deficiency (i.e., at high N:P supply ratios) (Suttle & Harrison 1988). In this situation Synechococcus suppressed the competing green alga.

In competition experiments with the two marine diatoms Skeletonema costatum (Grev.) Cl. and Phaeodactylum tricornutum Bohlin. the impact of temperature, light and nutrient supply ratio conditions on the outcome of resource competition was investigated. S. costatum supplanted Ph. tricornutum at low N:Si ratios and high N:P ratios in the medium (Pauw & Naessens-Foucquaert 1991). The optimal N:Si:P supply ratio for S. costatum proved to be 25:25:1 by weight.

Data from new experiments

In our experiments (Levich & Bulgakov 1993) we studied optimal N:P ratios in nutrient environments by cultivating several microalgal species in batch laboratory polycultures. We attempted to control the species composition of the simplest artificial algocoenosis by varying the initial N:P concentration ratio in the algal growth medium.

Laboratory clones of microalgae were obtained from the collection of the Institute of Plant Physiology, Russian Academy of Sciences as well as from the collection of the Department of Plant Physiology, Moscow State University. The laboratory algal communities used in this experiment were grown in Dauta medium (Dauta, 1982) containing several combinations of N (KNO3) and P (K2HPO4) concentrations. Algologically pure collection clones of algae were inoculated into flasks with sterile liquid medium under sterile conditions. The polycultures obtained were grown in a luminostat at constant luminescent illumination of 7.3 Wt/m2. The experiments were continued until all the members of the community reached the stationary phase of growth. Cell number and individual weights of cells, by means of which number was converted into biomass, were counted periodically under the microscope. Simultaneously, concentrations of biogenic nutritive elements in the medium were determined. N concentration was estimated using ion-selective electrode measurement adapted for microalgal suspensions (Bulgakov et al. 1985). P concentration was measured by a modified colorimetric method (Rinkis & Nollendorf 1982). Initial and final N and P cell quotas were estimated for all the species by the method of estimating phytoplankton requirements in biogenic nutrients at different stages of their growth (Levich 1989). In order to estimate the share of final biomass of each species that grew by mineral resources of the medium, the increment of the biomass by N and P located in cells in the beginning of the experiment was estimated by calculating initial cell quotas for all the species. Competition was estimated by the biomass of each species at the stationary phase of its growth.

Four species of Chlorococcales were used in the first set of experiments: Scenedesmus quadricauda, Chlorella vulgaris Beyer, Ankistrodesmus falcatus (Corda) Ralfs, and Ankistrodesmus sp. Polycultures of these four species grew in two media with the following initial N:P concentrations: Medium 1 — 11 mg L-1 N and 3 mg L-1 P (N:P = 3.5 by mass); Medium 2 — 50 mg L-1 N and 2.5 mg L-1 P (N:P = 20 by mass).

The 37 days of the experiment proved to be insufficient for all the species to attain their stationary phases. However, this circumstance did not hide the revealed effects which would have become still clearer if the stationary phase would have been achieved. In both media, as growth took place using both the environmental resources and cell storage, S.quadricauda was the most abundant species in the community. However, at N:P = 3.5 (first medium) its position was not so dominating (50 % of the total biomass), whereas when the N:P ratio increased up to 20 (second medium), the culture of S. quadricauda practically supplanted the other three species from the community (Figure 1A); the total of their relative biomass amounted to 8 %. In the medium with N:P = 3.5 only S. quadricauda and A. falcatus reached their stationary phase, while at N:P = 20, on the contrary, S. quadricauda was the only species which continued to grow by the end of the experiment. Thus, bringing the growth of a polyculture up to the srationary stage would most likely enhance the observed tendency. Comparison relative biomass grown on medium resources without cell stocks demonstrated that Ch.vulgaris dominated absolutely (100%) at N:P = 3.5, while S.quadricauda constituted 95% of the total biomass at N:P = 20 (Figure 1B).

In the second set of experiments the artificial community consisted of only two competing species of Chlorococcales: Scenedesmus quadricauda and Ankistrodesmus falcatus, and the N:P supply ratio was varied more broadly: Medium 1 — 4 mg L-1 N and 3.1 mg L-1 P (N:P = 1.3); Medium 2 — 14 mg L-1 N and 3.1 mg L-1 P (N:P = 4.5); Medium 3 — 34 mg L-1 N and 0.6 mg L-1 P (N:P = 57). When the cultures were sown in all the media, the initial polyculture biomass consisted of 53 % S. quadricauda and 47 % A. falcatus. As the N:P ratio increased, S. quadricauda increased its relative biomass from 78 % at N:P = 1.3; to 87 % at N:P = 3.7; and to 90.5 % at N:P = 57 by mass. The growth on cell stocks was small in comparison with total biomass growth; therefore the corresponding amendments did not bring about any significant alterations in the distribution of final biomass in all the three media.

Natural phytoplankton in vitro

Data from previous studies

Laboratory competition experiments have also been performed on natural phytoplankton consortia, rather than artificial mixtures of laboratory clones of algae. This permits to analyze the influence of biogenic element ratios not only on separate phytoplankton species but also on whole groups of species, united either by a systematic marker, or by size. Hence, the stationary laboratory condition enables a control of assigned environmental factors.

For example, Sommer (1983), varying the environmental Si:P molar ratio from 4:1 to 80:1, cultivated natural multispecies (over 30 species) lake phytoplankton populations in chemostat. Under Si-deficient conditions (Si:P = 4:1), the alga Ag. dominated. At the ratios 10:1-40:1 the most numerous were Koliella spiculiformis Hind, Synedra acus Kutz. and Asterionella formosa. Under P limitation (Si:P = 80:1) S. acus was the most successful competitor. Moreover, these shifts in species composition were consistent and independent of the initial inoculum composition. The position of Mougeotia thylespora in the experiments (lowest Si:P ratio) is in good agreement with its position in lake.

Suttle & Harrison (1988) also conducted a number of experiments with natural freshwater phytoplankton in laboratory conditions at N:P ratios equal to 5, 15 and 45 by atoms. At N:P = 45, dominance by the cyanobacterium Synechococcus was observed. At lower N:P supply ratios, however, the experimental cultures were dominated by two diatoms, Nitzschia holsatica Hust. and Synedra radians (Kutz.) Hust., and the green alga Scenedesmus sp.

Kilham (1986) analyzed the growth of phytoplankton from Lake Michigan in the laboratory at different environmental Si:P supply ratios. She also concluded that it is the resource supply ratio rather than the absolute quantity that determined the outcome of species competition. She showed that under P limitation diatoms replaced the representatives of all other taxonomic groups from a community; however, under conditions of Si limitation, dominance shifted to green algae. In media with a high Si:P supply ratio (313 and 74 by moles) diatoms were dominant, while green algae were dominant at low Si:P supply ratios (4.6 and 0.9). Cyanobacteria were competitively eliminated from the experiment, most likely because very high N-to-P supply ratios (100 by moles) were used in this study. When the medium was supplied with Si and P in a ratio of 20:1 by moles, green algae dominated, while diatoms dominated at Si:P = 80:1 (Grover 1989).

When an algal community from an acid lake dominated by the cyanobacterium Anabaena variabilis Kutz. was inoculated into polyethylene bags incubated in situ, the pH increase and the decrease of the N:P ratio in the environment by P supply resulted in a phytoplankton biomass growth without shifts in algal species composition (Wilcox & De Costa 1990). It was shown that the N:P ratio values higher than 25 by mass were unfavorable for the growth of cyanobacteria.

Egge & Heimdal (1994) added N and P with ratios 16:5, 16:1 and 16:0.2 by moles into the vessels with natural marine phytoplankton. Abundance of diatoms was reduced with N:P increase. In vessels with high N:P (16:1 and 16:0.2) abundance of predominant species Emiliania huxleyi (Haplophyta) increased from initial value 0.09´ 109 cells/m3 accordingly up to 20´ 109 and 37´ 109 cells/m3. At N:P = 16:5 E. huxleyi reached lesser abundance 5´ 109 cells/m3 and was superseded by Phaeocystis sp.

Data from new experiments

We conducted batch culture experiments aimed at studying the impact of different ratios of N and P on the species and size structure of an algocoenosis taken from a fish-breeding pond (Levich et al. 1997). Water from a fish-breeding pond (Astrakhan region) was placed in 2-liter flasks and NH4NO3 and Ca(H2PO4)2 were supplied in different quantitative combinations. Five values of the N:P mass ratio were obtained altogether, with the natural background taken into account: 2:1, 5:1, 20:1, 50:1, and 100:1 by mass. The initial phytoplankton biomass were the same in all the aquaria. All aquariums were situated 50 m off the pond which was used for water sampling; the illumination conditions (corresponding to natural illumination (corresponding to natural light conditions for 0 N) and temperature (about 250C) in them and in the pond were the same. The observations of algal growth dynamics were conducted within 8 days. Samples for estimating phytoplankton abundance and concentrations of biogenic components of mineral nutrition were withdrawn every day. The abundance and, simultaneously, masses of phytoplankton cells (the latter by measuring individual size) were determined under a microscope. The size structure analysis included a comparison of the fractions of certain size classes of algae in the biomass. For that purpose all the species found were divided into six size groups according to their volumes. The following classes of cells were obtained: (1) less than 0.1 ng; (2) 0.1 to 0.3 ng; (3) 0.3 to 1 ng; (4) 1 to 3.2 ng; (5) 3.2 to 10 ng; (6) more than 10 ng. The effective cell sizes corresponding to their volumes are as follows: (1) < 5.8 mm, (2) 5.8-8.3 mm, (3) 8.3-12.4 mm, (4) 12.4 -18.3 mm, (5) 18.3-26.7 mm, (6) > 26.7 mm. By an effective size is meant the diameter of a sphaeroidal cell having an identical mass.

N:P ratios >5 essentially changed the algocoenosis structure, resulting in the dominance of Chlorophyta (Fig.2). Chlorophyta had a maximum biomass at N:P > 20. However, biomass of its most predominated species Scenedesmus quadricauda having a single peak at N:P=20 decreased at N:P=50 and 100. Per cent of biomass for other Chlorococcales (Scenedesmus bijugatus, Coelastrum microporum, Crucigenia tetrapedia, Didymocystis planctonica, Chlorella sp.) was stimulated by N:P = 50-100. For Bacillariophyta and Cyanophyta the greatest biomass were achieved at the lower N:P ratios of 2-5 (Fig.2). The ratios between 5 and 20 were optimal for the growth of diatoms Stephanodiscus and Nitzschia. The cyanobacterium Microcystis developed best of all at ratios between 2 and 5. Higher N:P ratios gave lower biomass among Cyanophyta. On the contrary, biomass of Euglenophyta, after decrease at N:P ratio of 20, increased again at ratios of 50 and 100.

The abundance of algal size classes (in % of total biomass) are shown in Fig.3. Cells >10 ng were extremely rare in the biomass and are excluded. Cells from the range 1-3.2 ng occupied a predominant position in the community at N:P ratios of 20 and 50 while their fraction biomass decreased at higher and lower N:P ratios. Cells with masses between 0.3 and 1 ng were most abundant at the ratio of 5. The representation of the two smallest classes were very low in ratios from 2-50. However, at a ratio of 100 they restore their predominant position (Fig.3).

Natural phytoplankton in situ

Data from previous studies

Back in the thirties of the present century Pearsall (1930, 1932) was one of the first to observe that different chemical element abundance ratios, including N:P and Ca+Mg:Na+K, promote the formation of different phytoplankton communities in the nature.

Schindler (1977) for many years carried out an experimental study of small fertilized lakes. In one of the lakes, which for six years running had been fed with fertilizers with the atomic N:P ratio equal to 30, algae from the genus Scenedesmus dominated the plankton throughout the experiment. After that the N-to-P proportion in the fertilizers was lowered to the value of 11 and the plankton became dominating by cyanobacteria, mainly Aphanizomenon gracile (Lemm.) Elenk. In another lake the fertilizers were fed with the N:P ratio equal to 11 throughout the experiment. That resulted in the domination of the N-fixing cyanobacteria of the genus Anabaena. According to Schindler, at low N:P ratio blue-greens dominated not only in non-fertilized lake water but also when fertilizers with low N:P ratio were supplied into ponds. Thus, when the N:P ratio in the fertilizer was as low as 5 by moles, it turned out that the phytoplankton biomass increase in the experimental lake (by a factor of 4 to 8 as compared with a non-fertilized lake) took place almost entirely because of the cyanobacteria, mainly of the genus Anabaena (Findley & Kasian 1987).

Smith (1983), having analyzed the situation in 12 lakes of the world, found summer dominance by cyanobacteria when the epilimnetic total N : total P ratio had values less than 29:1 by mass. At TN:TP ratios greater than 29:1 by mass, the phytoplankton in these lakes was dominated by non-cyanobacterial species.

Later Smith (1986) summarized the data from 22 lakes worldwide. Multiple linear regression analysis suggested that TN, TP and light interact to determine the relative biomass of blue-green algae. At a fixed light level, blue-green relative biomass increases as light availability decreases. At a fixed light level, blue-green relative biomass also increases as the TN:TP ratio decreases.

Pick & Lean (1987), reviewing a number of studies on the influence of different nutrients on cyanobacteria, concluded that above TN:TP ratios of 30, cyanobacteria tend to become rare, but below this value they may or may not dominant. However, while experimental manipulation of N:P ratios in situ may often stimulate or supress relative cyanobacterial biomass, laboratory studies do not clearly link low N:P ratios with this algal taxon.

So far we have dealt with the impact of mineral forms of N and P on the phytoplankton structure; however, according to McQueen & Lean (1987), the increase of the annual percentage of cyanobacteria can be induced by the decrease of the ratio of the nitrate N to the whole amount of P. Conversely, when this ratio was greater than 5 by mass, a mass development of cyanobacteria was never observed.

Likewise, an apparent dependence of number of cyanobacteria on low N:P ratios and sufficient P supply was observed during an experimental study of phytoplankton in Kennedy Lake (warm-monomictic lake mixing completely from about November to March), British Columbia (Stockner & Shortreed 1988). A reverse effect was obtained in the South African hypertrophic impoundment (Haarhoff et al. 1992): measures aimed at water clearing from industrial sewage resulted in the N:P ratio increase in the water from 4 to 25 by moles. After that the entire phytoplankton biomass lowered, while the formerly dominating cyanobacteria (Microcystis aeruginosa) were replaced by green algae. An inverse dependence between the number of cyanobacteria and the value of the N:P ratio in Finnish lake ecosystems was also showed using statistical correlation analysis by Varis (1991).

The joint interaction of N, P and Si may exert influence on the composition of a natural algocoenosis as well. Long-term changes in the hydrochemistry of the Rhine delta, that expressed themselves in the decrease in the Si:N and Si:P ratios, promoted a mass development of dinoflagellates and cyanobacteria (Admiraal & Vlugt 1990).

During the last 10 years in the Northern Adriatic Sea desalinized by the Po river, and in the Northern Mexican gulf desalinized by the Mississippi river, the ratios of Si:N, N:P and Si:P significantly changed (Justic et al. 1995). As a result the species structure of the phytoplankton community has changed completely.

Michard et al. (1996) studied the chemical composition of water and mainly the variations of N:P ratios are the deterministic factors of the mass occurence of Microcystis aeruginosa in the hypereutrophic Villerest reservoir (located near the city of Roanne, France). The summer growth phases of this species observed both sampling years started as soon as N:P levels decreased below 5 by moles. The proliferation of Microcystis did not seem to be regulated by biological factors. Any manipulation to lower N-inputs will translate into low N:P ratios allowing Microcystis to become dominant.

Data from new experiments

We have been studying the impact of the N:P ratio on the natural phytoplankton composition for three years in fish-breeding ponds in the delta of the Volga river (Levich & Bulgakov 1992; Levich et al. 1996). The area of the ponds is 2-3 ha, the depth is about 1.5 m. From April to September P (Ca(H2PO4)2) and N (NH4NO3) were supplied into the ponds in certain proportions under two schedules: experimental and control ones. In the control ponds the lump doses of fertilizers were the same throughout the season and corresponded to 2 mg/l N and 0.5 mg/l P concentrations (N:P = 4) in the water. Nutrients were supplied into experimental ponds at N:P = 25:1-50:1 during the greater part of the season (the single doses for N varied from 0.24 to 0.95 mg/l during the season; those for P varied from 0.01 to 0.03 mg/l). Ca(H2PO4)2 and NH4NO3 were simultaneously supplied into the ponds from a boat, but they were not mixed and were distributed evenly over the whole pond area. Phytoplankton samples were taken from April to September in all ponds once in every 10 days. At the same dates, the concentration of N and P were evaluated. The samples were obtained from a depth of 1 m. The methods of phytoplankton biomass estimation as well as the methods of nutrient concentration measuring were the similar with the methods of our in vitro experiments (see previous section).

Beginning with June, i.e., immediately after a significant increase in the value of the N:P ratio against that under control, a higher biomass of the representatives of the Chlorococcales was observed in the experiment in contrast with the control. In 1988 the stimulating effect of experimental fertilization for Chlorococcales was observed later, in August and September. Therefore its season-average biomass was lower in the experiment. The biomass increase of the Chlorococcales in the experimental ponds in comparison with the control ones in the second half of the season is shown in Figure 4A. Simultaneously, the percentage of the Chlorococcales in the total biomass of the experimental ponds (with the exception of 1988) increased: sometimes the fraction of cell biomass of this order reached 60 %, Figure 4B). Among the dominant species of the Chlorococcales the most sensitive to an N:P ratio increase was Scenedesmus quadricauda. Stimulation of the growth of Chlorococcales led to a biomass growth of the whole Chlorophyta division, observed in the second half of the season. The increased N:P ratio more often than not failed to cause an improved growth of Bacillariophyta and Euglenophyta, as far as the total biomass of these divisions is concerned. However, the dominating diatom species, Melosira sp., increased its biomass in the experimental ponds during most of the period of the two seasons (1988-89). In 1987 this species did not belong to the dominating ones. A constant result of the fertilization in the experimental ponds was the suppressed biomass and lowered fractional abundance of cyanobacteria, beginning with the middle of the season (Fig. 5). This suppression is connected with the biomass decrease of the dominating genera of cyanobacteria in the experiment, viz., Merismopedia and Phormidium, which took place during the period from June till September. Changes in the size structure of the algocoenosis were indicated by an increased biomass of medium-sized cells, belonging to the classes 0.3-1 and 1-3.2 ng, in ponds with an increased N:P ratio.

Management of pond algocoenoses by regulation of its composition with biogenic nutrient inputs was carried out within the framework of a special experimental schedule which included fertiliziation of fish-breeding ponds in order to obtain a fish polyculture with prevalence of herbivorous species. The fertilization schedule was designed to increase both the total phytoplankton biomass and % of Chlorococcales, the most preferable food item for herbivorous fish, and to decrease that of Cyanophyta. Apart from differences in the N:P ratio, the experimental fertilization system included: 1) more frequent supply of mineral substances (once every 3 or 4 days instead of a standard schedule once in 10 days); 2) earlier in the season beginning of fertilization with 3) higher initial doses of fertilizers; 4) low N:P ratio (4:1) in fertilizers during April-June (April-May in 1989); 5) non-rhythmical seasonal dynamics of their supply adapted to enhanced food requirements of fish and zooplankton; 6) reduced absolute doses of P and increased those of N. The differences in the fertilization schedules between experimental and control ponds are shown in Table 1. The experimental schedule used in these experiments made it possible to increase the production of herbivorous fish by 30% and to reduce incomplete consumption of P by aquatic plants with simultaneous preservation of normal N and P levels in fish tissues.

Discussion

As has been shown above, variations in biogenic nutrients ratios in the medium influenced the species and size compositions of phytoplankton communities. However, the role of this factor in comparison with other parameters, such as variations in absolute concentrations of N and P, was open to question. The results of our pond experiments in situ described in the previous chapter (Levich & Bulgakov 1992) facilitated its analysis. It should be recalled that although changes in phytoplankton structure were monitored throughout the three-year observation period (1987-1989), none of experimental fertilization schedule components (with the exception of enhanced N:P ratio) was held constant.

In 1987, such components of the fertilization system as application of a enhanced initial dose and more frequent supply of nutrients were eliminated from it, but the phenomenon of regulation of the algal community structure remained. Absolute concentrations of the fertilizers supplied into experimental and control ponds varied throughout the observation period, while the effect of phytoplankton structure rearrangement manifested itself after the middle of the year, i.e., immediately after increasing of the N:P ratio. At the same time, the absence in 1989 of such component as low N:P ratio (4:1) in fertilizers introduced into the ponds in the spring led to a situation when redistribution of Chlorococcales and Cyanophyta biomass occurred as early as the beginning of June. However, biomass redistribution was not observed before late June-July in 1987 and 1988, when low N:P ratio (4:1) influenced phytoplankton growth for a sufficiently long time after fertilization. These data suggest that the relative biomass of taxons and size groups largely depended on relative concentrations of N and P and not on increased N concentration in water. Similar results were reported by other authors (Holm & Armstrong 1981).

It is known that at low N:P ratios in pond water and N limitation in the environment blue-green algae assimilating N from atmospheric air gain advantage over representatives of other taxa. Thus, in hypertrophic lakes minimization of N:P ratio preceded the blooming of N2-fixers (Blomqvist et al. 1990). Conversely, enhanced N input in reservoir inhibited the growth of phytoplankton, in which the N2-fixing blue-green alga Aphanizomenon flos-aquae was predominant (Elizarova & Korolyov 1990). It is known, however, that far from all blue-green algae possess N2-fixing properties.This capacity is a characteristic of heterocystous cyanobacteria (Bothe 1982), in the first place, representatives of Anabaena and Aphanizomenon genera. Some non-heterocystous algal species (e.g., Oscillatoria) can also fix N (Carpenter & Price 1976; Bryceson & Fay 1981). In our studies, these genera were never predominant in biomass (Levich & Bulgakov 1992). The difference between control and experimental ponds was determined by the decline of biomass of Merismopedia and Phormidium (in some cases, Aphanothece and Microcystis) genera that are devoid of N2-fixing ability. According to other authors (Varis 1992), limited N supply is favourable both for N2-fixing blue-green algae and representatives of the Microcystis genus. Thus, blue-greens, like other algal taxa, respond more intensely to relative quantities of biogenic elements than to their absolute concentrations.

Differences in the responses of different representatives of a microalgal community to changes in the resource supply ratio in the environment are determined by adaptive physiological mechanisms acting at the cell level. Such a mechanism is described by the concept of phytoplankton requirements for mineral nutrition components (Droop 1973; Rhee 1978; Tilman 1982; Sommer 1983; Levich 1989; Levich et al. 1997). By this concept, a species' requirement for N, P, etc., is equal to the diminution of the corresponding element in the environment related to a single cell or to a unit amount of biomass. In other words, it is the presence of a substance assimilated in a cell (a quota) if respiration, excretion, etc. are neglected. A quota is a specific quantity for a species. For a single species it may vary between maximum and minimum values in the process of population development. The minimum quota is such a minimum amount of a substrate inside a cell which it is still able to divide. According to Droop (1973), in the logistic growth curve of batch growth a reached minimum quota means that a stationary phase has been achieved. It is tempting to conclude that if a certain species possesses a certain ratio of minimum quotas of for instance N and P, this ratio indicates the degree of its competitiveness at one or another value of the N:P ratio in the water. The biogenic element concentration ratio, closest to the ratio of minimum quotas, is optimal for a given species. Similar ideas have been put forward by Rhee & Gotham (1980) who found optimal N:P ratios for nine microalgal species and stressed that these ratios correspond to the ratios of requirements for these elements.

Within the framework of the variational model of a phytoplankton community (Levich 1980), the theorem of maximum species abundances was proved using mathematical models (Levich et al. 1993; Alexeyev, Levich 1997). According to this theorem, relative abundances of species depend exclusively on the ratios of environmental resources that limit community growth. The resource ratios which provide the maximum per cent share of a species in a community are equal to the ratios of minimum cell quotas for this particular species (Droop 1973). As a matter of fact, the hypothesis concluded in the formulation of the theorem on the maximum of species abundances has stimulated us to conduct a series of experiments and search for more recent literary data concerning the role of nutrient ratios in species community structure of algocoenoses.

However, for multispecies communities the dependence of the results of competition for resources on the gradient of ratios of consumed resources are also adequately described by other models constructed with the use of different methodological approaches. Thus, the graphic theory of resource competition proposed by Tilman (1982) allows forecasting of co-existing species abundance in communities according to ratios of medium resources and species requirements in these resources (according to Tilman, the term “species requirements” is applied to a resource consumed by a population in the stationary phase of its growth when its increment is exactly equal to mortality inside the population). Analysis of such competition with the help of classical differential equations of the Lotka and Volterra type demonstrated (see also Abrosov 1977) that the areas of co-existence or elimination of species in a community are divided by threshold values of resource ratios in a medium. A direct computer-aided analysis using a phenomenological model of algocoenosis simulating the dynamics of intracellular concentrations of N and P (Jorgensen 1980), revealed conspicuous alteration of species ranking with a change in N:P ratio (Levich et al. 1997).

References to an “optimum” ratio of nutrient resources can be found in numerous reports by other investigators engaged in the study of laboratory and natural phytoplankton communities. Many of these observations have been described in the preceding sections of this review. Tilman (1982) calls attention to the striking coincidences between optimal resource ratios for individual species determined in laboratory conditions and those at which some species prevail in natural communities. In particular, citing the works by Smith (1983) who studied the influence of the N:P ratio on the dominance of blue-green algae in world lakes, Tilman characterized it as being “dramatic” for the taxonomic composition of lake phytoplankton. It should be mentioned in this connection that in empyrical studies the term “species-optimal” is used to define a resource ratio, the approximation to which makes a given species increase its biomass.

The mentioned above theorem postulating a maximum of species abundances defines more concretely the conditions and criteria of optimality: the optimal resource ratio is associated with species-specific physiological characteristics of organisms, such as cell quotas for these resources in the case of phytoplankton, and ensures the maximum relative biomass of a species in a community. It should be stressed once again that our formulation of the hypothesis postulating the contribution of resource ratios to the community structure does not by any means diminish the role of absolute concentrations of resources available for this community: whereas absolute concentrations of medium resources determine the total biomass of a community, the ratios of these resources determine the relative biomass of individual species in the community.

References

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Table 1. Fertilization pattern in experimental and control ponds

Elements of fertilization system

1987

1988

1989

 

experiment

control

experiment

control

experiment

control

N quantity applied in a season, kg ha-1

563

195

382

134

450

267

P quantity applied in a season, kg ha-1

16

48

39

34

11

67

N:P weight ratio in applied fertilizers averaged over a season

35

4

10

4

41

4

N:P weight ratio in applied fertilizers before the fish stocking

-

4

-

23

Interval between inputs of fertilizers, days

8

12

4

11

4

10

Beginning of fertilization

April

17

April

17

April

1

May

10

April

4

April

17

Remark. "-" Fertilization was begun after the fish stocking.