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Hydrodynamics as a limiting factor in theLake Baikal ecosystemV.A. Verkhozina a , O.M. Kozhova b & Yu.S. Kusner aa Geochemistry Institute of the Siberian Branch of Russia Academy ofSciences, Favorskogo 1, 664033, Irkutsk, Russiab Biology Institute of Irkutsk State University, Lenina 3, 664000, Irkutsk,RussiaPublished online: 07 Nov 2008.

To cite this article: V.A. Verkhozina , O.M. Kozhova & Yu.S. Kusner (2000) Hydrodynamics as a limiting factorin the Lake Baikal ecosystem, Aquatic Ecosystem Health & Management, 3:2, 203-210

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Aquatic Ecosystem Health and Management 3 (2000) 203-210

Aquatic Ecosystem Health & Management

www.elsevier.com/locate;aquech

Hydrodynamics as a limiting factor in the Lake Baikal ecosystem

V.A. Verkhozina a'*, O.M. Kozhova b, Yu.S. K u s n e r a

~Geochemistry Institute of the Siberian Branch of Russia Academy of Sciences, Favorskogo 1, 664033 Irkutsk, Russia bBiology Institute of lrkutsk State University, Lenina 3, 664000 lrkutsk, Russia

Abstract

The intense turbulent diffusion in the water body is one of the features of Lake Baikal, a deep-water, sea-like lake. In this paper we argue that the turbulent mixing of water determines the dynamics of intermediate gaseous nitrogen-containing products, and therefore may be regarded as the limiting factor for Lake Baikal as well as for other ecosystems of the oceanic type. © 2000 Published by Elsevier Science Ltd.

Keywords: Nutrient; Nitrogen cycle

1. Introduct ion

The purpose of this paper is to identify the essential factors that determine or limit the production of bacterio- and phytoplankton biomass in the ecosystem of Lake Baikal. The term 'limiting factor' is used here in the same sense as in Odum (1975), that is, in the steady state of an ecosystem, a substance or factor is called limiting when its availability is closest to the minimum requirements for the organisms of concern (Liebig's law of the minimum). It follows from this principle that the variation of biomass of organisms of a given species should be proportional either to the concentration of the limiting substance or to the inten- sity of the limiting factor (light, temperature and so on). Lake Baikal, located in southcentral Siberia, is the largest (by volume, 23 km3), deepest (1700m, maximum depth), and oldest (ca. 25 million years old) lake in the world. An understanding of factors limiting ecosystem processes will be crucial in the successful management of this unique aquatic resource.

As far as lake ecosystems are concerned,

* Corresponding author.

1463-4988/00/$20.00 © 2000 Published by Elsevier Science Ltd. PII: S 1463-4988(00)00019-1

assessment of limiting factors is particularly impor- tant in connection with the problem of anthropogenic eutrophication and management of lake resources (JCrgensen, 1985; Rast and Holland, 1988). Experi- mentation in small lakes has shown that the limiting factor during the growing season in many cases is the supply of one of a few nutrient elements. For example, long-term observations of several lakes in England have shown that the biomass of planktonic diatoms is proportional to the silicon concentration (Macan, 1970).

However, it is generally recognized that the most important nutrients influencing the quantity and species composition of phytoplankton are nitrogen and phosphorus (Smith, 1982; JCrgensen, 1985). Particular emphasis has been placed on Pas the primary limiting factor governing the eutrophication of small lakes as its ratio to the amount of other elements in living organisms is substantially higher than the corresponding ratio in the environment (Hutchinson, 1957; J0rgensen, 1985). This emphasis on P has led to a number of empirical models that set standards for avoiding the eutrophication of lakes and the permissible inputs ofP (Jorgensen, 1985; Rast and Holland, 1988). However, under certain conditions,

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other factors can become limiting (e.g. solar radiation, temperature, or an alternative element, such as N). This may occur when a given element has been essen- tially depleted by the biota, but other elements (such as P, sulphur and so on) have not, because their supply is 'super-stoichiometric'. As the average composi- tional ratio of the major nutrients, N and P, is main~ tained in biota at an N/P ratio of 9:1 (by weight), it has been observed in data compiled from many small lakes (Jcrgensen, 1985) that when the N/P ratio is in excess of 12:1, limitation by P may occur, with the chlorophyll concentration, Coal, proportional to the P concentration. When the N/P ratio is less than 4:1, N may become limiting, with Cchj proportional to the N concentration.

Based on these considerations and available data, Lake Baikal should not become limited by P. The seasonal dynamics of NH3-N and PO4-P in the upper layers of Baikal reflect phytoplankton metabolism (Votyntsev, 1961), with the minimum concentrations of these nutrients in the springtime during the ice break coinciding with maximum phytoplankton development. In years of massive proliferation of Melosira (a chain-forming diatom), which usually occurs once every 2 -4 years, NO3-N in the surface layer is exhausted, whereas PO4-P is never fully depleted, although the springtime surface concentra- tion can be less than 2 -4 mg m 3. After ice break-up, nutrient concentrations are restored within 3 - 4 months, mainly due to mixing with the underlying layers. Seasonal variation of nutrient concentrations are observed in depths shallower than 100 m, while in deeper layers nutrient levels are practically constant. The highest concentrations of NO3-N and PO4-P in the 0 - 2 5 0 m layer were observed in 1952 (62 and 33 mg m-3, respectively). The simultaneous presence of NH3-N and NO3-N has not generally been observed, although NH3-N has been detected at various depths with considerable fluctuation of its concentration. These observations have been supported by the results of more recent studies (Votyntsev et al., 1975).

These investigations suggest that there is a signifi- cant deficiency in the reserves of NO3-N in Lake Baikal relative to those of PO4-P. For example, over all the study years, the N/P ratio has never exceeded 6:1. Therefore, it is more likely that the Lake Baikal ecosystem is limited by N (Votyntsev et al., 1975). It

is important to point out as well that oceanic ecosys- tems are in many cases considered to be limited by N. As in Lake Baikal, NO3-N in some oceanic surface waters is completely consumed during the period of the maximum development of phytoplankton, while P is not exhausted (Raymont, 1963). The Baikal ecosystem also more closely resembles an oceanic ecosystem than a lake in the patchiness of its bacter- ioplankton (Verkhozina et al., 1988a,b), reflecting intense turbulent diffusion in the lake.

Given the limiting role (in the kinetic sense) played by N in the Baikal ecosystem, one might predict that the gain of the biomass of plankton should be propor- tional to the N concentration in the lake. However, fluctuations of phytoplankton biomass in Lake Baikal between 'good' and 'bad' years (Kozhova, 1987) are substantially larger than fluctuations of the concentra- tions of NO3-N despite seasonal fluctuations in chemical nutrients which more or less correspond with plankton dynamics. Several hypotheses may be proposed to explain the general lack of correlation between interannual fluctuation in phytoplankton biomass and overall nutrient levels. For example, fluc- tuations of phytoplankton biomass in lakes may be influenced by the annual input of nutrients with river inflows. Kozhov (1962) compared the periodi- city of high crops of springtime phytoplankton in Lake Baikal with riverine nutrient loading, the amount of precipitation and the level of the lake defined by the hydrological regime. No direct corre- spondence was found between these factors and the spring biomass of phytoplankton. In any case, such correspondence could hardly be expected to occur in Lake Baikal, whose volume exceeds 450 times the annual hydraulic load.

Differences in the thickness of the snow layer during the ice period might also affect interannual variation in phytoplankton by altering the light inten- sity below the ice and therefore the intensity of winter photosynthesis. However, no correspondence between year-to-year fluctuations of the crop of spring phyto- plankton and the thickness of the ice and snow layer has been established. The same is true with respect to variations in lake heat content (Votyntsev et al., 1975).

Thus, it appears that the Lake Baikal ecosystem, as a whole, is not kinetically limited by the biogenic elements or by simple physical parameters such as

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V.A. Verkhozina et al. / Aquatic Ecosystem Health and Management 3 (2000) 203-210 205

gasin

[ h ] i !

/ I gas out

[ h ]ill

I gas out

/ , h

• I dif

Z

h < < z 1¢

Atmosphere

Zone I

Zone II

Fig. 1. Box diagram illustrating the fluxes of materials between atmosphere, trophogenic (photosynthetic) layer (zone I), and deep water (zone II) considered in our model, h = thickness of the trophogenic layer, z = thickness of deep waters beneath the tropho- genic layer; in Lake Baikal, the depth of the trophogenic layer (h) is much less than the thickness of deeper layers (z >> h). Arrows illus- trate gaseous fluxes out of and into zone I (Igasin, Igasout) losses of organisms via sinking and diffusion ( Ib io) , and turbulent diffusion from zone II to zone I (Idif).

solar radiation and temperature. The same conclusion has been drawn for the oceans (Button, 1978). This implies that, within the existing range of the ratios of the biogenic elements, there is no proportionality between biomass and nutrient concentration or between biomass and the intensity of physical factors and biomass and so on.

In this paper, we argue that the turbulent mixing of water determines the dynamics of biomass and the dynamics of intermediate gaseous N-containing products and therefore may be regarded as the limiting factor for Lake Baikal, and perhaps for other ecosys- tems of the oceanic type.

2. Quantitative description of a theoretical model

In order to identify and quantitatively evaluate the limiting factors in the Lake Baikal ecosystem, let us consider the simplest 'two-zone' model of the ecosystem of a deep lake (Fig. 1). The gain of biomass of autotrophs and consumption of nutrients take place mainly in the photosynthetic zone (Zone I), with deeper waters (Zone II) serving as a nutrient reservoir. The supply of nutrients to Zone I (the flow in Fig. 1) is via turbulent diffusion between water masses of differing concentration, because owing to the consumption of biogenic elements in zone I, the

concentration of elements in Zone I is less than that in Zone II ([h] il < [h]iil). Surface depletion of nutri- ents is well established for Lake Baikal (Votyntsev et al., 1975). Because of turbulent diffusion and net sedi- mentation of biomass by gravity, there is a net flow from Zone I to Zone II. The ratio of water depth to the depth of the photosynthetic zone (z/h) increases and the stock of biogenic elements in Zone II becomes sufficiently great that it is not depleted due to diffusion into Zone I. The growing z/h ratio also implies a higher mixing intensity and therefore raises the diffu- sion coefficient. As a result, for a sufficiently large and deep lake: (1) the concentration of nutrients in photo- synthetic zone I remains relatively constant and inde- pendent of the biomass growth; and (2) the total quantity or 'store' of the biogenic elements in Zone II, proportional to the depth Z, would not be substan- tially depleted over the period of biomass growth. For a very large, deep lake, the limiting factor becomes not the actual concentration of a given nutrient in trophogenic Zone I, but the intensity of turbulent diffusion which establishes the exchange flow between Zones I and II. This is dependent exclusively on the concentration gradient and the value of the turbulent diffusion coefficient. Therefore, in shallow lakes, the 'store' of biogenic elements is quickly depleted owing to the shallow depth and where the intensity of turbulent diffusion is low, kinetic limita- tion by a given nutrient can occur.

In the model we describe here, the intensity of turbulent diftusion is the sole factor defining the dynamics of biomass growth. From the standpoint of this model, the probability of restoration of all biogenic elements is the same in a lake that is suffi- ciently deep and large. Although the importance of the extent of turbulent diffusion in determining the distri- bution of biomass in the Lake Baikal ecosystem has been indicated by numerous studies (Kozhov, 1962; Lurid, 1966; Votyntsev et al., 1975; Verkhozina, 1988a,b), there nevertheless appears to be a certain contradiction with the facts mentioned above that suggest a primary role of N vs. other potential biogenic elements as limiting factors in Lake Baikal (Votyntsev et al., 1975). A comparative analysis of the seasonal and spatial distributions of microorgan- isms active in the N-cycle has revealed that, in a shallow water body such as the Aral Sea, the greater quantity of bacteria inhabits the bottom layers. In

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Lake Baikal, most bacteria are found in the upper trophogenic layer (Verkhozina and Semenchenko, 1982), whose distribution is also characteristic of the oceans (Nealson, 1988). This observation also serves as an additional argument in favour of regarding Nas the primary chemical limiting factor.

From the standpoint of the hydrodynamic limitation model we present here, the prominent role of N in the ecosystems of large, deep lakes might follow from the following circumstances. The cycles of the two elements, N and P (the most probable factors limiting the quantity of phyto- and bacterioplankton), are essentially different. The aquatic N-cycle is an example of a quite complicated, multistage cycle involving gaseous substances, including ammonia (NH3) and nitrogen dioxide (NO2), whereas the P- cycle is a simpler sedimentation cycle.

In the aerobic surface layers of the pelagic ecosystem, an important reaction in the N-cycle involves the process of nitrification:

reaction states (la, lb) is, for Zone I:

d[B]/dt = a[NO3] - /3[B] -D~_u(O2[B]/Oz 2)

- vg(O[Bl/Oz)

d[NO3]/dt = - a [ N O 3 ] + D~_ii(a2[NO3]/az z)

+ K2 [NO2]

NH~- + 202 ~ NO2 + 2H20 (la)

2N02 + 0 2 -"+ 2NO3 (lb)

These reactions are important, because two of their constituents (NH3 and NO2) are liable to be lost to the atmosphere (Weiler, 1979). In large, deep lakes with intense turbulent diffusion these components may be carried away into the atmosphere and into the deeper strata of the lake, thus interrupting the cycle (the gaseous flows out of zone I, Igasout, in Fig. 1). Thus, because certain components of the N-cycle are gaseous, a part of them can diffuse into the atmo- sphere under intense water exchange.

3. The mathematical model, estimates from the model, and identification of 'deep' water bodies according to the model

Let us formulate an equation describing the models discussed above in order to account for limitation by an intermediate product, that is, in a form permitting approximations similar to Michaelis-Menten kinetics. The system of equations describing the growth of autotrophic plankton in a 'two-zone reactor' with turbulent mixing (Fig. 1) including the

(2a)

(2b)

d[NOz]/dt = -K2[NO2] - (D>n + D~_b( O2[NOe]/Oz 2)

+ K 1 [NH 3 ] (2c)

d[NH3]/dt = - K 1 [NH 3] - (DI_ n + Di_b)

× (02[NH3]O/Z 2) + Ko[N2] (2d)

where [ ] is the concentration of the respective chemical element or compound; [B] the biomass of autotrophic plankton; D~_, the coefficient of turbulent diffusion between Zones I and II; DI_ b the coefficient of diffusion of gaseous products from water into the air; a , /3 and Ki the respective reaction rate constants; Vg the rate of sedimentation of plankton by gravity. The last term on the right-hand side of Eq. (2d) has been included conditionally, as N-fixation is also a multistage process; hence, among other things, K0 << K 1 .

The equations for Zone II are:

[B] = 0; [NO3]II = constant; [NOe] = 0;

[NH4] = 0 (2e)

To obtain the required estimates, let us simplify the system of Eq. (2). To begin, let us replace the deriva- tives by their estimated values:

02[ ]/Oz e ~ [ ]/h2; O[ ]/Oz ~ [ ]/h (3)

Let us now estimate the values of the dimensional (i.e. dependent on the depth of the water body) coefficient of turbulent diffusion between Zones I and II (DH0. This will be done by employing the results of studies of seasonal fluctuations of dissolved 02 at great depths in Lake Baikal (Tolmachev, 1957) and the results of assessments of the coefficient of vertical turbulent diffusion in the oceans (Denman and Cargett, 1983). For Di-m as for any diffusion coefficient, the Einstein

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diffusion equation (ratio) is true:

D I _ I I = z2/2a (4)

where z is the displacement of a liquid particle in the course of diffusion, and is the time over which this displacement has taken place. By introducing the random walk velocity u = z into Eq. (4):

D I _ n = 1/2uz (5)

Introducing next the specific coefficient of the rate of vertical dissipation of turbulent energy, ~7, in accor- dance with the dimensional turbulence theory (Landau and Lifshits, 1986):

O°v = U3/Z (6)

and substituting the value of u from Eq. (6) into Eq. (5)

DMI = 1 /2 (~sv ) (Z 4/9) (7)

To assess the value of D1-tl of Eq. (4), let us use the data supplied by natural observations from Tolmachev (1957), which suggest that the time in which there is observed variation of the oxygen concentration at a depth of 1 km ( = 105 cm) is about 2 months (1 month = 5 x 10 6 s). Then, for z = 105 c m : DIII = 101°/2 X 5 X 10 6 = 10 3 cm 2 s 1 (Eq. (4)); u = 2 X 10-2cm s i (calculated from Eq. (5)); and therefore ev = 8 x 10 -11 cm 2 s -3 (calculated from Eq. (6)).

It is of interest to compare the above values for Lake Baikal with similar values observed in the oceans. According to Denman and Cargett, 1983, for the upper levels of the ocean under strong winds, e z = 2 X 10 - l I cm 2 s 3, and, under weak winds, ev = 2 x 10-13 cm 2 s-3. Thus, vertical mixing in Lake Baikal is at least as intense as in the upper ocean. In both cases, vertical mixing is significantly less intense than horizontal mixing, which is the physical argument in favour of the introduction of the two-zone model of Fig. 1 (Votyntsev, 1961; Verkhozina et al., 1988a,b).

We next assume that the characteristic value of the size of the trophogenic zone ! for Lake Baikal as h = 30 m ( = 3 X 103 cm) and that the magnitude of Dt_ b is the same as that of DI-II. The value DI_ b is likely to be even higher due to effective mixing in the hori- zontal direction close to the surface and therefore this assumption is a conservative one. Let us mention,

207

however, that under the ice in the wintertime there is greatly reduced flow into the air.

Using now the appropriate values of the diffusion coefficients and approximations (J0rgensen, 1985), let us evaluate the terms in the system of kinetic equa- tions (Rast and Holland, 1988) assuming that the Michaelis-Menten approximation is appropriate, that is

d[NH3]/dt = 0, d[NO2]/dt = 0 (8)

Assuming the characteristic time of reactions (1) to be about 3 h ( = 10 4 s), let us assume K 1 = K2 = 10 -4 s. With the value DI-II, h -2 equal to 10 2 s -1, it follows from Eqs. (2c) and (2d) that

[NH3] ---- Ko[N2]/(K 1 + 2D/h 2) << N 2

and thus the last term on the right-hand side of Eq. (2b) may be ignored particularly in the case of turbu- lent streams where K1 < Kl + 2D h -2. This conclu- sion would appear somewhat trivial, as the terms of Eq. (2b) could be assessed in the very first place from the fact that in Lake Baikal [NO2] << [NO3]. The conclusion is not trivial, however. The 'complete' , that is, initial, system of Eq. (2) which is now simplified was constructed solely from the values of the rate (velocity) constants and diffusion coefficients. Such is, for example, the conclusion suggested by Eqs. (2c) and (2d) that with the flow of the inter- mediate gaseous products into the air in the winter- time being absent, the concentrations [NH3] and [NO2] would rise by about a factor of 2 if K1 + 2D h -2. The agreement of this prediction with obser- vations confirms the absence of contradictions of the adopted approximations.

Assuming, based on data from Hutchinson (1957), that the sinking rate (Vg) for diatoms is less than 10 -2 cm s 1 and that the characteristic value of /3 is approximately 10-Ss 5 (the lifetime of autotrophic plankton being about 24 h), it can be seen that the diffusion terms have the same order of magnitude as the rest of Eqs. (2a) and (2b). Let us now denote the simplified system of equations:

d[B]/dt = ce[NO3] - /3 [B] - Dl_ n [B]/h 2 (9a)

d[NO3]/dt = - oz[NO3] = DI_II [NO 3]/h 2 (9b)

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Let us next estimate the coefficient ~ in system (9) from the stationary or 'equilibrium' quantity of biomass [B]e q where d[B]/dt = 0 and [NO3]eq = [NO3]H

V.A. Verkhozina et al. /Aquatic Ecosystem Health and Management 3 (2000) 203-210

4. Discussion of results and comparison with observations

(lO) o~ = (~ + Dl_n/h2)([B]eq/[NO3]eq).

We now have one equation left:

d[NO3l/dt = - [NO3] {/3([B]eq/[NO3leq) + (Dl_n/h 2)

× (([B]/[NO3]) -- 1)},

or, by substituting the value of the diffusion coeffi- cient from (Votyntsev, 1961), arriving finally at (with [B]eq/[NO3] << 1):

d[NO3]/dt = -([NO3][B]eq/[NO3]eq)(/3 + Di_[i/h2))

(11)

from which it follows immediately that limitation would be strictly of the diffusion type provided that

(Ol_n/h 2) > ]3; 1/2(,f~)(Z 4/3[h2)/h2 > [3 (12)

that is, in deep lakes with intense turbulent mixing. Otherwise, there would be purely kinetic limitation by the concentration [NO3]. By substituting a value for 13(3 x 10 5s 1), it can be seen that hydrodynamic limitation takes place when the depth of the water body is in excess of around 500 m:

(13) Z > (21~h2/'f~) 3/4 ~ 500 m

In addition to condition (12), it is also necessary that the condition of sufficiency of the supply of [NO3] in Zone II over the characteristic development time T of autotrophic plankton should be satisfied. For the situa- tion of hydrodynamic limitation, this condition of sufficiency can be expressed as:

(14) T < ([NO3leq/[Bleq)(2h/.f~)

With the numeric values substituted, it follows that this condition is not met if T exceeds 105 s, that is, the 'supply' or store would suffice only for a few days. This means that in large, deep lakes the essential role is played by the source of [NO3] in the water body, and the assumed constancy of NO3 concentration in Zone II (condition (2e)) is merely approximate.

The identification of the limiting stage of develop- ment of phytoplankton is particularly important in light of the growing importance of anthropogenic eutrophication of Lake Baikal. For 'conventional' lakes, in accordance with the limiting factor concept, eutrophication of water bodies is associated with the supply of nutrients, P and N in particular (Smith, 1982; JCrgensen, 1985; Rast and Holland, 1988). As demonstrated in this paper within the accuracy of the estimates made while deriving condition (12), the limitation of the Lake Baikal ecosystem (its southern and central basins at least) is primarily hydrodynamic and of the diffusion type. Condition (12) is probably met very rarely by lakes but it is practically always satisfied by the deep ocean. It also appears that the general assumption that the oceans are limited by N has the same physical background as that discussed in this article.

Our model suggests that the growth of the plankton biomass in Lake Baikal is limited by the supply of the biogenic elements into the trophogenic layers because of a deficiency of the intermediate compounds of the N-cycle, [NH3] and [NO2], which results from intense turbulent mixing. Thus, as in the oceans (Nealson, 1988), the main biomass of plankton is concentrated in the upper trophogenic layer and close to the bottom where N sources are available. This perhaps explains the agreement of numerous observations with predic- tions proceeding from the assumption of purely kinetic limitation of Lake Baikal by N (Votyntsev, 1961).

However, condition (12) is not met by the Baikal bays, and is also poorly met by the lake's northern basin, so that our model explains quite logically the enhanced growth of plankton in the shallower areas. Our model also suggests that intense spring blooms phytoplankton under the ice occur, because the inter- mediate gaseous products of the N-cycle are retained by the ice and not lost to the atmosphere.

Considerable interannual fluctuations of phyto- plankton biomass occur in Lake Baikal (the 'Melo- sira' years). However, biomass fluctuations are not generally accompanied by proportional changes in primary production (Kozhova 1987). This pattern is precisely described by Eq. (9) which predicts that

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large fluctuations should take place in biomass ([B]) but not in the ratio of its derivative to the biomass itself (d[B]/dt: [B]), because, as follows from the equa- tions, this ratio is only weakly dependent on the hydrodynamic diffusion terms of the equations, which are those that support these fluctuations. The agreement between the (production/biomass) ratio in Lake Baikal with values measured in water for the world's oceans was first noted by Votyntsev (1992). Phenomena similar to the 'Melosira' years are not generally found in most lakes, although such biomass and production dynamics have been observed in some drainage lakes of New Zealand where hydrodynamics also appear to be the natural determining factor (Duthie and Stout, 1986).

The trophic status of lakes can be defined by several criteria, including the species composition of the dominating algae, the biomass of phytoplankton, and the concentrations and ratios of the biogenic elements in the lake (J0rgensen and Vollenweiider, 1988). Based on its algal species composition, Lake Baikal could be classified as a eutrophic lake (with diatoms prevailing) or as a oligo- or mesotrophic lake based on its algal biomass and its concentrations and ratios of biogenic elements. Our model explains this discrepancy, which is generally not found in conven- tional lakes, by the fact that the limiting factor here is hydrodynamics and not the supply of biogenic elements per se. Heavy diatoms are retained in the trophogenic layer due to depth and intense turbulent mixing, and are not lost via sinking (see, e.g. Lund, 1966).

Long-term observations during the ice-covered and ice-free periods have shown that nitrifying activity is enhanced in regions of the lake subject to anthropo- genic influence, that is, in the area of discharge of waste water of the Baikal Pulp Combinat. Microor- ganisms entering Lake Baikal from this source are in concentrations exceeding the natural values (in the case of chemoorganotrophs, by two orders of magni- tude over a considerable area) and thus are capable of accelerating the N-cycle (Ilyaletdinov and Verkho- zina, 1982). Thus, because of hydrodynamic limita- tion of the lake's N-cycle, the biomass of plankton likewise grows substantially, a process which has drawn attention to effects of waste water of the Baikal Pulp Combinat for a number of years. It should be remembered that the N-cycle in Lake Baikal is not

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entirely closed, because, practically speaking, the process of denitrification is not prominent in Lake Baikal because of high O2 concentrations and low organic matter content. Even in deepwater Baikal sediments, denitrification has only been detected when nitrates are added (Verkhozina et al., 1988a,b). The relationship between water quality in the water body and the interaction of bacteria and algae has also been traced in experiments. When large quantities of organic matter and chemo-organo- trophic bacteria enter the lake, a bloom of phyto- plankton occurs due to the immediate utilization of nutrients by phytoplankton as soon as they are regen- erated (Omerad, 1978). All these factors should be fully accounted for in resolving the problem of permissible biogenic loading of Lake Baikal.

The model discussed here is not conventional in its treatment of limiting factors in lake ecosystems. However, the substantial influence of turbulence upon distribution of the biomass of phytoplankton of Lake Baikal has been discussed previously from a more qualitative perspective (Lund, 1966). The effect of the aggregate state of the products of the cycle of the biogenic elements has also been discussed earlier. The classical monograph of Hutchinson (1957) describes experiments examining the effect of addi- tions of radioactively tagged P O 4 o n the ecosystems of small lakes in Russia. A one-time reduction of the N/P ratio from the natural 10:1 to 3:1 resulted in an irre- versible effect on the biochemistry of the lake, which performs as a self-regulatory system. The discovery of this phenomenon, which has been ascribed to the sedi- mentation character of the P-cycle, is considered one of the most important in limnology. The crucial importance of the gaseous components of biochemical cycles has been demonstrated for carbon, as it has been experimentally confirmed that the main source of C even in small lakes is the CO2 of the atmosphere. This is even more so in the case of larger and more turbulent lakes (Schindler et al., 1973). The gaseous cycle in this case means that C is never the limiting element.

5. Conclusions

I. Owing to intense turbulent diffusion, phyto- plankton biomass in Lake Baikal is dependent on

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the total supply of NO3-N in the water body and the intensity of microbial processes of the N-cycle in the lake's trophogenic layer.

2. The growth of biomass in a water body with hydro- dynamic limitation is proportional to the growth of the concentration of NO3-N in the entire water body. Hence, in setting standards for antrophogenic influence on nutrient inputs, it is necessary to set the permissible percentage of biomass growth.

3. The effectiveness of systems using biological treat- ment of waste water is substantially impaired in the case of Lake Baikal (as well as in seas and oceans limited in N in the hydrodynamic manner) due to microbial acceleration of the N-cycle.

Acknowledgements

The authors express their gratitude to M.A. Grachev, Corresponding Member of the Academy of Sciences, for the stimulating discussion. J.J. Elser, Arizona State University, provided comments and assistance in improving the English translation.

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