SHAPING THE HYDROBIOLOGICAL CONDITIONS FOR LARGE RESERVOIRS

by

P.L. Pirozhnikov and M.P. Miroshnichenko,
Candidates of Biological Science

As a result of waterpower development on large and medium size rivers, a great number of reservoirs have been constructed in the U.S.S.R.

At the present time over 100 new reservoirs have been completed, 14 of them being very large (more than 100,000 ha each).

Reservoirs play a very important role in the national economy. They generate much electric power, improve navigation conditions, control floods, irrigate arid areas and obtain high quality fish products in quantities exceeding 15 to 30 times the original fish catches on the merging stretches of the rivers.

The most important prerequisite for creation of large fish stocks in reservoirs is rich fish food reserves comprised of valuable food species.

Therefore, when fisheries management development in reservoirs is considered great attention is paid to the study of the formation of food resources, as well as to the development of measures aimed at their increase and improvement.

Numerous hydrobiological investigations in this field carried out on rivers, various flood-land waters prior to flooding and on artificial reservoirs have: (1) permitted the tracing of the flora and fauna formation process; (2) shown the importance of plankton and benthos for feeding and growth of food and other fish; and (3) sustantiated biologically the enrichment of the reservoir fish food supply as a result of acclimatization of different invertebrates.

A comparative study of the plankton and bottom population formation in reservoirs built on different rivers and in different landscape zones has shown a considerable similarity in this process. The similarity lies in the fact that plankton and benthos develop in different reservoirs under conditions of a slower run-off and more or less stable temperatures. These factors contribute to an intensified development of phytoplankton and, on the whole, favorable conditions are formed for the mass development of zooplankton and, specifically, for an intensified propagation of those Cladocerans and Copepoda which are filtrators as far as feeding is concerned. Such an approach to the slower run-off and temperature regime factor was applied when plankton forecasts for the Volgogradskoye reservoir were made. The forecast proved true for this reservoir as well as for others (Pirozhnikov, 1954, 1961). The increase in the abundance of plankton constituents under conditions of the slower run-off due to damming rivers with hydroelectric dams and other hydraulic structures may be termed “the head effect” (Pirozhnikov, 1965).

The intensive development of zooplankton in reservoirs during the first years of their existence considerably exceeds its density in the merging water bodies. This phenomenon is due not only to a slower run-off but also to improvement of trophic conditions, i.e., an intensified development of phytoplankton and bacterial flora, this being connected in turn with biogenous matter leaching from the flooded soils and with biochemical decay of submerged vegetation. Later the abundance and standing crop of zooplankton stabilize on a certain level specific for each reservoir.

Plankton of large reservoirs is formed mainly from plankton algae and invertebrates of the rivers and numerous and diverse water bodies of the flooded area.

It is clear that, the higher the ratio of the initial area of merging water bodies to the reservoir area, the stronger its influence on the plankton formation process in the future reservoir. It is extremely important to remember that reservoirs are formed in areas of different landscapes. These may be pastures, arable lands forests and brushs, various lakes, backwaters and cut-off lakes. As a rule, the higher aquatic vegetation is poor in newly developed waters and more or less uniform conditions prevail in these throughout vast areas, particularly, in pelagic waters. As already mentioned, two processes essential for bioproductivity take place in reservoirs during the first years of their existence. They are water-soluble mineral and organic compound leaching from flooded soils and biochemical decay of submerged soils and vegetation. As a result, large quantities of nitrates and phosphates, as well as other compounds, find their way into the water. Consequently, it is followed by a mass propagation of Diatoms, Pyrophyta, Protococcales and Cyanophyceae. Thus in reservoirs, unlike the original sections of rivers, and extremely abundant primary standing crop develops, it being especially large in the large reservoirs which have flooded fertile lands, as in the southern districts of this country. The Kakhovskoye and Tsymlyanskoye reservoirs may be cited as examples.

In the Kakhovskoye reservoir the total phytoplankton standing crop is as much as 133 g/m²; the standing crop of Cyanophyceae being 132.7 g.

The phytoplankton distribution in every large reservoir is determined by hydrological features of different parts of the reservoir as well as by meteorological factors. In the upper part of the reservoir where there is current, phytoplankton consists mainly of diatoms. The ratio of the main groups constituting phytoplankton changes in the direction towards the dam, Cyanophyceae becoming prevalent. The specific phytoplankton composition of the downstream part of the reservoir is less varied than that of the upstream section. Seasonal features are well pronounced. In spring the plankton almost entirely consists of diatoms. In June-July a mass development of Cyanophyceae begins. This is due to lower flow speeds higher water temperatures and other factors.

The above features of phytoplankton formation may be illustrated by the Tsymlyanskoye reservoir.

Diatoms are the first to appear in the plankton and they vegetate longer than other algae (Pototskaya, 1965). By autumn they reach their peak stage (up to 23 million cell/l, weight up to 45 mg/l), Stephanodiscus astrea prevailing.

Every summer an intensive water “blooming” is observed in the reservoir, according to Pototskaya (1965) who carried out the investigations from the fifth to tenth year of the reservoir's existence. Water bloom is caused by mass propagation of Cyanophyceae (Aphanizomenon flosaquae, Anabaena flosaquae and Microcystis aeruginosa). The first two species were also abundant during the first years of the reservoir's existence. Maximum abundance of Cyanophyceae (almost monoculture Microcystis) reached 2,386) million cell/l, the standing crop being 521 g/m³. Such algae accumulations were observed in the coves of the downstream part of the reservoir and had been brought there by the wind.

The high content of biogenous compounds in the water, intensive vertical water exchange, high insolation and long-term vegetative period (6 to 7 months) were conducive to the extremely intensive propagation of Cyanophyceae in the Tsymlyanskoye reservoir. The mass development of phytoplankton observed in the majority of the Volga, Dnieper and other reservoirs is a very important bioproduction factor. Many phytoplankton components are important for plankton eating animals but much more important is the role of phytoplankton as the supplier of fine detritus, on the basis of which a rich saprophytic microflora develops. It forms the food supply for plankton Cladocera and Calanoidae and, in the southern reservoirs of the U.S.S.R. for mollusc larvae Dreissena polymorpha as well. It is remarkable that the maximum abundance of the reservoir bacteria is in the 2 to 5 m deep layer (Romanova, 1965).

Figures of the primary production of a number of reservoirs are given in Table 1. The amount of the primary production in the Tsymlyanskoye reservoir considerably exceeds that of other reservoirs, this being due to the higher content of biogenous substances (mineral phosphorus and nitrate nitrogen) of the submerged soils of this zone.

Later we shall discuss the zooplankton problem but now we should note that phytoplankton and planktogenous detritus may be of vital importance for the diet of some fish species. The main specimen of detritophage fish is the Hypohthalmichthys molitrix (Valenciennes), i.e. a species belonging to the warm-water Chinese fauna but absent in the fauna of Siberia, central Asia and Europe.

According to the recommendations of the State Scientific Research Institute of the Lake and River Fisheries, work is being confined to transplanting of Hypophthalmichthys molitrix into the southern reservoirs of the U.S.S.R.

The Soviet scientists N.A. Dsuban (1959), L.A. Luferova (1965) and others specify three main stages in the zooplankton formation process.

The first stage is the formation of an ecologically heterogeneous population in the extensive pelagic area of the new water reservoir. The population consists of conventional components of the river plankton as well as of a great number of littoral and thicket species of protozoon, rotifers, Copepoda and Cladocera characteristic of ponds and floodplain water bodies. The degree of the zooplankton variety in the systematic composition is demonstrated by the data obtained by N.A. Dsuban (1958) on the Tsymlyanskoye reservoir and L.A. Luferova (1964) on the Gorkovskoye reservoir. During the first years the zooplankton of the Tsymlyanskoye consisted of 95 forms and that of Gorkovskoye consisted of 115 forms. The zooplankton is still abundant and rich as far as the number of individuals and the standing crop are concerned.

TABLE 1. PRIMARY PRODUCTION OF SOME RESERVOIRS

The mass development of zooplankton in reservoirs during the first years of existence of the new waters is the result of a number of conditions favorable for planktonic invertebrates, namely the slower flow, lessening of mineral trash and debris, mass development of bacterioplankton and phytoplankton. In other words, the mass development of zooplankton is due to the head effect of the river and the effect of submerging lands with their various areas and waters (Pirozhnikov, 1957, 1961, 1956).

The next stage of the zooplankton formation is characterized by a more or less decrease in the number of species in the pelagic area and in the littoral zone. In the pelagic area, the number of shoreline and thicket forms decreases. In the littoral zone, the plankton forms peculiar to the pelagic area of large reservoirs decline. Rotifers become dominant, including Polyarthra trigla, Keratella quadrata, or K. cochlearis, Kellicottie longispina, Brachionus calyciflorus, Euchlanis dilatata, Conochilus, Asplanchna priodonta; among the Cladocera - Daphnia longispina, D. hyalina, Bosmina coregoni, B. longirostris, Leptedora kindti; among the Copepoda - Mesocyclops leuckarti, M. oithonoides, Cyclops strenmus, Diaptomus graciloides, D. gracilis.

These species are common and, as a rule, abundant in all large reservoirs of the European part of the U.S.S.R. The plankton component characteristic of the reservoirs built on the Dnieper, the Don and the Volga is larvae (veliger) of the mollusc Dreissena polymorpha.

The total number of species forming the reservoirs' zooplankton dropped to 40 to 42 species (Tsymlyanskoye reservoir) and to 64 species (Gorkovskoye) in 4 to 5 years. The second stage of the zooplankton formation in reservoirs ends in a more or less stable specific complex, and the third stage is set, which is characterized by more or less regular seasonal changes in plankton, quantitative indices being the same every year.

Thus, the zooplankton formation process in large reservoirs lasts for a comparatively short time, 3 to 4 years; but it may last as long as 6 to 7 years in some water bodies.

The total standing crop and production of zooplankton in reservoirs, as well as its quality as fish food, considerably exceeds that in merging water bodies. Thus, in the area of the Don prior to the damming at the station of Tsymlyanskoye, the zooplankton standing crop was 0.47 g/m³, while in the second year after the damming it was already 8.04 g/m³ in the reservoir. In subsequent years (Kaftannikova, 1965), the average zooplankton standing crop for the whole reservoir was more than 1 g/m³ and it was as much as 5 g/m³ in the coves. Data processing according to the systematic groups shows that the abundance of crustaceans increases considerably in the zooplankton composition (see Table 2), this being extremely essential for fisheries.

Crustaceans in the corresponding areas of the Volga, Dnieper and Don did not exceed 8,000 m³ of water.

TABLE 2. CRUSTACEAN ABUNDANCE IN SOME RESERVOIRS THOUSAND INDIVIDUALS/m³

In each large reservoir the upper, intermediate and lower zones are distinguished according to morphometric and biological features, the lower zone extending as far as the dam. The upper zone is under the direct influence of the river feeding the reservoir and river conditions are retained in it. The lower, or fore-dam zone, differs greatly from the upper one especially in lake-type reservoirs. It is usually characterized by maximum depth, slow flow and the mass development of diatoms and Cyanophyceae. The intermediate zone is of transitional character.

In accordance with this, zooplankton of more or less characteristic specific composition and component number is formed in each zone. Certainly the zones of the reservoir are closely interconnected hydrologically and biologically, especially during flood time. At this time (usually May-June), the zooplankton of reservoirs consists mainly of rotifers (chiefly Asplanchna priodonta, various Keratella and Synchaeta). But even at this time the abundance and the total standing crop of zooplankton increase in the direction towards the dam. This change is particularly pronounced in summer and autumn.

Mass development of rotifers continues in July and August in the upper zone and in some waters in the intermediate zone. The abundance and the total standing crop of Cladocera and Copepoda at this time increase rapidly in all the zones (see Table 3). Maximum peaks are observed most often in the intermediate zone but variations also occur.

The zooplankton of large reservoirs is an object of regular observations. These investigations show that some fluctuations in quantitative development of zooplankton (several groups or species) occur.

Thus, the summer zooplankton standing crops in the Kakhovskoye reservoir varied in 1956–59 from 0.31 to 15.7 g/m³ of water and in the Volgogradskoye reservoir from 0.53 to 4.94 g/m³ of water in 1959–61. These variations are due to the peculiarities of hydrometric conditions of each year, the duration of the vegetative period, the meteorological conditions of the year and so on. The importance of the water level conditions for the quantitative zooplankton development is illustrated by the data obtained on the Tsymlyanskoye reservoir. In high flood years the zooplankton standing crop may be considerably higher than in low level years (see Table 4).

The zooplankton is of great importance as a food supply for fish inhabiting reservoirs. The main groups forming the zooplankton, i.e., rotifers, Copepoda and, especially Cladocera, are the staple food for the fry of all species. Some species of fish: Abramis ballerus, bleak (Alburnus alburnus), white bream (Blicca bjoerkna) and others, feed on plankton crustaceans during their whole life. One may say that the abundance, growth and total standing crop of plankton-eating fish depend to a considerable degree on the extent of the quantitative development of zooplankton. High zooplankton production of our southern reservoirs is mainly responsible for the great abundance of Abramis ballerus, bleak, bream and other limnophilous fishes.

It is notable that these species develop in the new controlled waters much better than in uncontrolled rivers. We may take Abramis ballerus as an example. Its growth rate in the Tsymlyanskoye reservoir has almost doubled. For instance, the Abramis ballerus fingerling in the reservoir is 14.1 cm long, while before the Don was controlled it was only 8.1 cm. The adult fish weight has doubled. The high growth rate of a tremendous abundance of fry and adult fish planktonophages in the new reservoirs is evidence of such high fish productivity of these waters, which was never surpassed by uncontrolled rivers.

Reservoir bottom fauna formation is a longer and more complicated process. Analyzing the data on the benthos development in reservoirs, F.D. Mordukhai-Boltovskoi (1961), T.I. Ioffe (1961) and other researchers have shown that bottom fauna formation in large reservoirs on plain rivers goes through a number of stages.

An extremely specifically varied fauna inhabits rivers with additional waters (plain lakes, river backwaters and cut-off lakes).

Reservoir formation is accompanied by the decomposition of rheophil and phytophil biocenoses which had existed before. Mixing of ecologically different species takes place. Characteristic is the presence of soil fauna (Enchytraeidae and Lumbricidae); and Phyllopoda (Apus cancriformis and Leptestheria) in the case of the Tsymlyanskoye reservoir.

At this period the bentho-fauna is very irregularly distributed over the bottom of the water reservoir. Phytophil elements gradually become extinct and rheophil species remain in the upper zone of reservoirs, where river conditions remain, or form separate accumulations. The inhabitants of silt-sandy soils are affected least of all.

The duration of this stage depends on the reservoir filling time. It lasts through the whole winter, if filling is started in autumn and is reduced to 2 to 3 months when the reservoir is filled in spring.

The second stage of “temporary” biocenosis formation begins when water stops running completely or nearly so and is characterized by the mass colonization of the whole reservoir bottom by chironomid larvae as early as the first summer. These species adapt themselves to new waters quickly and become prevalent in the benthos due to spreading by air and intensive propagation (several generations per year). Chironomus f.l. semireductus, Clyptotendipes and Proclaclius are first to appear in the reservoirs. At the same time, molluscs (Viviparus, Unio, Dreissena) leeches and oligachaetes spread rapidly over the bottom of the reservoir. Big quantities of oligochaetes occur in the silted bed areas.

The development of various invertebrates in submerged forest and bush areas is typical. These are: pearlworts (Bryozoa), molluscs, crustaceans, chironomid larvae, etc.

Thus, during the first year of the reservoirs existence a mass benthos development is observed due to the favorable trophic conditions during the period of rapid biochemical decay of submerged vegetation and fast growth of saprophytic microflora.

In the second and third years chironomid larvae no longer dominate and are replaced by molluscs and oligochaetes. The bottom fauna becomes more homogeneous in different biotopes as far as abundance and specific composition are concerned. Only the beds of the main rivers are characterized by an abundance of oligochaetes.

The third stage of benthos formation begins when the homotropous fauna stops breading over the submerged land. Generally it sets in 2 to 3 years after the reservoir has been formed, the specific composition becoming less varied and the benthos standing crop is decreasing. Deterioration of the oxygen supply results in the extinction of many rheophil forms. The variety of phytophil species decreases because of the inadequate development of shoreline vegetation due to the fluctuations in the water levels. For instance, in the first year of the Tsymlyanskoye reservoir over 130 species (average standing crop 6.79 g/m²) were found, while in the third year the number of species fell to 65 (standing crop not exceeding 1.83 g/m²).

In the Volga reservoirs (Kamskoye, Rybinskoye and Gorkovskoye), no sudden changes in benthos occurred in subsequent years and the total standing crop of bottom invertebrates (big molluscs not included) is still relatively low. In the Rybinskoye reservoir the first year after filling, the benthos standing crop was 11.9 to 58 g/m²; in the sixth year 8.2; in the eighth to tenth years 8.9, and in the 12 to 13 years it dropped to 2.1 to 2.6 g/m². Evidently such dynamics of the benthos standing crop are closely connected with the growth in the abundance of fish benthophages.

Benthos development in the Volga reservoirs is determined by their location. For instance, in the Rybinskoye reservoir the conditions are unfavorable for bottom fauna because the reservoir is fed by stagnant waters and the silt formation process is extremely slow. The silt in the reservoir is chiefly peaty and consists mainly of ligninohumus substances which are difficult to assimilate.

According to the classification by T. I. Ioffe (1961), the above Volga reservoirs are of average productivity with the average standing crop of benthos 30 to 60 kg/ha.

In the southern reservoirs, Dnieprovskoye, Kakhovskoye and Tsymlyanskoye, the benthos development is intensive. For instance, in the Tsymlyanskoye reservoir the benthos standing crop rose to 25.8 g/m² in the fourth year after filling. The benthos increased progressively during subsequent years and was as high as almost 80 g/m² in the tenth year (1961). The abundance and total standing crop of benthos is likely to increase in the future, as the bottom silting and organic matter accumulation in the bottom sediments are still going on (see Table 5).

Situated in the black and partially chestnut soils zone, the southern reservoirs are notable for their high bioproductivity. As far as benthos goes, they are highly productive reservoirs, with the average benthos standing crop being over 120 kg/ha.

The total standing crop of benthos in the Kuibyshevskoye and Volgogradskoye reservoirs is considerable: 340 to 690 kg/ha. However, these waters are characterized by specific features essential for fisheries. The bulk of the benthos standing crop is formed by Dreissena which is not often eaten by fish. The benthos standing crop (Dreissena not included) in the Kuibyshevskoye reservoir is about 4 g/m² and in the Volgogradskoye 2 g/m².

The whole process of benthos formation in reservoirs on plain rivers takes 6 to 7 years.

TABLE 5. AVERAGE STANDING CROP (IN G/M²) OF BASIC GROUPS OF ZOOBENTHOS IN THE TSYMLYANSKOYE RESERVOIR IN DIFFERENT BIOTOPES, JULY 1961

A characteristic feature of the river reservoirs is summer fluctuations in water level and a considerable drop of the water level in autumn and winter. Drainage of large areas is fatal for the benthic organisms.

When the water level falls in winter, ice sinks to the bottom and masses of molluscs and chironomid larvae perish. Summer-autumn drainage of the shallow water zone results in the death of the majority of the organisms due to the moisture shortage. This is especially true for the southern reservoirs. A partial drainage of the shoreline zone of the reservoir, which occurs in the middle or end of summer, furthers the benthos development. The drained area becomes overgrown with moisture-loving vegetation which provides detritus and serve as a good substrate for spawning of wild carp and other carp type fish.

Another equally important feature characterizing the plain river reservoirs is the considerable seasonal fluctuations in the abundance and standing crop of the benthos. This is explained by the fact that secondary limnophilous organisms, particularly chironomid larvae, prevail in the benthos of reservoirs, the number of species being limited. The pupation of larvae and hatching of adult forms entails a sharp fall in the benthos standing crop. It occurs in reservoirs just in the feeding period of benthos-eating fish. In the Tsymlyanskoye reservoir in the spring of 1955 benthos standing crop was 5.04 g/m², in summer it fell to 1.12 g/m² but by autumn it increased four times and was 4.16 g/m². A food deficiency at the peak of fish feeding affects the growth rate of food fish. One or another fish species may also be short of sufficient food in the reservoir. A very interesting situation arose in the Tsymlyanskoye reservoir. Due to abundant plankton and benthos the carp fry grew very well and, because of their large size, they became inaccessible for the zander fry. The idea to acclimatize Mysidae there was suggested as a way out of the situation.

The sharp decrease in the benthos standing crop at the peak of summer fish feeding due to winged chironomid leaving the water, the food shortage for some valuable fish species at separate stages of their life and the absence or insufficiency of higher crustacean species in the fauna of the reservoirs, suggested work on the planned formation of invertebrate fauna in the reservoirs to enrich the food supply for the bottom fish.

Soviet scientists V.I. Zhadin (1940, 1947), P.A. Zhuravel (1947), F.D. Mordukhai-Boltovsky (1947), P.L. Pirozhnikov (1955), T.I. Ioffe (1958) and others have substantiated the expediency of transplanting and the possibility of acclimatization of valuable species of fish food in the reservoir. Some invertebrates have already been transplanted into the southern reservoirs. Lately this work was expanded considerably. From 1947 to 1962 over 110 transplantations of food species into 37 reservoirs were done. The food invertebrates were acclimatized in 27 reservoirs and in 11 reservoirs the transplanted species became naturalized i.e., began to propagate.

Forty species of invertebrates were used for acclimatization in the reservoirs, including seven Mysidae species, nine Gammarida species, six species of Cumacea, three species of Corophiidae, ten species of molluscs and two species of Polychaetes. Of these, 21 species were acclimatized in some of the reservoirs and nine were propagated on a large scale.

The successful acclimatization of valuable fish food species was conducive to a considerable increase in fish production in these waters.

In the Tsymlyanskoye reservoir, Mysidae (Mesomysis Kowalewsyi and M. intermedia) were acclimatized successfully.

Masses of these crustaceans now develop there and are consumed by the fry of Zander and other fish. Thanks to this measure, over 2,000 t of fish are caught in the reservoir annually. The transplanting of Pseudocum cercaroides into the Dnieprovskoye reservoir gave good results. Encouraging data was also obtained in the Kuibyshevskoye and Gorkovskoye reservoirs.

We should note in conclusion that reservoirs offer wide prospects for slower run-off and other hydrological and hydrochemical features of the reservoirs that result in an abundance of plankton and bring about favorable conditions for the development of benthos and nektobenthos.

Under such conditions the abundance of the relevant fish species may be very high. Therefore, fish catches in reservoirs may increase many times compared with those in merging river areas.

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