Diversity of Zooplankton and Zoobenthos in the Danube River, Its Tributaries, and Adjacent Water Bodies

Boris Russev, Anelia Petrova, Ivanka Janeva, and Stoice Andreev

I. The Danube River (Between the 845th and 375th Kilometer), Including Islands and Riparian Marshes

Scientific research on the benthic fauna of the Bulgarian section of the Danube River began with the work of Wohlberedt (1911), and was continued by several other investigators (Haas, 1911; Buttner, 1926-1928; Arndt, 1943; Drenski, 1947). However, specialized studies of species composition and distribution, and environmental impacts on the development of the zoobenthos, began only in 1956. Specialized research on the zooplankton began in 1958. Many studies on these topics have been published since (information on these publications is presented in the bibliography). In addition to these, we have used the more general publications of Naidenow (1965, 1978, and 1984) for zooplankton and Russev (1967, 1987, and 1979) for zoobenthos.

Zooplankton

One hundred and twenty-four species of zooplankton occur along the Bulgarian back of the Danube River. Two hundred and twelve species, chiefly Rotatoria, are known for the riparian and island marshes. The zooplankton community is dominated by the Rotatoria. The Rotatoria Brachionus calyciflorus Pallas occurs year-round, but constitutes 90% of the total biomass in the spring and autumn. Crustacea are relatively more numerous in the summer.

Protozoa

Only 17 taxa of Rhizopoda and Ciliata have been identified in zooplankton studies. They occur only in sporadic mass incidents. The most frequently occurring are representatives of the Arcella, Centropyxis, Lesquerisia, Vorticella, and Zootamnium.

Rotatoria

One hundred and twenty-three taxa have been established: 58 in the Danube River, the others in Danube's riparian and island marshes.

Cladocera

Sixty-eight species, including 9 forms and varieties, have been established. Thirty-three are found in rivers, 36 on islands and marshes, and 46 in low-lying coastal wetlands. Only 16 species are common to all three categories. However, 26 species are shared by the islands and marshes.

Copepoda

Thirty-four species have been established. Thirteen are found in rivers, 18 on islands, and 32 in riparian marshes. Nine species are common to all three categories. Seventeen species occur in both riparian and island marshes.

Crustacea

The Danube has 44 species of Crustacea. Thirty-four species occur in marshes and on riparian islands, while 78 species are found in riparian marshes and floodplains. In other words, there are only half as many Crustacean species in the rivers as in the standing riparian water bodies. This is the reverse of the ratio for the Rotatoria.

Research over the years on the quantitative distribution of zooplankton along the Bulgarian Danube has revealed two basic patterns in its development: a natural (undiked) and diked regime. A non-diked left bank and the absence of dams in the middle and lower stretches characterized the river until 1970. This was important in terms of the character of the plankton community, which is determined by the periodic or regular exchange of faunistic elements between the river and its adjacent water bodies.

The zooplankton in the river consist almost entirely of Rotatoria species, constituting on average 76% of the plankton numbers and 70% of the biomass. Cladocera, whose mean biomass barely exceeds 4% of the total, is the least represented. The Crustacea play a smaller role in the summer months, while the Rotatoria never fall below 90% of the total biomass values in the autumn.

Zooplankton distribution is uneven along the length of the river. The highest indices occur in the Rousse region, and the lowest in the river below Svishtov. Distribution is also uneven across the river's width, zooplankton being more prevalent towards the northern bank, where the spacious marshes of Romania drain. Zooplankton are most abundant in the autumn months (when over 95% of the annual quantities occur), followed by the spring. The lowest quantities occur in the summer months. In the autumn, the mean biomass weight amounts to 2.8 g/m³; in the spring 0.12 g/m³; and in the summer barely 0.009 g/m³.

Under the natural hydrological regime, the period of mass development of zooplankton does not coincide with the period when the main species that feed on them zooplankton - the Danubian fishes - hatch and grow. After the construction of dikes and dams along the river, the quantity of thelmatho-heleoplankton Crustacea declined, without, however, affecting the dominant Crustacea. Rotatoria comprise 72% of the biomass, although Cladocera can at times comprise up to 85% of the biomass.

Over the course of a years, numbers generally fluctuate between 134,000 specimen/m³ and 13,000 specimen/m3, while biomass may vary from 0.79 to 0.108 g/m³. The seasonal curve has an identical spring and autumn maximum and a very low autumn minimum. Thus the changes in the zooplankton curve throughout the year, and the changing role of Crustacea plankton in building up zooplankton biomass, show the transition from lotic to limnic conditions.

As noted above, zooplankton are relatively unevenly distributed along the width of the river. Frequently the quantity of zooplankton along the Bulgarian bank is lower than it is along the Romanian bank. Despite a reduction in the average annual flow after 1971, the conditions during the first half of the year favored the survival of the small fish, which are exceptionally planktonophagic in this period.

From a biological point of view, the river system under its natural hydrological regime should be seen as a dynamic unity consisting of the river, riparian marshes, and marshes on river islands in which an exchange of aquatic organisms occurs. The intensive rate of plankton development during the autumn months (when water levels rise in the river and marshes) can be explained by the presence of large quantities of plankton in the river. After the barriers between the river and adjacent marshes are ruptured , the concentration of plankton rapidly falls below that of the spring and summer, although hydrological conditions in the Bulgarian sector of the river may exhibit only small changes. Research has shown that the standing water bodies along the river and on the islands play a considerable role in the normal biological cycle of the river, and in particular in determining plankton density - a significant prerequisite in establishing a rationally managed fishery and maintaining water quality in the Danube.

Studies of the Danube in the section between the 845th and 541st kilometer show that when the biomass level is at 0.1 g/m³, zooplankton production amounts to 17,250 tons, or 40% of the secondary productivity in this section. This, however, represents only 6.2% of total biological productivity. Under these conditions, the river would produce 260 tons of fish - approximately 40% in the spring season, 28% in the summer, and 35% in the autumn.

Zoobenthos

A total of 305 zoobenthos species have been identified in the Bulgarian section of the Danube - 275 in the river itself and 94 in channels and marshes on the Bulgarian islands. However, a number of species that are more sensitive to changing conditions are now extinct. The larvae of mayfly Palingenia longicauda, which has long been extinct in the rivers of Western and Central Europe, is a typical case. In the Bulgarian Danube, it was common up to the 1970s. Today it is found only in the tributary system of the Tissa River. Forty-four species occur relatively frequently, and 16 species are dominant. The species composition can be broken down into the various groups as follows:

Hydrozoa

So far only 3 species have been found in the Bulgarian sector of the Danube.

Turbellaria

Five species, among them Dendrocoelum lacteum (O.F. Mueller), have been found in the Danube. They also occur in the channels of the islands in the Bulgarian Danube.

Nemertini

The species Prostoma graecense Boehm has been found.

Nematoda

Only two species of the Dorylaimidae family have been found.

Nematomorpha

Only the species Gordius aquaticus L. has been reported.

Polychaeta

Two species (Caspian relicts) have been found.

Oligochaeta

Forty-six species have been found in the Bulgarian sector of the Danube.

Hirudinea

Eleven species of leeches are known to inhabit this section of the Danube. Five of these are also found in the marshes of the Bulgarian islands on the Danube.

Gastropoda

Twenty-eight species have been found in the Danube, and 15 in marshes and channels of the Bulgarian islands on the Danube.

Lamellibranchia

Sixteen species have been found in the Danube, and 8 in the marshes and the channels of the Bulgarian islands on the Danube.

Hydrachnellae

Only six species have been reported, from marshes of the Bulgarian islands on the Danube.

Decapoda

Only the Caspian relict Astacus leptodactylus Esch. has been found.

Mysidacea

Six species have been found in the Bulgarian section of the Danube. Limnomysis benedeti Czern. has also been established in the marshes on the Bulgarian islands along the Danube. Two species of Isopoda have been found. Jaera sarsi Valk. is a Caspian relict.

Amphipoda

Twenty-three species have been found in the Danube, while 4 have been found in marshes and channels of the Bulgarian islands on the Danube.

Ephemeroptera

In the Bulgarian section of the Danube, 22 species have been found; 7 species are found in the marshes and channels of the Bulgarian islands along the Danube. However, many of them can no longer be found here.

Plecoptera

Only three species have been reported here, and these were found during the first period of studies (1956-1961).

Odonata

Six species have been found in the Bulgarian Danube, and 7 species in marshes and channels of Bulgarian islands on the Danube.

Heteroptera

Ten species occur in Danube, and 9 species in marshes and channels of the Bulgarian islands of the Danube.

Coleoptera

Nineteen species have been found in the Danube River, and 10 species in the marshes and channels of the Bulgarian islands on the Danube.

Trichoptera

Seventeen species have been found in the Danube River, but only one in the marshes and channels of the Bulgarian islands on the river.

Diptera

In the Danube 55 species have been found. Two species are found in marshes and channels of the islands of the Danube. Two of them belong to the Simuliidae family, and 47 to the Chironomidae family.

Bryozoa

Nine species have been found in the Danube River.

The following groups of benthic invertebrates have not been studied in the Bulgarian section of the Danube: Rhizopoda, Ciliata, Porifera, Tardigrada, Hydracarina, Ostracoda, Harpacticoida, Cumacea, Hydrozoa, Turbellaria, Nematomorpha, and Nematoda; the Diptera family has also been poorly studied.

Studies of the zoobenthos communities in the Bulgarian-Romanian section of the Danube throughout three seasons in 1964 showed that the mean biomass had fallen approximately 3.5 times when compared with data from 1958. This was observed in particular at the eastern end of the river section, and was attributed to the increasing influx of pollution from the tributaries and industrial towns situated along both banks of the river. A gradual rise in the general biomass was detected for the 1970-1973 period, and was attributed to the favorable influence of falling tur-bidity (i.e., lower rates of sedimentation due to a decrease in water-borne silt) as a result of the commissioning of the Iron Gates power complex.

These favorable impacts have continued, and even increased. In October 1982, the zoobenthos species composition in two western parts of this section of the Danube - Novo selo (834th kilometer) and above Lom (747th km) - was comparable to that in the 1970-1973 period, a finding supported by results in comparisons of carried sediments, turbidity, and zoobenthos biomass over various periods. However, the impact of the hydropower plant does not extend over the entire Bulgarian-Romanian sector. The tributaries from both sides of the Danube, despite an insignificant (10%) increase in water flow, carry substantially more sediments and insufficiently treated waste waters than in previous periods.

The Danube was studied during a period of very low water (between 1998.7 and 2732 m³/s) in October 1986. Fifteen groups and 89 taxa of invertebrates were found. Sixty-five were identified at the species level (species of the Chironomidae family have not been distinguished). The class Oligochaeta is represented by the highest number of taxa (42), followed by the order Amphipoda (12), the class Gastropoda (9), and the order Trichoptera (6). These figures are considerably lower than those given in general studies of species composition of the Danube River. The latter reflect many years of research at many locations along the length of the river.

In addition, many species (in particular, those in the orders Ephemeroptera and Plecoptera) have long been absent from the Bulgarian portion of the river.

The groups that are found most often are the Diptera (chiefly Chironomidae and Bezzia), with a frequency (pF = 71.43%), followed by Amphipoda (pF = 68.64%), Oligochaeta (pF = 51.79%), Gastropoda (pF = 35.71%), Trichoptera (pF = 28.57%), Lamellibranchia (pF = 25.00%), Turbellaria (pF = 23.21%), Isopoda (pF = 19.64%), Nematoda (pF = 16.07%), Polychaeta (pF = 8.93), Odonata (pF = 7.14%), and Hydrozoa (pF = 5.36%). The most frequently found species are Corophium curvispinum (pF = 30.36%), Hydropsyche bulgaromanorum and Isochaetides michaelseni (pF = 26.79%), Lithoglyphus naticoides and Dreissena polymorpha (pF = 23.21%), Pontogammarus obesus (pF = 21.43%), Pontogammarus sarsi (pF = 19.64%), Jaera sarsi sarsi and Dikerogammarus haemobaphes fluviatilis (pF = 17.86%), Propappus volki (pF = 16.07%) etc. Prevailing species are Corophium curvispinum and Pontogammarus sarsi (DF = 17.86%), followed by Hydropsyche bulgarumanorum (DF = 14.29%), Isochaetides michaelseni (DF = 12.5%), Propappus volki (DF = 10.71%), Lithoglyphus naticoides (DF = 7.14%), and Jaera sarsi sarsi, Pontogammarus obesus, Stenogammarus compressosimilis, and Otamothrix moldaviensis (DF = 5.36%). There are some species, that appear infrequently in the biocenoses, but when they do appear immediately become dominant and acquire a high "order of domination" (DT) ranking. This is the case with the species Dero obtusa, Potamothrix hammoniensis, and Stenogammarus compressosimilis (DT = 100%), Pontogammarus sarsi (DT= 90.94%), Propappus volki (DT = 66.65%), Corophium curvispinum (DT = 58.83%), and Hydropsyche bulgaromanorum (DT = 53.34%).

With regard to density, the representatives of Amphipoda are first - mainly Corophiidae (66.70%), followed by Oligochaeta (17.23%), Trichoptera (5.17%), and Chironomidae (2.91%). With regard to biomass, the most important groups are from the Gastropoda (79.90%) and Lamellibranchia (17.13%). If we ignore Mollusca (because of the weight of their shells), once again Amphipoda (57.47%), followed by Richoptera (26.73%) and Oligochaeta (8.98%), was the most significant as regards biomass.

The structural parameters of the biocenoses reveal quite varied conditions in this section of the Danube. Thus, the species diversity index is between 0.1 and 3.46; the general species diversity index, between 0.48 and 9.27; the equitability index, between 0.16 and 0.97; and the dominance index, between 0.13 and 0.89. Low values of these indices (and, in the case of the dominance index, high values) occur generally in parts of the Danube with large-grain sand bottoms (with no silt traces), which are unfavorable for animals. High values are found in areas with a dense lithofauna and (to a lesser degree) in pelorheophylic biocenoses. Studies of the bottom of the Danube indicate that the psamic biotope occupies 60% of this section, followed by gravels (27%), silts ( 5.5%), and psammic-silts ( 7.27%).

Lithorheophilic zoocenoses are the most abundant (in particular those of corofiumic silt), followed by eulythorheophilic zoocenoses. The average general density and biomass are higher than averages from the recent past. Psammorheophilic zoocenoses have the lowest values, with a total number of 1537 species per m² and mean biomass of 0.9 g/m² (molluscs have not been found). This biomass is about 4 times higher than the values from past years, which we believe is due on the one hand to exceptionally low water quantities during the course of the studies, and on the other hand to lower sedimentation rates and the probable enrichment of the bottom with organic substances - all of which create conditions for increases in the trophic base of the ecosystem.

Ecological Problems

The impacts of pollution and dam construction are the main ecological problems of the Danube that have been studied by Bulgarian biologists. In addition to generally employed saprobiologic methods to study the impacts of pollution, methods of determining the self-purification potential of the river through the use of biogenic aeration were also used, together with some original criteria - "percentage of habitation," "ratio between numbers and species composition of various zoobenthos groups," and "variation in biomass."

Zoobenthos studies, reflecting the state of the bottom of the river, provide a more precise and valid assessment of total pollution and self-purification characteristics as compared with studies conducted in other scientific disciplines. Thus, in the years 1956-1961, the Bulgarian Danube was oligosaprobic to oligo-betamesosaprobic. In the years 1964-1978, it was stable betamesosaprobic. In the years 1982-1988, this stable state was interrupted, with a certain tendency towards alphamesosaprobia , despite the exceptional autopurification potential of the great river (average water flow about 6,000 m3). This has occurred below the inflow from the tributaries and the larger cities on both banks of the river. Autopurification occurs over the course of about 1-5 km. Immediately below Rousse, however, the benthos fauna has been destroyed over a stretch of 4 km, while the zone of autopurification of the river (based on plankton studies) amounts to 15 km at a water temperature of 22-23°, 23 km at 16-19° and 31 km at 7-8°.

Pollution from the Arges, which carries the effluent of Bucharest to the Danube, results in a state of polysaprobia on the river, with a ten-fold decrease in the zoobenthos biomass. In order to protect against further pollution, the Danube should be given the strict status of inland rivers under Bulgarian legislation, which requires the treatment of industrial waste and effluent. This can be done only after a trilateral (or at least bilateral) convention on the preservation of the Danube's water quality is agreed to by Bulgaria, Romania, and possibly Serbia. The remaining countries of the Danube basin have not so far played a significant part in saprobic pollution of the Danube along its Bulgarian bank.

Studies have shown that saprobic pollution from even large cities such as Vienna, Bratislava, and Budapest is self-purified by the river at distances up to 40 km, and the Iron Gates dams I and II contribute to the self-purification of the Danube in the Bulgarian sector. It would, of course, be useless to provide funds for treatment plants on the Bulgarian bank of the Danube if this is not matched by efforts in Romania and Serbia. To ensure the appropriate development of such a convention, it should be drawn up by the International Organization for the Study of the Danube, as well as the former Comecom.

II. The Bulgarian TributAries of the Danube River

Various groups and species of invertebrates in the Bulgarian tributaries of the Danube have been studied by both Bulgarian and foreign specialists. In some cases, these studies have consisted of individual projects over selected sections of only the most commonly visited rivers, such as the Iskar and its tributaries from Vitosha Mountain. Later research has focused on the systematics of the groups. For example, Golemanski and Todorov (in print) report 33 species of Rhizopoda from the tributaries of the Beli Iskar.

Studies of the Ciliata fauna in most of the Bulgarian tributaries of the Danube have been conducted over a longer period, concentrating on species composition, the influence of various ecological factors on them, and their significance as bioindicators (Detcheva, 1971, 1972, 1974 -1975, 1975a, 1975b, 1979, 1982, 1983a, 1983b, 1991, in print). Ryanovska-Vasileva (1949) reported 3 species of Hydrozoa for the Iskar and Vit. More detailed studies of Oligochaeta from the Danubian tributaries have been conducted by Dimitrov (1972 and 1973), Uzunov (1976 and 1980), and Uzunov (1979). Buettner (1928), Petkov and Oshanova (1960), Drenski (1947) and Angelov (1959 and 1976) provided information on the distribution of some representatives of Gastropoda, while the latter two also worked on Lamellibranchia.

Other important studies include Valkanov (1938) on Isopoda; Schaeferna (1923) on Amphipoda; Boulgourkov (1961) on Decapoda; Petrova (1971 and 1985) on Hydrachnellae; Russev (1957, 1960, 1971, 1977), Jacob and Braasch (1984), and Braasch et al. (1985) on Ephemeroptera; Rauser (1962, 1965), Sowa (1970), Russev (1971), Braasch (1969, 1970, 1972), Braasch and Joost (1971a, 1971b, 1975, 1976) on Plecoptera; Beshovski (1965, 1967, and 1968) on Odonata; Gueorguiev (1957, 1961, 1962, 1965, 1971, 1987) on Coleoptera; Klapalek (1895 and 1913), Navas (1929), and Popov (1981) on Neuroptera; Josifov (1957) on Heteroptera; Botosaneanu and Sykora (1963), Kumanski (1968a, 1968b, 1969, 1971a, 1971b, 1972, 1973a, 1973b, 1985, 1988) on Trichoptera; Gulichka (1961 and 1966) on Blephariceridae; Dimitrov (1962, 1966, 1972, 1973) on Chironomidae; Konsulov and Paspalev (1925), Enderlein (1929), Kovatchev (1969, 1973a, 1973b, 1979) on the family Simuliidae; Patev (1924) and Grancharova (1968a, 1968b, 1971) on the distribution of Bryozoa. Hristovich (1892), Arndt (1943), and Kownacki and Kownacka (1973) reported data on the distribution of several groups of invertebrates, but only in some locations in one or several rivers.

Complex hydrochemical, hydrofaunistic, biocenological, and saprobiological studies have been carried out over periods of several months and years in all the Bulgarian Danube tributaries, and most of the results have been published. Parts of the Lom River were studied by Russev et al. (1991); of the Tsibritsa River by Russev and Janeva (1986), of the Ogosta River by Janeva (1991); of the Iskar River by Russev (1959, 1968), Gardanov et al. (1971), and Janeva and Russev (1989); on the Iskar tributaries from Vitosha Mountain by Russev (1961); on the Vit River by Russev (1977, 1979a 1979b, 1988); of the Osam River by Gardanov et al. (1971), Russev (1977), and Russev et al. (1984); of the Yantra River by Russev (1968), Russev and Janeva (in print); of the Rositsa River (a tributary of the Yantra) by Kovachev and Uzunov (1987); and parts of the Russenski Lom by Russev et al. (1987 and 1988). Russev (1968, 1970, 1975) contributed a general review of the saprobiological state of rivers in Bulgaria, including some of the tributaries of the Danube.

A total of 1610 taxa from 25 groups of benthic invertebrates have been established in the 13 tributaries of the Danube in Bulgaria.

Protozoa

Rhizopoda have been studied only in several small streams of the Iskar drainage; 33 have been identified. Ciliophora is represented in all tributaries by 224 taxa, including the Gymnostomata (61), Vestibulifera (9), and Hypostomata (21) in Kinetofragmophora; Hymenostomata (37) and Peritricha (20) in Oligohymenophora; and Spirotricha (67) in Polyhymenophora. Studies have looked at the frequency of some species in different rivers; their distribution over various seasons; and the influence of the major chemical parameters on distribution.

Coelenterata

Hydrozoa

Four species have been found; Hydra circumcincta is most common.

Vermes

Turbellaria

Five species have been found.

Nematomorpha

One unidentified specimen of the family Gordiidae was found in the Lom River.

Nematoda

Many representatives of this group in the tributaries of the Danube have not been identified.

Oligochaeta

Ninety-one species from 43 genera have been found. A comparison of the characteristics of the Oligochaeta fauna of the Danube (between the 845th km and the 375th km) and its Bulgarian tributaries has been carried out. Analysis of the coefficient of faunistic similarity (K%) has established that the Oligochaeta fauna of the Bulgarian sector of the Danube differs substantially (mean K = 44.65) from that of its Bulgarian tributaries. The similarity among the different tributaries is considerable (K = 65% for the Vit and Yantra, K = 74% for the Osam and Iskar, etc.); the mean value of K for the tributaries is 62.90%. This is explained by the relative similarity of various ecological factors within the Bulgarian section of the Danube and by the presence of Ponto-Caspian species; the tributaries, on the other hand, have been subject to considerable anthropogenic impacts, leading to sharp variations in many ecological factors.

Hirudinea

Of the 11 species that have been found, Cystobranchus fasciatus (Koll.) and Piscicola geometra (L.) are rare. Studies have also been conducted on the ecology of leeches and their significance as bioindicators in Bulgarian rivers.

Mollusca

Gastropoda

Thirty-five species have been found. Four of these - Acroloxus lacustris L., Amnicola steini Martens, Myxas glutinosa (Muell.), and Pseudamnicola sp. - are among the rare species in the Bulgarian tributaries of the Danube. The species Fagotia acicularis Fer., F. esperi Fer., Lithoglyphus naticoides Pfeif., Theodoxus transversalis Pfeif., and Th. danubialis Pfeif. came from the Danube.

Lamellibranchia

Seventeen species have been found. Because the sampling methodology has not always been suitable, their representation in the tributaries of the Danube is probably larger.

Arachnida

Hydracarina

Forty-nine species have been found. The number of species and their presence in the tributaries of the Danube are likely more extensive, however, as a great proportion of the gathered material has not been determined.

Crustacea

Isopoda

The species Asellus aquaticus L. occurs almost everywhere where the degree of saprobic pollution is classified as Alphamesosaprobic.

Amphipoda

A total of 7 species have been found. Caspian relicts from the Danube have probably not penetrated the tributaries. Indicator parameters of Gammarus arduus Kar. for establishing the saprobicity of rivers have now been determined and published.

Decapoda

Three species are found commonly in the country. However, their presence in tributaries of the Danube is probably greater than previously determined, as the methodology used in sampling has been unsuitable.

Insecta

Endemic, relict, and rare species, as well as species with aquatic forms during portions of their life cycle, are discussed in the reports on insects.

Ephemeroptera

Seventy-seven species have been found. Studies have examined aspects of their ecology and the significance of mayflies as indicators.

Plecoptera

Ninety-four taxa have been found. Their presence in the rivers Topolovets, Voinishka, Vidbol, Archar, and Skomlya is likely much higher than currently known, as these rivers have been insufficiently studied in their upper stretches.

Odonata

Thirty-one species have been found, mainly in the middle and lower stretches of the Danube tributaries.

Coleoptera

One hundred eleven species have been found.

Heteroptera

Twenty-nine species have been found so far.

Megaloptera

The species Sialis lutaria L. has been found in pure deep silt.

Trichoptera

One hundred thirty-one species have been found so far in the tributaries of the Danube.

Diptera

Three hundred twenty-eight have been found. These can be broken down according to the following families: Blephariceridae (3), Tipulidae (3), Cylindrotomiidae (1), Limoniidae(8), Psychodidae (7), Dixidae (1), Culicidae (2), Simuliidae (60), Chironomidae (217), Thaumaleidae (1), Ceratopogonidae (3), Stratiomuidae (5), Tabanidae (2), Athericidae (2), Rhagionidae (1), Syrphidae (2), Empididae (2), Ephydridae (2), Muscidae (1), Sciomyzidae (1). With the exception of the families Simuliidae and Chironomidae, identification has been carried only to the level of genera due to insufficient research and a lack of published literature. The main aspects of the ecology of the Simuliidae family have been studied. Comparison of the Simulid fauna in the various tributaries indicates that similarity is greatest among the rivers from the Stara Planina Mountains - the Vit, Osam, and Yantra. The Simulid fauna of the Iskar, which springs from the Rila Mountains, is more distinctive, as it includes species typical of the Boreo-alpine complex, well represented in the Rilas and less in the Stara Planina. The degree of similarity between the tributaries and the Danube itself is inconsiderable for the reasons stated above.

Tentaculata

Bryozoa

This group has been studied only in the Yantra River, where 5 species have been found.

Table 1 summarizes information on the provided distributions of the various groups of benthic invertebrate animals in the different tributaries of the Danube. The northwestern tributaries of the Danube - the Topolovets, Voinishka, Vidbol, Archar, Skomlya, and Tsibritsa Rivers - have been studied to a lesser degree. As a result, they are poorer in known species diversity. Moreover, the volume of water in these streams is much lower than in the other tributaries.

Data on the number of species in the various groups of aquatic invertebrates pertains to research conducted throughout Bulgaria. Today, species composition is considerably poorer due to the increases in effluents and industrial wastewaters. Thus, studies carried out on the Yantra River in the periods 1964-1967 and 1987-1988 showed a considerable decline in species composition. The groups Porifera, Rotatoria, Gastrotricha, Nematoda, Tardigrada, Ostracoda, Harpacticoida, and Cumacea have not been studied at all in the Bulgarian tributaries of the Danube. The groups Hydrozoa, Turbellaria, Nematomorpha, Hydracarina, and Diptera (with the exception of the families Simuliidae and Chironomidae) have been insufficiently studied.

Structural Characteristics of Benthos Zoocenoses

The species compositions of different rivers have been compared using a coefficient for species similarity (Soer-ensen, 1948). Analyses of the structure of benthos zoocenoses have been carried out by determining the index of individual diversity (H) (Shannon and Weaver, 1963); the general species diversity index (d) (Margaleff, 1958); the domination index (c) (Simpson, 1949); and the equitability index (e) (Pielou, 1966). The number of species in 20 main benthos groups have been determined. Analysis of composition and structure has been based on two main criteria: the mean annual water volume and the degree of disruption of the zoocenoses due to anthropogenic impacts.

Structural analysis of the biocenoses of the Bulgarian Danube tributaries brings out the following characteristic features (Janeva and Russev, 1991). Table 2 presents the coefficient values for species similarity among the Danube tributaries. In only two instances (between the Voinishka and Vidbol Rivers, and between the Ogosta and Rousenski Lom Rivers) are these values over 50%. In most cases, the similarity of species composition between the Vidbol, Voinishka, Topolovets, Archar, Tsibritsa, and Skomlya Rivers on the one hand, and the Lom, Osam, Vit, Ogosta, Yantra, and Iskar Rivers on the other, varies between 35%-50%. Exceptions are the lower levels of similarity between the rivers Vit and Rousenski Lom (30.24%) and Topolovets and Archar (21.58%). In general, the differences in species composition between rivers from the two groups were greater, with coefficients varying between 25% and 47%. Differences in species composition were most significant between the Vit and Tsibritsa Rivers (26.90%), the Yantra and Skomlya (21.20%) and the Iskar and Skomlya (18.75%). Differences in the volume of water in these rivers (resulting in variable current speeds) and in other ecological factors, explains the differences between their species compositions.

Analysis of the mean values of general and individual species diversity, together with dominance (table 3), allows us to group the studied tributaries of the Danube into three groups. Those in Group I (Voinishka, Vidbol, Archar, and Lom) have species diversity values above 3.0 and dominance values under 0.20. The rivers in Group II - Skomlya, Tsibritsa, Ogosta and Vit - are moderate species diversity and dominance values, within the intervals 2.97-2.65 and 0.27-0.23; in this group, only the Vit has a lower dominance value (0.20). The third groups includes the Iskar and Yantra Rivers, which have the highest dominance indexes and the poorest levels of species diversity. As far as evenness is concerned, it is clear that the tributaries of the Danube have high equitability indexes.

These analyses lead to the following conclusions.

• Most of the Bulgarian tributaries of the Danube (the Voinishka, Archar, Skomlya, Lom, and Tsibritsa) are characterized by a rich and varied species composition. Their biocenoses are stable and successfully neutralize the negative effects of anthropogenic interference.
  
  
• Most of the middle and lower stretches of the relatively large rivers (e.g., the Vit, Yantra, and Osam) are in an unfavorable position, however much their upper stretches and tributary systems augment their species composition;
  
  
• The Iskar, which receives the effluent of Sofia, is in a comparatively unfavorable situation.

The Saprobiological Status of the Danube Tributaries in Bulgaria

The saprobiological status of the various tributaries of the Danube in the period from 1956 to 1991 has been reported in many publications. These have been discussed earlier, and are listed in the bibliography. The most recent data from the saprobiological studies are given in Table 4.

Comparisons were drawn between the saprobiological state of the 13 Danubian tributaries in the upper and lower courses of the rivers based on a study conducted from June 1984 to April 1989, and carried out by Russev and Janeva (1991). The content of dissolved oxygen, oxidation, and the ammonia group were studied using standard methods, and the saprobiological index calculated after the method of saprobic valence described by Zelinka and Marvan (1961) and Rothschein (1962). Data for new bioindicators from Sladecek (1973), Russev et al. (1976), Russev (1979), Janeva (1979), Uzunov (1979), and Kovachev (1979) were also used in these comparisons.

The state of the tributaries in the foothills of the Stara Planina Mountains is relatively good, ranging from xenosaprobic to oligosaprobic. Thus, the saprobic index of the Ogosta River at the town of Chiprovtsi in April 1985 was 71.91; of the Cherni Vit River above the village of Cherni Vit in April 1989 - 72.67; of the Cherni Osam River above the village of Cherni Osam in April 1989 - 81.58; and of the Yantra River above the village of Yabalka in May 1987 - 84.88. The position of the Tsibritsa, Archar, and Iskar is similar (Table 4). For the remaining tributaries of the Danube, which are fed from springs arising on lower, more even ground (e.g., the Skomlya and Beli Lom) or which have been studied in more remote locations in the mountains, have saprobiological indexes within the beta-mesosaprobic portion of the spectrum (42.59 - 56.67). The content of dissolved oxygen during the measurements varied between 8.00 and 13.28 mg/l. Oxidation varied from 1.76 to 9.60 mg/l O₂; the ammonia group, from 0.00 to 0.20 mg/l O2. These tributaries, although lower in water volume, are clean since they do not run by larger settlements and industrial centers.

Naturally, the tributaries of the Danube are considerably more polluted just above the point where they join the Danube. The most eastern of the tributaries, as well as the Tsibritsa (with an average annual water flow of between 0.31 and 1.93 m³/sec) have a SRs of between 39.95 and 55.19 (Beta-alpha-mesosaprobic to beta-mesosaprobic); dissolved oxygen content values of between 8.8 and 15.04 mg/l; oxidation values between 1.50 and 12.0 mg/l O₂; and concentrations of the ammonia group of between 0.00 and 0.77 mg/l. The remaining tributaries (with average annual flows of between 4.93 and 54.9 m³/sec) have saprobiological indexes between 13.97 and 29.51 (polysaprobic and alpha-mesosaprobic); dissolved oxygen content between 0.9 and 11.7 mg/l; oxidation between 8.80 and 83.00 mg/l O₂; and concentrations of the ammonia group between 0.05 and 1.6 mg/l. Their higher pollution load is due to the presence of large industrial settlements at Lom, Miziya, Sofia, Bozhuritsa, Byala, Rousse, and elsewhere. In gen-eral, however, pollution from the Bulgarian tributaries is relatively slight, and concentrated along the Bulgarian riverbank. This reflects the fact that the Danube's water volume is more than 100 times greater than the largest of the Bulgarian tributaries (the Iskar).

III. Reservoirs

Within the drainage area of the Bulgarian section of the Danube, the reservoirs behind the Stamboliiski Dam, the Iskar Dam, and the Pancharevo Dam have been studied. The initial studies of temperature, oxygen regime, quantitative and qualitative composition of the zooplankton, zoobenthos, and ichthyofauna, and the feeding of some fish species were performed by Dimitrov (1957 and 1960). The zooplankton of the man-made lakes in Bulgarian was studied indepth by Naidenow (1962b, 1969b, 1972b, 1984, 1987) and Naidenow and Baev (1987). The data for species composition for the various reservoirs can be summarized as follows:

Protozoa

Four species found in the reservoir at the Stamboliiski Dam.

Rotatoria

Eight species at the Iskar Dam, and 36 in the Stamboliiski Dam.

Oligochaeta

Twelve species at the Stamboliiski Dam.

Hirudinea

One species at the Stamboliiski Dam.

Cladocera

Nine species at the Iskar Dam, 4 species at the Pancharevo Dam, and 15 at the Stamboliiski Dam.

Copepoda

Eight species at the Iskar Dam and 6 species at the Stamboliiski Dam.

Chironomidae

Twenty species at the Stamboliiski Dam.

No other groups have been examined beyond these 7 groups of common aquatic invertebrates.

Naidenow (1984) has compiled an original typology of the standing water bodies in Bulgaria. According to him, the quantitative development of the zooplankton is not always a suitable index for characterizing the type of water body, since its principal parameters vary within broad limits; the qualitative composition and dominant complexes are generally more stable.

Owing to their well expressed stages of evolution, the classification of glacial lakes, falling into four classes, requires precise measurement of their hydrologic and zooplanktonic characteristics. Oligotrophic water bodies comprise the higher mountain glacial lakes and some dam-lakes. Glacial lakes in the third stage of development are usually oligo-mesotrophic or alpha-mesotrophic.

Most of the man-made lakes in Bulgaria are mesotrophic. Contrary to natural lakes, however, they have never been in an oligotrophic state, and few have reached advanced states of eutrophication. Eutrophic standing water bodies include coastal wetlands, marshes and wetlands along the Danube, warm water fisheries, and some dam and microdam reservoirs. There are no dystrophic water bodies in Bulgaria; landslide lakes in the Rhodope Mountains are the nearest in characteristics to these.

Classification of freshwater basins is based not only on the presence of indicator species, but on qualitative and quantitative relationships between taxonomic groups of zooplankton and changes in the structure of zoocenoses. These are important considerations in that they determine the ratio among the fine filtrators, crude filtrators, and predator organisms in the system.

In the course of oligo-eutrophic succession, and parallel to the establishment of new biotopes and ecological niches, there is an increase in the number of zooplankton species. The level of species diversity reaches a maximum in late mesotrophy, after a subsequent fall in eutrophy and, in particular, hypertrophy.

IV. Lakes

Devetashki Lakes

The Devetashki lakes are situated in the karstic Devetaki plateau, between the Osam and Rositsa rivers. The zooplankton were examined in Sinovishte, Dedevetz, and Geranishte Lakes, as well as several smaller lakes (Stublata, Dakova Bara, Shekerova Bara, Lewskova Bara, Bonchova Varpina, Machkovetz, Ruzhina Bara, Starzhieva Bara, and Roupova Bara).

Twelve species of Rotatoria, 15 species of Branchiopoda, and 6 species of Copepoda were found, most of the species being common in Bulgaria. Copepoda was represented by 2 species of Calanoida, one of which (Arctodiatomus wierzejskii) was found on only one occasion. The Cyclopoida are poorly represented, with representatives of the subfamily Eucyclopinae being absent. Among Branchiopoda, all families (with the exception of Daphniidae) are represented by only 1 or 2 species. Typically, representatives of these groups that are common in the Thracian and Danubian plains are absent here.

Many of the basins with abundant zooplankton have not been used as fisheries. Stocking those that retain sufficient water year-round with fish, followed by semi-intensive feeding, could result in an annual yield of 25-50 kg fish per quarter acre (100-200 kg per acre) based on the existing trophic base. Qualitative analysis of the seasonal dynamics of dominants (Cyclops vicinus, Acanthocyclops vernalis, Daphnia pulex) indicates that conditions are favorable for stocking, as long as the titer of the plankton food base is maintained between 2-3 g per m³ (Naidenow, 1972).

Standing Water Bodies in the Ludogorie Region

A total of 93 water bodies were studied, 14 of them seasonal. This total also includes small natural water bodies, artificial reservoirs, microdam lakes, and waters collected behind dikes, often with diameters as small as 100-150 m and a depths between 0.5 and 2.5 m. As they are supplied only by rains in the dry hot summers, many of the shallower water bodies dry out completely.

Within these pools, a total of 24 species of Rotatoria, 73 species of Crustacea (37 species of Cladocera and 24 species of Copepoda), and 6 species of Coleoptera were found. In its composition, structure, and seasonal dynamics, this fauna differs substantially from that found in the hydrobiocenoses of the interior of the country, along the Black Sea coast, and along the Danubian banks.

The most characteristic elements from a faunistic-zoogeographic perspective are Lovenula alluaudi, Alonopsis ambigua, Daphnia ulomskyi and Pleuroxus uncinatus, which do not occur elsewhere in the country. L. alluaudi is most common along the southern coasts of the Mediterranean, but is also found in the countries of southern Europe and reaches the northernmost limits of its range in South Austria, Hungary, and the Danubian wetlands of Romania. However, its dominant role in the wetlands of Ludogorie has no parallel in the hydrofauna of other European countries. A. ambigua is a boreal element, and its occurrence in the Ludogorie region has as yet no satisfactory logical explanation.

Based on these observations, the hydrofaunistic complex of the Ludogorie clearly constitutes an unusual zoogeographic phenomenon. Markedly southern species, such as Lovenula alluaudi, Diaptomus mirus, Arctodiaptomus byzantinus, Arct. kerkyrensis, Daphnia similis, D. ulomskyi, and D. atkinsoni, appear simultaneously with species typical for Central and even Northern Europe, such as Macrothrix hirsuticornis, Alonopsis ambigua, Eudiaptomus zachariasi. In the context of the neighboring, well studied regions, the Ludogorie region appears as an isolated oasis, where a unique mixture of species from different zoogeographic regions, situated far away from one another, can be found. The reason for the formation of such a specific hydrofauna cannot at present be given.

In their studies of various small standing water bodies (ponds, small marshes, dendrothelms, etc.), researchers as early as the turn of the century included accounts of various types of aquatic invertebrates. These studies are included in the bibliography. However, since the data is obsolete in terms of nomenclature, and the existence of many of the small water bodies today is doubtful, we have not summarized here the findings of these reports.

V. Stigobiontic (Underground) Fauna of Bulgaria

Bulgaria is one of the European countries whose stigobiontic (underground) fauna is comparatively well studied.

The Protozoa are not well known in continental underground waters. Foraminifera, which are found in wells in southeast Bulgaria in the proximity of the Black Sea, are an exception. Some authors report similar finds from underground waters in the deserts of Central Asia (Mikhalevich, 1976). In all these cases, perhaps, we have an example of a marine organism from ancient seas that has adapted morphologically and physiologically to an underground manner of life. Also, some epibiontic Protozoa of the genera Podophrya, Tokophrya, and Vorticella are found on underground crustacea and the larval stages of underground Limnohalacaridae. No special studies have been carried out on the latter in Bulgaria.

Few groups of worms have penetrated into the underground waters. Oligochaeta is only represented by Haplotaxis bureschi (Michaelsen), which occurs in the karst regions of the western Stara Planina. The same species has been found in Yugoslavia and Romania.

Despite serious speleological studies, researchers have been unable to find the rare cave arachnelid Troglochaetus beraneki (known to occur in Central and Eastern Europe), nor Marifugia cavatica (known from Herzegovina).

Two Hirudinea species, of the Dina genus, have been found in caves in western Bulgaria (the western and central Stara Planina). They are rare species whose taxonomic status not clear. They can probably be considered relict forms from the end of the Tertiary, but at this stage this remains a hypothesis.

In Gastropoda, 10 species and 1 subspecies, from 8 genera, are known in Bulgaria, 8 of which are endemics (73%). Compared with the karst of the Adriatic, where Gastropoda are abundant, Bulgaria has fewer species.

The karst of the western Stara Planina appears to be the richest area for gastropod fauna, with marked occurrence of endemics. This may be due to the process of species formation after the end of the Tertiary, when considerable changes occurred in the configurations of water basins in these lands.

The basic components of the biocenoses in underground waters are stigobiontic crustacea and acarids. The underground Ostracoda in Bulgaria are insufficiently studied. Isolated reports come from southern and southeastern Bulgaria (Svetkov, 1966, 1972; Danielopol and Svetkov, 1979; Danielopol, 1980). Underground Ostracoda are typical for karstic and interstitial habitats. Eight species are known in Bulgaria, in 3 genera from 2 families. Three of them (37%) are endemics. According to Danielopol (1980), the representatives of the genus Kowalevskiella (which the author calls "living fossils") are ancient forms, thermophilic relicts from the end of the Tertiary.

The subclass Copepoda are typical with their great diversity in underground waters. No underground forms from the Calanoida order are known in Bulgaria. Bulgaria's underground waters are inhabited by 30 species (in 6 genera) from the Cyclopoida order; 19 species are stigobionts, 7 (36.8%) are endemic. Because studies have been unevenly conducted, it is not possible to indicate the territories in Bulgaria with the greatest numbers of endemic, relict, and rare species. Owing to the considerable extent of karst regions in the western Stara Planina, and the greater number of systematic studies that have been carried out there, this region appears to be the richest in species. The most recently studied regions are the Danubian plain and Strandzha Mountain. The genera Speocyclops, Acanthocyclops and Diacyclops are the most specialized for an underground way of life.

The Harpacticoida order is well studied in Bulgaria. Typical for underground waters, it has a marked degree of diversity and endemism. Harpacticoida are more common in underground interstitial waters than Cyclopoida, due most likely to their small dimensions. Forty-nine stigobiontic species, falling into 10 genera, are known in Bulgaria, 21 of which are endemic (42.8%). All members of the Parastenocaris genus are found underground. The genera Elaphoidella, Stygoelaphoidea and Nitocrella also have many stigobiontic representatives. The greatest numbers of endemic species are found in the regions of Dobrudzha, Strandzha, the Rhodope Mountains, and the western Stara Planina Mountains.

Within the large group Syncarida, the Bathynellacea are well studied in Bulgaria. They are among the most typical crustacean stigobionts, having evolved over a long period in underground environments. Seven species in three genera (Bathynella, Parabathynella, and Hexabathynella), are known. Five (71%) are endemics. Most of the endemics come from southern and southeastern Bulgaria, due probably to the changes in the Black Sea and Aegean basins towards the end of the Tertiary. They inhabit mainly the interstitial water, but also occur in karst waters.

In the order Amphipoda, stigobiontic representatives from the genera Bogidiella, Ingofiella and Niphargus are known and occur throughout Bulgaria. The first two genera are represented in Bulgaria by one species, and (unlike Niphargus) are not sufficiently studied. The genus Niphargus is represented by 7 endemic species.

The representatives of the Cirolanidae family in Bulgaria are rare endemic and relict forms. Two species have been found, and one subspecies of the genus Sphaeromides (100% endemism). The representatives of Sphaeromides have been described and are known only from caves in the western and central Stara Planina. As in other instances noted above, they originated in a period of species formation that occurred toward the end of the Tertiary and in the early Quarternary in the western Stara Planina.

The stigobiontic Asellota are well represented in the Balkan Peninsula. Three Bulgarian endemic species, from 2 genera, have been found (100% endemism). The genus Protelsonia is found in caves in the western Stara Planina, while the genus Balkanostenasellus is known from only one location - the karst springs near Assenovgrad.

Broadly represented are the microisopoda of the Parasellidae family. They belong to one genus (Microcharon) with 8 species, 6 of which (75%) are endemics. They inhabit mainly phreatic waters (wells), interstitial waters of river terraces, and (to a lesser extent) karst waters.

The Microcerberidae is another family of typical stigobionts of ancient origin. Two endemic species from one genus are known.

Only one endemic genus and species of terrestrial Isopoda (Genus Oniscidea, Bureschia bulgarica Verhoeff) has been described, from a cave near Polaten in the Teteven Region of the central Stara Planina. This is an amphibic species, and its inclusion in the underground fauna is disputed. Nevertheless, there are many species (in particular some soil acarids) that are potential stigobionts and indicate the course of stigobiontic evolution.

Aquatic acarids constitute a group of taxonomic categories that are not directly interrelated: Stigothrombiidae, Hydrachnellae, and Halacaridae. About 130 species, 56 of them stigobionts, have been found in the underground waters in Bulgaria. Eight of the stigobionts (14.3%) are endemics.

The Stigothrombiidae family was created to include the first Thrombiidae acarids found in underground waters. Many representatives of the Thrombiidae inhabit the wet areas between the subsoil and underground waters, representing different degrees of adaptation to the aquatic environment (Petrova, 1984). The species Stygothrombium racovitzai Mot. et Tan. is rare for Bulgaria.

In the group Hydrachnellae, the endemic relict genus and species Momonisia phreatica Petrova is of special interest (Petrova, 1974). It belongs to the ancient Momoniidae family, which has 4 other genera. Most of the representatives of this family are tropical land forms, living underground in moderate regions. We consider Momonisia phreatica a relict endemic form, a successor of the tropical fauna that inhabited Strandzha prior to the Tertiary (Petrova, 1984). The unique genus and species has been found only in a limited area of phreatic waters of the terrace of the Mladezhka River on Strandzha Mountain.

In the Halacaridae freshwater group, 10 species have been found in Bulgaria (as compared to 5 species in Romania and 4 in Yugoslavia). Soldanellonyx visurgis Viets was, until its discovery in Bulgaria, known only in Germany (Petrova, 1975). Also of particular interest is the discovery of the northern species Limnohalacarus wackeri (Walt.) in Bulgaria. Its southernmost occurrence up until its discovery in our country was in the high mountain lakes of Switzerland. While it is a surface-dwelling in Switzerland, it was found as an underground species with one single location (in wells at Gorni Chiflik in the Belogradchik Region) in northwestern Bulgaria.

Halacarellus is a marine psammobiontic genus found in the underground continental waters of Bulgaria. It too is of special interest, with 3 species in Bulgaria - H. capuzinus (Lohm.), H. subterraneus Schutz, and H. phreaticus Petrova (the Halacaridii of underground continental waters in Bulgaria thus total 13). The 3 species of genus Halacarellus in Bulgaria are a phenomenon from a genetic or ecological point of view. The first two species are known as psammobionts from the Atlantic Ocean, the Baltic Sea, the North Sea, and the Black Sea. In Bulgaria, they became established not only in the mesopsammal zone, but in underground continental waters along the Black Sea coast at a considerable distance from the sea. A new species - H. phreaticus Petrova - was also described from fresh water sources: springs, wells, and phreatic waters of river terraces 20-30 km from the estuary zone (Petrova, 1972).

The question is why these halacarid species of the Halacarellus genus, known from Boreal European and the Boreo-Atlantic zone of North America, are found outside of these areas only along the western Black Sea coastline in Bulgaria), and penetrate the continental phreatic waters. The resolution of the question must involve the geological history of the Black Sea basin - in particular the Miopleocene period, when the Bulgarian Black Sea coast took shape. Probably these ecologically pre-adapted forms (i.e., mesopsammic and brackish) were part of a fauna that survived from the time when the present day river valleys in their lower and even middle stretches were bays of the Neogene sea basins.

There can be seen in many groups of soil acarids (e.g., Thrombidiformes, Oribatei, Mesostygmata) a transition towards a semi-aquatic and aquatic way of life. The same tendency has been found in representatives of the astygmatic acarids, which belong to the genus Schwiebea (Angelkova and Petrova, 1986). In this case we are referring to Schwiebea barbei Cooreman, described from caves in France and found repeatedly in underground waters in Bulgaria (e.g., the Yambol-Elhovo valley, the Sofia Plain, Strandzha Mountain) in all of its life stages (larval, nymph, and adult). This is also the case with five other species of the same genus that have been found in deep-lying underground waters from the metro system of Vienna (Angelkova and Petrova, 1986). We believe that all these species show pre-adaptive and adaptive mechanisms for breaking away from the underground water medium and reaching the stigallum.

From this discussion, the following brief conclusions regarding the stigobiontic fauna can be drawn.

• A specific endemic fauna formed as a result of changes that occurred toward the end of the Tertiary in the region of the western Stara Planina Mountains, Strandzha Mountain, the Black Sea coast, and Thrace.
  
  
• The stigobiontic fauna of northern Bulgaria is much poorer in species composition than that of western Bulgaria, which can be explained by the long period of time that the Misian plate was covered by the sea.
  
  
• The occurrence of endemic and relict species is related to microevolutionary species formation processes within defined, strictly localized micro-regions;
  
  
• A marked degree of diversity, and active processes of species formation related to this diversity, is typical for neostigobionts of the Harpacticoida, Cyclopoida, and Microisopoda of the family Microparasellidae; of the genus Atractides; and of the aquatic acarid family Hydrobatidae.

The information in the Bulgarian literature on relict, endemic, and rare species of freshwater fauna are scant. Hence, the data in the attached appendixes lists and tables should be considered incomplete.

Table 1. Distribution of invertebrate fauna groups in the Bulgarian Tributaries of the Danube River

Invertebrate Fauna Groups	Total	Topo- lovets	Voy- nish- ka	Vid- bol	Ar- char	Skom- lya	Lom	Tsi- britsa	Og- osta	Iskur	Vit	Osum	Yan- tra	Rouss- enski Lom
1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
PROTOZOA
Rhizopoda	33									33
Ciliophora	0
Kinetofragmophora
Gymnostomata	61	4	7	5	3	7	10	2	19	27	12	26	20	27
Vestibulifera	9						2		3	4	5	4	3	5
Hypostomata	21	1	2			1	2	1	7	8	6	9	7	7
Olygohymenophora
Hymenostomata	37	11	6	10	5	9	19	6	19	6	15	25	1	20
Peritricha	20	1	3	1		2	7	1	6	14	6	8	5	10
Polyhymenophora
Spirotricha	67	9	6	8	6	5	14	7	20	33	28	35	23	30
COELENTERATA
Hydrozoa	4		1	1	1	1	1	1		4	1	2	2
VERMES
Turbellaria	5	1			1	1	2		5	5	3	3	3	1
Nematomorpha	1						1
Nematoda	1	1		1	1		1		1	1	1	1	1	1
Oligochaeta	91	18	18	18	14	14	26	14	33	48	60	45	57	27
Hirudinea	11	1	1	2	3		4	2	2	7	7	6	8	3
Gastropoda	35	1	5	3	4	2	6	6	8	25	20	16	22	14
Lamellibranchia	17	2	1		1	2	2	1	1	8	7	5	11	5
ARACHNIDA
Hydracarina	49		4		1		2		10	4	31	6	14	1
CRUSTACEA
Isopoda	2	1	1				1	1	2	1	1	2	2	1
Amphipoda	7	2	3	1	1	1	5	1	4	5	4	2	4	4
Decapoda	3							1	1	1	2		2
INSECTA
Ephemeroptera	77	18	27	21	17	9	46	24	40	53	56	50	61	19
Plecoptera	94		2	1	1	1	17	1	31	63	51	51	27	3
Odonata	31	3	4	5	6	6	9	8	8	14	7	15	17	7
Coleoptera	111	5	7	4	3	4	12	10	16	79	14	30	27	15
Heteroptera	29	6	7	4	2	1	8	13	5	6	6	11	14	6
Megalaoptera	2									1	1		2
Trichoptera	131	8	8	6	7	6	28	8	24	80	47	48	47	4
DIPTERA
Blephariceridae	3						2		1	3	2	2	1
Tipulidae	3	2	2	1	1	1	2	2	2	2	2	2	3	2
Cylindrotomiidae	1							1
Limoniidae	8	1	2	1	1	2	5	3	4	5	6	6	7	2
Psychodidae	7	1	1	1			1	1		6	1	4	4
Dixidae	1									1		1	1	1
Culicidae	2	1	1				1	1	1	1	1	1	2	6
Simuliidae	60	3	3	5	3	3	9	4	20	46	26	26	24	22
Chironomidae	217	1	3	1	7	3	73	20	51	81	120	63	66
Thaumaleidae	1									1				2
Ceratopogonidae	3	1	1	1	1	1	2	2	3	3	2	3	1
Stratiomyidae	5					1	1			1	2	5	5	1
Tabanidae	2	2	2		2	1	2	1	1	2	2	2	2	1
Athericidae	2	1		2			1		1	1	1	2	1
Rhagionidae	1												1	1
Syrphidae	2								1	2			1
Empididae	2									1	1
Ephydridae	2						1						1
Muscidae	1									1	1	1	1
Sciomyzidae	1											1
Indet.	1	1	1				1	1	1	1	1	1	1	1
Bryozoa	5								1	1			5
Total	1610	107	129	103	92	84	326	144	352	709	559	520	527	249

Table 2. Similarity of Species in the Bulgarian Tributaries of the Danube River (Sorensen, 1948)

River	Archar	Vidbol	Voy- nishka	Tsi- britsa	Topo- lovets	Skom- lya	Rouss- enski Lom	Lom	Osum	Ogosta	Yantra	Iskur
Archar
Vidbol	46,15
Voynishka	42,52	55,21
Tsibritsa	30,85	42,35	43,08
Topolovets	21,58	39,46	45,35	44,69
Skomlya	37,93	43,55	36,24	35,90	39,10
Roussenski Lom	33,72	38,89	39,02	39,38	41,27	31,32
Lom	35,29	33,33	38,40	32,89	36,86	30,71	36,92
Osum	44,09	40,38	41,18	37,46	36,94	35,18	34,97	46,98
Vit	30,77	31,29	32,60	26,90	27,72	27,86	30,24	38,82	43,34
Ogosta	30,10	35,51	41,00	32,30	34,98	30,00	39,53	50,33	37,99	41,06
Yantra	24,79	29,20	35,05	29,74	28,49	21,20	36,59	48,74	45,68	46,11	45,22
Iskur	37,35	37,98	31,16	30,23	32,79	18,75	40,74	39,58	42,58	34,57	45,04	34,95

Table 3. Structural Parameters of the Bulgarian Tributaries of the Danube River (Janeva and Russev, 1991)

Danubian tributary	Mean water quantity of many years m3/c	H	d	c	e
Vidbol	0,31	2,0-4,5 3,28	8,28-14,9 11,02	0,06-0,45 0,20	0,45-0,87 0,71
Archar	0,31	1,9-3,8 3,05	6,54-11,3 9,17	0,10-0,47 0,22	0,47-0,82 0,68
Skomlya	-	2,0-3,7 2,78	5,63-10,6 7,96	0,12-0,39 0,23	0,46-0,80 0,64
Voynishka	0,80	1,3-4,6 3,14	4,27-19,3 9,99	0,07-0,58 0,21	0,41-0,86 0,73
Topolovets	1,69	0,16-3,4 2,01	1,84-12,2 7,45	0,17-0,96 0,46	0,06-0,72 0,48
Tsibritsa	1,93	1,9-3,65 2,76	5,72-11,04 8,41	0,12-0,49 0,27	0,47-0,82 0,66
Lom	9,46	1,5-4,6 3,74	4,4-21,9 15,12	0,05-0,48 0,15	0,40-0,92 0,74
Vit	13,1	0,63-4,44 2,97	0,18-5,69 3,34	0,06-0,85 0,20	0,20-0,90 0,63
Ogosta	19,6	1,3-3,9 2,65	3,68-16,9 7,72	0,10-0,69 0,26	0,31-0,96 0,70
Yantra	40,1	0,10-4,2 2,08	1,16-12,8 4,80	0,08-0,73 0,38	0,1-0,87 0,61
Iskur	54,9	0,72-4,0 1,96	0,47-12,9 4,04	0,09-0,75 0,35	0,36-0,99 0,66

Table 4.

Danubian tributary	Length km	Catch- ment area km2	Mean water quantity of many years km3/S	Upper course					Lower course - above outfall
				Last year and month of research	O2 mg/l	Oxida- tion mg/l	NH+4	S R	Last year and month of research	O2 mg/l	Oxida- tion mg/l	NH+4	S R
Topolovets	68	583	1,69	10.1986	10,24	3,20	0,20	49,19	10.1986	10,72	2,24	0,24	39,95
Voynishka	60	277	0,80	10.1986	8,00	3,44	0,16	54,65	10.1986	8,8	11,6	0,2	45,85
Vidbol	62	330	-	04.1986	-	-	-	52,88	10.1986	11,04	2,40	0,77	48,75
Archar	59,4	365	0,31	10.1986	13,28	1,76	0,09	69,93	10.1986	15,04	12,0	0,12	46,84
Skomlya	41,6	162,8	-	10.1986	9,6	4,32	0,12	42,59	10.1986	10,72	2,88	0,2	55,19
Lom	92,5	1140	9,46	10.1986	9,6	2,64	0,14	47,77	10.1986	0,9	83,00	1,6	23,74
Tsibritsa	87,5	934	1,93	06.1984	-	-	-	56,67	06.1986	12,9	1,50	0,0	53,40
Ogosta	144,1	4231	19,60	04.1985	10,50	9,60	0,23	71,91	04.1985	5,2	2,60	0,75	29,51
Iskur	368,0	8646	54,9	04.1985	10,60	2,48	0,23	68,36	10.1986	8,48	8,80	0,16	29,29
Vit	188	3225	13,1	04.1989	9,58	-	-	72,67	10.1987	-	-	-	13,97
Osum	314,0	2824	12,5	04.1989	9,58	-	-	81,58	10.1988	11,7	-	-	23,21
Yantra	285,5	7862	40,1	05.1987	10,08	0,96	0,00	84,38	08.1987	10,22	12,00	1,14	26,00
Roussenski Lom	196,9	2947	4,93	06.1984	9,60	7,49	0,21	47,17	06.1987	8,1	16,00	0,05	32,92

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