by Yevgeny SHEIN, Dr. Sc. (Biol.), Department of Soil Science, Lomonosov Moscow State University; Anna FEDOTOVA and Lyudmila YAKOVLEVA, Drs. Sc. (Biol.), Astrakhan State University; Vladimir PILIPENKO, Dr. Sc. (Biol.), Director of the Innovation Natural Science Institute of Astrakhan State University

Talking about Astrakhan, a city in the upper delta of the Volga, invites the following associations in most people: fish, tomatoes, watermelons-the things this region is famous for. There are but few who know about complicated natural interrelations accounting for the biological diversity of this area and the significant role of soils in the life of the local flora and fauna.

One of the Volga branches delta in summer.

Map of Russia's Astrakhan Region: 1-shallows; 2-above-water plants; 3-wafer plants; 4-delta channels; 5-sea delta edge at the 18th cent., maximum rise; 6-sea delta edge in 1817; 7-sea delta edge in 1873; 8-numbers and boundaries of shallows in Volga coastal waters; 9-depth isolines.

Back in 1978 the Volga delta was artificially divided into two parts by a dam. During high water periods its eastern piscicultural part is fully flooded: fish spawn on its wide plains, new generations of fish grow and then leave for the Caspian Sea. The western agricultural part is intersected by irrigation systems that bring water to grow watermelons and tomatoes. Vegetables are grown in a semidesert under a scorching sun with almost no precipitation in summer and the upper layer of soil heated to 50-60 ºC! It is impossible for plants to survive here without water; this water and soil particles brought in by the great river Volga from the north in fact determine the life pulse of these territories.

PULSE OF FLOODPLAIN ECOSYSTEMS

The geographical structure of the Astrakhan flood-plain formed on the Khvalynsk saline clays-deposits of the Caspian Sea (Khvalynsk Sea in Old Russia), is unique. Such clays form soils characterized by a high concentration of salts through the whole profile. The maximal quantity of salt is concentrated in low fertility salt marshes where only very specific plants--so-called halophytes--can grow. If you look at the vertical section of such soil (or profile), you will see accumulations of white crystals to a depth of l -1.2 m sparkling in the sun now and then. They are found right on the surface as well.

When fresh water covers the floodplain, some of the salt is carried away into lower soil horizons. Meadow plants will grow on this overlayer. But soon hostile substances will get to the surface through capillaries, and halophytes prevail again. Such is the life cycle of ecosystems of the piscicultural part of the Astrakhan delta.

The flood level and its duration directly depend on the Volga's flow and the level of the Caspian Sea. When the Caspian Sea is high, the time of spring and summer floods becomes longer, and the water level higher, thus leading to the redistribution of readily soluble salts throughout the Astrakhan region. Thus, in low, long flooded areas of the delta-unique natural spawning

grounds-toxic ions are washed away from the soil profile, and only relatively safe sulphateion concentrations are on the increase. Consequently, the structure of vegetation is changed thereby. By contrast, on higher ground we can observe accumulation of all ions except for hydrocarbonates. But the low scanty spring floods of 2002 and 2006 partially reduced the said differences and preconditioned all-round salinization.

The living organisms can hardly adjust to such kind of soil variability; that is why the local fauna is very specific. But local inhabitants--numerous fish--are able to procreate only under these unique conditions. How are they unique, these conditions, in terms of spawning and fish-growing?

The point is that a significant part of the food chain of the regional ichthyofauna is formed in the grass cenoses of the Volga's delta. But over these 35 years the vegetation cover in the Volga-Akhtuba floodplain and Volga's delta serving as a spawning substrate has been developing under the conditions of artificial stream flow control. Research data show: the structure and composition of the phytocoenoses are gradually changing. For example, near the Sumnitsa Shirokaya channel a typical European halophyte saltwort well-known to local dwellers was listed in 14 geobotanical reports in 1990, in 2001--only in one. At present, the number of thero-phyte (annual grasses living in the form of seeds during unfavorable seasons) species went down from 17 to 6; in 2001 we did not see any spiny pribrezhnitsa (grass family). On the contrary, the share of hemicryptophytes and geophytes* increased from 11 to 41 and from 15 percent to 33 percent respectively. There appeared such hydrophilous plants as Stachys palustris, Scirpus lacustris, Eleo-charis and Typha angustifolia. What is the cause of this phenomenon? The changed water-salt regime of soils only.

WHEN WATER IS HIGH

In the lower reaches of the Volga the water level varies by 5 meters from low to high. In spring it keeps on the same high level for 3 months. All around, as far as we can see, there are flooded meadows and some herbaceous plants rise above the water and tremble in the flow or when touched by big fish. Local fishing practices are quite peculiar: a fisher with a spear in hands stands on a small island and watches a trembling cattail where the Volga carp spawns. It is a very big fish: just imagine, sometimes even one-meter water depth (!) is too shallow for it, and its back fins stand out above water. The fisher cautiously comes closer to his prey that is very alert even in the mating season and often escapes. But it still could be traced by trembling plants, and the fisherman may (catch) his fish, especially if he is experienced enough.

In spring and in early summer a vast area between the Volga and the Caspian--the delta proper--is in blue (surface of numerous channels) and dark green (neighbor-

*Hemicryptophytes--plants with buds growing on the surface (sometimes a little bit higher) in periods unfriendly for vegetation and protected by scales, fallen leaves and snow cover; for example, species of the buttercup, dandelion, bugle, etc. Geophytes hide buds in the soil. Among them are onion, tulip. May lily, many cereals and sedge grasses. This life-form is prevalent in arid steppes.--Ed.

ing bushes, cat-tail and willow reeds) colors. Depending on the level of water in the river and in the sea, this territory is either flooded or drained, while in the lower reaches of the Volga, the kingdom of reed, you can't understand whether it grows on solid soil where you can go on foot or in channels with a muddy bed where you may go under. For example, on the map you are on firm ground, while you in real life you make way through 3-meter high footstalks.

To sum up: regulated flow of the great river has had an ambivalent effect on the ecosystem of the Lower Volga. Transgressions of the Caspian Sea have only intensified initial transformations. In the 1990s the spawning grounds were overgrown with stiff vegetation that covered almost 70 percent of separate areas. The reed was quite at home, while the upper delta was overgrown largely with soft meadow plants. But every cloud has a silver lining: the present conditions are favorable for proliferation of the crucian carp, the leader among tiny fresh-water fishes. This fish feels quite well in the lush thicket of water plants.

In high water only roads and separate high hills looking like islands in floodplains are seen above the water surface. These hills are unique natural formations of the Pre-Caspian area called Baer knolls in honor of Karl Ba-er, elected foreign honorary member of the St. Petersburg Academy of Sciences in 1830, who was the first to describe them. Scientists will not agree on their origin. But before we get down to the existing hypotheses, a few words about the outstanding researcher of the 19th century whose name lives on in the name of these relict features.

BAER KNOLLS

Baer formulated fundamental laws in such areas of natural science as geomorphology and anthropology. Most biologists consider him a patriarch of modern embryology--it was he who proved that all animals, both higher and lower, exhibit similar embryonic growth stages. Almost all textbooks of geography and geomorphology cite the Baer law formulated in the course of his field expeditions in southern Russia. The scientist noticed a curious regularity: the right bank of a river running from the north to the south is usually steep by contrast with the flat left bank. He understood why: owing to the rotation of the globe, water flows under the action of inertial forces erode the right bank. In the past geological ages (i.e. over thousand and millions of years) longitudinally oriented rivers like the Volga, Dnieper, Don, Ob, Irtysh and Lena formed such asymmetric valleys; it is also true of the Danube and Nile.

Traveling in these areas, Baer discovered specific geo-morphological formations typical only of the Pre-Caspian Lowlands and described them in 1856: asymmetric wave-like elevations from 5 to 25 m high, 1.5-1 km wide and several kilometers long, often extended sublatitudinally and were separated by depressions of different width (from 0.5 to 5 km and more). Dr. Boris Fyodorovich of the Institute of Geography explained their formation by eolian wind-borne processes. In the 1940s, after studying the contemporaneous ridgy relief of sandy deserts in the Central Asia and comparing it with the Volga area elevations, the researcher drew

analogy between these forms and viewed Baer knolls as fossil ridgy sands. In the 1930s Dr. Vladimir Baturin of the Institute of Fuel Minerals did not challenge these views, as sandy rocks forming Baer knolls contained finest loamy pads typical of the present-day deposits of deserts and sea shores.

However, soil and geobotanical studies of the 2000s carried out within the framework of a Russian Fund of Basic Research project led scientists to other conclusions. Baer knolls are built of shale, not sandstone--the point is that saline soils always break into very small pellets consolidated by salts similar to sandstone. Yet another interesting thing related to these topographic forms. One would think that only depressions in this part of the Volga delta are saline. We have already said: salts contained in these soils are dissolved during every spring flood and then reach the surface through capillaries. The knolls stay dry and ions cannot get to their top (the knolls are as high as 3-5 storey buildings). Nevertheless, these specific elevations are covered with a patchwork of solonchaks, saline and nonsaline semi-desert soils (brown semidesert soils) with relevant vegetation. What is the cause of this phenomenon? Perhaps, it could be explained by fluctuations of the water level of the Caspian Sea*.

Baer knolls and their slopes are covered with numerous ephemerals and ephemerids (tulips, onions, irises, mouse-ear, Androsace and other species), as well as typical xerophytes--goat's-wheat (a genus of perennial ramose bushes of the buckwheat family), saltwort and

* See: M. Mitina, B. Malashenkov, "Dynamics of Hydrometeorological Characteristics of North Caspian", Science in Russia, No. 4, 2009.--Ed.

anabases (succulent bushes of the goosefoot family). During floods these plants are usually attacked by huge herds of great and small cattle from private and collective farms. As a result, after intense and long-lasting pasture periods, plant associations with dominating Artemisia lerchiana and saltwort lose most of the perennial species, while poisonous and inedible plants, such as leafless anabasis and harmala shrub (perennial of the zygophyllaceous family) stay on. Annuals prevail both in number and mass.

Thus, specific landscapes of the Volga delta, Baer knolls and stretches in between as well, different saline soils, specific vegetation and fish living on flood plains are an impressive example of interrelation ship among the above components. Changes in the soil cover lead to transformations of the local vegetation and spawning conditions. This means we cannot explain what happened to the reproduction of fish in the Volga delta without an ecosystemic approach.

CASPIAN UPS AND DOWNS

The western part of the Volga delta is not flooded each year. But floods often reach as far as these territories, filling in depressions and old dry river beds, running into eriks and ilmens--formations typical of this very area. The eriks are temporary channels drying out in a low-water season. The ilmens are elongated lakes located in clusters in this part of the delta. (The name "ilmen" likely comes from the name of the Northern Novgorod lake called Ilmen located along the way "from the Varangians to the Greeks".) This is where the delicious Astrakhan tomatoes and watermelons grow: abundant

sunlight and heat, and water from the ilmens and temporary channels! However, the soils are very different there and not all of them are suitable for vegetable farming due to salinization.

This process can be seen with the naked eye: on both sides of the road you are walking or driving on you see ilmens overgrown with cat-tail; all of a sudden, a lake glittering in the sunlight with its snow-white salt cover! Sometimes the concentration of salts in the upper soil layer is so high that the surface gets covered with small crystals, and insects and vegetable remains sticking out.

Let's recall the geological structure of the territory: if the above-mentioned Khvalynsk loams and saline domes are located close to the surface as a bed of the ilmens, they soon mineralize to form picturesque "winter" landscapes.

A saline lake amidst blossoming solonchaks is really beautiful: white, violet, red, light brown and dark gray layers are interstratified and can tell us the story of its formation. Depending on the level of subsoil waters, floods and aquatic erosion processes, the salt content rises or falls. The behavior of the Caspian is the prime cause thereof. Traces of its rises and transgressions could be seen in the soil cover.

Meanwhile, from the early 20th century, especially in the 1930s, due to human activities, the water flow of the Volga fell significantly, and the Caspian Sea level was 3.5 m down. The intensive development of the sub-water delta changed to fast dry land encroachment. Numerous sand islands and crescent-shaped sand bars came up here and there. Joining, they gave rise to the contours of ilmens. Later on, the lakes dried out and turned into vast boggy lowlands. Concurrently with the regression of the shoreline, the area of ilmens decreased while the Pre-Caspian deserts expanded. In 1956-1958 due to a persistent fall in the sea level the average annual horizontal expansion of the delta made up 185-190 cm, and the all-time low of the water level over the last 400 years was registered in the 1970s.

By 1959 a cascade of water storage basins on the rivers making part of the Volga basin was complete in the main. From 1978 on its flow was finally taken under control (a dam and a bifurcation gate were constructed), and so hydroelectric complexes changed the hydrological regime in the low reaches. The depth and duration of spring and summer floods changed as well. What with less intensive floods, a slow fall in the ground waters level could be expected. But this did not happen. The point is that alongside poor floods in the upper estuary area, the frequency and duration of fresh water winter floods in the central depressed area increased, extruding soil and ground waters from soluble salts.

Since 1978 the sea level has increased by 2.5 m. About 320 thous. ha of the Russian seaside have been flooded. A quarter of the Volga's delta is covered by water; as for the rest of the territory, the higher level of ground waters has triggered progressive swamping, hydromorphism and soil gleying processes (a biochemical process of oxide reduction). The non-flooded area has contracted to 10 percent and is localized in the delta where Baer

knolls are rising. Some experts have forecast that by 2010 the Caspian Sea level would increase by yet another 1 m; however, in the early 21st century the sea leveled off and its level even fell slightly.

PRECARIOUS BALANCE

Thus, the Caspian Sea level is decreasing, the delta is drying, floods are out, and nothing seems to stop the encroachment of saline soils and solonchaks. But then the sea advances again, vast territories are flooded, violent spring and summer floods are back again, and soluble salts are washed out of the upper horizons. The local vegetation is also changing in the wake of the fickle environment.

Interestingly enough, the higher floods on the one hand, and aridization on the other, create favorable conditions for plant species diversity. With the reduction or cessation of spring flooding in the western part of the Volga delta steppe plants appear everywhere or at least on high terraces. So-called steppificated solon-chak meadows are formed. Then salts are washed out from top soil horizons, the newly formed herbage is replaced by semidesert feather grasses, wheat grasses, artemisia, leban and other similar communities like that. With such vegetation growing on dry ilmens, the soil gets light gray.

At present, swamp vegetation is increasing its share in the lowland plains, while salt-resistant plants are predominant at knoll tails. It is clear that these changes are closely related to the evolution of the soil cover, and the next link in this chain of interconnections is the vital activity offish.

Yet one thing gives rise to concern: many floral species of this unique region have become rare. Each year the number of aggravating conditions is increasing, and the most acute problem is in the expanding anthropogenic impact on natural habitats, especially the habitats of relics and endemics, confined to specific soils, water conditions and relief. There are 25 rare plant species registered in the Volga delta and the Volga-Akhtuba floodplain, nine of which are in the Red Data Book of Russia. Fortunately, none of these species are listed as extinct.

Thus, the ecological balance between local hydrology as well as soil and vegetation transformations and, consequently, fish reproduction conditions in the Volga delta is rather flimsy. Our task is to keep on this delicate, flexible and very sensitive balance--an environment where the famous Astrakhan watermelons, tomatoes and fish grow.