The Danube Delta As A Living Environment And Its Biological Characteristics

excerpt The principal characteristic of the Danube Delta, a region in which water predominates over land, resides in the preponderance of aquatic organisms. From a scientific point of view, nor are the non-flooding land areas of the Delta lacking in interest, such as the Letea and Caraorman sandbanks, which have developed a fauna and flora characteristic of the east-European steppes, with slight Mediterranean influences. Between areas in which the aquatic environment constantly dominates and the high, non-flooding sandbanks, there is a series of land areas which are more often or more rarely non-flooding. During the evolution of the Delta, these areas have developed a specific fauna and flora, with multiple possibilities of adaptation to the alternation between life on dry land and under water. The Black Sea, in the area neighbouring the Delta, is strongly influenced by the waters of the Danube, which create specific hydrographical and hydrochemical conditions for it. It likewise forms a highly interesting biotope insofar as the variation of its biological components is concerned, determined by the instability of the environment in this region (tides, temperature, winds, salinity). This influence of the Danube waters can be traced above all during the great river floods, when a series of fresh water forms are carried out to sea. Thus, during the flood of 1942, carp were carried together with the fresh waters from the mouths of the Danube as far as Constantza and Mamaia. From the point of view of conditions for life, the Danube Delta can be divided into the following six major biotopes, with a number of subdivisions: 1. Permanent waters, which can be stagnant ("ghioluri") or flowing (the arms of the Danube, natural distributaries and channels)2. Non-flooding dry land3. Reed beds and floating reed islets4. Interior sandbanks, whether alluvial or sandy5. The coast with its flanking sandbanks6. The sea area, directly influenced by the waters of the Delta It can therefore be seen that the Delta also reveals itself to us in biological terms as a region of remarkable variety. Here, the same as everywhere else on earth, in each biotope can be found corresponding biocoenoses (the totality of species living together within an environment). Fresh stagnant waters are represented by numerous ghioluri, japse (shallow, flooded depressions of limited duration), which change their form and position after each flood, and a number of relatively deep pools of water, remnants of abandoned arms of the Danube. According to their hydrographical features, the lakes of the Delta can be grouped in three categories:· Ghioluri well supplied with water during the whole year, with the distributaries for supply and evacuation being permanently operative. Such are the ghioluri in the east of the Caraorman sandbanks, those on the Dranov Island, and the fresh water part of Razelm Lake· Ghioluri insufficiently supplied with fresh water, partially stagnant and overrun by vegetation and floating reed islets: Parches, the Gorgova ponds etc.· Ghioluri that no longer communicate with the Danube (except exceptionally, during very high floods: Tatanir, Somova), due to the complete stagnation of the supplying distributaries. The waters of the first category provide optimal conditions for the development of valuable fish, such as pike perch, carp, and bream, which find here food in abundance, not only in the sapropelic mud on the bottom of the pond, but also around the bank and in the thickets of submerged vegetation: pond weed, frog lettuce, water milfoil. In the partially stagnant and poorly supplied ghioluri, pike, perch disappear and bream become scanty. Carp continue to develop well, alongside perch, pike, and roach. The ghioluri that no longer have any link to the river are usually shallower and completely overrun with vegetation. Due to the unfavourable hydrological conditions, the species of fish that remain have a slow development: they degenerate and are threatened with mass mortality, especially in summer. Here can be found only small fry, tench, pike and perch; No species of fish live beneath the floating reed islets. In terms of chemistry, it can be argued that, due to the almost permanent agitation of the water and the wealth of phytoplankton, the water of the Delta's lakes, even those without any direct supply of river water, is generally sufficiently oxygenated, which is of major importance for the totality of chemical and biological processes in these lakes. It is only beneath the floating reed islets that the water is vitiated and lacking in oxygen. In winter, when it freezes, the water of the lakes is usually deficient in oxygen. From May until August inclusively, it is slightly supersaturated with oxygen in the daytime, due to the process of photosynthesis. At night, when the assimilation of chlorophyll ceases, the oxygen almost entirely disappears, being consumed by the respiration of aquatic, animal and plant organism. This occurs above all a short time before sunrise. The water of the ghioluri is generally poor in some of the elements indispensable to life, such as nitrogen, phosphorus, and calcium. Flood waters, which contain compounds of the latter, are beneficial, because they enrich the waters of the ghioluri with precisely these elements. During the season of high temperatures, the water of the ghioluri decays due to the intense decomposition of organic matter produced by the aquatic vegetation. During this time, the fish flee, if they can, into the reeds that surround the lakes. If, during this time, there is no supply of fresh water, then the phenomenon of "summer stagnation" occurs, causing a high rate of fish mortality. The further the Delta's lakes are from the arms of the river, the fainter is the influence of the Danube waters on their chemistry and biology, and they are therefore less productive. Beneath the large expanses of floating reed islets, where the light cannot penetrate and the possibilities of aeration are non-existent, the water is completely vitiated. With the exception of sulpho-bacteria, these waters are completely lifeless. The ponds, being on the one hand shallower and smaller in surface area, and, on the other, covered in aquatic vegetation, often present phenomena of water putrefaction, with their natural consequence: mass mortality of those fishes which have not been able to escape (during the summer stagnation mentioned above). Less exposed to these dangers are the deep pools of water, but which have an active silting, which rapidly brings them, in their evolution, to the stage of ponds, then marshes. Typical are some pools of water near the Ciamurlia ghiol, south of Crisan. Briny waters are to be found in the Razelm-Sinoe complex. Their salinity is less than that of the average in the Black Sea (17g/l). The chemical and biological make-up of these waters is conditioned by the basin's links with the Danube and the Black Sea: the water level of the Danube determines the quantity of fresh water brought by the channels, while the wind increases evaporation and determines the direction of the currents at Portita. Lake Razelm is more strongly influenced by the waters of the Danube, for which reason its salinity has dropped to 2 g/l. On the other hand, Lake Sinoe is influenced to a much greater extent, for which reason its salinity is higher (up to 16 g/l, and in years of drought even 20 g/l). Sulphates vary at a similar rate to chlorides: in the northern part of the northern briny system (Razelm), they exceed the admissible limit for freshwater (100 mg/l). Calcium is to be found in greater quantity than in fresh water, due to the deposits on the bottom. The total hardness of the water is great: The Ph shows optimal values for aquatic life (7-8). Phosphates exist in sufficient quantities. However, nitrates do not, for which reason the development of phytoplankton is poor. Salt waters, represented by the Sinoe-Smeica lagoon system, constitute a medium characterised by a very high salinity. The fish life is much poorer than that of the above environments, being composed of migration forms (grey mullet and endemic forms, gobies and flounders). Moving waters are represented by the arms of the Danube, the distributaries and natural channels, as well as manmade canals. Leaving aside the Sulina Arm, which for the most part is a manmade canal, between the Chilia Arm and the St Gheorghe Arm there is an evident morphological distinction. The former is characterised by three ramifications and few meanders, thus it is a faster flowing course, while the latter has many meanders and is slower moving. Together with the flowing of water from the interior into the arms of the Danube, a phenomenon that occurs from summer onward, representatives of the flora and fauna of the Delta ghioluri are borne by the current into the Danube. A biological characteristic of the fluvial arms is the presence of a number of species of fish, such as the Vimba vimba and orfe (Idus idus), as well as the reappearance of the sterlet (Acipenser ruthenus), which now breeds here intensively, as proven by the large quantities of fry that are caught between Chilia-Periprava and Gorgova-Crisan. As we approach the mouths of the arms, the quantity of sea sturgeon and other marine species increases. The mouths of the arms present a rather different biological aspect to the rest of the river, due to the flux and reflux of marine and fresh-water forms, which succeed each other and alternate according to the dominant hydro-meteorological situation. The distributaries and channels present, during the course of the year, different biological aspects. At high waters, they present the character of Danube water; At low waters, they evacuate standing water into the Danube, bringing with them stagnant water forms. The flooding lands in the Delta have their own well-defined character. When they are under water, they are a place for the laying of fish roe and for fish food. During the time of low waters, some aquatic life forms withdraw, while others enclose themselves in a cyst. Dry-land forms, however, return to active life, so that on such lands we witness an alternation of aquatic and terrestrial creatures. The fluvial, alluvial sandbanks, situated on either side of the Delta arms and distributaries, are partly covered with willow woods or, when the silting phenomena have been too intensive, lack arborescent vegetation, but are favourable for agriculture, producing rich harvests. The continental sandbanks, formed from deposits of loess, more rarely sandy, are situated in the middle and the west of the Delta. They are covered by large surface areas of saline soils and the vegetation characteristic of the latter. In general, however, the continental sandbanks have a steppe character, with all the range of steppe vegetation. A unique formation is constituted by the marine sandbank forests of Letea and Caraorman, which cover a large part of their surface area. The seacoast, contained between the arms of the Delta, resembles in biological terms the marine sandbanks in the interior, being made up of the same, compacted marine sands. At their maximum height, the floodwaters manage to infiltrate these coastal sandbanks. For this reason, the play of fresh and salt water that alternates here determines the occurrence of specific flora, formed mostly of plants resistant to high salinity and resembling those on the marine sandbanks. The coast to the right of the Razelm-Sinoe complex is different to that described above, however. Here, the shore, formed mostly of saline soils, has very poor vegetation. Almost the only plants met here are the jointed glasswort (Salicornia herbacea) and sea kale (Crambe maritima). Both categories of coast serve as a resting place and refuge for a biotope that is very interesting for the variation of its biological component, determined by the hydrochemical variation of the marine environment. In the immediate vicinity of the mouths of the Delta, there has long been preserved a faintly saline character, with the predominance of fresh water, due to the large masses of water carried by the river. Thus, in the small coastal bays (situated between the mouths of the two arms), there predominate species of freshwater fish adapted to a certain salinity (up to 5 g/l). They live here throughout the year, withdrawing into the river arms only when there is a cold current from out at sea, which also has high salinity.

SRSC, Bucharest, 1959

by A. C. Banu; L. Rudescu