Danube river mouth

The outer shelf zone is located at depths greater than 60 m its marginal part is characterized by steeper slopes than those in the inner and central zones. The greatest depths here reach 60-100 m its smallest width (down to 10 km) is observed in the east, while in the west, off the Danube River mouth, it reaches 60 km. Here, the shelf edge is confined to 130-150 m depth contours. 

Results of large-scale investigations of the Danube River mouth are discussed in [6-9]. The most comprehensive analysis of the present-day peculiarities of hydrological and morphological processes in the Danube delta is made in [8]. 

During the last 60-100 years [5,21,22], the majority of rivers under consideration were subject to the human-induced decrease in the water runoff and sediment load. The decrease in the water runoff was mainly due to the water withdrawal for economical needs and losses through evaporation from the free-water surface of reservoirs. At the Danube River mouth, however, the climate-induced increase in the water runoff over the recent 30 years has exceeded the human-induced decrease in it. Human-induced decrease of water runoff among large rivers was the greatest in the Dnieper, Don and Kuban rivers and comprised 1.2-1.3 times. 

The Danube River mouth area is the largest among other river mouths in the region of the Black Sea and the Sea of Azov. This mouth belongs to the open deltaic type. The Danube mouth area includes the near-delta river reach, the... 

The mean vectors of the winter and summer currents in the surface (0-40 m), subsurface (50-75 m), and intermediate (100-300 m) layers of the Black Sea are presented in Fig. 2. They confirm the cyclonic character of the BSGC in the winter and summer. Even at depths of 1,000 and 1, 500-m (not shown), southwest and southeast of the Crimea, where the currents feature relatively low velocities and diverse directions, in nine and five of the total 19 cases, cyclonically and anticyclonically directed vectors, respectively, were observed (in the remaining five cases, the vectors had quasimeridional directions). In the surface layer near the shore, anticyclonic vector directions were observed only three times twice in the wintertime off the Danube River mouth (see Fig. 2a) and once in the fall west of the Bosporus (not shown). These regions are known as areas of quasi-stationary NSAEs (see Fig. 1). 

At that time, on the northwestern shelf of the Black Sea, weak currents up (to 0.10 ms-1) along the outer front of the desalinated area off the Danube River mouth were observed. As was noted in [23], the MRC features almost no... 

In addition to the results presented above, we should also note the studies of the climatic BSGC [56] based on the basic Russian prognostic model [57]. The distinctive features of [56] were related to the dependence of the coefficients of horizontal turbulence on lateral velocity shears and to the specifying of the monthly climatic temperature and salinity field at the surface [29] instead of the heat and moisture fluxes. Despite the relatively coarse horizontal calculation grid (about 22 km), this allowed the authors to reproduce [56] a relatively distinct MRC jet and the known NSAEs off the Turkish and Caucasian coasts and off the Danube River mouth. The results of the tuning in [56] of the Munk-Anderson s formula for the coefficient of the vertical turbulent exchange from the point of view of reproduction of the actual CIL were used in [53,54]. 

In Romania, oceanographic studies of the Black Sea are conducted by the Marine Research Institute in Constanta. Their target is the shelf zone. For a long time the Romanian-Ukrainian team had been engaged in studies of the Danube River delta, which is under the jurisdiction of both countries. In 1979-1999 environmental monitoring in the Danube mouth area was conducted. 

Some information on evolution, structure and regime of the Danube, Dniester, Dnieper, Rioni, Don and Kuban river mouths at the period up to the first part of the 20th century is given in [4]. More complete and present-day characteristics of the above-mentioned river mouths are considered in [5]. 

The rate of the RSLR in the Black Sea and the Sea of Azov and at their river mouths (Table 3) varies over a wide range [3,8,24-26]. These data show that actual eustatic sea level rise increased with time and by the beginning of the 21st century reached in the Black Sea 4 mm year x. Besides, these data confirm the assumption that the land subsidence plays very important role in the RSLR in river mouth areas and can reach 3-6 and even 12 mm year-1. The most values of the RSLR and subsidence are typical of the coastal parts of the Danube and Rioni deltas (Table 4). 

The mixing zone of river fresh water with salinity less than 0.5%o and sea brackish water with salinity up to 18%> occupies a bend up to 20-30 km in width during high-flow period and western winds and of 3-5 km during the low-flow period and eastern winds [7,30]. The Danube water spreads mainly on the surface with layer from 1-3 to 5 m [9]. The Danube River water runoff and its distribution between delta branches play a very important role not only in hydrological and hydrochemical regime of the delta and mouth nearshore zone but also in formation of ecological conditions in the northwestern part of the Black Sea as a whole [7,9,30]. 

PAH concentrations in the canal water were almost the same during the first and the second sampling (15.10 and 14.10 pg L, respectively). Danube River water collected during the first sampling downstream from the canal mouth contained PAHs in concentrations significantly exceeding permissible values for river waters (6.30-1.54 and 2.19-1.30 pg L , in the surface layer and near the bottom, respectively). 

The open nearshores of the majority of the rivers under study are deep. Mean slope of the open nearshore bottom at the Rioni mouth is 6.5%o ( old delta) and 14.3%o ( new delta) [3]. Less deep are the open nearshores of the Danube and Kuban rivers [3,5]. The open nearshores of this kind are typical of the Dniester, Dnieper and Don mouths [5]. 

During the high-flow periods, waters with small salinity can reach the Zmeiny Island in the east, Bulgarian coast in the south and the Dniester mouth in the north. During very significant spring-summer river floods, the area of the Danube influence occupies 70% of the northwestern part of the Black Sea. The total area of this direct river influence, defined according to the freshwater phytoplankton presence, is not less than 105 km2 [9]. 

Observations show that the area of hypoxia directly depends on volume of the Danube water runoff during spring-summer flood. Besides, this area depends on the time of the flood peak [9,31]. If the flood peak takes place in April, the river fresh waters are driven out of the northwestern part of the Black Sea in the south direction under the influence of predominated northern winds in this time, and hypoxia is absent. Other situation takes place if the flood peak falls on May or June, when under the impact of the southern winds, main mass of river fresh water remains in the northwestern part of the Black Sea. In this case, hypoxia forms later and in the area between the Danube and Dniester mouths [9,31]. 

It is possible that eutrophication processes and such adverse events as algal blooms and hypoxia will be more frequent in some sea areas in the future. Conditions like that can take place in the northwestern part of the Black Sea in the vicinity of the mouths of the Danube, Dnieper and Southern Bug rivers and in the Taganrogskiy Bay of the Sea of Azov. 

Bottom sediments in the coastal zone of the sea may be polluted with copper, zinc, nickel and cadmium. The highest levels of toxic heavy metals are found in the mouths of rivers. The bottom sediments in the Black Sea have a high mercury level—from 0.28 to 0.40 pg/1. In the coastal waters of the Krasnodar Territory the mercury level is 0.15-1.55 pg/1, while its maximum concentrations are registered in the Danube and Dnieper mouth areas. The Danube alone brings annually up to 50-60 tons of mercury, while the Dnieper brings up to 5 tons. The distribution of heavy metals in bottom sediments in the Russian shelf of the Black Sea is not uniform. Their greatest quantities are accumulated in sediments in the deepest part of the shelf where their concentration is 3-5 times higher than in sediments in the shallower part. Toxic metals contained in sea water in the dissolved and suspended forms are ac-... 

The samples were collected from locations up- and downstream from the waste-water canal of the industrial complex at the place of its discharge into the Danube, as well as from the canal itself. Sampling was performed along the left riverbank and in the middle of the river course, at the river section from the mouth of the river Tamis (1,154 km) up to 1,148 km of the river Danube course in the following manner location I, representing a pure , control section was situated upstream from the canal (1,153.4 km) downstream from the canal - locations 11 (1,152.6 km). 111 (1,151.5 km, left riverbank) and V (1,151.5 km, in the middle of the river course by a sand river island) and location IV in the canal itself. Investigations at these locations included only the closer zone of the accidental pollution (Fig. 2). 

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