transformations in the caspian sea ecosystem under the fall and rise of the sea level

ISSN 0001�4370, Oceanology, 2010, Vol. 50, No. 4, pp. 488–497. © Pleiades Publishing, Inc., 2010.Original Russian Text © V.V. Sapozhnikov, N.V. Mordasova, M.P. Metreveli, 2010, published in Okeanologiya, 2010, Vol. 50, No. 4, pp. 524–533.

488

INTRODUCTION

During the past seven decades, profound changestook place in the Caspian Sea ecosystem, first underthe level fall until 1978 and then at the rise of the leveland the degree of its stabilization within the lastdecade. As a closed basin separated from the WorldOcean, the Caspian Sea responded much sensitively toall the changes that occurred over the drainage basinduring these seven decades. The following events tookplace: all the great rivers were dammed at many sites,the spring floods decreased to 40% of the annual run�off and “winter floods” of 35–40% of the runoffappeared, and the runoff was chemically changed.Whereas 95% of the runoff and 96% of the chemicalsubstances were formerly transferred with the springflood, nowadays, when all the water reservoirs areblooming, not mineral forms of phosphorus and nitro�gen (phosphates and nitrates) but ammonium, urea,organic forms of nitrogen and phosphorus, and greatamounts of allochthonous organic matter are sup�plied. The composition of the plant community istransformed. During the last years (2007–2008), theabundance of peridinians was four times as high as thatof diatoms. In 1921, with launches from Batumi, thediatom Rhizosolenia as well as the bivalve Mytilaster,were imported [2]. It is thought now that Rhizosoleniadid not survive the transfer of launches from Batumi toBaku, but this diatom appeared in 1934 in the courseof the acclimatization of two mullet species [6]. Allthese facts affected the Caspian Sea ecosystem.

MATERIAL AND METHODS

For the quantitative estimation of the changes inthe Caspian Sea ecosystem, the authors had available

the data by Bruevich [2] obtained at a relatively highsea level in 1933–1934, facultative information of sev�eral expeditions of the Russian Federal ResearchInstitute of Fisheries and Oceanography (VNIRO),the data of the expedition of the Institute of Oceanol�ogy of the Russian Academy of Sciences (IO RAS)with the participation of the VNIRO specialists(1983), the data of the VNIRO expedition on boardthe GS 194 vessel, and the information on the annualsurveys of R/Vs of the Caspian Scientific–ResearchInstitute for Fisheries (KaspNIRKh) from 1996 tillnow.

Since 1995, the surveys were carried out using NeilBrown CTD probes and, later, with an SBE 25 probeequipped with an SBE 43 sensor for oxygen and withthose for the chlorophyll, pH, and Eh values. Thesamples were collected by means of a GoFlo bottlesampler cassette with a Neil Brown probe and usingNiskin bottle samplers with an SBE 25 device. Todetermine the hydrochemical parameters, the expedi�tions used classic procedures of chemical analysesdescribed in guidebooks [11, 16]. The analyses weremade with a Technicon flow autoanalyzer (USA) anda KFK colorimeter. For the biochemical analyses, aHitachi fluorescence spectrophotometer was used, aswell as TOC 500 and TOC 5000A organic carbon ana�lyzers of high�temperature combustion in an oxygenflow with special platinum catalysts.

The measurements for the dissolved oxygen werecarried out in hydrophobic plastic flasks by means ofan automatic oxygen titrator manufactured by Jan�kons Co (England).

Transformations in the Caspian Sea Ecosystem under the Fall and Rise of the Sea LevelV. V. Sapozhnikov, N. V. Mordasova, and M. P. Metreveli

Russian Federal Research Institute of Marine Fisheries and Oceanography, Moscow, RussiaE�mail: [email protected]

Received August 8, 2007; in final form, September 24, 2009

Abstract—By the data of the surveys performed in 1976, 1983, and annually from 1995 to 2006, the successivetransformations of the ecosystem were traced in the Central and Southern Caspian Sea: from the anoxic con�ditions in 1933–1934 to the oxic state during the low level period in 1978, then the hypoxia increase and accu�mulation of nutrients registered anew after 1995, and to the hydrogen sulfide appearance in the near�bottomlayers of the Southern Caspian Sea. Some irreversible changes in the silicon distribution in the Central Cas�pian Sea were revealed (the concentration increase to 217 µM at the depth of 780 m, which considerablyexceeded the absolute values for the World Ocean).

DOI: 10.1134/S0001437010040053

MARINE CHEMISTRY

OCEANOLOGY Vol. 50 No. 4 2010

TRANSFORMATIONS IN THE CASPIAN SEA ECOSYSTEM 489

DISCUSSION OF THE RESULTS

The initial conditions in the Caspian Sea ecosys�tem were stated by the expedition lead byS.V. Bruevich in 1933–1934 at the sea level stand ofthe –26.5 m mark (Fig. 1) and published in the classicwork Hydrochemistry of the Central and Southern Cas�pian Sea [2]. The distributions of dissolved oxygen,hydrogen sulfide, silicon, phosphates, and nitratesover the sections Divichi–Kenderli and KurinskiiKamen’–Ogurchinskii Island across the depressionsof the Central and Southern Caspian Sea are pre�sented in Fig. 2. One must look closely at it because allthe comparisons below are related to this figure. It isimportant to note that the silicon concentration in thedeep�water depression of the Central Caspian Sea wasover 130 µM, whereas it amounted only to 105 µM inthe Southern depression (even at the presence ofhydrogen sulfide). The oxidation of organic matterproceeded up to the complete utilization of the oxygenand, partially, owing to sulfates. Recall that the concen�trations of silicon, ammonium, and phosphorus in theBlack Sea amount to 350, 100–110, and 9.5–10.5 µM,respectively, at a depth of 2000 m in the hydrosulfidezone [4].

In 1934, very high phosphate concentrations (up to2.2 µM) were found in the Southern Caspian Sea atthe deepest station (980 m) in the near�bottom layer(Fig. 2). The content of nitrates was the maximum inthe layer of 150–250 m, being equal to 12 µM in theCentral Caspian Sea; at that, a rapid decrease of thiscontent to 2.2 µM towards the bottom was registeredthere and to 0.05–0.5 µM in the Southern CaspianSea.

The next stage of the level decrease (–28.2 to –28.4 m)and of a certain stabilization of it at these values wasregistered in 1958–1962 (Table 1). It is well seen fromthe table how the oxygen content increased in the deepwaters at the depths of 200 m and below. In the layer of600 m, the concentration of oxygen was over double(up to 3.49 ml/l).

In 1971, under the level stand at the mark of –28.4 m,before the drastic decrease to –29.0 m, the concentra�tion of oxygen at the depth of 600 m was even4.12 ml/l, i.e., 3 times as high as that in 1933. By 1971,the concentration of phosphates at the level of 600 mdecreased from 1.6 µM in 1933 to 1.02 µM.

The next year when the measurements for O2, P,and Si were performed (unfortunately, only to a depthof 200 m) was 1976 with the level mark of –28.5 m. Atthat time, it was found that the relative content of dis�solved oxygen in the layer of 200 m amounted to 70–

Lev

el,

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–291940 1960 1980 2000

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Fig. 1. Caspian Sea level variations from 1930 to 2008.

Table 1. Vertical distribution of the oxygen and phosphates in the Central Caspian Sea in the summers of different years (averagedata)

Layer, m1934 1958–1962 1971

O2, ml/l O2, % O2, ml/l O2, % O2, ml/l O2, % PO4, µM

0 5.94 98 5.81 100 5.82 104 0.39

10 5.97 97 5.71 97 5.75 102 0.39

25 5.78 86 5.81 85 5.82 93 0.33

50 6.07 74 5.81 75 5.49 72 0.45

75 – – – – 5.57 71 0.56

100 6.09 74 5.56 70 5.44 68 0.78

150 – – 4.88 65 5.81 72 0.97

200 4.23 51 5.06 62 4.58 56 1.12

300 – – 4.37 54 3.96 48 1.38

400 2.17 27 4.30 53 3.74 45 1.32

500 – – 3.84 46 3.19 38 1.18

600 1.42 17 3.49 41 4.12 48 1.02

700 – – 3.34 39 4.73 55 1.05

750 – – 3.55 42 – – –

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Fig. 2. Distribution of dissolved oxygen (O2, ml/l), silicon (Si, µM), mineral phosphorus (P–PO4, µM), and nitrates (N–NO3, µM)over the sections Divichi–Kenderli and Kurinskii Kamen’–Ogurchinskii Island in August 1934 [2].

80% with 1.6–1.7 µM of phosphates and 20–40 µM ofsilicon.

Unfortunately, the authors have no data availablefor 1978, when the level decreased to the minimummark of –29.0 m.

In 1983, when the level increased sharply to themark of –27.8 m, the expedition of the IO RAS withthe participation of the VNIRO specialists was per�

formed. In the Central Caspian depression, the con�tent of oxygen in the deep waters (below 300 m) wasstill quite high and varied from 3.6 to 4.8 ml/l,although the oxygen concentration in the South Cas�pian depression decreased below 2.5 ml/l (deeper than400 m). The amounts of silicon and phosphates in theCentral Caspian Sea amounted to 128 and 1.0 µM,respectively, i.e., differed little from those in 1971–



1976. In the deep waters of the Southern Caspian Sea,the concentration of silicon increased to 130 µM,which was higher than that in 1933 (100 µM); the con�tent of phosphates increased to 1.2 µM (against 1.02 µMin 1971), and the distribution of nitrates became thesame as that observed by Bruevich (Fig. 3). The maxi�mum of nitrates is located in the layer of 200–400 mand amounts to 12 µM, but, above all, the decrease ofthe nitrates towards the bottom is very small (a littlebelow 10 µM in the near�bottom layers), whereas thenitrates in 1933 decreased down to zero.

The next survey was carried out in 1995, when thelevel increased drastically to –26.5 m, i.e., to the samemark as that of 1933. Unfortunately, in the CentralCaspian depression, we were able only to measure thecontent of oxygen, which appeared to be equal to0.4 ml/l. Thus, it has become evident that the rapidformation of hypoxia takes place, and the Caspianecosystem should to return to the conditions thatexisted in 1933 at the like high level stand (–26.5 m).Even the first cruise of 1995 showed that the concen�tration of silicon decreased by 20–30% compared to

1933–1934, when the classic studies by Bruevich werecarried out. High concentrations of organic matterwere also registered (10–15 mg C/l). Hence, someirreversible changes took place. Most probably, this isthe effect of the regulation of all the great rivers when12 water reservoirs on the Volga River; 3 on the KuraRiver; and 2 on each of the Terek, Sulak, and Samurrivers were built. The initial stages of the decrease ofthe phosphate supply to the Northern Caspian Seawere registered by Vinetskaya [3] and Zenin [5]; thesilicon supply decrease was noticed by Barsukova [1].Moreover, Vinetskaya showed that this immediatelyaffected the resources of breams, pike perches, androaches in the shallow�water Northern Caspian Sea.

At the Laboratory of marine ecology of theVNIRO, a resolve was made to perform the annualmonitoring of the conditions in the Caspian Sea eco�system to trace all the stages of the transformation ofthe hydrochemical structure from the anoxic condi�tions of the deep waters in the depressions of the Cen�tral and Southern Caspian Sea to the subanoxic and,probably, anoxic state. Moreover, it became possible to

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Fig. 3. Distribution of silicon (Si, µM), mineral phosphorus (P–PO4, µM), and nitrates (N–NO3, µM) over the sections Divi�chi–Kenderli and Kurinskii Kamen’–Ogurchinskii Island; R/V Parallel’, August 1983.

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trace the changes in the Caspian ecosystem caused bythe regulation of all the great rivers flowing into theCaspian Sea by 1995. The total number of hydroelec�tric power plants with water reservoirs on small riversand confluents amounted to 120 by that time.

In the Black Sea, the damming of all the great riversalready caused pronounced changes in the ecosystem:a drastic decrease of the silicon, phosphates, andnitrates in the surface layer (0–50 m) by factors ofalmost 11, 5, and 8, respectively [4, 10]. In turn, thiscaused an intense bloom of peridinians. The bloom ofGonialax Poliedra near Varna Bay reaches the degree of“red flood” and was observed by the authors during thecruise of R/V Akademik Knipovich in 1989.

All the water reservoirs on the rivers becameblooming; therefore, instead of nitrates and phos�phates, great amounts of dissolved organic matter(15–22 mg C/ml), ammonium, urea, and organiccompounds of nitrogen and phosphorus were suppliedto the sea. The silicon supplied by the rivers mainly asparticulate matter and colloids fell to the bottom sedi�ments of the water reservoirs. All these facts resulted inthe profound changes in the hydrochemical basis ofthe bioproductivity in the Black Sea and in the consid�erable transformations of the ecosystem as a whole. Toutilize the excess of allochthonous organic matter, theecosystem promoted the intense development ofmedusas, ctenophores, and noctilucales capable ofbinding and removing the excess of organic matterwhen it died off. All these changes adversely affectedthe anchovy and sprat fishery.

In the Caspian Sea basin, all the great rivers wereregulated at just the same time as those in the BlackSea, but no considerable changes in the ecosystemwere observed until the drastic increase of the level in1995. This is caused by the fact that, at the low levelstand, all the changes in the surface layer associatedwith the runoff were leveled under the winter verticalcirculation and mixing down to the bottom in theCentral and Southern Caspian depressions. Not untilthe isolation of the surface layer (0–50 m) by the levelincrease and the following desalination did the ecosys�tem changes begin to accumulate in the euphotic layerto cause the transformations in the plant community.The abundance of peridinians increased by almostfourfold, the ctenophore Mnemiopsis appeared, andthe rate of organic matter destruction increasedsharply (BUO1 = 0.78 ml/l) owing to the excess ofallochthonous organic matter and the intensifieddevelopment of microheterotrophs. A decade later,the Caspian Sea experienced all the stages of the eco�system transformation according to the Black Seamodel, including the appearance of Mnemiopsis.

The basin’s eutrophication resulted in an equilib�rium shift in the trophic relations to bacteria and pro�tozoa, which may rapidly utilize the organic matterand transform their metabolism, i.e., go faster when anenergy source is more abundant in the ecosystem. Inthis case, the microheterotrophs transformed the

environment to adversely affect the subsequenttrophic chain links. Thus, the energy transformationproceeds at lower trophic levels, whereas higher levelsare lacking nutrition. The process of the transforma�tion of the trophic relations formed many new econ�iches. The species capable of increasing their abun�dance, not needing an exacting environment, andadaptable to a wide variety of nutrition have the advan�tages. A considerable role is also played by the basin’sclosedness and the relative species poverty of its fauna.Thus, the conditions are formed for the appearance ofintroduced species such as the ctenophore Mnemiopsisleidyi.

In 1995, the expedition over the Northern andCentral Caspian Sea was performed by the VNIRO onboard the R/V GS 194, which showed that the hydro�chemical structure of the Caspian Sea was profoundlytransformed since the time of low level stand. Hypoxiaformation started in the deep waters of the CentralCaspian Sea, where the oxygen content was 0.4 ml/l orbelow. In the Central Caspian Sea, the effect of a “bio�logical pump” appeared, which caused a drasticdecrease (almost to analytical zero) of the amount ofphosphates and nitrates in the euphotic layer in thesummer time and the accumulation of phosphates (upto 1.6–1.8 μM) and silica (up to 140–160 μM) in thenear�bottom layer. The concentration of nitratesincreased to 12–14 μM in the maximum layer (200–500 m) and then decreased at the bottom to 7 μM.

The same processes also took place in the deep�water part of the Southern Caspian Sea, where theconcentration of oxygen decreased to 0.2–0.4 ml/land the content of phosphates and nitrates increased(in the maximum layer). The increase of the silicaconcentration was considerably slower than that in theCentral Caspian Sea.

In the course of the annual surveys in the Centraland Southern Caspian Sea, the authors succeeded inregistering a continuous decrease of the dissolved oxy�gen in the deep waters of the depressions of the Centraland Southern Caspian Sea to values of about 0.2 ml/l.At that, one must note that these values were registeredby an oximeter installed on a Neil Brown probe and,then, since 1995, by a flow�through sensor for dis�solved oxygen (SBE 43) installed on an SBE 25 CTDprobe. In addition, the absolute values were deter�mined by the Winkler procedure, which allowed us tocalibrate the oximeter data.

The vertical distribution of the temperature, salin�ity, and dissolved oxygen formed by 2001 in the deep�water depressions of the Central and Southern Cas�pian Sea is presented in Figs. 4 and 5. In the SouthernCaspian depression, the near�bottom values of theoxygen were equal to 1.4 ml/l at the 880 m depth,whereas these values in the Central Caspian Sea at thedepth of 700 m were 0.7 ml/l or below. The precedingsummer of 2000 was very hot and the surface waters ofthe Southern Caspian Sea were warmed to 30°C.Owing to the intense evaporation and heating, the



water of salinity to 12.9‰ may be kept at the surface.The abnormal salinity increase by 0.2‰ in the surfacelayer is compensated for by the temperature growth toalmost 30°C. When it is cooled to 6°C in the winter, aconventional density value of σt = 10.2 units isreached, which provides the mixing down to the bot�tom [14].

The fact that the salinity component of the convec�tion is prevailing in the Southern Caspian Sea wasmentioned even by Knipovich [8].

In 2002, the concentration of dissolved oxygen inthe Derbent Basin remained at the same level, whereasthe oxygen content in the Southern Caspian depres�sion decreased by 1 ml/l and amounted only to 0.35–0.4 ml/l. Evidently, the winter vertical circulation didnot reach the bottom in the Southern Caspian Sea. Inthe summer of 2001, the surface South�Caspian waterswere not sufficiently warmed (25°С), which decreasedthe evaporation intensity. Because of this, in August2002, the salinity in the surface layer of the central part

of the Southern Caspian Sea was as low as 12.7‰, whichnegatively affected the depth of the vertical mixing per�meation. The violation of the mixing processes wasimmediately reflected in the increase of the near�bottomconcentrations of nutrients compared to 2001 [15].

The conditions observed in the water mass of thedeep�water depressions of the Central and SouthernCaspian Sea are quite steady. For the complete deple�tion of the oxygen and the hydrogen sulfide appear�ance in the near�bottom waters of the Central CaspianSea, several successive warm winters are required, dur�ing which the convection should not reach the bottom.In the Southern Caspian Sea, the formation of hydro�gen sulfide requires, evidently, several rather cool sum�mer periods during which no highly saline surfacewaters are formed.

Thus, according to the mechanism described, thecomplete depletion of oxygen and appearance ofhydrogen sulfide in the Central and Southern CaspianSea requires abnormal conditions with a low chance of

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Fig. 4. Distribution of the temperature (T°C), salinity (S‰), dissolved oxygen (O2, ml/l), silicon (Si, µM), mineral phosphorus(P–PO4, µM), and nitrates (N–NO3, µM) over the section Divichi–Kenderli across the deep�water depression of the CentralCaspian Sea; R/V Issledovatel’ Kaspiya, October 2001.

494


SAPOZHNIKOV et al.

occurring. Evidently, just because of this, for all thereliable cases of hydrogen sulfide discovery, small con�centrations of it were registered. Lebedintsev in 1904[9] registered concentrations of hydrogen sulfide equalto 0.24 ml/l in the depression of the Southern CaspianSea. Knipovich [7] found 0.3 ml/l of hydrogen sulfidein the deepest layers of the Central Caspian Sea duringthe expedition of 1914–1915. Bruevich, during theexpedition of 1933–1034, revealed hydrogen sulfide(0.29 ml/l) in the near�bottom layer of the SouthernCaspian depression [2].

At the values of the biochemical utilization of oxy�gen we measured in the subsurface and deep waters(BUO5 and BUO1), the former value was subtractedfrom the latter and then divided by 5 to obtain thediurnal average BUO value of about 0.01 ml/l, whichwas more appropriate for the equilibrium BUO valuesin the deep waters. (Of course this is not a rigorous cal�culation but rather a qualitative estimation of the orderof magnitudes.) At BUO1 values as such, it is clear that1 ml/l of dissolved oxygen should be utilized in

100 days; i.e., the rates of the oxygen utilization aretoo high, even including the water temperaturedecrease towards the bottom [12]. Were it not for theperiodic aeration of the deep waters, all the oxygenwould have been utilized a long time ago and thehydrogen sulfide content would be considerably high.During the rather short�time period of one or twodecades, the content of hydrogen sulfide in the deepwaters of the Caspian Sea should reach values compa�rable to the hydrogen sulfide concentrations in thenear�bottom waters of the Black Sea.

However, the absence of a thick thermocline and ofa reverse halocline in the Caspian Sea in the winterprovides the sufficient aeration of the deep waters, andthe appearance of at least small amounts of hydrogensulfide requires a combination of abnormal conditionslimiting the winter vertical circulation.

It becomes understandable why, formerly, even atquite a long�term high level stand, high amounts ofhydrogen sulfide were never registered.

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Fig. 5. Distribution of the temperature (T°C), salinity (S‰), dissolved oxygen (O2, ml/l), silicon (Si, µM), mineral phosphorus(P–PO4, µM), and nitrates (N–NO3, µM) over the section Kurinskii Kamen’–Ogurchinskii Island across the deep�waterdepression of the Southern Caspian Sea; R/V Issledovatel’ Kaspiya, October 2001.



A steady occurrence of hydrogen sulfide wouldcomplete the transformation of the hydrochemicalstructure of the Caspian Sea started under the levelincrease to –26.5 m. The concentrations of silica andphosphates should be maximally increased in thedeep�water depressions of the Central and SouthernCaspian Sea. Instead of nitrates, which should becompletely utilized under the thiodenitrification,quite high amounts of ammonium nitrogen shouldappear. Thus, a further depletion of the surface layer innutrients should take place; i.e., the new primary pro�duction should decrease and, hence, the nutritionconditions for sprats and other pelagic fishes shouldbecome worse.

Knowing about the oxygen, one may easily proceedto the nutrients that accumulated in the deep�waterdepressions of the Central and Southern Caspian Sea.However, in the euphotic layer, the concentrations ofphosphates, nitrates, and silica decreases every year.This “pumping” of nutrients from the surface to thedeep layers causes a gradual decrease of the new pri�mary production. To a degree, the decrease of the newprimary production owing to the spring bloom of algaeis compensated for by the increase of the primary pro�duction due to recycling. However, it should beremembered that the production due to the recyclingof nutrients does not reach the higher trophic levelsand, respectively, in no way promotes the improve�ment of the nutrition conditions for pelagic fish spe�cies, especially for anchovy sprats.

In 2001, by the survey data of the R/V Issledovatel’Kaspiya, the profiles were plotted for the vertical dis�tribution of the temperature, salinity, oxygen, silicon,ammonium, nitrates, phosphates, and organic phos�phorus over the sections Divichi–Kenderli and Kurin�skii Kamen’–Ogurchinskii Island passing through thedeep�water depressions of the Central and SouthernCaspian Sea [14]. It is well seen from these sectionshow the accumulation of nutrients in the deep watersand depletion in them in the surface waters are pro�ceeding (Figs. 4 and 5).

The result of these processes is evident from thesurvey data of 2006. In a similar section across thedepression of the Central Caspian Sea, the concentra�tion of silica in the near�bottom layer amounted up to217 μM, i.e., to the absolute maximum for the entireWorld Ocean (Fig. 6). The concentration of siliconincreased gradually, as seen from the results given inTable 2. The vertical distribution of phosphates andnitrates showed no significant changes.

The vertical distribution of nitrates shows a growthof the maximum to 16 μM at the middle depths of200–400 m. The distribution of phosphates allows oneto see a drastic increase of their amount in the near�bottom layer (to 3.6 μM). The sample was quite trans�parent with a slight odor of hydrogen sulfide. Mostprobably, this increase is caused by the formation ofsubanoxic or anoxic conditions in the near�bottomlayer at an oxygen concentration of 0.2 ml/l. Evi�

dently, under conditions as such, the phosphate trans�fer is started from the bottom sediments with the inter�stitial waters’ phosphate concentrations being higherby an order of magnitude.

In the section Kurinskii Kamen’–OgurchinskiiIsland passing through the maximum depths of theSouth Caspian depression, the view is even moreinteresting (Fig. 6). The concentration of siliconamounts to halved values (116 μM) compared to thedeep waters of the Central Caspian Sea, and this factrequires further studies and an explanation. The con�tent of oxygen in the near�bottom layer is below0.1 ml/l and coexists with the evident presence ofhydrogen sulfide (about 0.2–0.4 ml/l by the organo�leptic evaluation).

The presence of hydrogen sulfide caused the almostcomplete disappearance of nitrates (0.6 μM) in thenear�bottom layer at the 990 m depth; however, at thedepth of 800 m as well, the concentration of nitrates isstill quite low (1.6 μM), which testifies to the effect ofthe thiodenitrification process even at a distance of200 m from the bottom. Evidently, the concentrationof phosphates increased instantly to 3.0 μM in thehydrosulfide near�bottom layer. The maximum layerof nitrates is now located deeper (in the layer of 400–600 m), and their concentration is as high as 16.5 μM.To compare, it is interesting to consider the like sec�tions executed by Bruevich in 1933–1934 (Fig. 2). Thevertical distributions of oxygen and hydrogen sulfide inthe Southern Caspian Sea were much similar in 2006and in 1933, but the accumulation of silicon in 2006amounted up to 117 μM against about 100 μM byBruevich. The concentration of nitrates in the maxi�mum layer was 12.4 μM by Bruevich and 16 μM in2006, etc.

Summarizing the experience of the continuous 13�year monitoring of the variations in the hydrochemicalstructure and the changes in the whole ecosystem ofthe Caspian Sea, one must note the following:

—With the beginning of the level increase and thedesalination of the surface layer, a steady stratificationis formed that does not allow the processes of the win�ter vertical circulation to reach the deep waters in thedepressions of the Central and Southern Caspian Sea.

—The hypoxia formation is started in the deepwaters, and the nutrients “pumped” there owing to thebiological pump effect are accumulated;

—The most effective is the accumulation of silicain the deep�water depression of the Central CaspianSea, where its concentration is as high as 217 μM, i.e.,about twice as much as that in the depression of theSouthern Caspian Sea (116 μM). At that, the silicaaccumulation continues, although the concentrationlevel registered by Bruevich (120 μM) is alreadyalmost doubled.

—The values of the phosphates and nitrates in thelayer of their maximum are practically equalized to thoseobserved in 1933–1934 at the level stand of –26.5 m.

496


SAPOZHNIKOV et al.

Divichi Kenderli0

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800O2, ml/l

September 2006

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8090100

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PO4, µМSeptember 2006


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Fig. 6. Distribution of dissolved oxygen (O2, ml/l), silicon (Si, µM), mineral phosphorus (P–PO4, µM), and nitrates (N–NO3, µM)over the sections Divichi–Kenderli and Kurinskii Kamen’–Ogurchinskii Island; R/V Issledovatel’ Kaspiya, September 2006.

—No steady hydrogen sulfide presence and nitrateabsence are still observed in the deep�water layers ofthe Southern Caspian Sea (600–900 m).

The Caspian Sea level is now standing at the markof –27.1 against –26.5 m during the surveys byBruevich (1933–1934). It turns out that a little differ�ence as such is sufficient for steady anoxic conditions

to still not be formed in the depressions of the Centraland Southern Caspian Sea.

The concentrations of silica accumulated in theCentral Caspian Sea, which exceed almost twice thesilicon content by Bruevich, evidently resulted fromthe winter bloom of the diatom Rhizosolenia under thedrawing off the water reservoirs for electric power gen�



eration at the hydroelectric power plants during thecold period. As was ascertained, Rhizosolenia wasimported to the Caspian Sea even after the expeditionsby Bruevich (in 1934) at the time when no winterfloods took place. Producing great amounts of biom�ass, this diatom binds enormous quantities of siliconand, because it is not eaten up by any organism, theaggregations of these algae sink to the deep waters anddecompose there, and their siliceous frameworks aregradually dissolved.

Should steady hydrosulfide contamination in thedeep water appear and the acidity of these waters increase(the pH value decrease), the dissolution of the siliceousframeworks would proceed even faster, and we should beable to register in them very high values for all the nutri�ents characteristic of anoxic basins [13].

ACKNOWLEDGMENTS

This study was supported by the Russian Founda�tion for Basic Research (project nos. 05�05�64081 and05�05�64254).

REFERENCES

1. L. A. Barsukova, “Biogenic Runoff of the Volga River in1963–1964,” Tr. Kasp. Nauchno�Issled. Inst. Rybn.Khoz. 23, 242–255 (1967).

2. S. V. Bruevich, Hydrochemistry of the Middle and South�ern Caspian Sea, (Akad. Nauk SSSR, Moscow, 1937)[in Russian].

3. N. I. Vinetskaya, “Hydrochemical Regimen of theNorthern Caspian Sea after the Regulation of the VolgaRiver Runoff,” Tr. KaspNIRO 24, 78–99 (1968).

4. M. E. Vinogradov, V. V. Sapozhnikov, and E. A. Shushkina,The Black Sea Ecosystem (Nauka, Moscow, 1992) [inRussian].

5. A. A. Zenin, Hydrochemistry of the Volga River and ItsReservoirs (Gidrometeoizdat, Leningrad, 1965) [inRussian].

6. A. F. Karpevich, Theory and Practice of Acclimation ofAquaatic Organisms (Pishchevaya promyshlennost’,Moscow, 1975) [in Russian].

7. N. M. Knipovich, “Hydrological Studies in the Cas�pian Sea in 1914–1915,” Tr. Kasp. Eksped. 1914–1915(Pervaya Gos. Tipogr., St. Petersburg, 1921).

8. N. M. Knipovich, The Caspian Sea and Its Harvesting.RSFSR (Gos. Izd., Berlin, 1923) [in Russian].

9. A. A. Lebedintsev, “Journal of Hydrological and Mete�orological Observations Made by the Caspian Expedi�tion of 1904,” in Tr. Kasp. Eksped. 1904, Vol. 3 (Tipo�litografiya Yakor’, St. Petersburg, 1913) [in Russian].

10. Yu. R. Nalbandov and V. R. Vintovkin, “Hydrochemi�cal Conditions of the Aerobic Zone of the Black Sea inAutumn 1978,” in Ecosystems of the Pelagic Zone of theBlack Sea (Nauka, Moscow, 1980), pp. 50–61 [in Rus�sian].

11. A Guide to Chemical Analysis of Sea and Fresh Watersduring Environmental Monitoring of Fishery Water Bod�ies and Harvest�Promising Areas of the World Ocean(Izd. VNIRO, Moscow, 2003), p. 202 [in Russian].

12. V. V. Sapozhnikov, “Comprehensive Hydrochemicaland Biochemical Investigations Carried Out by theVolga�Caspian Expedition on Board Vessels Antaresand GS�194 from August 4 to September 10, 1995,”Okeanologiya 36 (1), 148–151 (1996) [Oceanology 36(1), 137–140 (1996)].

13. V. V. Sapozhnikov and M. V. Sapozhnikov, “VerticalDistribution of Principal Nutrients in the Black Sea,”Okeanologiya 42 (6), 831–837 (2002) [Oceanology 42(6), 789–794 (2002)].

14. V. V. Sapozhnikov, D. N. Katunin, N. P. Bespartochnyi,et al., “Hydrological and Geochemical Studies of the Cas�pian Sea in Cruises of R/Vs Midiya and Issledovatel’Kaspiya (October 11–25, 2001),” Okeanologiya 42 (4),634–638 (2002) [Oceanology 42 (4), 608–612 (2002)].

15. V. V. Sapozhnikov, D. N. Katunin, and K. B. Kirpichev,“Hydrological and Geochemical Studies of the Cas�pian Sea in a Cruise of R/V Issledovatel’ Kaspiya(August 23 to September 8, 2002),” Okeanologiya 43(4), 529–534 (2003) [Oceanology 43 (4), 498–502(2003)].

16. Hydrochemist’s Handbook: Fishery, Ed. by V. V. Sapozhni�kov (Agropromizdat, Moscow, 1991) [in Russian].

Table 2. Silicon concentration variations in the near�bottomlayer of the Central Caspian depression for the last decade(1998–2008)

Years Layer, m Concentration of Si, µM

1998 775 982000 760 1452001 764 1632002 704 1602004 600 2052005 660 179

770 2142006 600 165

780 2172007 763 1752008 720 171

transformations in the caspian sea ecosystem under the fall and rise of the sea level

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