Июн 02 2002

The Extremely Late Bottom Hypoxia

THE EXTREMELY LATE BOTTOM HYPOXIA

ON THE NORTH-WESTERN SHELF OF THE BLACK SEA

AT THE END OF NOVEMBER 2001

S.I. Kondratiev 1, Yr.D. Stepanyak 2

1 Marine Hydrophysical Institute of UNAS, Sevastopol, Ukraine

2 Southern branch of Ukrainian State Geological Research Institute,

Simferopol, Ukraine

ABSTRACT. Hypoxy-conditions (the dissolved oxygen content less then 2 ml/l) with the smallest O2 concentration 1.44 ml/l (20 % saturation) were observed 22.XI.2001 in the bottom waters at the 25 – 28 m depth near 300E at the 45010’N section not far from the Danube delta. The thickness of the hypoxy-layer is estimated 1 - 3 m as well as in September 1997. The area of hypoxia is also characterised by relatively higher concentrations of nutrients and low pH-value.

In accordance with the requirements for the driller of the gas platform on the Ukrainian part of the North Western shelf, performed by "Chernomorneftegas" company, a set of research cruises was fulfilled there. The main aims of research are to study the influence of gas-platforms boring on the surrounding area; to compare the concentrations of the hydrochemical components before and after the platform disposal. The platform "Olimpyiskaya" was installed near the 44°44’ N and 30°25’ E at the beginning of 2001. Two ecological expeditions were carried out around this place at the end of November 2000 and 2001. Hydrological and hydrochemical characteristics were analysed at nine stations, located as the "envelope" in 3 - 4 miles from each other. The positions of these stations and the stations performed later at the "secular" cross-section at 45010’N are presented at Fig.1.

The watersampling from the bottom and surface layers was performed by the CTD-sond "Shick-1", equipped also by the "in situ" O2-analyser with the vertical accuracy of 0.5 m. The concentrations of nutrients (phosphates, silica, nitrates, nitrites) were analysed in correspondence with [1]. The O2 concentrations in bottom and surface layers, necessary for the O2-analyser calibration, were determined by Winkler method [2]. The mean values of these

characteristics with 95 % confidence interval are presented in the Table 1. The comparison of the analogous values, determined two years before at the same hydrological season (November 2000), shows the drastic changes in some values in November 2001. The reduction of O2-dissolved concentrations in bottom waters from safe 5 - 6 ml/l in 2000 to the dangerous 3 ml/l in 2001 is the first to be mentioned. The reduction of O2 content was corresponded by the reduction of pH value from 8.0 to 7.7 and the growth of nutrients contents: twice for phosphates and silica and three times for nitrates.

The minimal determined concentration of the dissolved oxygen for these nine casts is 2.18 ml/l (or 33 % of saturation), which is slightly more then the 2.0 ml/l limit for the hypoxy-conditions. The oxygen concentrations smaller then 2.0 ml/l are not sufficed for life-support of some kinds of sea organisms and cause the mass-mortality.

In attempts to determine the reason of this ecological condition deterioration in the bottom waters near the platform "Olimpyiskaya" we had examined eleven additional casts of the so-called "secular" cross-section along 45°10’ N from the Danube delta 29°50’ E till the longitude 32°00’ E (Pic.1). The positions of the stations № 1 - 3 at this Picture were determined not by the longtitude (it was almost the same), but by the real distances between the stations № 1 – 3 and the "properly" located station № 4. The main question is whether the observed phenomena was result from the gas-platform impacts only or it was connected with the ecological conditions of the all NW shelf.

Our assumption about general changing for the worse the hydrochemical conditions of the entire NW shelf in summer-autumn 2001 was confirmed by the peculiarities of the vertical distribution of almost the all hydrochemical parameters across the 45°10’N cross-section, Pic. 2 a-d. The most dangerous is the low absolute O2 content in the bottom waters at the longtitude 30° E, this area is filled black at the Pic. 2a. The filled area is also characterised by the low O2 saturation, less then 30 %, and low pH values, less then 7.6. Also the higher concentrations of all the nutrients were observed in bottom waters on the 45°10’N section at 30°E, Pic.2 b-d. It should be noted that in the bottom waters at the station № 4 there were the highest silica 23.4 mM, phosphates 0.7mM and nitrates 7.0 mM content, the smallest oxygen concentration 1.44ml/l (only 20 % saturation) below hypoxy level, and caused by it the smallest pH = 7.48. Oxygen concentrations below hypoxy level 1.68 ml/l and 1.93 ml/l were also observed at nearest № 3 and № 5 casts.

Table 1. The mean values over 9 casts near the platform "Olimpyiskaya" with 95 % confidence interval of each characteristic

Period

Temperature 0C

Salinity ‰

Oxygen ml/l

Oxygen %

Surface waters

2000

11.03±1.73

18.11±0.03

6.5±0.1

94±4

2001

12.43±0.19

17.45±0.34

6.5±0.2

97±2

Bottom waters

2000

10.92±1.58

18.21±0.01

5.6±0.3

84±6

2001

8.35±0.19

18.23±0.26

3.0±0.4

41±6

Period

pH

Phosphates mM

Silica mM

Nitrates mM

Nitrites mM

Surface waters

2000

8.26±0.02

0.03±0.03

1.5±0.7

0.38 ±0.17

0.10 ±0.04

2001

8.14±0.02

0.17±0.07

11.0±7.3

0.06 ±0.13

0.03 ±0.03

Bottom waters

2000

8.04±0.14

0.17±0.15

8.7±2.0

0.84 ±0.43

0.10 ±0.04

2001

7.71±0.05

0.37±0.11

13.4±5.2

2.88 ±0.77

0.01 ±0.01

The last ours investigation of the hydrologo-hydrochemical parameters vertical distribution over the cross-section 45°10’ N was performed on September 1997 (details are presented in [3]). Meteorological conditions in autumn 1997 provided two different hydrological situations near Danube Delta: penetration of surface seawater westward on September and penetration of Danube waters eastward with the upwelling as the result on October. In both cases influence of Danube input on the distribution of hydrochemical parameters in bottom waters was marked approximately till the 30.2° E. At the aquatory between Danube Delta and 30.2° E bottom waters contained rather more nutrients than surrounding waters. But minimal oxygen content was high enough to be estimated as non-dangerous. It should be noted here that seasonal termocline on September 97 was not destroyed completely, at the 20 – 25 m depths water temperature decreased from 15°C to 21°C, the thickness of termocline on September was approximately 3 m.

In November 2001 we have collided with absolutely another hydrological situation. The lack of oxygen in bottom waters can’t be explained by the presence of seasonal thermocline, the remnants if it were situated rather deeper, at 40 – 45 m depth. So, on November 2001 bottom waters at 25 – 28 m depths were not sealed by pycnocline, nevertheless the dissolved oxygen content there was less than safe 2 ml/l. We believe that the observed in 2001 hypoxia is the result of the penetration of the polluted bottom waters from the Danube-Dnieper region in the accordance of the Black Sea waters circulation.

The explanation of this situation first of all requires the estimation of the thickness of hypoxi-waters. Due to presence of dissolved oxygen analyzer in the STD-sond it is possible to estimate the thickness of bottom hypoxy-waters at casts № 3 – 5 as 1 – 3 m with the accuracy of about 0.5 m. This is absolutely the same 1 – 3 thickness of bottom layer of hypoxy-waters observed at September 1997.

It should be noted that the area of bottom hypoxia on September 1997 was restricted at north by the latitude 45°20’ N, so the cross-section 45°10’ N was out of hypoxia. That time we have believed that oxygen depletion in the bottom waters was caused by O2 consumption for the mineralisation of fresh-precipitated bottom sediments. The same explanation should be done now. This means that the water temperature 10° is sufficient to the rather active processes of bottom sediment mineralisation.

Notoriously noted hypoxy-phenomena in the bottom waters of NW shelf have begun in the early seventies of the last century. As a rule they were observed in the summer-autumn period between June and October. The recently observed hypoxia at the end of November (22.XI.2001) is probably the latest in Autumn among the known observations. During the expedition we did not have any information about the hydrochemical peculiarities on the NW shelf in summer 2001. After the preliminary analysis of our data we only can

guess how strong and extensive could be summer hypoxia that to manifest such event at the end of November. However, the real situation on the NW shelf was even worse [4]. Not only bottom hypoxia, but the anoxia (absolute absence of oxygen) with the hydrogen sulfide concentrations up to 1.7 ml/l [4] was observed in the bottom waters of NW shelf in July 2001.

So late preservation of the oxygen deficiency in the bottom waters was probably caused by the rests of summer stratification with the remnants of thermocline at the depths 45 - 50 m. This situation was the result of extremely hot summer 2001, provided so strong heating of surface waters that the remnants of thermocline were observed even in November.

Thus, during investigation the ecological conditions near the gas-platforms, hypoxia in the bottom waters was observed in later Autumn (November 22, 2001). Probably, it was the latest seasonal event among all known observations on the North Western shelf.

Reference

1. Methods of hydrochemical investigation mean biog. elements. M: VNIFC, 1978, 213 p. (in Russian).

2. Methods of hydrochemical investigation of ocean. M: Nauka, 1978, 124 p. (in Russian).

3. Kondratiev S.I., Gevorgiz N.S. Changes distributions hydrochemical parameters of apvelling influence in Danube delta area in autumn 1997 y.// Ecological safety coastline and shelf zone and complex using resources of shelf. Sevastopol: ECOSI-Gidrofisica.-1999 y., p 111-125 (in Russian).

4. Ukrainskyi V.V., Popov U.I., Orlova I.G. “Summer-autumn Regime of the Dissolved Oxygen in the Eutrofic Conditions of the North-Western Shelf of the Black Sea//Ecological Problems of the Black Sea, Odessa, 2001, p. 332-336 (in Russian).

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