T S Osadchaya - Organic carbon and oil hydrocarbons in bottom sediments of sevastopol bay - страница 1

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МОРСЬКИЙ ЕКОЛОГІЧНИЙ ЖУРНАЛ

УДК 551.46(262.5)

ORGANIC CARBON AND OIL HYDROCARBONS IN BOTTOM SEDIMENTS OF SEVASTOPOL BAY (THE BLACK SEA)

© T. S. Osadchaya1, E. I. Ovsyaniy2, R. Kemp3, A. S. Romanov2, O. G. Ignatieva2

1 A. O. Kovalevsky Institute of Biology of the southern Seas, National Academy of Sciences of Ukraine,

Sevastopol, Ukraine

2 Marine Hydrophysical Institute,National Academy of Sciences of Ukraine, Sevastopol, Ukraine 3 Institute of Biological Sciences, University of Wales, Aberystwyth, UK

Поступила 17 февраля 2003 г.

The paper summarizes data available from the literature and recently obtained about interannual variations of the content of organic carbon, chloroform bitumenoids and oil hydrocarbons found in bottom sediments of Sevastopol Bay (Black Sea). For the recent years the bottom sediment has been receiving a rich inflow of organic matter. Localities where organic carbon content is especially high appeared owing to special sedimentation conditions and unevenly distributed anthropogenic load on the bay's water area. As the result, such sediments have reductive properties which foster accumulation of bitumenoids and oil hydrocarbons.

Key words: the Black Sea, bottom sediment, organic carbon, bitumenoid, oil hydrocarbon, pollution

Сделано обобщение предшествующих и полученных новых данных о межгодовых колебаниях в распределении органического углерода, хлороформных битумоидов и нефтяных углеводородов в донных отложениях Севастопольской бухты. За последние годы отмечается явное обогащение донных осадков органическим веществом. Наличие локальных районов, отличающихся высокими концентрациями органического углерода в осадках, обусловлено различными условиями осадкообразования и неодинаковой степенью антропогенной нагрузки на различные участки бухты. Как следствие, такие осадки имеют восстановительные условия, способствующие накоплению битумоидов и нефтяных углеводородов.

Ключевые слова: Черное море, донные осадки, органический углерод, битумоиды, нефтяные углеводороды, загрязнение

Sevastopol Bay is an inland seawater area subject to continual human impact. River inflow overuse, poor water exchange owing to the recently constructed pier, eutrophication provoked by the untreated or insufficiently treated sewage discharged from numerous and diverse sources of pollution have badly changed ecosystem of the bay. The adverse changes in the environment have affected the marine organisms at every taxonomic level as well as hydrological

and hydrochemical characteristics of the bay [7, 10, 13, 15].

Bottom sediments, in which most of the substances entering the bay accumulate and transform, are of primary importance for the ecology of the bay. Pollutants are injurious to biota through qualitative and quantitative disturbances they create in communities structure [8]. Simultaneously, biogeochemical processes going in the environment lead to decay and transformation of the received pollutants. Having accumulated in excess, organic matter of the bottom sediment triggers imbalance between production and destruction that leads to higher trophic level of the water body. Strong autumn and winter storms may make the sea water turbid down to the sea bottom, and the stirred up bottom sediment may bring about repeated pollution of the marine environment.

Suspended matter brought into the bay by the inflowing Chernaya river and from the adjoining catchment area, abrasion of the sea bottom and coast and organic matter as the result of vital activity of hydrobionts are the main sources which contribute to bottom sediment accumulation. The ratio between the catchment area and the area of Sevastopol Bay is estimated 56 that indicates how significant the terrigenous component is to sedimentation.

In Sevastopol Bay sedimentation goes at relatively high rate - about 2.4 mm/yr as a study based on 137Cs measurements has shown [2]. This estimate closely approximately the sedimentation rates characteristic of the northwestern Black Sea and the Azov Sea - 2.3 and 2.5 mm/yr, correspondingly [1, 4].

The morphometric structure and hydrodynamic features special for the bay are the factors which strongly influence granulometric fraction distribution in the uppermost layer of the

Морський екологічний журнал,2, Т. II. 2003

bottom sediment. The presence of dynamic anticyclonic formation with downwelling sea water in the centre [10, 12] has effect on the dynamics and distribution of facies of the present bottom sediment and on accumulation and transfer of substances composing the sediment. Flood events are typical of the river Chernaya that explains qualitative and dimensional heterogeneity of the terrigenous substances entering the bay. In the farest part of the bay depositing of large - grained fractions is owing to the particulate matter brought in abundance with the river influx. The river stream deceleration towards the centre of Sevastopol Bay and some special features of the hydrodynamical regimen determine settling of finegrained fractions. Sandy bottom with inclusions of shell debris stretches along the coastline of Northern Quarters and is found in Artilleriyskaya bay and in the central part of Southern bay. Finegrained aleurite and pelite fractions concentrate in the central area of Sevastopol Bay.

Thus, most of the terrigenous substances brought with the river flow settles in the inner part of the bay. The coastal water is less turbid than the sea water of the bay, and the outflow from the bay is usually greater than the inflow from the sea. Therefore, the suspended matter transfer is predominately directed towards the open sea. Sometimes, when the terrigenous matter is abundantly supplied by the river flow and sea water from the surface of the open sea is transferred along the coast into the bay, the flux of suspended matter goes opposite direction.

Material and methods. Samples of the bottom sediments were collected during a benthic survey (32 stations altogether, Fig. 1) conducted in July 2001 and preceded by a hydrological hydrochemical survey covering the same station

grid.

Fig. 1. The map of the stations in Sevastopol Bay where samples of bottom sediment were collected from the upper 0 - 5 sm layer

Рис. 1. Схема станций отбора проб поверхностного слоя (0 - 5 см) донных осадков Севастопольской бухты

Samples were taken with a modified Petersen dredge having the catchment area 0.025 m2 from the upper 0 - 5 layer of the sediment. For Corg determination the samples were placed into special dark glass jars with airtight stoppers and for measuring Eh and pH into polyethylene jars. Eh and pH were measured immediately after sampling with the use of needle- shaped platinum electrodes coupled with an argentum chlor electrode and a combined glass electrode [16]. Organic carbon was determined in air-dry samples using Tyurin method modified by Orlov [9]. Granulometric analysis was made by siege method which provided fractionation of the sediment through dry dispersion. Natural moisture content was measured using traditional weighing technique [17]. Chloroform bitomenoid was determined by gravimetric method after extraction with chloroform, and hydrocarbons were extracted by using CCL4 and determined by means of an infrared spectrophotometer IRS-29

[7].

Results and discussion. Organic carbon most reliably indicates total organic matter (OM) content in natural waters; the share it contributes average about 50 %. In 2001 a study of organic matter in the bottom sediment of Sevastopol Bay pointed out that the content of Corg varied from 0.97 to 11.6 % (stations 24 and 5) with the average 4.4 %. The content of organic carbon in the sediments covering most of the sea bottom was estimated as 3 - 10 %, it reduced to 1.5 -3 % only in the mouth of the bay. Bottom sediments in which Corg was relatively low were found on the less than 1/5 sea bottom area (Fig.

1).

The obtained estimates vary in the range close to that registered during the 1980s. At the same time, compared with the 1980s, the organic carbon content is increasing in the recent years

(Table) that may be owing to the new pier, which has narrowed the entry into the bay and high eutrophication of the sea area during the 1970s

[13].

Corg maximums (> 4 %) were measured in the Southern part (Oil Harbour - Heat Power Plant (HPP), Fig. 2).

Table. Interannual variation of the content of organic carbon, chloroform bitumenoid (Bchl) and oil hydrocarbons in the upper layer of bottom sediment in Sevastopol Bay

Таблица. Межгодовое изменение содержания органического углерода, хлороформного битумоида (Ахл) и нефтяных углеводородов в поверхностном слое донных осадков Севастопольской бухты

Years

Number of

Q,rg, %

Bchl, g/100g

Hydrocarbons, g/100g

 

stations

average

variation range

average

variation range

average

variation range

1982

19

3,37

 

1,18 - 5,89

1,54

0,26-4,92

1,18

0,14 - 4,61

1985

20

4,17

 

1,16 - 7,18

1,80

0,01-5,70

1,44

0,03 - 4,77

1988

20

5,35

 

2,53-10,04

1,35

0,22-4,13

1,24

0,76 - 3,04

1991

20

3,06

 

0,61 - 5,84

1,25

0,04-4,58

0,56

0,02 - 2,23

1994

22

3,00

 

1,56 - 6,64

1,46

0,05-3,29

0,55

0,002-2,45

1997

19

-

 

-

1,14

0,03-2,88

0,45

0,03 - 1,25

2001

33

4,38

 

0,91-11,58

1,10

0,10-3,20

0,42

0,01 - 1,54

44°6Г

44°62'-

44o63L

44o64L

33o51'

33o52'

33o53'

33o54'

33o55'

33o56'

33o57'

33o58'

33o59'

33o60'

Fig. 2. The distribution of organic carbon concentration ( % ) in the upper 0 - 5 sm layer of bottom sediment in Sevastopol Bay

Рис. 2. Распределение концентраций углерода органического (%) в поверхностном слое (0 - 5 см) донных осадков Севастопольской бухты

An exception is the central part in which the maximums were measured in the fine-grained sediments near Kylen bay and Holland bay (in the south and in the north, correspondingly). Comparative analysis has shown relationship between   the   organic   matter   content and

granulometric composition of the bottom sediment.

An exception is the central part in which the maximums were measured in the fine -grained sediments near Kylen bay and Holland bay (in the south and in the north, correspondingly). Comparative analysis has shown relationship between the organic matter content and granulometric composition of the bottom sediment. The Corg estimates consistently increase from the silty shell debris to the silty sand, yielding maximum in the aleurite-pelite silt (Fig. 2). This suggests that organic carbon in the bottom sediment of Sevastopol Bay distributes following the pattern acknowledged for the majority of continental water bodies - Corg content depends on the type of bottom sediment [1].

The specific distribution of organic carbon (Fig. 2) is also determined by the local hydrological processes and the main sources of organic matter delivery into the bay. High Corg content estimates measured in the seawater area of Oil Harbour - Heat Power Plant (HPP) are owing to two factors: the allochthonous organic matter brought in with the river inflow and settled at the river - sea geochemical border, and the autochthonous organic matter settled as the result of bioproduction. Effluent discharged into the bay also contributes to the organic matter supply. It has been reported [11] that Sevastopol Bay annually receives 300 - 350 t of allochthonous organic substances brought with the untreated or insufficiently treated seawage discharged from more than 30 outlets and totally estimated as 3 million m3.

In Southern bay considerable portion of organic matter is supplied owing to abundant primary production generated in summer. During the summer stagnation, oxygenation of the surface sea water is estimated 200 % and more

that indicates a large-scale phytoplankton bloom: during the season the phytoplankton produces biomass almost four times as great as in the conventionally unpolluted sea water near of the mouth of the bay [5]. This enrichment of the bottom sediment and near - bottom sea water with the organic matter leads to drastic near -bottom oxygen depletion. Dissolved oxygen content was the lowest (about 1.4 ml / l) in the innermost part of Southern bay and in Inkerman inlet.

Fortunatelly, the high biological production, abundant influx of detritus and organic substances from the sewage, the hypoxic seawater environment during the stagnation season have not resulted in hydrogen sulphid contamination of the bay as yet. However, these factors fostered development of an extensive area of reduced sediments (Eh + 20 + - 130 mV) in the upper 0 - 5 sm bottom layer over more than half of the bay [12]. Fig. 3 shows correlation between Eh and Corg content which charaterizes the diagenesis going in Sevastopol Bay. High (> 3 %) concentrations of organic carbon may be found both under oxydation and under reduction conditions. Difference in the changes of physical and chemical properties (pH, Eh) of the sediments are probably related to the organic matter genesis and transformation. Maximum estimates about 12 % of organic carbon deposited in the reduced sediments from the innermost part of the bay were found to concentrate at the river -sea geochemical barrier, where organic substances brought in with the river influx settle, transform through benthic organisms and bacteria vital activity and are finally deposited. In the anaerobic environment organic matter is the basic and the only energy source supplying energy for the reductive transformation [1].

Fig. 3. Relationship between redox potential and organic carbon content in the upper 0 - 5 sm layer of bottom sediment in Sevastopol Bay Рис. 3. Зависимость величины редокс потенциала от количества органического углерода в поверхностном слое (0-5 см) донных осадков Севастопольской бухты

It has already been reported [14] that the direct relationship between Corg content and sulphate reduction rate is found in the majority of cases, i.e. in the reduced sediments some portion of the easily hydrolized organic matter is used for sulphate reduction. High organic matter concentrations measured in the reduced bottom sediment from Sevastopol Bay (Fig. 3) point out that this relationship applies to bottom sediment both from the open ocean and from coastal areas of the sea. In Southern bay sulphate reduction rate is the highest that can be deduced from Eh negative estimates and the increased (8.01 - 8.55) pH values. The elevated pH estimates are the result of extra number of hydrocarbonate ions produced during organic matter mineralization.

The expansion of the sea bottom area covered with the reduced sediment and the resulting disturbed gas regimen contribute to prolonged preservation of a wide variety of pollutants. Highly toxic and ruinous to biota, oil hydrocarbons pose special danger [6]. Chloroform extracted bitumenoids (Bchl) are the most reliable indicator of the level of oil pollution of sea bottom sediments; their portion in the organic matter of polluted sediments may be as large as 50 % [3].

Close correlation between bitumenoids and Corg (Fig. 4) proves the determining role organic substances perform in formation of bitumenoids in the bottom sediment.

Data obtained from the regular monitoring of Bchl distribution over Sevastopol Bay which has conducted by Department of Marine Sanitary Hydrobiology (IBSS) since 1973, were used in determining localities differing in the level of pollution with oil and oil products from relatively safe to dramatically endangered [8].

Comparison between the recent data from the survey conducted in 2001 and the available literature points out that as earlier, concentrations of the pollutants were highest in Southern bay (1.79 - 3.20 g%, sts. 17 - 21, correspondingly) and in the centre of the eastern part (1.1 - 2.5 and 2.1 - 1.9 g%, sts. 10 - 12 and 14 - 16 correspondingly). The estimates of Bchl decreased towards the mouth of Sevastopol Bay (Fig. 5).

Conclusions. The river influx, municipal and industrial sewage received and the local hydrodynamic and lithodynamic conditions are the factors determining the character of organic matter distribution in Sevastopol Bay. Effect produced by the natural (river) and the anthropogenic (urban and industrial) effluent is different. In summer, the organic matter brought in with the river inflow decreases to minimum

2 4 6

Bbjmenmds, %

Fig. 4. Correlation between Corg and bitumenoids for the upper 0 - 5 sm layer of bottom sediment in Sevastopol Bay Рис.    4.    Отношение    органического   углерода и битумоидов в поверхностном слое (0 - 5 см) донных осадков Севастопольской бухты

owing to the corresponding sanitary norms, while that discharged with the municipal seawage increases to maximum in accord with the seasonally increased recreation load. High content of organic substances in the bottom sediments of Sevastopol Bay stimulates the intensity of reduction processes going through the depth of sediment; it is also the basic factor which determines the content of bitumen accumulated in these sediments.

Acknowledgements. The field studies in 2001 were financially supported through the INTAS 99­01390 grant. The authors thank L.N.Kiryukhina (IBSS) for the permission to use the archive data on organic carbon and oil pollution.

Fig. 5. The distribution of bitumenoid concentrations (%) in the upper 0 - 5 sm layer of bottom sediment in Sevastopol Bay

Рис. 5. Распределение концентраций битумоидов (%) в поверхностном слое (0 - 5 см) донных осадков Севастопольской бухты

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Морський екологічний журнал,2, Т. II. 2003

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T S Osadchaya - Organic carbon and oil hydrocarbons in bottom sediments of sevastopol bay