Long-term evolution and trends of the hydrological and hydrochemical parameters in Bulgarian Black Sea waters during the period 1992-2000

Abstract

The dynamics of chemical parameters in the Bulgarian Black Sea over the 1990s reflects the complex relations in the ecosystem itself and the influence of the Danube water discharge, which is a major climatic and anthropogenic factor for the Western Black Sea. Analyses of hydrological (temperatures, salinity) and hydrochemical (dissolved oxygen, oxygen saturation, nitrite nitrogen, nitrate nitrogen) data, collected during the period 1992–2000 in the 30-miles zone in front of the Bulgarian Black Sea coast, up to 150 metres depth, were carried out in the framework of the DANUBS project. Over 1990s gradually the winters were becoming warmer, the springs and autumns colder, and the summers short and hot. The long-term averages show spatially a minimum of salinity at 10 miles in front of Cape Galata, whereas in front of Cape Emine salinity gradually increases from the coast towards the 30-miles offshore. In the late 1990s, very low summer concentrations or even complete absence of inorganic nitrogen were recorded in front of the Bulgarian Black Sea coast. Seasonally, oxygen has varied in broad terms, however on average the surface waters were saturated or slightly oxygen super-saturated.  Down on the vertical, there was a regular decrease of oxygen concentrations.

Content Table

• Abstract
• Introduction
• Material and Methods
• Results and Discussion
• Conclusions
• Acknowledgements
• References
• Links

Introduction

The Black Sea is of global interest on several levels. It has experienced the worst environmental degradation of all of the world’s oceans. The situation has become so severe that it has affected the health, well being, and standard of living of the people in the immediate area. The Black Sea’s area is 431 200 km². About 160 million people live in the Black Sea catchment basin, including 80 million only in the Danube River basin. The environmental crisis and subsequent dramatic changes in the Black Sea’s ecosystem and resources are a direct effect of both natural and anthropogenic causes: an enormous increase of the nutrient and pollutant load from three major rivers - the Danube, Dniestr, and Dniepr; from industrial and municipal wastewater pollution sources along the coast; and from dumping on the open sea.                                                                                                                                                                                                                  Major trends of the observed basic parameters are presented in this survey. Emphasis is given to the level of eutrophication of the Bulgarian Black Sea, regulated through nutrient inputs by the Danube transformed waters.

Material and Methods

Figure1. Map of observations

Observations were performed in front of Capes Galata and Emine, at transects of 5 stations each (Figure 1), perpendicular to the Bulgarian coast during the period 1992-2000 (Dineva, 2002). Stations were selected to reflect gradients in properties, with a control station at 3-miles in front of Cape Galata during the period 1995-2000. Sampling was performed at standard depths (0, 10, 25, 50, 75, 100, 150 m).

Processing of samples was done via unified methods for marine waters.

Results and Discussion

Fig 2. Long-term evolution and trends of the sea temperatures (˚С) at standard depths in the 30-miles zone in front of the Bulgarian Black Sea coast during the period 1992-2000

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Figure 3. Long-term seasonal changes and trends of the sea surface temperatures (SSTs, ˚С) in the 30-miles zone in front of the Bulgarian Black Sea coast during the period 1992-2000

In the 1990s (Figure 2), there were no significant deviations of sea temperatures from the long-term averages. However (Figure 3), gradually the winters were becoming warmer, the springs and the autumn colder, and the summers short and hot (Dineva, 2002)

Figure 4. Long-term monthly dynamics of Danube runoff (m³.s-¹) over the period 1992-2000

Danube water discharge (Cociasu et al., 1997; Report, 1998; Dineva, 1999; Dineva, 2002) is a major climatic and anthropogenic factor for the Western Black Sea (Figure 4).

Figure 5.  Monthly dynamics of Danube run-off (m³.s-¹) and sea surface salinity (SSS, psu ) of the Black Sea in front of the Bulgarian coast (Cape Galata, 3-miles offshore) over 1995 – 2000

Depending on the level of the river runoff and the current system, the pathway of Danube transformed waters along the Bulgarian coast seasonally shifts and passes at a different distance offshore. The Danube transformed waters, passing along the Bulgarian coast, influence strongly the seasonal and inter-annual variability of salinity, mainly in the surface layers. In the period 1995-2000 (Figure 5), as a result of the May maximum (in average) of the Danube runoff, a June minimum of SSS was observed in the Bulgarian Black Sea waters (Dineva, 2002).

Figure 6. Long-term evolution and trends of sea salinity (psu) at standard depths in the 30-miles zone in front of the Bulgarian Black Sea coast over the period 1992-2000 

Under Danube river impact, sea salinity was decreasing through the 75 m Black Sea layer in front of the Bulgarian coast over the 90s (Figure 6).

Figure 7. Long-term evolution and trends of oxygen saturation of water (%) at standard depths in the 30-miles zone in front of the Bulgarian Black Sea coast over the period 1992-2000

There was a regular decrease of oxygen saturation with depth – both in coastal and open-sea waters (Figure 7).

Figure 8. Long-term evolution and trends of nitrite nitrogen (μg/l) at standard depths in the 30-miles zone in front of the Bulgarian Black Sea coast over the period 1994-2000 

Figure 9. Long-term evolution and trends of nitrate nitrogen (μg/l) at standard depths in the 30-miles zone in front of the Bulgarian Black Sea coast over 1996-2000

Figure 10. A                                  Figure 10.C

Figure 10.B

Figure 10. Monthly dynamics of Danube runoff, m³.s-¹(B.), NO₂-N, μg/l (A.), and NO₃-N, μg/l (C.) in the surface Black Sea waters along the Bulgarian coast (Cape Galata – 3 miles offshore) over 1995 – 2000

The long-term evolution and trends of nitrite nitrogen and nitrate nitrogen are presented on Figure 8 and Figure 9. Generally, Figure 10 depicts positive correlations between Danube river discharge (Figure 10 B.), and concentrations of nitrite nitrogen (Figure 10 А.) and nitrate nitrogen (Figure 10 C.) in the surface Black Sea waters along the Bulgarian coast. 

Conclusions

Rising winter and summer temperature trends, autumn temperature reducing tendency, and less reducing spring temperatures were established in the surface Black Sea waters in front of the Bulgarian coast over the period 1992-2000. The balancing of the temperature changes in the 10 m layer was due to season changes of the year’s cycle. Waters on 25 m depth were considerably cooling, with 2.5˚C over 1996-1998. There was impressive decreasing temperature trend on horizon 25 m during the period 1992-2000. Rising temperature trends were established in the 50-100 m sea layer. These were waters from the Cold Intermediate Layer (CIL), which become warmer in the frame of isotherms of 8˚C.

Danube run-off increasing has caused sea salinity decreasing through the 75 m Black Sea layer in front of the Bulgarian coast over the 90s. The vertical stability of the layers is minimal in the winter, when the direct interaction of waters during storms could approach down to 75 m – a mechanism of Danube transformed waters’ impact onto deeper sea waters as well, unlike usually.

The oxygen saturation of sea water was with increasing trends through the 50 m photosynthetic layer over the period 1992-2000, and also there was a significant improving on 150 m depth.

The nitrite nitrogen was with increasing trends throughout the 150 m layer. The nitrate nitrogen was decreasing, and only on 75 m depth its concentration was strongly increasing due to the favorable conditions for doing of nitrification processes through.

Acknowledgements

The results presented stem from the project “Nutrient Management in the Danube Basin and its Impact on the Black Sea” (daNubs) supported under contract EVK1-CT-2000-00051 by the Energy, Evironment and Sustainable Development (EESD) Programme of the 5th EU Framework Programme. Details on the project can be found on the project homepage: http://danubs.tuwien.ac.at/

References

1. Cociasu A, Diaconu V, Popa L, Buga L, Nae I, Dorogan L and Malciu V. (1997). The nutrient stock of the Romanian shelf of the Black Sea during the last three decades. In Sensitivity to Change: Black Sea, Baltic Sea and North Sea. NATO ASI series, Series 2: Environment Vol. 27, Kluwer Academic Publishers, Dordrecht/Boston/London, pp.49–63.
2. Dineva S. (1999). Hydrochemical and Hydrophysical monitoring of the Bulgarian Black Sea aquatory. Proceedings of the Institute of Fishing Resources, Varna, Bulgaria, 25, pp.143-151.
3. Dineva S. (2002). Chapter I. Hydrology and Hydrochemistry, Report over 1992-2000. In:  Bulgarian Report over 1990s, Nutrient Management in the Danube Basin and its Impact on the Black Sea (daNubs Project), Contract EVK1-CT-2000-00051 by the Energy, Environment and Sustainable Development (EESD) Programme of the 5^th EU Framework Programme, WP7, Effects in the Western Black Sea, Institute of Fisheries and Aquaculture, Varna, Bulgaria (Partner N5).
4. Report on the ecological indicators of pollution in the Black Sea. (1998). Danube River Pollution Reduction Programme and the Black Sea Environmental Programme, Romanian Marine Research Institute, Constantza, Romania.

5. Links

Slava Dineva
Institute of Fishing Resources, 4 Primorski Blvd., P.O.Box 72, BG – 9000 Varna, Bulgaria, Tel/Fax: +359 52 63 20 66,

E-mail: dineva_slava@abv.bg

Water Science & Technology [Water Sci. Technol.] Vol 51 No 11,  pp 19–26. 2005