Recent State of Lake ()

Galina Shtereva, Boryana Dzhurova Institute of Oceanology, Bulgarian Academy of Sciences, Varna, BULGARIA

Abstract The paper deals with the dynamic of hydrochemical parameters in Beloslav Lake-one of Bulgarian coastal lakes. It is strongly affected by human activities. An estimation of water quality (WQ) and anthropogenic impact on the lake is of the first importance for a sustainable management in context of EU Water Framework Directive/60/2000. On the base of seasonal monitoring data for nutrients (nitrite, nitrate, phosphate), dissolved oxygen, BOD and oxidizability the lake could be considered as a high eutrophicated area. High polluted rivers discharge into the lake and cause negative changes of WQ, including hypoxia in the near bottom waters. The paper discusses different periods of anthropogenic influence depending on human activities and different kind of pollution sources. The comparison of the data after 2000 with the data from period of intensive eutrophication (80s) reveal a slight decreasing tendency of nutrients in relation of the collapse of the economy during the last 10-15 years.

Key words : Beloslav Lake, Water quality, eutrophication, nutrients, oxygen

Introduction Beloslav Lake is one of the important Black Sea coastal lakes affected by human activities (industry, agriculture, transport and urbanisation). It is located in NE part of Bulgaria. It has a surface area 3.9 km2, mean length 4 km, mean width 1 km and volume 9*106 m3. The lake eutrophication is a significant and an ever increasing problem has been well documented (Rozhdestvensky, 1977; Velikova, Petrova, 1999; Shtereva et al. 2001). The drivers for enhanced eutrophication are nutrient inputs from point sources and non-point sources namely agriculture and urban activities. In fact the load of nutrients to the lake system has greatly increased with time through human activity (Andreev, 1984; Rozhdestvensky, 1996; Stoyanov, 1991) with increase especially in nitrogen (N) and phosphorus (P) loading about several times above that in “pristine” state. The Chemical Industrial complex plays a major role in emission of nutrients and other pollutants. Two Waste Water Treatment Plants (WWTPs) with mechanical and biological treatment discharge into the lake with total volume about 24 l/s. Except chemical industrial complex, two ports are located in Beloslav Lake banks: Ferryboat Port and Port Varna-Zapad. The second one is a pollution source too since in-bulk cargo is handled there. The pollution of both rivers River and Provadijska River is one of the major problems in the area. Provadijska River with catchment 2132 km2 discharges into Beloslav Lake 72.44 103 m3 per year. Devnya River discharges into Provadijska 96.53 103 m3 per year (Dimitrova et al., 1988).

Methods The study is based on seasonal monitoring of surface and bottom waters on two stations in Beloslav Lake during 2000-2002 period. The disposition of the investigated basin is presented at Fig. 1.

43° 14'

43° 13'

43° 12' Varna Zapad Port AKE AV L VARNA OSL VARNA LAKE BEL BAY 43° 11' Ferry a jsk di port va r ro ve 43° 10' P Ri 0 km 5 km 43° 09' 27° 38' 27° 40' 27° 42' 27° 44' 27° 46' 27° 48' 27° 50' 27° 52' 27° 54' 27° 56' 27° 58'

Figure 1. Map of the investigated area

The samples were collected by Nansen sampling bottle from surface and from bottom waters and were analyzed for salinity, nutrients (phosphate, nitrate, nitrite, silica), pH, dissolved oxygen (DO), alkalinity (Alk), biochemical oxygen demand (BOD5) and oxidizability (Mn).

Results Hydrological and hydrochemical regimes of Beloslav Lake are determined by river inflow and also by water exchange with Varna Lake, i.e. by the indirect influence of Varna Bay sea waters. In this regard salinity in surface waters varies in range 7.55-15.32 ‰ with minimum in the west part of the lake, where the influence of fresh water from the rivers is more significant. A main share of fresh water incoming into the lake is that of the Provadijska River. The area close to the river mouth demonstrated low similarity with other stations. Concerning bottom waters (depth 12 m) comparable concentrations with surface in autumn-winter are measured in regard to homogeneity of the water column. As a consequence of river discharge the lake WQ does not correspond to the Bulgarian standards (Regulation № 7) not only for nutrients. The measured concentration for BOD and total P often exceed Limiting permissible concentration (LPC) for II category water body. The close to river mouth aquarory is characterized with high suspended solids and nutrients content and pH>9. The rivers discharge waters with nitrogen content (especially ammonia) and pH over permissible concentration. The seasonal distribution of the chemical parameters is presented at Fig. 2 and Fig. 3.

DO, m l/l OS, % 16 250

14 200 12

10 150 8 100 6

4 50 2

0 0 November March May August Figure 2. Seasonal distribution of DO and OS in surface waters

The surface water OS exceeded 100% almost during the all seasons. Rather higher saturation was registered in spring-summer months. A low concentration of DO (<2.0 ml/l) in the bottom waters (depth>11 m) and saturation of about 30 % are typical for the summer period due to the limited vertical water circulation and oxygen consumption as result of the organic matter oxidation. Besides, the limited water exchange with Varna Lake prevents the oxygenated water entrance and leads to worsening of oxygen conditions at the near bottom layer due to the induced oxygen deficit. It should be mentioned that anoxia is not registered during the recent period in contrast to 80-s.

NO3, mg/l NO2, µg/l PO4, µg/l TP, µg/l Si, µg/l 18 160 250 2000

16 140 1800 14 200 1600 120 12 1400 100 150 1200 10 80 1000 8 60 100 800 6 600 40 4 50 400 20 2 200 0 0 0 0 Nov. M ar ch M ay Augus t November March May August

Figure 3. Seasonal distribution of nutrients in surface waters The events of hypoxia/anoxia and fish mortality as consequences of water contamination and eutrophication have been reported (Rozhdestvensky, 1977; 1996). The described changes result in dramatic alterations in the chemical and biological regime (Velikova, Petrova, 1999). Hypoxia in bottom waters and high organic load determine the changes in benthos community. As a consequence of worsened state of the bottom a part of the investigated lake is characterized by very poor benthic community (Konsulova et al., 2000). For example the area adjacent to the port in western part shows a severe level of disturbance in respect of very poor environmental quality. After Varna Zapad Port construction the changes in hydrology and hydrochemistry were with relatively positive character regarding lower nutrients content in sea water coming from Varna Bay. The increased depth of the western part of the lake and widening of the navy canal leaded to increased water entrance from Varna Lake. Changes of hydrology influence thermal and salinity regime. Due to more intensive water exchange with Varna Lake Beloslav Lake is converted to saline basin. A slight increase of the parameters salinity, alkalinity and pH the lake is presented (Fig. 4).

S, ‰ Alk pH 13.60 2.70 8.30

13.40 2.60 8.20 13.20 2.50 8.10 13.00 2.40 8.00 12.80 2.30 7.90 12.60 2.20 12.40 2.10 7.80 12.20 2.00 7.70 1975- 1980- 2000- 1975- 1980- 2000- 1975- 1980- 2000- 1979 1984 2002 1979 1984 2002 1979 1984 2002

DO, ml/l OS% Oxidizability 10.00 160.00 3.60 9.00 140.00 3.50 8.00 120.00 3.40 7.00 6.00 100.00 3.30 5.00 80.00 3.20 4.00 60.00 3.10 3.00 40.00 3.00 2.00 1.00 20.00 2.90 0.00 0.00 2.80 1975- 1980- 2000- 1975- 1980- 2000- 1975- 1980- 2000- 1979 1984 2002 1979 1984 2002 1979 1984 2002

Figure 4. Comparison between recent and previous periods of investigation

The comparative analysis of data for chemical parameters revealed that the periods of intensive eutrophication and pollution 1975-1985 caused dramatic shifts in hydrochemical regime of the lake (Andreev, 1984; Rozhdestvensky, 1996; Kazakov et al., 1995). Long-term pollution provoked biodiversity changes as well as changes in plankton and benthos population structures (Konsulova et al. 2000; Velikova , Petrova. 1999). During the 80-s an increased chemical production, port and navy activity were established. An increase not only of nitrogen and phosphorus content responsible for eutrophication but a significant pollution with metals and petroleum hydrocarbons (TPH) are recorded (Andreev, 1984; Stoyanov, 1991; Shtereva et. al, 2000). On one hand increase of saline water quantity with marine origin and reduce of fresh water quantity because increasing in water consumption for industry on other hand lead to increased role of sea waters. In this connection the changes in main cations and anions concentrations were available also. Increasing of salinity and alkalinity in the investigated period is an evidence of positive role of water exchange with transformed seawater coming from Varna Bay. Higher oxygen content in recent period indicates better oxygen conditions. As it is evident the mean OS value for 1980-1984 periods is lower than 90 %. Hypoxia conditions were initiated not only in summer but as early as spring. A total dissolved oxygen depletion in bottom waters and hydrogen sulphide existence was reported by Rozhdestvensky (1977). In contrast to 80-s, when oxygen deficiency was established not only at the bottom (Rozhdestvensky, 1977; 1996), entire water column was well oxygenated during the last period except summer. Similar critical summer situations in bottom water in 1990-s have been documented (Stoyanov, 1991; Shtereva et al. 2000; Shtereva, Dzhurova, 2001). In spite of observed low oxygen content in bottom waters in 90-s, the comparison shows a relatively improvement of oxygen conditions during the recent period (Fig. 5).

DO- s DO- b OS- s OS- b

12 180 10 160 140 8 120 100 6 80 4 60 40 2 20 0 0 1974-1979 1980-1984 1999-2001 1974-1979 1980-1984 1999-2001

Figure 5. Comparison of DO(ml/l) and OS(%) in surface (s) and bottom (b) waters by periods

As it is shown at Fig. 4 an increasing trend of organic matter is established. This fact corresponds to the oxygen deficiency in the bottom waters because still significant organic loading to the lake. A decreasing trend of nutrients was observed in comparison to the period of progressive eutrophication. Figure 6 illustrates about 2 fold decrease of phosphates and nitrites content in comparison to 1970-s.

P-PO4,µg/l N-NO2µg/l 0.35 250 0.30 200 0.25 0.20 150

0.15 100 0.10 50 0.05 0.00 0 1975- 1980- 2000- 1975- 1980- 2000- 1979 1984 2002 1979 1984 2002

Figure 6. Comparison of nutrients content in surface waters by periods

Occurring changes in economy after 1990 (decrease of industrial and agricultural production, particularly of fertilising intensity) are cause of relative water quality improvement in the lake (Shtereva, Dzhurova, 2003). These changes with positive aspect are good sign in connection with main aim of Water Framework Directive: achieving good status for all waters until 2015. The lake became buffer zone and receiving the load of nutrients and other pollutants it plays an important role for protection of marine environment (Varna Bay). Besides, the closed character of the investigated area and insufficient intensive exchange with Varna Lake-Bay water system limit the spreading of contaminants or produced organic matter and in this way predetermine their retention. According to conceptual framework DPSIR (Driving forces-Pressure-State-Impact-Response) the main drivers in Beloslav Lake region are industry, agriculture, urbanisation and shipping which caused significant pressure on the aquatic system. The worsened state of the lake after long-term pressure of drivers was presented. Consequences are high level of eutrophication, hypoxia/anoxia events, blooms and other negative effects for the lake ecosystem (impact). Decreasing of industrial and agricultural production leads to relatively positive changes of environment. The established nutrients decreasing trend in last years is not stable. The Beloslav Lake environment needs comprehensive and immediate measures (response).

Conclusions: During the last decade a trend of improvement was observed. Reduced content of nutrients in the Beloslav Lake as a result of the economy collapse is a positive sign of recovery initiation. So far, the trend is not stable and the eutrophication level is still high. In summer dramatic diminution of oxygen in bottom layers still occur. Summer is suggested as a season most vulnerable to the evolution of anthropogenic eutrophication.

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