TENTH INTERNATIONAL CONFERENCE ON MARINE SCIENCES AND TECHNOLOGIES

VULNERABILITY OF AND VARNA LAKES TO EUTROPHICATION

Valentina DONCHEVA, Snejana MONCHEVA, Nataliya SLABAKOVA

Abstract The main goal of the study is to assess vulnerability of Varna and Beloslav Lakes to eutrophication based on the screening models applied for water column and sediment characteristics. Simplified ‘screening’ models that have been applied for assessment the status of the lakes are Comprehen- sive Studies Task Team (CSTT)model (1994, 1997) for the water column status, and biogeochemical indicators for the sediment quality (Viaroli et al, 2004). According to applied models Beloslav and Varna lakes could be classified as hypereutrophic with high turbidity where the phytoplankton is light limited due to self-shading effect of the high biomass. Based on up- take of available nutrients by phytoplankton the resulting biomass characterizes the lakes as hypereutrophic. A reduction of the phytobiomass in the lake would require more than 20 times decrease of WWTP discharge. The sediments are defined as of high to excessive vulnerability risk. Key-words: eutrophication, Beloslav Lake, Varna Lake, screening model, CSTT, chlorophyll, sediment.

Introduction from a sampling grid of nine stations in the lakes Understanding the increased eutrophication pres- (Regional Inspectorate of Ministry of Environment) sure over the coastal systems has challenged the need and additionally, our own data for chlorophyll and for integration of many complex processes. Screening nutrients (IO-BAS - Varna) were used (Fig. 2). The models are designed to provide an overview of the background station 301 is selected from the monitor- trophic status based on a few diagnostic variables [1], ing grid of IO-BAS. The Varna wastewater treatment to act as a link between data collection, interpretation plant (WWTP), which discharges about 1800 l/s into and coastal management. the Varna Lake from Varna city (more than 300000 in- The goal of the present paper is to assess vulnerabil- habitants), is included in the balance as a source. Water ity to eutrophication of system Beloslav Lake - Varna mass and nutrients balances were estimated according Lake based on screening models, applied to the water to [4]. PAR (photosynthetic available radiation) data column and upper sediment layer. was obtained from [5].

Material and methods The CSTT is a simple model for identifying the waters that were at risk of eutrophication. (Fig.1). The model calculates the maximum phytoplankton chlorophyll concentration that could occur in summer for a given nutrient loading, based on uptake of avail- able nitrogen and phosphorus by phytoplankters. This concentration than is compared to a predetermined threshold to determine trophic state. The model was proposed by the UK’s Compre- hensive Studies Task Team [2, 3] in conformity to the requirement of the Urban Waste Water Treatment Directive (UWWTD) implementation. Fig. 1 Schematic presentation of the CSTT’s The input data for nutrient concentrations are based model for eutrophication-after [1] on averaged data for the 10-year period (1992-2002)

Assoc. Res. PhD Valentina Doncheva, Assoc. Prof. PhD Snejana Moncheva, Assoc. Res. Natalia SLABAKOVA, Institute of Oceanology, tel.370 485 e-mail: [email protected], [email protected], [email protected] 261 TENTH INTERNATIONAL CONFERENCE ON MARINE SCIENCES AND TECHNOLOGIES

Tentative assessment of the potential vulnerability level (PVL) of transitional waters based on integrated measurement of status variables in the surficial sediment (upper 5 cm). (1) modified from [6, 7] Table1 Vulnerability low Moderate high excessive Individual Rank 3 2 1 0 PVL 12 8 4 0 Fig. 2. Sampling stations in the system Beloslav Water Retention < 1 1-7 7-15 > 15 lake-Varna Lake - Varna Bay. Time (WRT, d) Granulometry (G) sand sandsilt siltclay Clay Total Organic Car- As major variables to monitor sediment character- bon (TOC, % dw) < 0.5 0.5 - 2.5 2.5 - 5 > 5 istics biogeochemical indicators were employed, inte- upper 5 cm (1) Carbone (CaCO , grated in index according to the criteria of UNESCO/ 3 > 40 20 - 40 5 - 20 < 5 % dw) IOC and [6, 7]. Originally the index of vulnerability is a sum of individual ranks of 6 parameters - Water Retention Time, Granulometry (G), Total Organic Results and discussion Carbon (TOC, % dw) in the upper 5 cm, Carbonate The main CSTT procedure is relevant for zones in (CaCO , % dw), Reactive Fe (μmol cm-3), Acid Volatile 3 which discharged nutrients have residence times of a Sulphide (AVS, mol cm-3). Each rank is ranging from few days, sufficiently long to influence phytoplankton 3 - low vulnerability to 0 - excessive vulnerability and growth and biomass under of favorable conditions. the maximal value of the index is 18. Due to lack of Beloslav Lake has a residence time 6 days, Varna Reactive Fe and Acid Volatile Sulphide data, the index Lake 17.5 days and if unified in 1 box about 15 days is modified for 4 parameters - the maximal value for in average. index for low potential vulnerability level (PVL) is 12 In the CSTT eutrophication model applied the (Tbut able 1). parameters used to convert nutrients into worst case The data for sediment characteristics were obtained chlorophyll enrichment are according to [1, 2, 3]. from Environmental Impact Assessment Reports [8, There are three key steps in calculation of the 9, 10, 11] and new data collected within the project vulnerability of an area under investigation to eutro- TWReference net (Fig. 3.). phication [12]. The first step is to calculate the equilibrium con- centration of nutrients in the area that will result from a specified discharge, taking into account other nutrient inputs, and given the rate at which receiving-waters exchange with the sea (containing a known concentra- tion of nutrient)

1. S eq = S o + (s i /(E.V)) Fig. 3. Location of the stations for sediment sampling. Where S o - boundary or background concentration of nutrient; si is the local input of nutrient (in mmol d-1 from rivers, WWTP discharges, etc.; E: exchange or dilution rate; V: volume Next step is to calculate the concentration of chloro- phyll that would result if all nutrients were completely used during the phytoplankton growth, the chlorophyll concentration in the water coming from the adjacent sea, and consider if the total exceeds a threshold:

262 TENTH INTERNATIONAL CONFERENCE ON MARINE SCIENCES AND TECHNOLOGIES

2. X max =X o + q.S eq ((maximum) chlorophyll Thus based on uptake of available nutrients by concentration.); phytoplankton the resulting biomass characterizes the lakes as hypereutrophic (Table 2). The models results Xo: summer concentration of chlorophyll in sea:q are close to the measured concentrations in the area. In coefficients converting phytoplankton at yield of chlo- a number of cases the reported phytoplankton biomass rophyll per mmole of nitrate or phosphate; q = 1.1 mg expressed as Chl. a exceeded 100 mg.m-3 [14, 15]. chl (mmol N)-1; q = 30 mg chl (mmol P)-1; The maximal summer values measured in the canal, Finally it was determined if algae can in fact use these connecting Varna Lake and Varna Bay for the period nutrients - if receiving waters are too turbid to not allow 2000-2003 varied within the range between 10 and 50 plant growth, or diluted too fast to sustain an increase mg.m-3, in average 33 mg.m-3. in the abundance of phytoplankton. During the late spring 2001 the maximal value of chlorophyll was 90.64 mg.m-3, and during the summer 3. μ(I) = αb.(I-Ic) of 2005 - 52.8 mg.m-3. The maximal concentration measured in spring 2002 was 72.53 mg.m-3. Where :μ(I) is light-controlled phytoplankton growth; αB=0.006 is photosynthetic efficiency per day per ir- Input data and model results with and without radiance unit (d-1I-1) at low illumination. Photosynthetic WWTP input estimation (denominator) efficiency is the ratio of the rate of photosynthetic for- Table 2 mation of organic carbon for the available PAR Parameter Symbol Nitrogen Phosphates I is Photosynthetically Available Radiation (PAR) aver- volume box m3.106 V 195.5 195.5 aged over 24 h for a typical summer’s exchange rate per day E 0.04 0.04 _2 Ic 5 mE m s is compensation PAR, the illumination at nutrient input [kmol/ 70.61 0.9 si which photosynthesis and respiration are in balance day] /231.34 /21.05 when totalled over 24 h [1]. seawater concentra- S 8 0.3 The average illumination of the vertically mixed tion [mmol/m3] 0 Seq = S0 layer within which phytoplankton grow was calculated equilibrium concen- 17.03 0.42 + tration [mmol/m3] /37.58 /2.99 from: (si/(E*V)) -kD.h I=m.I0.(1-e )//kD.h≈m=I0/kD.h chlorophyll yield h : thickness of the surface mixed layer, metres (from N and P) q 1.1 30 [mgChl mmol nutri- I0 : 24-hr mean photosynthetically available radiation (PAR) at sea-surface, μE m-2 s-1 ent] seawater chlorophyll X0 5.5 5.5 kD: diffuse attetuation of (downwards) PAR in the conc. mg/m3 mixed layer, m-1 maximum chlorophyll Xmax = X0 24.2/ 18 m : correction for surface reflection and additional conc. mg/m3 + q*Seq 46.8 /95.3 near-surface losses rough estimate of the coefficient maximum permitted EQS 10 10 may be made from: chlorophyll mg/m3 kD = f/zS Where zS is the Secchi disc depth (metres), and the fac- tor f can be (taken) assumed as 1.4 typical for turbid Optic characteristic of Beloslav and Varna Lakes coastal water [13]. Table 3 Secchi disk depth data is averaged from monitor- Parameter Symbol Value Units average PAR ing data of IO-BAS, and a depth of 1 m is accepted I 20.5 μE m-2s-1 for the Lakes. in box Photosynthetic It is assumed that if µ(I)> E+L’the phytoplankton α 0.006 d-1 (μE m-2s-1)-1 efficiency can utilize the nutrients effectively, and if µ(I)< E+l compensation Ic 5 μE m-2s-1 the light for the phytoplankton growth is insufficient irradiance [1], where L is the local loss rate of phytoplankton due light-controlled μ(I) = α*(I-Ic) 0.093 d-1 to consumption by plankton or benthos. If the latter is growth rate not-known an assumption of 0 or 0.1 could be applied. loss (E + L) 0.14 d-1 Also if maximal summer concentration of chlorophyll exchange E 0.04 d-1 3 default value of exceeds the maximum permitted level of 10 mg/m the L 0.1 d-1 basin is categorized as eutrophic. biological loss

263 TENTH INTERNATIONAL CONFERENCE ON MARINE SCIENCES AND TECHNOLOGIES

UWWTP emissions exceed riverine input about 2. Granulometry (G) - determines diffusive proper- three times for nitrogen and 20 times for phosphorus. ties of oxygen and nutrients. The result suggests that in order to reduce phytobio- Out of the 30 measurements, 88% of Beloslav Lake mass in the lake would require more than 20 times indicated clay and 12 % silt substrate and in Varna decrease of WWTP discharge. lake - 70% of the samples were classified as silty clay The light-controlled growth rate µ(I) is less than or clayey silt bottom. the local loss as sedimentation, degradation and graz- 3.Organic carbon (Corg) - Organic matter (OM) ing (Table 3). Phytoplankton is light limited due to the content in the surface sediments originally is used as self-shading effect of the high biomass. Phytoplankton an indicator of the risk for species diversity reduction can’t assimilate nutrients which conditioned intensifi- in benthic macrofauna communities [6]. cation of the sedimentation rate of organic matter, its According to IOC list of benthic indicators [6] degradation and accumulation in the basin. In the lakes organic carbon <1% is indicator of healthy ecosystem. respiration dominates over production. 15% of sediments in Varna Lake and 6% in Beloslav The area of Beloslav Lake is characterized by poor Lake have Corg less than 1% associated with sandy benthic community - two of the six sampled points were sediments along the banks. About 50% of the data was inhabited by a single species, one - by two species and in the range 1-3% Corg.- indicating disturbed environ- the other three were classified as moderately to highly ment. In both Lakes about one third of the samples the disturbed [16]. The ecological status of macrozoo- organic carbon content was <3% - indicator of heavily benthic community inhabiting Varna Lake was better disturbed environments. The average concentration of and varied from highly and moderately disturbed to organic carbon in the lake sediments was in the range undisturbed [16]. The classification of ecological status 2.5-5% (2.9 and 2.7%) and both lakes could be classi- according to different indices: Shannon diversity index, fied as with high risk of anthropogenic pressure. In the AMBI and the multivariate AMBI (M-AMBI) rank the impacted zones in front of TPS and WWTP the organic Varna Lake site as bad [17]. carbon exceeded 5%. Zoobenthos species richness of Polychaetes, Mol- 4. Calcium Carbonate has been also proposed as lusks and Crustaceans was reduced associated to the potential buffers that control P and S reactivity and increased frequency of hypoxia/anoxia events, resulting mobility [22]. Due to soda production in the area close in mass mortality and deterioration of benthic com- to Beloslav Lake the mixing of high alkaline industrial munities [18, 19, 20]. The dominance of Polychaetes waste and river waters resulted in accumulation of opportunistic species tolerant to environmental stress carbonate deposits. The average carbonate content was increased to 79 % of the total average in the late 80-ies 46% ranging from 54 to 18%. The carbonates decreased - early 90-ies. Lifeless bottom areas “dead zoobenthic eastward to Varna Lake- average 25%. The correspond- zones” were identified along the lakes and lake-bay ing ranks are 3 - for Beloslav Lake, 2- Varna Lake. interface [21]. According to the rank values Varna Lake and The biogeochemical indicators that have been Beloslav Lake is classified as high to excessive high used for assessment of the vulnerability of the Lakes vulnerability (Table 4) and Beloslav Lake as moder- area are: ate to high because of the higher residence time and 1. Water Retention Time of the water - it might play carbonate content. Beloslav Lake possibly has the an important role in sustaining eutrophication because same rank as Varna Lake because the residence time high WRT is an indicator of potential water stagnation was overestimated in the simple balance model which and anoxia. Anoxia triggers accelerated phosphate re- does not account for the narrow connection between lease from sediments and increased nutrient buildup. the two lakes. In principle carbonates are artificially This high nutrient supply could fuel high biological deposited. The difference of organic carbon content in productivity and oxygen demand, enhancing oxygen the sediments of two Lakes is negligible. depletion and sulfide buildup via sulfate reduction. The respective ranks of Beloslav Lake and in Assessment of the potential vulnerability level of Varna Lake are 2 and 0, correspond to moderate and Beloslav-Varna Lakes excessive vulnerability. The real WRT for Beloslav Table 4 Lake is probably higher because the simplified box WRT Granulometry TOC Carbonates Index Vulnerability model does not account for the narrow connection with Beloslav 2-1 0 1-0 3 6-4 Moderate to Lake high the neighbouring Varna Lake, thus the more realistic Varna Lake 0 0 1-0 2 3-2 high to exces- rank would be between 1 and 2 (moderate and high sive vulnerability).

264 TENTH INTERNATIONAL CONFERENCE ON MARINE SCIENCES AND TECHNOLOGIES

Conclusions al., Eds. (UNESCO-IOC, Paris, 2001), pp. 177-182. According to the applied models Beloslav and [6] H y l a n d JL, Karakassis I, Magni P, Petrov Varna lakes could be classified as hypereutrophic with A, Shine JP, 2000. Ad hoc Benthic Indicator Group high turbidity where the phytoplankton is light limited - Results of Initial Planning Meeting”, 6-9 Dec 1999. due to the self-shading effect of the high biomass. UNESCO. The sediments are defined as of high to excessive [7] V i a r o l i P., M. Bartoli, G. Giordani, P. Magni, vulnerability risk. D. T. Welsh.2004. Biogeochemical indicators as tools The results suggest that simplified ‘screening’ for assessing sediment quality/vulnerability in transi- models might be a relevant tool for definition, assess- tional aquatic ecosystems. Aquatic conservation, vol. ment and prediction of eutrophication. In addition these 14: S19-S29. models could be successfully applied for cost-benefit [8] E n v i ro n m e n t a l Impact Assessment Study o analyses of amelioration management scenarios. Beloslav-Varna and Varna Bay 1991. Acknowledgment: [9]E n v i r o n m e n t a l Impact Assessment Study The paper was supported by INTERREG IIIB of , 1998a, 613 pages. CADSES project TW RefernceNet Project (№ 3B073). [10]E n v i r o n m e n t a l Impact Assessment Study The authors would like to thank Dr. G. Shtereva and of Varna Port, 1998b, 280 pages. the IO-BAS chemists for the sediment analyses. [11]E n v i r o n m e n t a l Impact Assessment Study of Municipality, 2001, 209p pages. References [12] T e t t P., V. Edwards. 2002. Review of Harmful [1] T e t t, P., Gilpin, L., Svendsen, H., Erlandsson, C. Algal Blooms in Scottish coastal waters. Report to P., Larsson, U., Kratzer, S., Fouilland, E., Janzen, C., SEPA, p.120. Lee, J.-Y., Grenz, C., Newton, A., Ferreira, J. G., Fer- [13] H o l m e s, R.W., 1970. The Secchi disk in tur- nandes, T. & Scory, S. , 2003. Eutrophication and some bid coastal zones. Limnology and Oceanography 15, European waters of restricted exchange. Continental 688-694. Shelf Research, 23: 1635-1671 [14] M o n c h e v a S., V. Petrova-Karadjova, [2] CSTT, 1994. Comprehensive studies for the pur- A.Palasov. 1995. Harmful Algal Blooms along the poses of Article 6 of DIR 91/271 EEC, the Urban Waste Bulgarian Black Sea Coast and Possible patterns of fish Water Treatment Directive. Published for the Compre- and zoobenthic mortalities. In: Harmful Marine Algal hensive Studies Task Team of Group Coordinating Sea Blooms, P Lassus, G.Arzul, E. Denn, P. Gentien [eds], Disposal Monitoring by the Forth River Purification Lavoisier Publ. Inc.,193-198. Board, Edinburgh., 42 pp. [15] V e l i k o v a V., Moncheva S., D. Petrova, 1999. [3] CSTT, 1997. Comprehensive studies for the pur- Phytoplankton dynamics and red tides (1987-1997) in poses of Article 6 & 8.5 of DIR 91/271 EEC, the Ur- the Bulgarian Black Sea. Water Science and Technol- ban Waste Water Treatment Directive, second edition. ogy, vol. 39, 8, 27-36. Published for the Comprehensive Studies Task Team [16] T r a y a n o v a A., 2003. Ecological status of of Group Coordinating Sea Disposal Monitoring by the macrozoobenthic communities in Beloslav and Varna Department of the Environment for Northern Ireland, Lakes during the autumn of 1999. Proceeding of Insti- the Environment Agency, the Scottish Environmental tute of Oceanology, Vol. 4: 140-148. Protection Agency and the Water Services Association, [17] T r a y a n o v a A., S. Moncheva, V. Doncheva. Edinburgh, 60 pp. 2007. Macrozoobenthic communities as a tool for as- [4] G o r d o n, D.C. Jr., P.R. Boudreau, K.H. Mann, sessment the ecological status of Varna lagoon. Tran- J.-E. Ong, W.L. Silvert, S.V. Smith, G. Wattayakorn, sitional Waters Bulletin. TWB, Transit. Waters Bull. F. Wulff And T. Yanag, 1996. Loicz Biogeochemical 3(2007), 33-36. Modelling Guidelines, LOICZ Reports and Studies 5, [18] K o n s u l o v a, Ts. 1992. Macrozoobenthic LOICZ, Texel, The Netherlands, 96 pp. communities present state in Varna and Beloslav Lake [5] M o n c h e v a S., V. Doncheva, L.Kamburska. adjacent to Black Sea. Rapp. Comm. Int. Mer Medit., 2001. On the long-term response of harmful algal Volume 33:pp. 43. blooms to the evolution of eutrophication off the Bul- [19] S t o y a n o v, A.1991. Negative Changes of Hy- garian Black Sea coast: are the recent changes a sign of drochemistry of Beloslav Lake - Varna Lake - Varna recovery of the ecosystem the uncertainties. In: Pro- Bay.-In: Proc.of II . Ecol. Conf., 29-30 Oct.1991,Varna. ceedings of IX International conference on “Harmful 38 46. Algal Blooms”, Hobart, Tasmania; G.M.Hallegraff et [20] T r a y a n o v a A., K. Stefanova, T. Trayanov, ,

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U. Niermann. 2002. Zooplankton and macrozoobenthic Proceedings of 2nd Intern. Conf. Port Development and communities of the Varna -Beloslav Lake System 1906 Coastal Environment, Volume 1: 109-120. - 2001, how economy and industry affected the ecol- [22]H e i j s SK, Jonkers HM, van Gemerden H, Schaub ogy: a case study. Proceedings of Second International BEM, Stal LJ, 1999. The buffering capacity towards Conference ”Oceanography of the Eastern Mediter- free sulphide in sediment of a coastal lagoon (Bassin ranean and the Black Sea: similarities and differences d’Arcachon, France): the relative importance of chemi- between two interconnected basins” October, 2002, cal and biological processes. Estuarine Coastal Shelf Ankara, Turkey, pp. 799-805. Sciences, 49: 21-35. [21] K o n s u l o v a, Ts., V. Todorova, G. Shtereva, A. Trayanova.2000. Benthic macrofauna status - a relevant tool for environmental impact assessment in port areas.

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