Assessment of the effects of variation in external nutrient loads on the Curonian lagoon ecosystem

*Artūras Razinkovas1, Ali Ertürk2 and Petras Zemlys1

1Coastal Research and Planning Institute, Klaipeda University, H. Manto 84, LT 92294, Klaipeda, Lithuania

2Istanbul Technical University, Department of Environmental Engineering, 34469 Maslak, Istanbul, Turkey

* Principal author, email: art@corpi.ku.lt

978-1-4244-2268-5/08/$25.00 ©2008 IEEE

Abstract- Curonian lagoon, the largest coastal lagoon in Europe is characterized by the strong seasonal and interannual variation in nutrient loads and water quality parameters is characterized by the dominance of the Nemunas rivers discharges. Coupled hydrodynamic and NPZD model was applied to assess the impacts of the foreseen reduction of nutrient loads prescribed in the State Plan to Improve the Quality in the Curonian lagoon, which was approved in 2006. The simulation of 1999-2000 was used as the background scenario. The NPZD model applied takes into the account the recently evaluated atmospheric nitrogen fixation by the cyanobacteria during the summer “blooms” and combined effects of temperature and wind climate.

I. INTRODUCTION

The Curonian (or Kuršių marios in Lithuanian) Lagoon, being the largest lagoon in Europe, lies along the Baltic coast of Lithuania and the Kaliningrad oblast (province) of Russia (Fig 1). It is separated from the Baltic by a narrow (~1-3km) sandy spit, the Curonian Spit. Total area of the Lagoon is approximately 1584 km2. The border between the two countries divides the Lagoon into a smaller, northern portion in Lithuania (413 km2) and a southern portion in Russia. Total volume of water of the Lagoon is approximately 6.2 km3, and the average depth, about 3.8 m. The lagoon is an important water body for Lithuania, especially for the western part because of its productivity, contribution to fishery and the recreational value. Curonian lagoon is a transitional ecosystem, where the physical and biogeochemical processes are the result of terrestrial, oceanic and atmospheric systems interaction. Curonian lagoon is an estuarine coastal lagoon, dominated by the Nemunas river discharges, which make up to 90 % of the total runoff. The Lagoon has been heavily polluted from a combination of shipping, military and industrial sources. Pathogenic organisms characteristic of untreated sewage loads are also abundant. Concentrations of petrochemicals and heavy metals in Lagoon waters have been very high. Not surprisingly, fishing and bathing in the Lagoon have declined significantly in modern times. The resent adoption of the EU Water Framework Directive along with the earlier Nitrate and Wastewater treatment directives facilitated the development of national programmes focusing on the reduction of nutrient and organic loads into the lakes, rivers and coastal waters. In Lithuania, nearly 80% of the territory belongs to the Curonian lagoon watershed [8].

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Fig. 1 Curonian (Kuršių marios) Lagoon

As the water quality in Curonian lagoon is general concern for both fishery and recreation (both are dominant industries in the area) in 2006 Lithuanian government approved a programme aimed to improve the water quality in the Curonian lagoon (The programme to improve the water quality in the Curonian lagoon, 2006). In that document most emphasis was put to combat the point and disperse pollution to reduce both the nitrogen, phosphorus and organic material. The objective of that programme is to decrease the pollution from the point sources in the total nitrogen loads by 810 tons, phosphorus loads by 85 tons and organic matter expressed as BOD7 by 1050 tons. To combat the non-point pollution sources objective is to reduce the nitrogen loads by 15% and phosphorus by 8%. As a result of that approach proportionally higher reduction in nitrogen than phosphorus compounds is expected (Fig. 2), bringing the N:P ratio od the total loads from 16.7 to 15.2 (Fig. 3)

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Fig. 2. Expected targets of total nitrogen and total phosphorus (tons/y) decrease by 2010 as compared to 2004 baseline.

However, nutrient loads being mostly controlled by the human activity also could be heavily modified by the climatic variation in the hydrological cycle. As a primary factor the discharge volume itself could be a good measure of the nutrient loads into the lagoon. As the variation in total nitrogen and total phosphorus concentrations in Nemunas river are much lower that the variation in the discharge volume both the short term (interannual) and long term discharge variations could be critical for the nutrient loads to the Curonian lagoon. Moreover, seasonal runoff variation patterns could also modify significantly the absolute amounts and the proportional share of nitrogen and phosphorus entering the lagoon. According to Tilickis [9] differences in groundwater versus surface runoff in the drainage basin of the Nemunas river are strongly influenced by the soil infiltration properties and, consequently by the climatic factors (wet vs dry years).

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Fig. 3. Changes of the N:P ratio of the riverine nutrient loads by 2010 as compared to 2004 baseline

In this paper our objective was to calculate how the reduction of both phosphorus and nitrogen according to plan approved by the government will affect the water quality in the lagoon and to discuss how foreseen climatic changes in the region could interact with the nutrient load reduction plans.

II. DATA AND METHODS

The model used in the analysis is the NPZD model [1; 2] consisting of two submodels; a transport model and a coupled ecological model comprising inorganic nutrient pool, organic substances and three groups of phytoplankton and zooplankton. Detailed description of the model is given in Erturk et.al. [1; 2].

As the initial setup of the model consisted of the real meteorological, hydraulic and nutrient forcing data. The advanced NPZD model was calibrated and validated for the years 1999 and 2000, for a calibration period which is almost three times of the calibration period of the simple nutrient dynamics model. According to flow records from the rivers, the year 2000 is significantly drier, than the year 1999 and the lagoons temperature is less in the year 2000. Therefore during the calibration process, the model was also partly validated [2].

To assess the impact the initial set of the forcing parameters (Nemunas discharges) was changed to follow the foreseen reduction in nitrogen and phosphorus loads (by 14% and 6% correspondingly) by changing the Nemunas river discharge nutrient concentrations proportionally over the year, while keeping the initial meteorological conditions similar as they were in the initial calibrated model for the years 1999 and 2000. Produced daily concentrations of nutrients and phytoplankton groups were compared to ones of the initially calibrated model.

III. RESULTS AND DISCUSSION

The simulation results revealed that at the reduction of nitrogen loads by 14% and phosphorus loads by 6%, significant DIN and DIP concentrations decrease in the lagoon during the vegetation period will be observed only in the spring and the autumn (Fig. 4).

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Fig. 4. Decrease of the DIN and DIP concentrations as compared to the baseline scenario

Total phytoplankton biomass, however, tended mostly to decrease during the springtime, with the maximum decrease in April (Fig. 5).

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Fig. 5. Decrease of total phytoplankton biomass as compared to the baseline scenario

Regarding the structure of the phytoplankton community, there could be seen decrease of the diatom concentrations during the

spring bloom and cyanobacteria concentrations during the summer bloom, while so called “green algae” assemblage, representing all other taxonomic groups demonstrated even increase in the autumn (Fig. 6). However the relative change in the total concentration calculated over 2 year period were not so impressive. It is only nitrate nitrogen who expected to decrease over 10%, while cyanobacteria, which development is of major concern for the water quality in the lagoon is projected to be reduced only by 4 %.

However, developed scenarios don’t account for the expected trends in the interannual climatic variation, which expected to be the major factor regulating both cyanobacteria bloom and seasonal succession of the phytoplankton communities in the lagoon[5; 6]. Increase of the so called “green algae” group abundance, is a generally positive change pointing towards the possibility to control the primary production by grazers [3]. Moreover, foreseen climatic changes in the Baltic Sea region will cause not only temperature, salinity and water level alterations, but will also significantly change the riverine discharges [4]. Therefore a detailed downscaling of regional climatic and drainage basin models for the Nemunas river basin needed to assess the combined effect of both the implementation of nutrient reduction programme and foreseen global change consequences.

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Fig. 6. Changes in phytoplankton community structure as compared to the baseline scenario

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Fig. 7. Changes in percentages as compared to the baseline scenario

REFERENCES [1] Erturk, A. 2008. Modelling the Response of an Estuarine Lagoon System to Nutrient Inputs, Doctoral Dissertation in Biomedical Sciences, Ecology and

Environmental Sciences (03B), Klaipeda University, Lithuania. [2] Ertürk A., A. Razinkovas, P. Zemlys, R. Pilkaitytė and Z. Gasiūnaitė. 2008. Linking NPZD and foodweb models of an estuarine lagoon ecosystem, this volume. [3] Gasiūnaitė Z.R., A Razinkovas. 2004. Temporal and spatial patterns of the crustacean zooplankton dynamics in transitional lagoon ecosystem. Hydrobiologia,

514: 139-149 [4] Graham, L.P. 2004: Climate change effects on river flow to the Baltic Sea. Ambio 33: 235–241. [5] Pilkaitytė, R., A. Razinkovas, 2006. Factors controlling phytoplankton blooms in a temperate estuary: nutrient limitation and physical forcing, Hydrobiologia, 555

(1): 41-48. [6] Pilkaitytė, R., A. Razinkovas, 2007. Seasonal changes in phytoplankton composition and nutrient limitation in a shallow Baltic lagoon, Boreal Environmental

Research, 12(5): 551-559. [7] Razinkovas A., Bliudziute, L., Erturk, A., Ferrarin, C., Lindim, C., Umgiesser, G. and Zemlys, P. 2005. Curonian lagoon: a modeling study- Lithuania, in:

Modeling Nutrient Loads and Response in River and Estuary Systems. Report No. 271., R.C. Russo ed., Committee on the Challenges of Modern Society, North Atlantic Treaty Organization., Brussels, pp. 194-222.

[8] Razinkovas A., Dailidiene, I., Pilkaityte, R. 2008. Reduction of the land-based discharges to the Curonian lagoon in a view of a climate change perspective. In: Gonenc (ed.) Sustainable Use and Development of Watersheds, Springer, in press.

[9] B. Tilickis, 2005. The changes of water chemical compositions in the Lithuanian watersheds. Klaipeda University Press. 199 pp. (in Lithuanian).