Estimation of the local riverine pollution load

to the lagoon on the Southern Baltic coast (The Puck Lagoon)

Robert Bogdanowicz, Roman Cieslinski, Jan Drwal, Artur Cysewski

Department of Hydrology, University of Gdansk, Dmowskiego 16a, 80-952 Gdansk, Poland

Keywords: Baltic Sea; rivers, nutrients, pollution, eutrophication

1. Introduction

The lagoons on the southern Baltic coast belong to the most eutrophic marine ecosystems in northern Europe. The main factor influencing the elevated concentrations of nutrients in these lagoons is nitrogen and phosphorus load transported by the rivers. The Puck Lagoon, located in northern Poland, is a north-west, inner part of the Gulf of Gdansk. This Lagoon is under significant pressure from the pollution sources distributed in its drainage basin. During the last thirty years an excessive nutrients load discharged to the Lagoon from coastal towns and transported by rivers to the western, the most shallow, part of the Lagoon, led to increased eutrophication and finally to a breakdown in the environmental balance. In order to stop further deterioration of the valuable ecosystem of the Lagoon, since the beginning of the nineties of the XX century, large investment in sewage infrastructure has been undertaken. Despite all of these concrete actions, algae blooms and other eutrophication symptoms were still frequently observed in the coastal waters. Therefore, the main objective of the study was to identify and characterise the sources (rivers and their catchments) responsible for the elevated nutrients levels in the coastal waters.

2. Method and materials

Most of the studies dealing with the problem of water pollution and eutrophication of the Puck Lagoon were based on the data from the state monitoring system. The only water quality sampling point in the drainage area of the Lagoon is located in Wejherowo, in the Reda River, over 20km from the stream outlet (Fig. 1). Therefore, only about 80% of the area of the catchment is under control. Moreover, there are some other, smaller rivers, transporting pollution to the marine waters, which are not controlled at all. In order to assess the overall riverine load of nitrogen and phosphorus discharged from the land into the Lagoon, in the beginning of 2006 a project started, aiming at systematic, monthly measurements of nutrients loads in the local rivers entering the Lagoon. The following paper presents the results of the first year of the realisation of this research.

Nutrients concentrations in four main streams entering the Puck Lagoon have been measured at water quality sampling points situated closest to the rivers outlets. The location of control points allowed for estimation of pollution load from the whole drainage area, which was not possible in standard monitoring. The research was concentrated on total nitrogen and total phosphorus analysis, but other forms of nitrogen (ammonium, nitrates, nitrite and organic nitrogen) and phosphorus (orthophosphates) were also taken into account. The research was oriented on pollution load estimation, therefore water quality sampling and river flow measurements were carried out simultaneously. In order to compare the results of the research with the estimates based on regular, state monitoring, the data from the Institute of Meteorology and Water Management and the State Inspectorate of Environmental Protection were also utilised. The paper does not deal with the problem of nutrients exchange between the Lagoon and the rest of the Gulf of Gdansk. Although it is recognised as a very important factor influencing water quality (Bolalek et al., 1993), it was out of the scope of the research being presented, which focused solely on inland sources of pollution.

Fig. 1. Study area

3. Characteristics of the study area

3.1 The Puck Lagoon

The Puck Lagoon is separated from the rest of the Gulf of Gdansk by the submerged part of an 8.6 km long sandbank, and the sand spit which length depends on sea water level. The Lagoon is shallow with a mean depth of 3.1 m and covers an area of 103 km². Water exchange between the Lagoon and the outer part of the Gulf of Gdansk is considerably obstructed, taking place only through artificial underwater channels (Nowacki, 1993). Water balance of the Lagoon is also affected by the inflow of water from the rivers draining a basin of nearly 1000 km². The influence of fresh water inflow is reflected in low salinity of water in the Lagoon which mean values range from 0.5 up to 7 PSU (5.32 in the central part of the Lagoon). According to EU Water Framework Directive, the Lagoon has been classified into the category of transitional waters (Krzyminski et al., 2005).

The hydrographic conditions of the Puck Lagoon are very favourable to the development of life. The biodiversity of the Lagoon counts among the greatest of all the waters in the Polish coastal zone. The analysis of water quality in the Lagoon has indicated high levels of phosphorus, especially in the forefront of the largest rivers outlets, namely Reda and Plutnica. High nutrients content has been also reported in the bottom deposits (Kruk-Dowgiallo, 2004). The Puck Lagoon is a unique biotope for macrophytes. In the sixties of the last century, 60 % of the biomass in the bottom vegetation consisted of brown algae and the eelgrass Zostera marina. However, increasing discharges from land resulted in a continuous deterioration of the bottom vegetation, and in the end of the 1970s, the former dominating species started to be replaced by filamentous brown algae (Lundberg, 2005).

3.2 The drainage basin

Over 150,000 people live in the region, mostly along the coast. The area is very close to Gdansk agglomeration, with the population of nearly one million inhabitants. During the summer, the region is very popular among the tourists. They concentrate mainly along the Hel Peninsula, especially in towns of Jastarnia and Hel (Fig. 1). These towns which count about 4000 of permanent inhabitants, host in summer over 20 000 visitors each. These changes result in strong fluctuations of effluent discharges and of pollution load. Average monthly nitrogen and phosphorus load, discharged from the Jastarnia sewage treatment plant into the marine waters, during two summer months (July and August) may be even ten times higher than in winter months.

The largest river entering the Lagoon is the Reda River with a catchment area of nearly 500 km² and an average annual flow of about 5 m³ s^-1. Water resources of the Reda River and the Zagorska Struga basins count among the highest in northern Poland, while the Gizdepka River cachment is characterised by very low specific runoff (Tab. 1). Annual changes of water flow of the Reda River in 2006 showed well-pronounced seasonal pattern, with high discharges in early spring and low discharges in summer (Fig. 2). The highest flows occurred in March and the lowest flows - in August. The hydrological regime of the Reda River can be classified as the groundwater, rainwater and snow-fed regime. The seasonal flow changes in other rivers were similar to the Reda River pattern, except for the Plutnica River, which discharge was under strong influence of artificially induced water supply, regulated mainly by the work of pump stations.

Tab. 1. Selected characteristics of the main rivers in the Puck Lagoon drainage basin

River	Catchment area [km^2]	Distance of the sampling point from the river outlet [m]	Mean specific runoff in 2006 [dm^3×s^-1×km^-2]
Zagorska Struga	105,2	1050	9
Reda	485,5	1000	9
Gizdepka	37,2	770	4
Plutnica	84,0	350	8

Water conditions in the Puck Lagoon drainage basin are influenced by relatively high annual rainfall in the western and the southern part of the basin (>700mm), and low rainfall on the coast (<550 mm). There are also significant differences in permeability of soils in the analysed catchments. (Tab. 2). The highest percentage of highly permeable deposits is characteristic for the southern part of the drainage basin (the Zagorska Struga and the Reda River catchments) while the northern part is dominated by low permeability soils. In the Plutnica River catchment there is also a significant proportion of the turf soils.

The Reda and the Zagorska Struga river basins are in 40% forested, with the domination of mixed (coniferous and deciduous) forest complexes. The Gizdepka basin contains large proportion of farming areas. Over 15% of land comprised of meadows and pastures is typical for the Zagorska Struga and the Plutnica River catchments (Tab. 3).

Fig. 2. Seasonal changes of monthly flow in relation to average annual discharge in the selected rivers in 2006

Tab. 2. Soils permeability in the selected catchments

Catchment	High permeability soils [%]	Low permeability soils [%]	Impermeable soils [%]	Varying permeability soils [%]
Zagorska Struga	48	29	5	18
Reda	45	39	2	14
Gizdepka	21	63	1	15
Plutnica	11	64	1	24

Tab. 3. Land use in the selected catchments

Catchment	Arable land [%]	Forest [%]	Meadows [%]	Wasteland [%]	Urban area [%]	Water [%]
Zagorska Struga	31,4	40,0	20,6	5,0	1,3	1,7
Reda	39,5	40,0	11,5	2,0	5,0	2,0
Gizdepka	60,4	35,0	2,0	1,5	0,0	1,1
Plutnica	40,8	37,7	16,2	2,0	1,9	1,4

4. Results

The completion of the first stage of the reclamation programme of the Puck Lagoon, led to the development and the upgrade of all coastal wastewater treatment plants, and to the extension of sanitary sewerage in the drainage basin. At the beginning of the nineties of the last century, pollution load discharged from point sources was over ten times higher than the load transported by the largest river in the catchment. During the last fifteen years pollution load from sewage treatment plants (STP) decreased significantly, more than ten times. The main reason of this was the modernisation of the largest plant in the region (Debogorze STP) in 1994-1995, when total nitrogen concentrations in the effluent dropped from 40 mg Nּdm^-3 to 15 mg Nּdm^-3 and total phosphorus concentrations were reduced from 5 mg Pּdm^-3 to 1 mg Pּdm^-3 (Bogdanowicz, 2007).

Fig. 3. Nitrogen and phosphorus loads discharged into the Puck Lagoon from the selected rivers

Despite all of these point-source oriented actions, nutrients levels in the Lagoon remained still high. One of the reasons for this could be still high load of nutrients transported by the rivers (Fig. 3). An important fact is also a very high proportion of inorganic nitrogen forms (especially nitrates) in total nitrogen, and orthophosphates in total phosphorus load, because these nutrients species are, in particular, responsible for stimulating eutrophication processes.

Most of the nutrients load, especially in case of nitrogen, originated from the Reda River catchment, however the load from smaller rivers couldn’t be neglected (Tab. 4). It was also estimated that the nutrients load measured in 2006 at the sampling point located at the Reda river outlet was significantly higher (about 15% - TN and about 10% - TP) than the load estimated at the gauging station up the river in Wejherowo. Therefore, the presented results have indicated, that the overall riverine load transported to the Lagoon may be over 60% higher than the reported one.

Tab. 4. Contributions of the selected rivers to the total nitrogen (TN) and total phosphorus (TP) load discharged into the Puck Lagoon in 2006

River	TN [%]	TP [%]
Zagorska Struga	15	12
Reda	72	65
Gizdepka	3	5
Plutnica	10	18
Total	100	100

Area specific load of total inorganic nitrogen (TIN) in 2006 was slightly higher in the Reda River than in the Zagorska Struga catchment, while the load from the Gizdepka River catchment was almost two times lower (Tab. 5). Nitrogen export coefficients were strongly, positively, related to specific runoff values (Fig. 4). There was no such relation in respect to phosphates load. In case of orthophosphates, the highest load in 2006 was estimated for the Plutnica drainage basin, where it was over two times higher than in the Reda River and the Zagorska Struga basins (Tab. 5). It appeared, that the important factor influencing the load in this case, was the permeability of soils in the catchment (Fig. 5).

Tab. 5 Area specific loads of total inorganic nitrogen (TIN) and orthophosphates phosphorus (PO₄-P) in the selected catchments in 2006

Catchment	TIN [kg×km^-2×year^-1]	PO4-P [kg×km^-2× year^-1]
Zagorska Struga	460	18
Reda	467	21
Gizdepka	261	27
Plutnica	357	46
Total	442	24

Fig. 4. Relationship between specific runoff and TIN area specific load in the selected catchments in 2006

The analysis proved that changes in transport of nutrients from all the analysed rivers showed well-expressed seasonal pattern that corresponded well with hydrological regime of the rivers. Nitrogen load was higher in winter half-year (November-April), especially during snow-melting period in March (Fig. 6). The lowest loads occurred in summer and in autumn, mostly in November and August, when river flows and nitrogen concentrations were the lowest. During winter, on average, about 60% of the annual load of total nitrogen was transported by the rivers to the Lagoon. These seasonal changes were less pronounced in case of phosphorus (Fig. 6), because of high TP load recorded in summer in the Plutnica River, which was probably caused by pump-induced high flows and the construction work being then conducted in the lower stretch of this river (Tab. 6).

Fig. 5. Relationship between share of low permeability soils and PO₄-P area specific load in the selected catchments in 2006

Tab. 6. Share of winter half-year (November-April) load in annual nutrients load in the selected catchments in 2006

Catchment	Winter half-year share (TN) [%]	Winter half-year share (TP) [%]
Zagorska Struga	61	54
Reda	62	57
Gizdepka	60	55
Plutnica	57	40
Total	61	53

Fig. 6. Seasonal changes of total nitrogen (TN) and total phosphorus (TP) monthly load in relation to average annual load in the selected rivers in 2006

5. Conclusions

The results of the research have proved that the nutrients load transported to the Puck Lagoon by the rivers was considerably higher than it is reported in the official documents, which are based on standard monitoring procedures. Because of the high vulnerability of the Lagoon, the load transported even by the small rivers may pose a threat to its ecosystem. The influence of rivers on water quality of the Lagoon could be more pronounced than the impact of discharges from point sources (sewage treatment plants) also because of the fact, that this input was directed into the shallowest part of the water body, characterised by much slower water exchange with the open sea.

The actions oriented on the reduction of still high specific loads of nitrogen and phosphorus should now concentrate on the further development of sewage collection systems, on the building up and managing stormwater systems in the urban areas, and on the reduction of the emissions from agricultural diffuse sources in the rural parts of the drainage basin.

References

Bogdanowicz R., 2007, Managing water quality in the Puck Lagoon drainage basin (NATURA 2000 site), in: Proceedings of the Second International Conference on Waters in Protected Areas, CWPC, EWA, Dubrovnik, Croatia, 129-133.

Bolalek J., Falkowska L., Korzeniewski K., 1993, Hydrochemistry of the Puck Bay, in: Korzeniewski K. (ed.), The Bay of Puck, FRUG, Gdansk; Poland, 222-302 (in Polish).

Kruk-Dowgiallo L. (ed.), 2004, Impact of the selected pollution sources on the environment of the Bay of Puck, IM, Gdansk, Poland (in Polish).

Krzyminski W., Kruk-Dowgiallo L., Zawadzka-Kahlau E., Dubrawski R., Kaminska M., Lysiak-Pastuszak E., 2004, Typology of Polish marine waters. Coastline Reports 4, 39-48.

Lundberg C., 2005, Eutrophication in the Baltic Sea – from area-specific biological effects to interdisciplinary consequences, Abo Akademi University, Abo, Finland.

Nowacki J., 1993, Morphometry of the Puck Bay, in: Korzeniewski K. (ed.), The Bay of Puck, FRUG, Gdansk; Poland, 71-78 (in Polish).