Riverine input of nutrients to the Gulf of Riga — temporal and spatial variation

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Abstract

Riverine transport is the, most important pathway for input of nutrients to the Gulf of Riga. The present study focused on updating existing estimates of the riverine nutrient contributions and on improving the favailable information on temporal and spatial variation in such input. The results show that the gulf received an average of 113,300 tons of nitrogen, 2050 tons of phosphorus and 64,900 tons of dissolved silica (DSi) annually during the time period 1977–1995. There was large interannual variation in loads, e.g., a factor two difference was found between the two most extreme years (1984 and 1990); this was attributed mainly to natural variation in water discharge. The seasonal distribution of nutrient loads exhibited a distinct pattern for practically all studied constituents, especially nitrate. Loads were high during the spring-flow and relatively low during the low-flow summer period. Examination of the spatial variation of nutrient loads showed that the Daugava River alone accounted for approximately 60% of the total riverine load. The highest area-specific loads of nitrate and phosphate were observed in the agriculturally dominated Lielupe River, and the highest loads of organic-nitrogen (org-N) and total phophorus (tot-P) were found in the Pärnu River. However, the values for all the studied rivers and constituents were rather low (phosphorus) or moderate (nitrogen and silica) compared to those reported for many other drainage areas of the Baltic Sea. This was true despite the inefficient sewage treatment and intensive agriculture in the studied basins in the 1970s and 1980s.

Introduction

The Gulf of Riga, with a surface area of 19,000 km2 and a volume of 420 km3, has been described as one of the most polluted areas in the Baltic Sea Yurkovskis et al., 1993, Andrushaitis et al., 1995. Most of this pollution load can be attributed to activities in the drainage basin of the Gulf of Riga, which covers 135,700 km2, or more than seven times the surface area of the gulf itself (Fig. 1). The first decades of the post-war period were characterised by rapid industrial development, urbanisation and growing use of fertilisers, pesticides and herbicides in agriculture Ojaveer, 1995, Löfgren et al., 1999, and the first signs of a deterioration of the environment of the Gulf of Riga were noticed as early as the 1950s (Ojaveer, 1995). The primary production rate has been reported to be twice that observed in the Baltic proper (Elmgren, 1984), and algal blooms and contamination with faecal microorganisms have been prevalent. Moreover, the Gulf of Riga is particularly vulnerable to pollution, because it is relatively shallow (mean depth 23 m, maximum depth 51 m) and the exchange of water with the Baltic proper through the two straits is rather limited.

Quantitative estimates of pollution loads that have been reported in literature vary. For example, our research team has recently demonstrated that the nitrogen load is higher than previously assumed (Stålnacke, 1996). In light of the described situation, we performed a systematic evaluation and estimation of the seasonal and spatial distribution of the nutrient loads to the Gulf of Riga, as well as studies of the long-term nutrient fluxes. More precisely, our main task was to estimate the monthly riverine loads of nitrogen, phosphorus and silica to the Gulf of Riga during the time period 1977–1995. Special emphasis was placed on nutrient fluxes in the 1990s, in particular 1993–1995, when intensive studies of the Gulf of Riga itself were carried out within a comprehensive research programme (Wassmann and Tamminen, 1999). The estimated riverine loads were also compared with input via other pathways. In addition, the ratios of dissolved inorganic nitrogen (DIN) to phosphate phosphorus (PO4–P) and dissolved silica (DSi) were examined, as well as relationships between the concentrations of the studied nutrient fractions and water discharge.

Section snippets

Study area

Over 4.5 million people live in the drainage area of the Gulf of Riga, and 40% of the land (mainly lowland) is used for agricultural production (Table 1). The population is relatively uniformly distributed among and within the river basins with the exception of a few large cities. The catchment of the Lielupe River has the largest cover of farmland in the area (56%). Further background information on the Gulf of Riga area has been published by Tsirkunov et al. (1992) and Ojaveer (1995).

Water quality and water discharge data

Average riverine flow and input of nutrients

The annual inflow of freshwater to the Gulf of Riga, calculated as a mean value for 1977–1995, was estimated to 36.2 km3 yr−1, or approximately 1150 m3 s−1 (Table 4). This corresponds to 9% of the total water volume of the gulf and is three to four times larger than the mean annual precipitation on the surface of the gulf. The average water discharge in 1977–1995 was found to be somewhat higher than the long-term average for 1921–1975 (Mikulski, 1982) and 13% higher than the long-term average

Concluding remarks

Economic recession in the catchment area during the 1990s, with decreased industrial and agricultural production, has been reported to have caused a 50% decrease in nutrient loads to the Gulf of Riga between 1987 and 1993 Helcom, 1996, Helcom, 1997. Our study confirmed that the riverine loads of phosphorus and nitrogen were somewhat lower in 1993 than in 1987. However, analysis of whole time series of annual loads did not provide any clear evidence of recent downward trends (Fig. 4). The same

Acknowledgments

The authors are grateful to the Latvian Hydrometeorological Agency in Riga, the Estonian Ministry of Environment and the Estonian Meteorological and Hydrological Institute in Tallinn for providing data on water quality and water discharge. We also acknowledge Mrs. Patricia Ödman for language revision and the two anonymous referees for valuable and constructive comments on the manuscript. This work was financially supported by the Nordic Council of Ministers and the European Union programme

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