Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Vol. XXXIX, No. 3 Institute of Oceanography (113-120) University of Gdańsk ISSN 1730-413X 2010 eISSN 1897-3191 Received: July 03, 2009 DOI 10.2478/v10009-010-0036-2 Original research paper Accepted: June 07, 2010 The variability in concentrations of chosen nitrogen and phosphorus forms in the Oder River estuary in 1999-2002 Agnieszka Tórz1, Arkadiusz Nędzarek West Pomeranian University of Technology in Szczecin Department of Hydrochemistry and Water Protection ul. Kazimierza Królewicza 4H, 71-550 Szczecin, Poland Key words: Oder River estuary, nitrogen, phosphorus, mineralization level Abstract The Oder River creates one of the largest estuaries in Poland. The estuary can be divided into three zones, each with a different hydrochemical regime: the Pomeranian Bay (salt water habitat), the Szczecin Lagoon (brackish water habitat), the Oder River up to the Widuchowa section (transitional environment – brackish and typical freshwater). The Pomeranian Bay was characterized by the lowest concentration of nutrients throughout the research series in comparison to the other regions. It had a low nitrogen to phosphorus proportion and the highest stability of variability in total nitrogen concentration. The Szczecin Lagoon was characterized by a higher concentration of nutrient loads in comparison to the Pomeranian Bay, by a lower nitrogen and phosphorus proportion than the Oder waters, and by the high stability of variability of concentrations of mineral nitrogen and total nitrogen. As the "nutrient trap" for the Oder estuarine system, the Oder River, with its highest nutrient concentration, is characterized by the highest stability of variability of total nitrogen concentration. 1 Corresponding author: [email protected] Copyright© by Institute of Oceanography, University of Gdańsk, Poland www.oandhs.org 114 A. Tórz, A. Nędzarek INTRODUCTION Estuarine systems comprise a specific environment where inland and sea waters mix together. This causes high variability in hydrochemical composition that is unprecedented in both inland and sea water habitats. Because of the heterogeneous quality of their waters and their specific individual features, estuaries are complex water bodies. Nutrient circulation and the dynamics of variability of the nutrient concentration are, both temporally and spatially, of particular importance in estuarine waters. Processes by which inland water is enriched with nutrient substances are well known and widely discussed by many authors. Nevertheless, research into the potential relevance of nutrient dynamism in the estuary for the habitat has been conducted on a large scale only from the 1980s onward (Bonsdorff et al. 2002; Elliot, de Jonge 2002). The Oder's estuarine system consists of an uncommon estuary (because of its salinity level and mixing characteristics) which, because of its economic importance, is under considerable anthropogenic stress. The multicomponency of the interior estuary of the Oder River makes the transformation of its hydrochemical system particularly complicated. The aim of the present paper is to define the dominant species of nitrogen and phosphorus in the regions of the estuary of the Oder River, with different levels of mineralization in particular seasons. STUDY AREA Approximately 2 km below the Widuchowa water-gauge (704.1 km), the Oder divides into the South and West Oder. A weir is situated in this area. The two river branches spread in the area of Międzyodrze, and are characterized by many canals. In the northern part of Międzyodrze, the East and West Oder merge through the Skośnica waterway (the Klucz-Ustowo canal), from which (730.5 km) the East Oder changes into the Regalica River and discharges into the flow-through Dąbie Lake (area approx. 56 km2; average depth 2.5–3 m). The West Oder, after flowing through Międzyodrze, divides and forms Szczecin's Oder and Parnica rivers. The Parnica joins the Regalica (739.6 km), and they flow together into Dąbie Lake. Szczecin's Oder joins the Regalica in its estuarine part of Dąbie Lake, and via Iński Nurt forms the Domiąża. Afterwards, it comes into the Roztoka Odrzańska basin and then to the Szczecin Lagoon. The northern complement to that water web is the connection with the Pomeranian Lagoon via the Świna, Dziwna, and Peene rivers (Buchholz 1991, Robakiewicz 1993, Andrulewicz 1997). Copyright© by Institute of Oceanography, University of Gdańsk, Poland Nitrogen and phosphorus forms in the Oder River estuary 115 MATERIALS AND METHODS The present study was conducted from July 2000 to August 2003, with samples taken approximately every second month. Water samples for hydrochemical analysis were collected with a Ruttner sampler. Sampling collecting times included all seasons characteristic for the temperate climate. As for the execution of the established aim, the research sites on the Oder's estuary were marked and grouped in three areas: the Oder (6 sites from Widuchowa to Police), the Szczecin Lagoon (4 sites), the Pomeranian Bay (3 sites: in Świna and Dziwna mouths, and the last site 3 nm away from Świnoujście - the most northerly site). During laboratory analysis, standard methods were used (Standard Methods 1995, Hermanowicz et al. 1999). In the water samples, chosen species of nitrogen (mineral and total nitrogen) and phosphorus (dissolve reactive and total phosphorus) were analyzed. All data collected were averaged for particular seasons, and minimum, maximum and dominant values were calculated. The variability ratio was calculated as well (Table 1). Analysis of variance (ANOVA) was performed in order to define the relevance of seasonal variability in particular areas (Fig. 1). Table 1 Designation of characteristic index values in the estuarine waters of the Oder, including peripheral sites of respective areas during the whole study time. N P P N P P N P P factor year time N total N total N total min. react. total min. react. total min. react. total area ODRA SZCZECIN LAGOON POMERANIAN BAY Dominant 0.353 0.539 0.132 0.183 0.258 0.324 0.141 0.227 0.142 0.134 0.063 0.189 Maximum 0.753 0.875 0.213 0.339 0.325 0.411 0.150 0.280 0.142 0.187 0.087 0.211 Minimum 0.316 0.527 0.053 0.164 0.185 0.258 0.054 0.155 0.075 0.101 0.028 0.084 Average 0.519 0.683 0.113 0.233 0.248 0.330 0.116 0.219 0.106 0.152 0.056 0.153 Standard deviation SPRING 0123 0.106 0.046 0.044 0.028 0.039 0.028 0.031 0.019 0.024 0.015 0.032 Variability index 23.7 15.5 40.7 18.9 11.3 11.8 24.1 14.2 17.9 15.8 26.8 20.9 Dominant 0.355 0.870 0.050 0.186 0.252 0.542 0.020 0.131 0.084 0.221 0.009 0.131 Maximum 0.663 1.332 0.100 0.279 0.403 0.647 0.070 0.246 0.217 0.313 0.039 0.159 Minimum 0.165 0.323 0.019 0.128 0.197 0.376 0.014 0.119 0.074 0.135 0.003 0.054 Average 0.373 0.765 0.050 0.203 0.283 0.494 0.034 0.177 0.116 0.198 0.021 0.112 Standard deviation SUMMER 0.113 0.234 0.018 0.025 0.042 0.058 0.013 0.040 0.028 0.035 0.010 0.026 Variability index 30.3 30.6 36.0 12.3 14.8 11.7 38.2 22.6 24.1 17.7 47.6 23.2 Dominant 0.348 0.523 0.082 0.243 0.281 0.381 0.070 0.257 0.101 0.176 0.041 0.140 Maximum 0.907 0.998 0.256 0.360 0.310 0.474 0.140 0.276 0.148 0.232 0.079 0.200 Minimum 0.249 0.365 0.050 0.157 0.187 0.275 0.030 0.124 0.054 0.097 0.030 0.096 Average 0.526 0.701 0.112 0.251 0.251 0.383 0.080 0.217 0.107 0.173 0.046 0.142 Standard deviation AUTUMN 0.139 0.186 0.044 0.049 0.033 0.048 0.031 0.045 0.027 0.035 0.013 0.029 Variability index 26.4 26.5 39.3 19.5 13.2 12.5 38.8 20.7 25.2 20.2 28.3 20.4 Dominant 0.430 0.598 0.162 0.301 0.305 0.349 0.162 0.239 0.128 0.170 0.072 0.123 Maximum 0.831 1.050 0.244 0.347 0.336 0.412 0.168 0.275 0.135 0.192 0.091 0.212 Minimum 0.389 0.477 0.100 0.173 0.202 0.273 0.114 0.218 0.065 0.092 0.048 0.118 Average 0.606 0.741 0.163 0.274 0.256 0.334 0.141 0.247 0.102 0.142 0.065 0.164 Standard deviation WINTER 0.162 0.204 0.036 0.047 0.032 0.036 0.018 0.015 0.020 0.029 0.013 0.029 Variability index 26.7 27.5 22.1 17.2 12.5 10.8 12.8 6.1 19.6 20.4 20.0 17.7 Units - dominant, maximum, minimum, average – [mg dm-3], variability index – [%] www.oandhs.org 116 A. Tórz, A. Nędzarek Fig. 1. The seasonal concentration variability of total nitrogen and phosphorus in respective areas of the Oder Estuary (statistically significant dependency: ANOVA for α=0.05, p<0.01).