Introduction. Study area. Renata Dondajewska

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1 Limnological Review Effect 7, 4: of restoration of a lowland dam reservoir on river water quality 185 Effect of restoration of a lowland dam reservoir on river water quality Renata Dondajewska Adam Mickiewicz University, Department of Water Protection, Umultowska 89, Poznań, [email protected] Abstract: The investigations, conducted in 2003, aimed to assess the changes in water quality in the river Cybina under the influence of the dam reservoir Antoninek. The reservoir was restored by means of removal of emergent vegetation and part of sediments. Physicochemical and biological parameters of water were measured at the inflow and outflow of the reservoir. The data were compared with results of research conducted before the restoration (i.e. in ). The removal of vegetation and part of sediments changed the character of the reservoir. After restoration it was dominated by phytoplankton instead of helophytes. As a result of this, concentrations of dissolved oxygen, ammonia N, and nitrite N increased. Like in the 1990s, also in the summer of 2003 the total P and N concentrations increased between inflow and outflow. The lack of helophytes enabled fast growth of phytoplankton and filamentous algae, leading to increased chlorophyll a concentration and dry weight of seston in summer months. Key words: nitrogen and phosphorus loads, shallow reservoir, restoration Introduction Cascades of dam reservoirs are built in order to limit the nutrient loads delivered by tributaries to lakes located further downstream. The phosphorus and nitrogen loads in waters of such a cascade are limited thanks to intensive phytoplankton growth. Phytoplankton accumulate those elements in their biomass and next they precipitate and are deposited in bottom sediments. To enable the fast growth of phytoplankton and thus to allow the cascade of reservoirs to perform their function, they should have the following properties according to Pütz and Benndorf (1998): (i) a low mean depth and a large reaction space (upper layer of water, 0-3 m in depth) in relation to the total volume, (ii) a mean water residence time of a few days and (iii) a surface outflow as a consequence of a constant storage level. In montane and submontane reservoirs, the trophic state of water is truly lowered in the course of water flow through a cascade of preliminary reservoirs (Wróbel and Bombówna 1976, Stachowicz and Czernoch 1992). However, in the case of lowland rivers, the favourable influence of preliminary reservoirs on water quality is debatable. For example, Gołdyn (2000), who studied a cascade of reservoirs on the river Cybina in Poznań, found that waters of this river were enriched in some forms of nitrogen and phosphorus in some periods. The deterioration of water quality at the outflow was due mainly to the influence of emergent vegetation, which caused oxygen depletion in water and mobilization of nutrients adsorbed on compounds deposited in bottom sediments. The objective of this study was to assess the changes in water quality in the river Cybina under the influence of water flow through the small dam reservoir Antoninek, which was subject to restoration. My results were compared with results of earlier research (Gołdyn 2000), conducted in , i.e. before restoration. Study area The reservoir Antoninek (Fig. 1) is shallow, formed by a dam on the river Cybina in Poznań city (western Poland). The Cybina is a right-bank tributary of the Warta and is about 41 km long. Its catchment covers km 2 and is dominated by farmland, which

2 186 Renata Dondajewska conducted in 2003, the surface area of the reservoir was 7.2 ha, the volume was 36,000 m 3 and mean depth was 0.5 m. The mean rate of water flow in the Cybina is 0.67 m 3 s -1, and mean residence time of water in Antoninek is about 0.6 days (Gołdyn 2000). However, during this study its residence time of water varied from 0.65 in spring to 160 days in summer. Fig. 1. The localisation of research stations (after Buczkowski and Zgrabczyński 2000, changed) accounts for 77% of the area, while woodland accounts for 14% (Kasprzak and Lemański 1991). Many natural and artificial water bodies are situated in the course of the Cybina and its tributaries. Within the administrative borders of Poznań city, there is a cascade of four preliminary reservoirs, including Antoninek. Their role is to intensify the processes of self-purification of river water before it enters the Maltański Reservoir, which is used for recreation. The reservoir Antoninek was created in and as a result of a fast colonization by helophytes, the open water was limited to only 3% of its original area (Gołdyn 2000). Sedimentation of suspended matter supplying the reservoir with tributary water, as well as deposition of autochthonous organic matter, caused a remarkable reduction of water depth. For this reason, restoration a removal of aquatic vegetation and part of the accumulated sediments - was carried out in years When this study was Methods Water quality in the river Cybina was analysed for 7 months, between April and October 2003, at two stations: (1) near the inflow of the river to Antoninek and (2) at the outflow from Antoninek. Water quality was characterized by the following physicochemical and biological parameters: water temperature, ph, conductivity, concentration of dissolved oxygen, biochemical oxygen demand (BOD 5 ), water flow, dry weight of seston, chlorophyll a, ammonium N, nitrite N, nitrate N, organic N, total N, soluble reactive phosphorus (SRP), and total P. The analyses were conducted according to Polish Standards (Hermanowicz et al. 1999). Daily loads of nutrients were calculated on the basis of the measured concentrations of individual parameters and the rate of water flow at the time of sample collection. For all the analysed parameters, means and standard deviations were calculated, and significance of differences between the two stations was assessed with the Wilcoxon signed ranks test (Table 1). In , water quality in the Cybina was Table 1. Means, ranges and standard deviations of parameters analysed in 2003 Parameter Inflow Outflow range mean±sd range mean±sd P z Difference Temperature ( C) ± ± ph Conductivity (μs cm -1 ) ± ± Dissolved oxygen (mgo 2 l -1 ) ± ± BOD 5 (mgo 2 l -1 ) ± ± Ammonium N (mgn-nh 4 l -1 ) ± ± Nitrite N (mgn-no 2 l -1 ) ± ± Nitrate N (mgn-no 3 l -1 ) ± ± Organic N (mgn l -1 ) ± ± Total N (mgn l -1 ) ± ± Soluble reactive P (mgpo 4 l -1 ) ± ± Total P (mgp l -1 ) ± ± Chlorophyll a (μg l -1 ) ± ± Dry weight of seston (mg l -1 ) ± ±

3 Effect of restoration of a lowland dam reservoir on river water quality 187 analysed at the same stations by Gołdyn (2000), so his results from the same study period (April-October) were used for comparison. Results Among all the analysed physico-chemical and biological parameters, six can be regarded as the most important and this will be section focused on them. Water oxygenation was good in spring, both in inflowing and outflowing water, but markedly lowered in summer, especially in the inlet. Between the inflow and outflow of the reservoir, oxygen conditions improved substantially during nearly all sampling sessions (Fig. 2A). The values of biochemical oxygen demand (BOD 5 ), reflecting the organic matter content, were during most sampling sessions (except late September and early October) lower at the outflow than at the inflow. In inflowing water the highest amounts (over 10 mg O 2 l -1 ) were noticed in spring and summer, while in the outflow in spring and autumn. A wider range of total P concentrations was observed in outflowing water. The highest values were recorded in June and July, while in the inlet in June and September-October. Between the inflow and outflow, total P concentrations decreased more often, especially in the end of August. Daily total P loads decreased in reservoir mainly in April and August- October (Fig. 2B). Total N concentration in most samples collected at both stations varied from 2.3 to 5.0 mg N l -1 (Fig. 3A). Daily total N loads were higher (over 100 kg N d -1 on average) between April and mid-august, but later they fell below 2 kg N d - 1. Chlorophyll a measurements revealed large changes in its values. The highest were recorded in spring, the lowest in early summer (outlet) and autumn (inlet). Daily chlorophyll a loads were also greatest in spring (over 1 kg d -1 ) and lowest in autumn (below kg d -1 ). Between the inflow and outflow of Antoninek, chlorophyll a concentrations and loads decreased in the first part of the study period, but starting from late July they markedly increased (Fig. 3B). Dry weight of seston at both stations alternately increased and decreased and its values there ranged from 1.54 to mg l -1. At the beginning of the study period, the values were very similar at both stations. Later on, in the first part of research period, they were higher at the inflow, while in the second part the tendency was opposite. Between the inflow and outflow, daily loads of seston decreased in spring but increased in summer and autumn. Discussion The removal of emergent vegetation had a positive effect on phytoplankton growth in the reservoir, so consequently it caused a substantial increase in chlorophyll a concentration and dry weight of seston in some months in summer, as compared to the values recorded in the 1990s (Gołdyn 2000). Before the restoration, high chlorophyll a concentrations were ob- Fig. 2. Concentration of dissolved oxygen (A) and daily total P loads (B) at the inflow and outflow of the reservoir in 2003

4 188 Renata Dondajewska Fig. 3. Total N concentrations (A) and concentration of chlorophyll a (B) at the inflow and outflow of the reservoir in 2003 served at the inflow but they greatly decreased (by up to 164 µg l -1 ) in reservoir as a result of water shading by emergent vegetation (Gołdyn and Szeląg-Wasielewska 2005). In 2003, chlorophyll a concentration and dry weight of seston also declined in the reservoir in the first half of the study period. On the one hand, this could be due to the influence of zooplankton, which might adapt to the new conditions more quickly and exerted a stronger grazing pressure in spring than in summer, when conditions were more favourable for phytoplankton growth (extended residence time of water). On the other hand, the decline could partly result from competition with filamentous algae of the genus Spirogyra, which was abundant at the bottom of the reservoir in spring. Thanks to intensive growth of filamentous algae and phytoplankton, water oxygenation increased. However, oxygen deficits were observed at higher temperatures, due to mineralization of organic matter in bottom sediments and the intensive loading of organic matter from phytoplankton and filamentous algae. Research conducted at the same stations in showed that concentrations of dissolved oxygen decreased between them as a rule due to (i) high intensity of organic matter decomposition and (ii) limited phytoplankton growth (Gołdyn 2000). Deposition of the suspended organic matter delivered by the river in bottom sediments resulted in statistically significant decrease in BOD 5 values between the inflow and outflow, both in the 1990s and during this study. Some short-term rises in concentrations of suspensions and organic matter as a result of stormwater run-off were not detected at the inflow but affected the values recorded at the outflow of Antoninek. Phosphorus release exceeded its accumulation in the reservoir in the summer 2003, what was associated with higher temperatures, which stimulated the mineralization of organic matter in bottom sediments (Søndergaard et al. 1999, 2003). The role of temperature in phosphorus release from sediments was also noted by Kleeberg and Kozerski (1997). Those authors found a relationship between temperature rise in sediment (at the depth of 0-10 cm) and an increase in phosphorus release, resulting from intensification of biological processes. Another important factor was the decline in oxygen concentration. Waters of shallow lakes are usually well mixed by wind, which improves the oxygenation of the whole water volume, also at the sedimentwater interface. In summer, however, winds are usually not strong, while the demand for dissolved oxygen is high due to microbial growth, so sediment surface may be deoxygenated in some periods (Kleeberg and Kozerski 1997) and in anoxic conditions phosphorus ions are released from iron compounds sensitive to changes in redox potential (Kajak 2001, Søndergaard et al. 2003). More intensive phosphorus release in anaerobic conditions was recorded also in the shallow Lake Kaiavere (Estonia), which was dominated by phytoplankton. Phosphorus release was twice as high in anaerobic than in aerobic conditions (Kinsand and Noges 2003). The domination of phosphorus accumulation over its release from sediments was usually observed in winter, thanks to the good oxygenation of water and the low intensity of microbiological processes (Gołdyn 2000).

5 Effect of restoration of a lowland dam reservoir on river water quality 189 This was also confirmed by Salvia-Castellvi et al. (2001), who studied a shallow preliminary reservoir Misére. They observed seasonal differences in phosphorus retention and the highest accumulation occurred in winter. Removal of fertile sediments and the better oxygenation of water in 2003 did not exert any radical effect on the process of phosphorus release. Only the intensity of this process in individual months changed. In the 1990s it was the most intensive in May or June, whereas in 2003 in July and August (Fig. 4). Short-term rise in concentration and load of phosphorus in end of July in outflowing water was probably a result of stormwater run-off (Fig. 2B). Fig. 4. The value of total P accumulation (negative values) or release (positive values) in the reservoir in and 2003 A decreasing trend in total N concentrations between studied stations was observed in 2003 and it was probably associated precipitatiion of suspended matter. Thanks to better oxygenation, ammonium N concentrations did not increase, so in summer the increase in total N concentration and loads at the outflow was not as high as in the 1990s (Gołdyn 2000). Results of the Wilcoxon signed ranks test show that the differences in values between the inflow and outflow of Antoninek were statistically significant only for five parameters (Table 1). The difference in water temperature resulted from the influence of sunlight on the open water table. In the case of ph, the observed difference resulted from the fast growth of phytoplankton and filamentous algae, which assimilated carbon dioxide from water. Conductivity increased because of more intensive mineralization of organic matter. Significant differences in dissolved oxygen and BOD 5 were mainly due to water oxygenation by phytoplankton and filamentous algae, and sedimentation of organic matter, measured as BOD 5. In the case of the other parameters, the values at the outflow were sometimes higher and sometimes lower than at the inflow, so they were not significant over the whole study period. Conclusion The comparison of results from 2003 with results from the 1990s showed that restoration of the reservoir contributed to some improvement of water quality in Antoninek. The removal of emergent vegetation and part of bottom sediments changed its character from a wetland to a water body dominated by phytoplankton and filamentous algae. Their high abundance resulted in improved water oxygenation and mineralization of organic matter delivered to the lake. The improvement was also due to the smaller loads entering the reservoir. The decrease in concentrations of phosphorus compounds in spring was partly due to better oxygenation. In summer, however, phosphorus loads increased between the inflow and outflow, what was mainly caused by intensive mineralization of organic matter in the shallow, warm bottom of the reservoir. In 2003, sedimentation of suspended matter was lower because of the lack of emergent vegetation, while chlorophyll a concentrations increased in summer as a result of phytoplankton growth. Acknowledgements This study was supported by a grant from the State Committee for Scientific Research, Warsaw, Poland (KBN, no. 3T09D 01527). References Buczkowski W., Zgrabczyński J., 2000, Projekt budowlanowykonawczy systemu stałego doczyszczania ciągu wodnego rzeki Cybiny, remont stawu Antoninek, Biprowodomel Biuro Projektów Wodnych Melioracji i Inżynierii Środowiska Sp. Z o.o., Poznań. Gołdyn R., Szeląg-Wasielewska E., 2005, The effects of two shallow reservoirs on the phyto- and bacterioplankton of lowland river, Pol. J. Environ. Stud. 14(4): Gołdyn R., 2000, Zmiany biologicznych i fizyczno-chemicznych cech jakości wody rzecznej pod wpływem jej piętrzenia we wstępnych nizinnych zbiornikach zaporowych, Wyd. Nauk. UAM, ser. Biologia 65, Poznań.

6 190 Renata Dondajewska Gołdyn R., Kozak A., Kostka K. 1994, Causes of changes in the water quality of the restored Maltański Reservoir in Poznań, [in:] Gołdyn R. (ed.) Protection of the water of the catchment area of the river Cybina, Poznańskie Towarzystwo Przyjaciół Nauk, Pr. Komisji Biologicznej 74: Hermanowicz W., Dojlido J.R., 1999, Fizyczno-chemiczne badanie wody i ścieków, Wyd. Arkady, Warszawa, p Kajak Z., 2001, Hydrobiologia-limnologia. Ekosystemy wód śródlądowych, Wyd. Nauk. PWN, Warszawa, p Kasprzak K., Lemański J., 1991, Przyrodniczo-techniczne aspekty zagospodarowania doliny rzeki Cybiny, Politechnika Poznańska, Materiały dla studiów doktoranckich i podyplomowych 19: Kisand A., Nőges P., 2003, Sediment phosphorus release in phytoplankton dominated versus macrophyte dominated shallow lakes: importance of oxygen conditions, Hydrobiologia : Kleeberg A., Kozerski H.P., 1997, Phosphorus release in Lake Groβer Müggelsee and its implications for lake restoration, Hydrobiologia 342/343: Pűtz K., Benndorf J., 1998, The importance of pre-reservoirs for the control of eutrophication of reservoir, Water Sci. Technol. 37(2): Salvia-Castellvi M., Dohet A., Vander Borght P., Hoffman L., 2001, Control of the eutrophication of the reservoir of esch-sur-sűre (Luxembourg): evaluation of the phosphorus removal by predams, Hydrobiologia 459: Søndergaard M., Jensen J.P., Jeppesen E., 1999, Internal phosphorus loading in shallow Danish lakes, Hydrobiologia 408/409: Søndergaard M., Jensen J.P., Jeppesen E., 2003, Role of sediment and internal loading of phosphorus in shallow lakes, Hydrobiologia : Stachowicz K., Czernoch M., 1992, Charakterystyka ekologiczna zbiorników zaporowych na Sole, Instytut Gospodarki Przestrzennej i Komunalnej, Warszawa Wróbel S., Bombówna M., 1976, The cascade type of dam reservoir and the eutrophication, Limnologica 10: