TRANSFER OF ORGANIC MATTER IN A LAKE INCLUDED IN THE POWER PLANT COOLING SYSTEM (WIELKOPOLSKO-KUJAWSKIE LAKE DISTRICT, POLAND)

Julita Dunalska¹, Bogusław Zdanowski², Konrad Stawecki², Wojciech Marchlik^3
1 Chair of Environmental Protection Engineering, University of Warmia and Mazury in Olsztyn
² Deptment of Hydrobiology, Institute of Inland Fisheries in Olsztyn
³ Department of Microbiology, University of Warmia and Mazury in Olsztyn

ABSTRACT

For assessment of the organic matter mobility rate, dissolved and particulate organic carbon were used. Quality of the organic matter was determined on the grounds of the relative indicator of aromatic properties (SUVA₂₆₀) and the absorbance ratio at 260 nm (A₂₆₀) and 330 nm (A₃₃₀). The study has revealed that the dominant form of organic carbon is the dissolved form (DOC). The main source of organic matter is the autochthonous production, intensifying in spring, which is exhibited by the growth of the heterotrophic bacteria number and intensive phytoplankton bloom. Organic matter in the lake is transferred both spatially and vertically. The rate of these processes is determined by, respectively: forced water flow resulting from the cooling waters dump and sedimentation of organic matter on organic and inorganic suspensions, which is favoured by the carbonate-calcareous character of the Lake Mikorzyńskie waters. Organic matter adsorption to calcium carbonate and destruction running with the heterotrophic bacteria participation are one of the major factors limiting the heavy degradation of the lake waters.

Key words: organic carbon, spectrophotometry, heated waters, lake, eutrophication.

INTRODUCTION

Dumping cooling waters to water reservoirs is a drastic form of man´s intervention into the natural environment. It causes not only increase in temperatures and water flow in the reservoirs but also contributes to their eutrophication. Increase of trophic level is mainly related to the effects of water evaporation in a system and input of various allochthonous pollutants to waters as utilization of reservoirs is accompanied by considerable transformations in their watersheds and caused by industrialization and air pollution (Zdanowski 1994). A precondition to restrict the negative changes is to recognize production and destruction of organic matter.

To evaluate the dynamics of organic matter turnover in surface waters organic carbon is used [6,22]. Total organic carbon (TOC) occurs in the form of dissolved organic carbon (DOC), particulate organic carbon (POC) and volatile organic carbon (VOC). Dominant is the dissolved form; its content is determined by the balance between external input and production vs. export and consumption [23]. Much lower portion of TOC in waters comprises the particulate form (POC). Transformations in the POC pool are initiated by physiological mortality of organisms and mortality caused by consumption and paratism. Also important is the role of bacteria that built POC into their biomass [4,13], as well as the role of filtrating and grinding organisms. Content of the individual forms of organic carbon allows an estimate of the rate of organic matter transfer in lakes and to recognize their trophic state.

The goal of this study was to inquire into spatial and vertical changes in the DOC and POC concentrations and the organic matter quality and eventually to determine the productivity of Lake Mikorzyńskie waters when included in the cooling system of the power plant.

MATERIALS AND METHODS

Lake Mikorzyńskie, with the surface area of 251.8 ha, max. depth of 36.5 m, mean depth of 11.5 m, is situated in the Wielkopolsko-Kujawskie lake district, 20 km north of the Konin city, in the drainage basin of Lake Gopło [10], (Fig. 1). It is the deepest lake in the open cooling system of the "Konin" and "Pątnów" power plants [17]. The lake is naturally connected to Lake Ślesińskie on the north and Lake Pątnowskie on the south and comprises an element of the Warta-Gopło water route. Lake Mikorzyńskie receives heated waters throughout the year from the Mikorzyński Channel that runs along its south-eastern shore and discharging into its middle part. Dump waters are put into the lake by a free inlet which results in a mixing of the cooling waters and the lake waters in the surface layer; eventually, horizontal temperature gradients occur. In summer when the cooling system operates in a so-called "distant" cycle, Lake Mikorzyńskie is additionally fed with the heated waters from Lake Ślesińskie.

The examinations were carried out in monthly intervals, April through November 2003, on 5 sampling posts (SP), (Fig. 2). Water samples were taken with a "Toń 2" sampling tube from the surface layer (0.5 m under surface) and from the bottom layer (0.5 m above bottom). During the summer stagnation also the metalimnion waters were examined.

TOC content was marked in unfiltered samples, and DOC after filtration on 0.45 um Millipore filter. POC was calculated as the difference between TOC and DOC concentrations. Determinations were done on organic carbon analyser Schimadzu TOC-5000, after prior acidification of the samples with 2N HCl to about 2 pH in order to remove CO₂.

The quality of dissolved organic matter (DOM) was determined based on the properties of the selected fragments of 200-400 nm absorption spectrum. The examinations were done in filtered samples (0.45-µm Millipore filters). The spectra in the range 200-400 nm were obtained with the help of a double-channel Shimadzu UV-1601PC spectrophotometer and 10-mm quartz cells, and with demineralised water as reference.

Additionally determined were:

• total count of bacterial plankton (TBC) and biomass of bacterial plankton (BB) by the direct analysis method of bacteria stained by acridine orange on polycarbonate black membrane filters (0.2 um) [10]. Bacteria were analysed under epifluorescent microscope Axioskop (OPTON) with automatic display analysis by MultiScan (CSS) [20],

• transparency, temperature, oxygen content, individual forms of nitrogen (ammonium, nitrite, nitrate, organic), total phosphorus and ortophosphates, iron, manganese, chlorides, total and calcium hardness. Analyses were conducted in accordance with the Standard Methods (1980).

The statistically significant relationships were computed between the concentration of organic carbon and the hydro-chemical parameters and bacterial parameters, assuming the level of significance at p=0.05.

RESULTS AND DISCUSSION

Water temperature in the examined period varied between 4.3°C and 26.8°C. Oxygen content was the highest in spring in the surface water layers (19.0 mg O₂/dm³) and in the following months it stabilized at the level of approximately 8 mg O₂/dm³. Near the bottom, after the thermocline had set up, oxygen content was quickly decreased to traces at the end of the summer stagnation; in the hypolimnion, hydrogen sulfide was detected. The surface waters of Lake Mikorzyńskie were alkaline (mean value in spring 8.6; in summer 8.5; in autumn 8.3). Calcium concentration varied from 60.1 to 70 mg Ca^2+/dm³. Carbon dioxide was not detected, except for trace amounts marked in autumn. Bicarbonates ranged from 232 to 311 mg/dm³ and electric conductivity from 475 to 588 mS*cm-¹. Mean content of the nutritive elements in the whole water mass equalled: total P 0.21 mg/dm³, orthophosphates 0.16 mg/dm³, total N 1.52 mg/dm³, ammonium N 0.28 mg/dm³. The content of TOC changed in the range: 6.1-11.2 mg/dm³, and of SUVA₂₆₀ in the range: 23.3-30.7. Absorbance ratio at 260 nm and 330 nm was similar and contained in the range 2.2-2.5.

The highest amounts of TOC were determined in spring on sampling site 1 (10.2 mg/dm³ on average). The dominant fraction, as in the whole lake, was DOC. Close relation between the high DOC concentrations and the lowest throughout the study values of SUVA₂₆₀ can make an evidence that in spring in the northern part of the lake, the easily available autochthonous organic matter dominates (Fig. 3a,b,c). An additional source can be the allochthonous substrate running off the drainage basin. According to [19], an elevated number of heterotrophic bacteria in spring, as in the "clean" thermally unpolluted lakes, can be related to a post-thawing enrichment. Utilization of these substances in the production of autochthonous organic matter and consequently the rate of its degradation and the environmental effects, are considerably higher in the konińskie lakes. The elevated temperature (correlation between DOC and temperature was statistically significant only in the spring time and equalled r=0.66) supports intensive production of autochthonous organic matter in the whole lake. The surveys by [6] revealed that in higher temperature and at high content of humic substances (HS), the enzymatic activity of planktonic bacteria increases. Transfer of organic matter across the lake stimulates such activity which is confirmed by a significant correlation between POC and BB (r=0.8), and between SUVA₂₆₀ and the Shanon index (r=0.6). Intensive growth of aquatic bacteria is accompanied by spring phytoplankton blooms which is exhibited by the surface waters´ super-saturation with oxygen (170%), trace amounts of orthophosphates and ammonium N, and the lowest water transparency (1.6 m on average). Primary production in Lake Mikorzyńskie is regulated to some degree by an abiotic factor. Chemical composition of the waters favours the phenomenon of phosphates precipitation on the humic compounds. HS-Fe complexes the absorb phosphates can considerably restrict the availability of this element to algae [9,16]. Intensity of these processes grows proportionally to Ca²⁺ increase [5]. In Lake Mikorzyńskie, occurrence of this phenomenon is confirmed by the significant correlation between DOC and Ca (r = -0.69), and PO₄ (r = -0.53). An additional factor restricting the phytoplankton growth can be potassium availability; statistical correlation analogous to the case of phosphates (r = -0.52).

The carbonate-calcareous character of the waters of Lake Mikorzyńskie waters clearly favours the process of the organic matter sedimentation. Adsorption to organic and inorganic suspensions can be one of the possible transport ways [14,15]. The statistical correlation between DOC and Ca (r = -0.69), and bicarbonates (r = -0.53) seem to favour such way of transfer. This process can immobilize great amounts of both allochthonous and autochthonous organic matter in the bottom sediments, and at the same time diminish epilimnion phosphorus reserves available to phytoplankton. Such natural protective mechanism is typical of clean mesotrophic lakes, ecotonal river-lake zones, or lakes intensively grown over by submerged plants [1,8,11]. [25] reports that in the konińskie lakes, characteristic of naturally higher trophy, this mechanism functions as a result of mineral pollution.

Taking into consideration that along with the waters´ trophic state deterioration DOC concentrations increase [6,24], it can be concluded that unlike in spring, in summer, the higher primary production is displayed only in the middle part of the lake. The elevated value of DOC (7.2) and the high DOC/POC ratio intensifying autolysis processes and microbiological degradation of the dead organisms (Fig. 3b, Fig. 4). In the discussed case, the input of the heated waters from the Mikorzyński Channel discharging into the middle part of the lake is very important.

Assuming the indexes applied by Górniak and Zieliński (Lake Wigry), it has been concluded that the sections in the lake under direct impact of the imported waters (SP 1 & 3) and near the outlet (SP 5) are more eutrophic, like those near the bottom, and the concentrations of phosphate and ammonium ions, compared to DOC, were much higher (Fig. 5). A little better was sampling from sites 2 & 4 which possibly result from natural mechanisms protecting the lake against detritus deposition on the bottom. Both sampling sites are located over the deepest spots in the lake. Settling organic matter, produced in the surface water layers, encounters a physico-chemical barrier caused by the water density gradient in the metalimnion, which eventually slows down the process of particles settling. Such occurrence favours intensive mineralisation. DOC released to the water in the metalimnion can be simultaneously used by hydrobiont assemblages, creating a biochemical barrier to organic matter deposition. However, the main role in decomposition and utilization of organic matter in the whole lake is played by heterotrophic bacteria. The survey by [21] revealed that the destruction process by bacteria in the heated channels comprised up to 80% of total destruction. In the bottom deposits, in the zone of the heated and deoxygenated waters input, the mineralisation processes comprised 50%, and outside the dumping zone up to 30%. Intensive processes of autolysis and dead organisms´ degradation in the waters of Lake Mikorzyńskie can be observed already in summer and after the growing season, in autumn. The evidence are the growing values of the DOC/POC ratio, as compared to spring (Fig. 4). As the reduction of POC concentration was not accompanied by DOC concentration increase, it can be concluded that resistant to degradation fractions of DOC remain in the surface waters and are displayed by the high values of SUVA₂₆₀ (Fig. 3c). Intensive bacterial activity contributes to intensive bacterial utilization of the degradable DOC components. Bacteria use only the peripheral, aliphatic organic compounds, bound to biopolymers of humic acids or free fulvic acids [3,6]. Bio-conversion of the organic matter dissolved in water by heterotrophic bacteria is the reason that mainly the hard to assimilate organic matter remains in the water as shown by the SUVA₂₆₀ value increase.

In accordance with the DOC concentration range (from 2 to 25 mg/dm³) given by [12], Lake Mikorzyńskie can be classified as HS-poor (5.1-7.6 mg/dm³). Assuming the criteria provided by [2], the mean DOC value (6.3 mg/dm³) qualifies the lake as transitional meso-eutrophic, while the range of the DOC concentrations is characteristic of a eutrophic type. Despite the qualities of a eutrophic lake, the high activity of heterotrophic organisms in creating the overall ecological structure of plankton and the effective mineralisation of organic matter, substantially restrict degradation of the examined waters.

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