Simulation of heavy metal effect on fresh-water ecosystems in mesocosms and estimation of water body self-purification properties

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1 Hydrological, Chemical and Biological Processes of Transformation and Transport of Contaminants in Aquatic Environments (Proceedings of the Rostov-on-Don Symposium, May 1993). lahspubl. no. 219, Simulation of heavy metal effect on fresh-water ecosystems in mesocosms and estimation of water body self-purification properties Yu. V. TEPLYAKOV & A. M. NEKANOROV Hydrochemical Institute, 198 Stachki av., Rostov-on-Don, Russia Abstract Behaviour of copper, mercury and cadmium at various concentrations and under different conditions were studied in experimental ecosystems (mesocosms) with volumes between 4.5 to 13 m 3. Kinetic coefficients of metal accumulation rate in bottom sediments, higher water plants and molluscs, as well as self-purification rates of water were calculated. It was shown that the presence of different components of an aquatic ecosystem together with the properties of the water are very important in model ecosystems used for the assessment of self-purification capacity of various water bodies. INTRODUCTION Self-purification capacity is defined as a capacity of aquatic ecosystems to decrease pollutant concentrations in water as a result of a combined effect of various factors. Each water body has a specific limit of self-purification capacity which, if exceeded, may lead to irreversible changes. Until now, there was no common methodology to estimate self-purification capacity of a water body. Through development of experimental conditions which simulate a natural water body (beginning with simulation in a laboratory), results of different investigations become more adequate. However, the investigations appear more complicated, expensive, and time consuming. Self-purification rate coefficients have been obtained from laboratory experiments for various organic substances (Zenin et al., 1977; Kaplin, 1973). Field experiments with mesocosms are considered to be the next step for estimation self-purification capacity (Zilov & Stom, 1990; Nikanorov & Teplyakov, 1990). For applications of experimental results, the investigations may be divided into laboratory studies (microcosms) and large enclosures or experimental ponds. Numerous investigations have been conducted using model ecosystems to solve environmental problems, primarily in toxicology. Each water body is unique in terms of hydrodynamic and biochemical processes, depending on its type, physicogeographical conditions, etc. Different areas of the same water body may have significant spatial inhomogeneity in both biotic and abiotic conditions. As far as self-purification capacity is concerned, a water body may be considered a system having a certain catalytic properties for chemically- and biologically-induced reactions under significant inhomogeneity in physical processes. Since pollutants may be both conservative and non-conservative, self-purification capacity of a water body may be considered as a function of retaining capacity of different pollutants. A

2 294 Yu. V. Teplyàkov & A. M. Nikanorov retaining capacity is defined by the amount of substance the system may transform or remove from water per time unit. Using mesocosms as a small part of the whole water body for an integrated and realistic estimation of the self-purification capacity, it is necessary to consider the effects of presence of various types of vegetation, bottom sediments, plankton types, intensity of pollutant introduction, exposure times, and type of loadings on transformation or decomposition rates of a pollutant. To collect proper data and solve some methodological problems in modelling using mesocosms, investigations have been carried out on various water bodies in the Hydrochemical Institute, Rostov-on-Don, Russia. Selection of heavy metals (copper, mercury, cadmium) used as pollutants in the investigation was based on the fact that mechanisms of their removal from water medium are less complicated than those for organic substances. Among conservative pollutants, metals are the most toxic and most resistant to biological removal (Nikanorov & Zhulidov, 1991). Their toxicity to aquatic ecosystems decreases in the following sequence: Hg > > Pb > Cd > Cr > Zn > Ni (Moore & Ramamurti, 1987). MATERIALS AND METHODS Mesocosms of an unified design allowing isolation of ecosystems of various volumes and conditions of a water body were used as a model. Cylindrical mesocosms of 2-3 m diameter that cut a volume of water from the surface to the bottom have been used in the experiments with a polyethylene film as the isolation material. The film was secured to demountable stainless-steel mesocosm containers buried in the bottom sediments. Characteristics of the water bodies, mesocosm volumes, and experimental conditions are given in Table 1. Generally, the load was applied in a classical schedule: a single introduction of heavy metals at the beginning of the experiment (C 0 is initial concentration of metal ions in water). In several experiments a constant load schedule, i.e. daily introduction of heavy metals, was used (C is an increase of metal concentration in water after mixing). Metal concentrations in water (expressed as total concentrations determined by atomic-absorption spectrometry) were estimated in filtered and nonfiltered water samples using 0.45 /xm membrane filter. Integrated depth water samples were collected as well as water samples from different areas within the mesocosms. A glass tube 1.6 m long, 6 cm diameter with a valve at one end was used as a sampler. Bottom sediment cores were collected at three mesocosm areas. The upper 2-cm sediment layer was subsampled from each core, and the subsamples were mixed in a glass jar to obtain a composite sample. Similar procedure was used for estimation of heavy metal concentrations in macrophyte and mollusc samples. RESULTS AND DISCUSSION Selected experiments were repeated to estimate their reproducibility. Figures 1 and 2 show results of experiments using copper and cadmium additions. The results of these and earlier experiments with mesocosms suggested that the results were reproducible,

3 Heavy metals and fresh-water ecosystems 295 Table 1 Initial data for mesocosm experiments. Water body, time of experiment Mesocosm no. Water volume (m 3 ) Installation depth (m) Regime of addition introduction, heavy metals in Notes Krivoye Lake, in the River Don flood plain, Sept-Oct 1990 I II III IV V C 0 = 145 C 0 = 140 -daily C = 29 C = 144 No bottom sediments Control mesocosm Mertvy Doniets River, July-August 1991 I II III IV V c = ioo -daily C = 20 Hg-single C 0 = 8.3 Hg-single Co = 8-0 No bottom sediments Control mesocosm Kirzhach River, July-August 1991 I II III IV V VI Cd-single C = 50 C 0 = 48 Cd-single C = 52 C 0 = 154 C 0 = 149 No bottom sediments No bottom sediments Control provided that the quality of the water in the mesocosm is controlled and the biotic and abiotic homogeneity maintained. The experiments were conducted simultaneously (Fig. 1). The volume of the second mesocosm was 2.7 times greater than that of the first ( m 3 and 12.7 m 3 for the first and second mesocosm, respectively). The rate of copper concentration decrease in the mesocosm water and increase in bottom sediments were similar in repeated experiments (Table 2). Dynamics of heavy metal concentration in model ecosystems and their compartments were used to calculate the following kinetic characteristics: water self-purification rate (K, day" 1 ), heavy metal accumulation (fig g' 1 day" 1 ) in bottom sediments, macrophytes and molluscs. The results are shown in Table 2. A first-order reaction equation (Kaplin, 1973) was used in calculation of K: K. hs (1)

4 296 Yu. V. Teplyakov & A. M. Nikanorov m 0)0 \ 1 to 6c VC 2c y ^ /> y ts- So ZS- Fig. 1 Dynamics of copper concentration in water (1, 2) and bottom sediments (3, 4). Lake Krivoye: 1 and 3 mesocosm I; 2 and 4 mesocosm II (Table 1). I copper concentration in water, /xg l" 1 ; II observation time, days; III copper concentration in bottom sediments, mg kg" 1 of dry weight. So i Ho 3c Zo ta 1 v -~^^. ^*~ Z ^^r~~~~~^_ * --* III 'X Fig. 2 Dynamics of cadmium concentration in water (1, 3) and bottom sediments (2, 4). Kirzhach River: 1 and 2 mesocosm I (Table 1); 3 and 4 mesocosm II. I cadmium concentration in water, /xg l" 1 ; II observation time, days; III cadmium concentration in bottom sediments, mg kg" 1 of dry weight. where: C 0 is initial heavy metal concentration in water; C t is heavy metals concentration after time t. Because of a low frequency in observations and lack of sampling at each location within the mesocosms, heavy metal accumulation rates for bottom sediments, molluscs, and higher aquatic plants were calculated as maximum based on initial and maximum observed concentrations. The presence of all components of an aquatic ecosystem in the model system is one of the most important factors that determine significance of the results. The mesocosms must contain all biotic and abiotic components, preferably in the same

5 Heavy metals and fresh-water ecosystems 297 Table 2 Kinetic parameters of heavy metals migration in mesocosms. Water body Mesocosm, metal Heavy metal decrease coefficient in water K (day 1 ) Heavy metal accumulation rates: Bottom Macrophytes Molluscs sediments (Ceratops.) (V. depressa) Krivoye Lake I, II, HI, mesocosm without bottom sediments Mertvy Doniets River I, II, III, IV, Hg Hg mesocosm without bottom sediments Kirzhach River I, II, III, IV, V, Cd Cd Cd mesocosm without bottom sediments ratios as in the simulated natural water body. Figures 3 and 4 show experimental results obtained in mesocosms with and without bottom sediments. The mesocosms without the sediments were designed differently than those with the sediments having a polyethylene film tied at the bottom to forma a sack opened upward to the water surface, rather than attached to a steel base in the sediments. In each experiment, the absence of bottom sediments and higher aquatic plants caused a significant decrease in the self-purification rate of the water (Table 2). In various experiments the value of K decreased from 3.7 to 9.7 times. As can be seen from Table 2, the values of K differ in the experiments conducted on various water bodies. A single introduction of copper, for example, produced values of Pranging from in Lake Krivoye, to day" 1 in Mertvy Doniets and Kirzhach Rivers. The data are to be analysed considering biochemical parameters which is beyond the scope of this report. The self-purification rate of the water correlates with the biomass of sestone, higher aquatic plants, concentration of dissolved organic matter, and bottom sediments composition. The greatest self-purification rates were observed in the experiments with mercury (K = 0.15 day" 1 ). However, in case of a single introduction of mercury at the beginning of the experiment (C 0 = 8.3 y.g T 1 ), the main contributors to the process were suspended matter (Nikanorov & Zhulidov, 1991) and dissolved organic matter (DOM). In constant-loading experiments,.stmay have different values being lower when compared to copper and cadmium. However, the above results are preliminary. An impact of pollutant load type is an important, though poorly investigated, issue in field experiments aimed to estimate the self-purification rate. The method of pollutant introduction is often considered in terms of ecosystem functional stability (resistant and flexible) (Odum, 1986). The majority of studies to assess self-purification rates has been conducted in the laboratory, involving organic matter and single introduction of pollutants at the beginning of the experiment. Equation (1) is often used to calculate K, however sometimes the equation involves a reaction order n which is not 1 (Topnikov & Vavilin, 1992).

6 298 Yu. V. Teplyàkov & A. M. Nikanorov Fig. 3 Dynamics of copper concentration in mesocosms with (1) and without (2) bottom sediments. Lake Krivoye: September-October 1990: I copper concentration in water, fig I' 1 ; II observation time, days. Fig. 4 Dynamics of cadmium concentration in mesocosms with (1) and without (2) bottom sediments. Kirzhach River: June-July 1990: I cadmium concentration in water, fig l" 1 ; II observation time, days. Many authors (Kaplin, 1973; Topnikov & Vavilin, 1992) note that the K coefficient is not constant and decreases with time. This is related to metal association with DOM at the beginning of the process with subsequent metal accumulation in the suspended matter. The experiments show the major difference in heavy metal concentrations in filtered and non-filtered water samples on the first day of the experiment were followed by decreases of the difference so that the curves almost overlap. At the beginning, after heavy metal introduction, a portion of plankton dies thus binding metal ions in complexes (Moore & Ramamurti, 1987) and promoting a rapid heavy metal concentration decrease on the first day of the experiment. Natural aquatic systems are often under different conditions exposed to a constant load of different pollutants. This applies specially to conservative, poorly degradable pollutants, eliminated from the aquatic phase mainly by adsorption on bottom

7 Heavy metals and fresh-water ecosystems 299 Fig. 5 Dynamics of cadmium concentration in mesocosms with initial (1) and daily (2) load. Usman River: October 1990: I - copper concentration in water, ng l" 1 ; II observation time, days. sediments or accumulation by higher water plants and molluscs. Several additional experiments with heavy metals have been conducted. Specific concentrations of heavy metals have been introduced to the water with subsequent daily additions that increased metal concentrations in the specific value C n following mixing. Results of one such experiment with copper (the Usman River, Voronezh Biospheric Preserve) are shown in Fig. 5. Straight line 1 indicates hypothetical increase of metal concentration in water in the absence of processes that introduce a decrease of the concentration. Following a simple recalculations, equation (1) has been used to calculate K. Table 3 shows the results of the calculation compared to those obtained by experiments using single-addition of copper to the mesocosms. Each experimental series in constant-load mesocosms demonstrates somewhat greater values of K. Distribution of heavy metals in model ecosystems depends on the rate of metal binding with various ecosystem components. This explains why K depends on comparative kinetics of metal absorption. It may be noted that heavy metals accumulation rates in macrophytes and molluscs are of the same order which suggests compatibility of data for accumulation of the metals. The rates of copper accumulation in bottom sediments of the Lake Table 3 Experimental results under various loading regimes. Water body Mertvy Doniets River Usman River Mesocosm volume (m 3 ) Loading type, heavy metal concentration G*g I" 1 ), C 0 = 100, C = 20, C 0 114, C 0 15 Water self-purification rate coefficient, K (day 1 )

8 300 Yu. V. Teplyakov & A. M. Nikanorov Krivoye and the Kirzhach River are similar, though lower than those in the Mertvy Doniets River by one order of magnitude. In planning experiments in mesocosms, one has to bear in mind that the model ecosystem and selected experimental method depend on study objectives. For example, in ecotoxicological investigations of hydrobiota response in natural habitat, the model ecosystem requires less strict simulation of a complex natural system than in prediction-type investigations of pollutant transformation and migration. This explains why field investigations of the self-purification have their peculiarities and limitations in application of obtained results. In the experiments with heavy metals in mesocosms, several chemical and biological parameters representing ecosystem conditions have been monitored. The analysis of structural parameters (phytoplankton, zooplankton, and bacterioplankton species composition and biomass), functional characteristics (phytoplankton production, biochemical oxygen demand BOD 5 ) indicated that for 3-10 m 3 -large mesocosms, experiment duration of one to two months may be considered satisfactory for non- and weakly-circulating water bodies. Mixing is the main factor limiting the time of sufficient simulation of riverine systems. Mesocosms demonstrate intensive form of lacustrine ecosystems. Therefore for mesocosms without mixing, regardless of their volume, the recommended experimental period is from two weeks to one month. Sometimes, the experiments involve mesocosm designs aimed to estimate the contribution of a specific component of studied ecosystem, such as the experiments with mesocosms involving only a certain volume of water containing plankton (Burdin, 1973; Sanders, 1985). In another study, Kelly (1984) generalized bottom sediment contribution using "benthic chambers" of various design and volume. To estimate contribution of higher aquatic plant activities, one may use the mesocosms in a form of enclosures placed along the shore line, as macrophytes occur mainly in the shallow water near the shore. CONCLUSIONS The following conclusions result from mesocosm experiments for estimating the self-purification capacity of water bodies. Rate coefficients for copper, mercury, and cadmium obtained in the experiments are comparable to those of decomposition rate (concentration decrease in water) for the intermediate group of organic substances (0.05 < K < 0.3; Zenin et al., 1977) at the boundary with the biologically resistant substance group (K < 0.05). If the model ecosystem contains a specific volume of natural water, the value of K decreases 4-10 times. To evaluate the capacity of an aquatic ecosystem to neutralize pollutants, particularly heavy metals, it is recommended to conduct mesocosm experiments in the constant-loading mode, changing load levels in different mesocosms. Such experimental method allows to estimate pollutant accumulation kinetics in ecosystem components in more details and to reveal maximum accumulation values. Since mesocosms represent approximated models of a water body, a few common rules are to be considered in experimental design: to obtain information comparable to natural aquatic ecosystems, the model ecosystem needs to include all major components of the water body;

9 Heavy metals and fresh-water ecosystems since water body properties are not uniform, more precise results may be obtained by conducting the experiment at different areas of investigated water body; - the experiments conducted during different seasons improve the validity of obtained results; - to carry out simultaneous experiments when technical and economic capabilities are not restricted to increase reliability of the results. REFERENCES Burdin, K. S. (1973) Development of standard laboratory and field ecosystem investigations to estimate the impact of pollution on marine environments (in Russian). Chelovek i biosfera, Moscow 3, Kaplin, V. T. (1973) Organic matter transformation in natural waters (in Russian). Abstract of Thesis for the Doctor of Sciences (Chemistry) Degree, Irkutsk. Kelly, J. R. (1984) Microcosms for studies of sediment-water interactions. In: Ecotoxicological Testing for the Marine Environment, vol. 2: Bredene, Belgium: State Univ. Ghent and Instr. Mar. Sci. 42, Moore, J. & Ramamurti, S. (1987) Heavy Metals in Natural Waters. Impact Control and Estimation (in Russian). Mir, Moscow. Nikanorov, A. M. & Teplyakov, Yu. V. (1990) Problems of pollutant transformation investigation by physical modelling methods (in Russian). In: Metodologiya Ekologicheskogo Normirovaniya (Proc. All-Union Conference, Kharkov, April 1990), vol. 1, Nikanorov, A. M. & Zhulidov, A. V. (1991) Metal Biomonitoring in Fresh-water Ecosystems (in Russian). Gidrometeoizdat, Leningrad. Odum, Yu. (1986) Ecology (in Russian), vol. 1. Mir, Moscow. Sanders, F. (1985) Use of large enclosures for perturbation experiments in benthic ecosystems. Env. Monit. Assess. 5(6), Topnikov, V. E. & Vavilin, V. A. (1992) Comparative estimation of river self-purification models (in Russian). Vodnye Resursy 1, Zenin, A. A., Sergeeva, O. V. & Zemchenko, G. N. (1977) Coefficients of pollutant transformation (decomposition) in water (in Russian). Obzornaya Informatsiya VNIIGMI - MTSD 1, 43. Zilov, E. A. & Stom, D. I. (1990) A model experiment in water toxicology (in Russian). Gidribiologicheskii Zhurnal26(1),

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