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8/9/2019 Simulation of Heavy Metal Effect on Fresh-water Ecosystems in Mesocosms and Estimation of Water Body Self-puri

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Hydrological, Chemical and Biological Processes of Transformation and Transport of Con taminants in AquaticEnvironments (Proceedings of the Rostov-on-Don Symposium, May 1993). lAH SPu bl. no. 219 ,199 4. 2 9 3

Simulation of heavy metal effect on fresh-waterecosystems in mesocosms and es t imat ion of water bodyself-purif icat ion propert ies

Yu. V. TEPLYAKOV A. M. NEKANOROVHydrochemical Institute, 198 Stachki av., Rostov-on-Don, Russia 344104

Abstract Behaviour of copper, mercury and cadmium at variousconcentrations and under different conditions were studied inexperimental ecosystems (mesocosms) with volumes between 4.5 to

13 m3

. Kinetic coefficients of metal accumulation rate in bottomsediments, higher water plants and molluscs, as well as self-purificationrates of water were calculated. It was shown that the presence ofdifferent components of an aquatic ecosystem together with theproperties of the water are very important in m odel ecosystems used forthe assessment of self-purification capacity of various water bodies.

INTRODUCTION

Self-purification capacity is defined as a capacity of aquatic ecosystems to decreasepollutant 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-purificationcapacity of a water body. Through development of experimental conditions whichsimulate a natural water body (beginning with simulation in a laboratory), results ofdifferent investigations become more adequate. However, the investigations appearmore complicated, expensive, and time consuming. Self-purification rate coefficientshave been obtained from laboratory experiments for various organic substances (Zeninet al., 1977; Kaplin, 1973). Field experiments with mesocosms are considered to bethe next step for estimation self-purification capacity (Zilov & Stom, 1990; Nikanorov& Teplyakov, 1990). For applications of experimental results, the investigations maybe divided into laboratory studies (microcosms) and large enclosures or experimentalponds. Numerous investigations have been conducted using model ecosystems to solveenvironmental problems, primarily in toxicology. Each water body is unique in termsof hydrodynamic and biochemical processes, depending on its type, physico-geographical conditions, etc. Different areas of the same water body may havesignificant spatial inhomogeneity in both biotic and abiotic conditions.

As far as self-purification capacity is concerned, a water body may be considereda system having a certain catalytic properties for chemically- and biologically-inducedreactions under significant inhomogeneity in physical processes. Since pollutants maybe b oth conservative and non-conservative, self-purification capacity of a water bodymay be considered as a function of retaining capacity of different pollutants. A


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294 Yu. V. Teplykov A. M. Nikanorov

retaining capacity is defined by the amount of substance the system may transform orremove from water per time unit.

Using mesocosms as a small part of the whole water body for an integrated andrealistic estimation of the self-purification capacity, it is necessary to consider theeffects of presence of various types of vegetation, bottom sediments, plankton types,intensity of pollutant introduction, exposure times, and type of loadings ontransformation or decomposition rates of a pollutant.

To collect proper data and solve some methodological problems in modelling usingmesocosms, investigations have been carried out on various water bodies in theHydrochemical Institute, Rostov-on-Don, Russia. Selection of heavy metals (copper,mercury, cadmium) used as pollutants in the investigation was based on the fact thatmechanisms of their removal from water medium are less complicated than those fororganic substances. Among conservative pollutants, metals are the most toxic and most

resistant to biological removal (Nikanorov & Zhulidov, 1991). Their toxicity to aquaticecosystems decreases in the following sequence: Hg > Cu > Pb > Cd > Cr > Zn> Ni (Moore & Ramamurti, 1987).

MATERIALS AND METHODS

Mesocosms of an unified design allowing isolation of ecosystems of various volumesand conditions of a water body were used as a model. Cylindrical mesocosms of 2 3 mdiameter 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 wassecured to demountable stainless-steel mesocosm containers buried in the bottomsediments.

Characteristics of the water bodies, mesocosm volumes, and experimentalconditions are given in Table 1. Generally, the load was applied in a classicalschedule: a single introduction of heavy metals at the beginning of the experiment (C0is initial concentration of metal ions in water). In several experiments a constant loadschedule, i.e. daily introduction of heavy metals, was used (C is an increase of metalconcentration in water after mixing). Metal concentrations in water (expressed as totalconcentrations determined by atomic-absorption spectrometry) were estimated infiltered and nonfiltered water samples using 0.45 /xm membrane filter.

Integrated depth water samples were collected as well as water samples fromdifferent areas within the mesocosms. A glass tube 1.6 m long, 6 cm diameter witha valve at one end was used as a sampler. Bottom sediment cores were collected atthree mesocosm areas. The upper 2-cm sediment layer was subsampled from eachcore, and the subsamples were mixed in a glass jar to obtain a composite sample.Similar procedure was used for estimation of heavy metal concentrations inmacrophyte and mollusc samples.

RESULTS AND DISCUSSION

Selected experiments were repeated to estimate their reproducibility. Figures 1 and 2show results of experiments using copper and cadmium additions. The results of theseand earlier experiments with mesocosms suggested that the results were reproducible,


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Heavy metals and fresh-water ecosystems

Table 1 Initial data for mesocosm experiments.

295

Water body,time ofexper iment

Kr ivoye Lake ,in the RiverDon f loodplain,Sept-Oct 1990

Mer tvy Donie t sRiver,Ju ly -Augus t1991

Kirzhach River,Ju ly -Augus t

1991

M e s o c o s mno .

I

II

III

IVV

I

II

III

IV

V

I

II

III

IV

V

V I

Water vo lume

(m3)

4 .0

4.7

4.7

4.712.7

4.7

4.7

4.7

4.0

4.7

4.0

4.0

3.0

3.7

3.0

4.0

Installationdepth(m)

-.

1.7

1.7

1.71.8

1.7

1.7

1.7

-

1.7

1.4

1.4

-

1.3

-

1.4

Regime of addi t ionintroduct ion,heavy metals in

Cu-singleC0 = 145Cu-singleC0 = 140Cu-dailyC = 29

Cu-singleC = 144

Cu-singlec = iooCu-dailyC = 20Hg-singleC0 = 8.3Hg-singleCo = 8-0

Cd-singleC = 50

Cu-singleC 0 = 48Cd-singleC = 52Cu-singleC0 = 154Cu-singleC0 = 149

Notes

No bo t tomsediments

Controlmesocosm


Controlmesocosm


No bo t tomsedimentsControl

prov ided that the quality of the water in the mesocosm is controlled and the biotic andabiotic homogeneity maintained. The experiments were conducted simultaneously(Fig. 1). The volume of the second mesocosm was 2.7 times greater than that of thefirst (4.7 m3 and 12.7 m3 for the first and second mesocosm, respectively). The rateof copper concentration decrease in the mesocosm water and increase in bottomsediments were similar in repeated experiments (Table 2).

Dynamics of heavy metal concentration in model ecosystems and theircompartments were used to calculate the following kinetic characteristics: waterself-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-orderreaction equation (Kaplin, 1973) was used in calculation of K :

K . hS (1)


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296 Yu. V. Teplyakov A. M. Nikanorov

1

m

0 0

to

6c

VC

2 c

\

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 l1;II observation time, days;II I copper concentration in bottom sediments, mg kg1 of dry weight.

So

Ho

3c

Zo

ta

i

v ~ ^ ^ .

1 ^ * ~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 cadmiumconcentration in bottom sediments, mg kg1 of dry weight.

where:C0 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 locationwithin the mesocosms, heavy metal accumulation rates for bottom sediments,

molluscs, and higher aquatic plants were calculated as maximum based on initial andmaximum observed concentrations.

The presence of all components of an aquatic ecosystem in the model system isone of the most important factors that determine significance of the results. Themesocosms must contain all biotic and abiotic components, preferably in the same


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Heavy m etals and fresh-water ecosystems 297

Table 2 Kinetic parameters of heavy metals migration in mesocosms.

Water body

KrivoyeLake

MertvyDonietsRiver

KirzhachRiver

Mesocosm,metal

I,II,HI,

I,II,III,IV,

I,II,III,IV,V,

CuCuCu

CuCuHgHg

CdCdCdCuCu

Heavy metaldecrease

coefficientin water K (day1)

0.008

0.057

0.075

0.110.130.150.04

0.080.090.010.110.12

Heavy metal accumulation rates:

Bottom Ma crophytes Molluscssediments (Ceratops.) (V. depressa)

mesocosm without bottom sediments2.07 3.852.45 3.9

0.58 - 3.30.33 - 1.250.05 - 0.02mesocosm without bottom sediments

0.290.31mesocosm without bottom sediments2.421.92

ratios as in the simulated natural water body. Figures 3 and 4 show experimentalresults obtained in mesocosms with and without bottom sediments. The mesocosmswithout the sediments were designed differently than those with the sediments havinga polyethylene film tied at the bottom to forma a sack opened upward to the watersurface, rather than attached to a steel base in the sediments.

In each experiment, the absence of bottom sediments and higher aquatic plantscaused a significant decrease in the self-purification rate of the water (Table 2). Invarious 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 conductedon various water bodies. A single introduction of copper, for example, producedvalues of Pranging from 0.057-0.075 in Lake Krivoye, to 0.11-0.12 day1 in MertvyDoniets and Kirzhach Rivers. The data are to be analysed considering biochemicalparameters which is beyond the scope of this report. The self-purification rate of thewater correlates with the biomass of sestone, higher aquatic plants, concentration of

dissolved organic matter, and bottom sediments composition. The greatestself-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 ofthe experiment (C 0 = 8.3 y.g T

1), the main contributors to the process were suspendedmatter (Nikanorov & Zhulidov, 1991) and dissolved organic matter (DOM). Inconstant-loading experiments, .STmay have different values being lower when comparedto copper and cadmium. However, the above results are preliminary.

An impact of pollutant load type is an important, though poorly investigated, issuein field experiments aimed to estimate the self-purification rate. The method ofpollutant introduction is often considered in terms of ecosystem functional stability(resistant and flexible) (Odum, 1986). The majority of studies to assessself-purification rates has been conducted in the laboratory, involving organic matterand 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 ordern which is not 1 (Topnikov & Vavilin, 1992).


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298 Yu. V. Teplykov 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 Kcoefficient is not constant and decreases with time. This is related to metal associationwith DOM at the beginning of the process with subsequent metal accumulation in thesuspended matter. The experiments show the major difference in heavy metalconcentrations in filtered and non-filtered water samples on the first day of theexperiment were followed by decreases of the difference so that the curves almostoverlap. At the beginning, after heavy metal introduction, a portion of plankton dies

thus binding metal ions in complexes (Moore & Ramamurti, 1987) and promoting arapid 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 degradablepollutants, eliminated from the aquatic phase mainly by adsorption on bottom


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Heavy metals and fresh-water ecosystems 299

Fig. 5 Dynamics of cadmium concentration in mesocosms with initial (1) and daily(2) load.Usman River: O ctober 1990: I - copper concentration in water, ng l 1; II observation time, days.

sediments or accumulation by higher water plants and molluscs. Several additionalexperiments with heavy metals have been conducted. Specific concentrations of heavymetals have been introduced to the water with subsequent daily additions that increasedmetal concentrations in the specific value Cn following mixing. Results of one such

experiment with copper (the Usman River, Voronezh Biospheric Preserve) are shownin Fig. 5. Straight line indicates hypothetical increase of metal concentration in waterin the absence of processes that introduce a decrease of the concentration. Followinga simple recalculations, equation (1) has been used to calculate K . Table 3 shows theresults of the calculation compared to those obtained by experiments usingsingle-addition of copper to the mesocosms. Each experimental series in constant-loadmesocosms demonstrates somewhat greater values of K Distribution of heavy metalsin model ecosystems depends on the rate of metal binding with various ecosystemcomponents. This explains why K depends on comparative kinetics of metalabsorption. It may be noted that heavy metals accumulation rates in macrophytes andmolluscs are of the same order which suggests compatibility of data for accumulationof the metals. The rates of copper accumulation in bottom sediments of the Lake

Table 3 Experimental results under various loading regimes.

Water body Mesocosmvolume(m3)

Loading type,heavy metalconcentrationG * g I 1)

Waterself-purificationrate coefficient,K (day1)

Mertvy Doniets River 4 74 7

Cu, C0 = 100Cu, C = 20

0.110.13

Usman River 3.43.5

Cu, C0Cu, C0

11415

0.090.11


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3 Yu. V. Teplyakov A. M. Nikanorov

Krivoye and the Kirzhach River are similar, though lower than those in the MertvyDoniets River by one order of magnitude.

In planning experiments in mesocosms, one has to bear in mind that the modelecosystem and selected experimental method depend on study objectives. For example,

in ecotoxicological investigations of hydrobiota response in natural habitat, the modelecosystem requires less strict simulation of a complex natural system than inprediction-type investigations of pollutant transformation and migration. This explainswhy field investigations of the self-purification have their peculiarities and limitationsin application of obtained results. In the experiments with heavy metals in mesocosms,several chemical and biological parameters representing ecosystem conditions havebeen monitored. The analysis of structural parameters (phytoplankton, zooplankton,and bacterioplankton species composition and biomass), functional characteristics(phytoplankton production, biochemical oxygen demand BOD5) indicated that for3-10 m3-large mesocosms, experiment duration of one to two months may be

considered satisfactory for non- and weakly-circulating water bodies. Mixing is themain factor limiting the time of sufficient simulation of riverine systems. M esocosmsdemonstrate intensive form of lacustrine ecosystems. Therefore for mesocosms withoutmixing, regardless of their volume, the recommended experimental period is from twoweeks to one month.

Sometimes, the experiments involve mesocosm designs aimed to estimate thecontribution of a specific component of studied ecosystem, such as the experimentswith mesocosms involving only a certain volume of water containing plankton (Burdin,1973; Sanders, 1985). In another study, Kelly (1984) generalized bottom sedimentcontribution using benthic chambers of various design and volum e. To estimatecontribution of higher aquatic plant activities, one may use the mesocosms in a formof enclosures placed along the shore line, as macrophytes occur mainly in the shallowwater near the shore.

CONCLUSIONS

The following conclusions result from mesocosm experiments for estimating theself-purification capacity of water bodies.

Rate coefficients for copper, mercury, and cadmium obtained in the experimentsare comparable to those of decomposition rate (concentration decrease in water) forthe 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 themodel ecosystem contains a specific volume of natural water, the value of K decreases4-10 times.

To evaluate the capacity of an aquatic ecosystem to neutralize pollutants,particularly heavy m etals, it is recommended to conduct mesocosm experiments in theconstant-loading mode, changing load levels in different mesocosms. Suchexperimental 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 commonrules 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;


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Heavy metals and fresh-water ecosystems 301

- since water body properties are not uniform, more precise results may be obtainedby conducting the experiment at different areas of investigated water body;

- the experiments conducted during different seasons improve the validity ofobtained results;

- to carry out simultaneous experiments when technical and economic capabilitiesare not restricted to increase reliability of the results.

REFERENCES

Burd in, K. S. (1973) Development of standard laboratory and field ecosystem investigations to estimate the impactof pollution on marine environments (in Russian). Chelovek i biosfera, Moscow 3, 66-74.

Kaplin, V . T. (1973) Organ ic matter transformation in natural waters (in Russian). Abstract of Thesis for the Doctorof Sciences (Chemistry) Degree, Irkutsk.

Kelly, J. R. (1984) Microcosms for studies of sediment-water interactions. In: Ecotoxicological Testing for theMarine Environment, vol. 2: Bredene, Belgium: State Univ. Ghent and Instr. Mar. Sci. 42, 77-85.

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 physicalmodelling methods (in Russian). In: Metodologiya Ekologicheskogo Normirovaniya (Proc. All-UnionConference, Kharkov, 16-20 April 1990), vol. 1, 46-47.

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.San ders , F. (1985) U se of large enclosures for perturbation experiments in benthic ecosystems. Env. Monit. Assess.

5(6), 55-99.Top niko v, V. E. & Vavilin, V. A. (1992) Com parative estimation of river self-purification models (in Russian).

Vodnye Resursy 1, 59-75.Zenin, A. A., Sergeeva, O. V. & Zemchenko, G. N. (1977) Coefficients of pollutant transformation

(decomposition) in water (in Russian). Obzornaya Informatsiya V NIIGMI - MTSD 1, 43.Zilov, E. A. & Stom, D. I. (1990) A model experiment in water toxicology (in Russian). Gidribiologicheskii

Zhurnal26(1), 67-69.


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simulation of heavy metal effect on fresh-water ecosystems in mesocosms and estimation of water body...

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