INFLUENCE OF WATER HARDNESS ON ACCUMULATION AND ELIMINATION OF CADMIUM IN TWO AQUATIC MOSSES UNDER LABORATORY CONDITIONS

C. Gagnon, G. Vaillancourt, L. Pazdernik
Laboratoire de Recherche sur les Communautés Aquatiques, Université du Québec à Trois-Rivières
Département de Chimie-Biologie, C.P. 500, Trois-Rivières,
Québec, G9A 5H7, Canada

ABSTRACT

This study investigated the effect of water hardness on the accumulation and elimination pattern of Cd by two aquatic mosses, Fontinalis dalecarlica and Platyhypnidium riparioides, under laboratory conditions. The two mosses were exposed to nominal Cd concentrations of 0, 0.8, 2 and 10 µg•L^-1, which are comparable to concentrations found in nature. The influence of three water hardness (very soft 15 mg•L^-1, soft 45 mg•L^-1 and hard water 90 mg•L^-1 as CaCO₃) was measured while maintaining the alkalinity and pH constant during the provide duration here. The Cd accumulation by the aquatic mosses was rapid, showing the potential of accumulation and sensitivity of this biomonitor. Even when the actual Cd concentration in the water was low (concentrations <0.15 to 6.82 µg•L^-1, the uptake of Cd was very fast and mostly linear. Accumulation rates of Cd were significantly different when the mosses were in very soft (15 mg•L^-1) as compared to hard water (90 mg•L^-1 as CaCO₃). The elimination of Cd followed a very slow process for the two species studied. The elimination rates of Cd from the mosses were not influenced by water hardness.

INTRODUCTION

The acute toxicity and accumulation rate of Cd in the aquatic biotope depends on many environmental factors, of which one is water hardness. Some previous studies have examined the influence of Ca⁺⁺ (Pickering and Puia 1969, Wehr 1983, Wehr and Whitton 1983) and Mg⁺⁺ (Wells and Brown , 1990) on the accumulation of heavy metals by the aquatic mosses. However, the present study is unique because the concentrations of Cd and water hardness used are representative of conditions found in Quebec freshwater.

Consequently, this study investigated the accumulation andelimination rates of Cd by Fontinalisdalecarlica and Platyhypnidium riparioides under controlled conditions of the laboratory. The Cd concentrations were 0 (control), 0.8, 2 et 10 µg•L^-1, which are similar to those found in nature ( Goulet and Laliberté 1982). This study also examined the influence of three different degrees of water hardness (10-15, 40-50 and 80-100 mg*L^-1 as CaCO₃), at constant alkalinity (80-100 mg•L^-1 as CaCO₃) and pH (7.30).

The objectives of this study were to evaluate the accumulation and elimination rates of Cd by two aquatic mosses, F. dalecarlica and P. riparioides, when exposed to three different water hardness by testing the hypothesis that Ca⁺⁺ and Mg⁺⁺ compete with Cd during its accumulation by the mosses.

MATERIALS AND METHODS

Mosses Used

These experiments were conducted using two bryophytes Fontinalis dalecarlica Schimp. ex B.S.G. and Platyhypnidium riparioides (Hedw.) Dix., from the Mastigouche Reserve, Quebec (46°40'N; 73°19'W).

Experimental Design

The experiments concerning the accumulation rates were carried out in a semi-open dynamic system; a 25 L aquarium was connected by three pipes to a drum filled with 190 L of water. Before starting the accumulation experiments, the systems were exposed to Cd in order to saturate their surfaces (Thain 1984). The elimination rates of Cd was followed using only the 25 L aquaria equipped with a simple air pump circulating system, which maintained the water homogeneity.

Methods of Contamination

The nominal Cd exposure concentration in the study were 0 (control), 0.8, 2 and 10 µg•L^-1 (with replicates for the 10 µg•L^-1 concentration). A commercial Cd reference solution (1000 mg•L^-1 of Cd ± 1% with 2% HNO₃, Fisher Scientific) was used to inject Cd in each system. Each contamination level was reproduced in three types of reconstituted water; very soft (10-15 mg•L^-1 as CaCO₃), soft (40-50 mg•L^-1 as CaCO₃) and hard water (80-100 mg•L^-1 as CaCO₃) (Table I), for a total of 15 experimental set- ups. Like it was said previously, the water hardness selected in this study were mostly soft water because it is representative of Quebec streams. A buffer solution of NaOH (1M) and KH₂PO₄ (1M) was added tomaintain a constant pH (EPA 1975, ASTM 1987).

Exposition Schedule for Accumulation and Release

The accumulation patterns of Cd by the bryophytes was followed for 28 days. Three samples of both mosses (0.3 g dry weight) were removed at intervals of 0, 1.5, 3, 6, 12, 24 h and every other day thereafter.

After the 28 days of the accumulation phase, the contaminated mosses were immersed in Cd free water in order to determine their elimination capacity. The sampling schedule for the elimination of Cd from the mosses was the same as that for the accumulation phase.

Analyses of Cd in Moss and Water

The Cd measurements were made following the method described by APHA (1989). Digestion blanks and standard additions were used to complete the quality control scheme. Quantification limit of the instrument used (Varian SpectrAA 10/20 with graphite furnace equipped with a deuterium lamp) was 0.15 g•L^-1 of Cd (three times the limit of detection).

Statistical Analysis

In order to compare the Cd accumulated by the aquatic mosses, a bioconcentration factor (BCF) (Eq. 1), inspired from the works of Bryan (1983) and Mouvet (1987), was computed. The percentage of elimination (%E) (Eq. 2) was calculated from the Cd lost between the value at the beginning of the release and the average value of the last two measured values . The Wilcoxon test for paired-samples was used when a comparison of the BCF and the %E was carried out on the two aquatic mosses.

Where

The simple linear regression values were calculated using the accumulation or elimination rates (slope) of each system. The analysis of variance (ANOVA) was calculated to know if the accumulation and elimination rates by the mosses were significantly different from zero. After that, the rates were compared using the analysis of covariance, which allows one to determine if the hardness of the water influences the accumulation or elimination rates of Cd by aquatic mosses. The entire statistical analysis was executed using SPSS X software (Norusis 1983).

RESULTS

Accumulation Profile

The accumulation by the mosses in contaminated system was significant (ANOVA p>0.05). The concentration values of Cd accumulated by the bryophytes as a function of time are expressed as the mean of the three different subsamples analysed independently (µg•g^-1 dry weight).

There is not much variation in the controls, the small excesses observed initially were attributed to slight initial contamination of the experimental system. At 0.8 µg•L^-1, there was a small Cd accumulation in the mosses. At 2 µg•L^-1, the Cd accumulation was obvious. At 10 µg•L^-1, the increase of the Cd concentration in the mosses was very noticeable. The Cd concentration in the aquatic mosses varied more at the end of the experiment, at least for soft and hard water where a steady state conditions was partially observed. These results were consistent with previous studies as far as the Cd absorption rate by aquatic mosses (Claveri 1995).

Elimination Profile

The Cd elimination rate found was much slower than the accumulation rates. The elimination by the mosses was significant (ANOVA p>0.05) and was similar to that reported by other studies (Mouvet 1985, Mersch et al. 1993b).

During the same time span, the aquatic mosses had accumulated more Cd than they eliminated. Since the elimination process was very slow, contamination by Cd produced a memory effect in the mosses, a necessary requirement for the environmental studies in nature which allows one to measure punctual and/or sporadic contaminations (Mouvet et al. 1993).

Bioconcentration factor (BCF) and Percentage of Elimination (%E)

The maximum BCF(s) at 10 µg•L^-1 of Cd were 158 000 for F.dalecarlica and 137000 for P.riparioides, factors which are very high for a 28 day study (Table I).

For comparison, Tessier et al. (1994) calculated a theoretical BCF for two molluscs using a two-compartment rate model. Their results showed that the gastropod (Viviparus georgianus) and the pelecypod (Elliptio complanata) subjected to a contamination of 10 µg•L^-1 of Cd for 60 days attained a theoretical steady state BCF maximum of 35 000 and 6 000 respectively. Thus, these results show that the aquatic mosses tested are more sensitive to the level of cd contamination of natural water and perform outstandingly even at the very low Cd concentrations used in this study.

The BCF decreases as the water hardness increases for all concentrations tested and for both species studied (Table I). These results demonstrate the competition between the ions responsible for water hardness (Ca⁺⁺, Mg⁺⁺) and the total accumulation of Cd by aquatic mosses. The significant differences observed between the BCF of the two bryophytes (Wilcoxon paired n=12 z=-2.353 p<0.05), with P.riparioides generally bioconcentrating more Cd than F. dalecarlica was also reported by André and Lascombe (1987).

The maximum %E were 39.4% and 47.5% for F.dalecarlica and P.riparioides respectively (Table I). There were no differences between the %E for the two aquatic mosses (Wilcoxon paired, n=12, z=-0.078, p=0.948). which eliminated Cd roughly at the same rate.

Accumulation Rates

The accumulation rates and the standard errors for each system are presented according to the Cd contamination and the hardness types in Table II. The replicate aquaria have been combined for subsequent analysis. A covariance test was applied to verify the influence of water hardness on accumulation rates (Table II). The analysis shows a significant difference in the controls which is consistent with intra-species variations. Nevertheless, the accumulation rates for the controls were markedly smaller than those exposed to Cd contaminations.

The accumulation rates for the mosses subjected to different Cd concentrations were influenced by water hardness (Table II), and there was a significant difference between the rates of the mosses exposed in very soft ( 15 mg•L^-1 as CaCO₃) and hard water ( 90 mg•L^-1 as CaCO₃). However, cd uptake observed in soft water was often similar to those measured in very soft and hard water It is important to mention that the range of the water hardness selected in this study was closely related to thoseobserved in Quebec streams. A nalogous studies performed on other organisms used a larger range of water hardness, generally 0-250 mg•L^-1 as CaCO₃ ( Markich and Jeffree 1994, Wurts and Perschbacher 1994).

Elimination Rates

The elimination rates that were not significantly different according to the covariance test are identified in Table III. Generally, there were no significant differences of the elimination rates between very soft, soft and hard water. Hence, the water hardness did not have a significant effect on the elimination rates for both mosses.

DISCUSSION

The first goal of this study was to asses the performance of aquatic mosses as a biomonitor. The mosses accumulated Cd and demonstre the exceptional integrator potential of this sensor. Even at low Cd concentrations (below 6.82 µg•L^-1 of Cd), the observed Cd accumulation by the mosses was higher in contaminated systems compared to the controls, which demonstrates the sensitivity of the aquatic mosses. Even through the accumulation was very strong for both mosses studied, P.riparioides accumulated more intensively.

The water hardness influenced the accumulation rates so that there was a marked difference between very soft water and hard water. The observed laboratory accumulation is considered to be greater than the accumulation expected to be found in nature because in vivo there is a lower Cd bioavailability due to more dissolved and suspended organic matter which inhibits its availability (Mouvet 1985). A difference of 45 mg•L^-1 as CaCO₃ between soft water and hard water was not sufficient to statistically separate the accumulation in the mosses. At smaller differences in hardness, the accumulation differences noted between the mosses resulted from the natural fluctuations in the intra-species variability or other surrounding factors, such as water current (Claveri 1995), pH, alkalinity, synergy phenomenon, among other. Note however that this difference of hardness (45 mg•L^-1 as CaCO₃) was estimated at a hardness range lower than 100 mg•L^-1 as CaCO3 and may not hold for very hard water. Further studies in the natural environment will be essential to establish the validity of this value.

This study was carried out to determine the effect of the water hardness on Cd accumulation and elimination in mosses. Water hardness integrates the two important environmental ions (Ca⁺⁺and Mg⁺⁺ ions) and several studies have shown that Ca and Mg decrease heavy metal accumulation by various organisms (Wells and Brown, 1990, Markich and Jeffree 1994), whereas other studies used only on the protector effect of the Ca⁺⁺ ions (Pickering and Puia 1969, O'Shea and Mancy 1978 ). In hard water Ca⁺⁺ and Mg⁺⁺ ions play a protective role as they compete with Cd ions. This competition should result in the availability or the preference of the plants for Ca⁺⁺ and Mg⁺⁺ ions, thus decreasing the number of available sites for Cd. Cadmium, a non- physiological element, takes advantage of its high complexation capacity with the surface sites of the cell wall and of its chemically similar properties to other physiological elements, such as Ca and Mg, to pass through the plasmic membrane.

The results of the elimination phase showed that Cd elimination was very slow. Hence the mosses showed a significant memory effect for Cd. Because of this memory effect, these mosses represent a good choice of sensor for environmental studies.

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