Figure 1 shows the response of different algal groups during exposure of an algal assemblage to Cu, and also during recovery from Cu stress. The chlorophytes were the least sensitive to Cu, whereas cyanophytes were the most sensitive and bacillariophytes occupied an intermediate position. The total biovolume of chlorophytes at 0.5 and 1 µM Cu was almost similar to that of the control up to the day 15, but it declined subsequently. However, the total biovolume was reduced right from the beginning of the experiment till the end of exposure to 2.5 µM Cu (day 25). The total biovolume of cyanophytes declined sharply after Cu (1.0 and 2.5 µM) treatment, and these organisms disappeared by the end of the exposure period except for the assemblage exposed to 0.5 µM Cu. Nevertheless, cyanophytes were poorly represented at 0.5 µM Cu. Reduction in total biovolume of bacillariophytes was noticed in the presence of Cu but the reduction was lesser than that for cyanophytes. When the assemblage was transferred to the basal medium, bacillariophytes recovered faster than the others. Chlorophytes showed a slow recovery after transferring Cu-treated assemblage to the metal unenriched medium. The members of Cyanophyta could survive only at 0.5 µM Cu treatment, and showed a little recovery after withdrawing Cu stress.
Figure 2 shows the changes in the total biovolume of different algal groups exposed to different concentrations of Zn, and also upon transfer to the basal medium. Green algae appeared most tolerant to Zn followed by diatoms and blue greens. In comparison to the control, the total biovolume of chlorophytes increased during exposure to 1 µM Zn as well as during recovery. However, further increase in concentration of Zn led to inhibition. The total biovolume of cyanophytes sharply reduced after 5 and 10 µM Zn treatments, but did not differ much from the control at 1 µM Zn. The total biovolume of diatoms declined in presence of Zn in a concentration-dependent manner. The total biovolume of diatoms recovered slowly depending on concentration of Zn used for the treatment.
Table 1 contains the list of algal species encountered in the assemblage. Chlorophyta was most well represented followed in decreasing order by Cyanophyta and Bacillariophyta. Euglenophyta was represented by only Phacus sp. The table also shows that three species disappeared during the course of the experiment even in the control. Metal treatment caused disappearance of a number of species in the assemblage. Higher the concentration of a metal, greater was the disappearance of species. Maximum disappearance of species was encountered at 2.5 µM of Cu and 10 µM of Zn. The members of Cyanophyta disappeared at higher concentrations of both the metals, whereas none of the diatom species could disappear even at the highest tested concentrations of test metals.
Figure 3 shows the response of four species of Scenedesmus (Chlorophyta) to elevated levels of Cu. The figure makes it amply clear that S. opolinensis was tolerant to Cu as the size of its population always remained higher in Cu-treated cultures than in the control. The higher population of this species was evident during the exposure as well as the recovery period. S. quadricauda and S. dimorphus were only slightly inhibited by Cu in a concentration-dependent manner. However, S. bijugatus displayed a marked sensitivity to Cu. Its population declined during exposure to Cu in a concentration-dependent manner. Another noteworthy feature of this species has been its ability to quickly recover from metal stress after its transfer to the basal medium not enriched with Cu.
Figure 4 shows the total biovolume of some other green algal species in the presence of Cu and after transfer to the basal medium. Ankistrodesmus falcatus, Zygnema sp., and Chlorella vulgaris were very much sensitive to Cu, with their total biovolume declining in a concentration-dependent manner. At 2.5 µM Cu, C. vulgaris and Zygnema sp. disappeared; 1.0 µM Cu also caused the disappearance of Zygnema sp. After transfer to the fresh basal medium, A. falcatus, C. vulgaris, and Zygnema sp. started to recover at different rates depending on the concentration of Cu given during the exposure period. On the other hand, Cosmarium bioculatum was highly tolerant to Cu, showing increase in its total biovolume with increasing Cu concentration. After withdrawing Cu stress, the total biovolume of C. bioculatum remained unchanged or begun to decline very slowly but it still remained considerably higher than the control.
The members of Cyanophyta were very much sensitive to Cu, and they disappeared during the course of Cu exposure. Only Aphanocapsa banarensis could survive in 0.5 µM Cu, and it started to recover after the withdrawing of Cu stress (Fig. 5).
Figure 6 shows the total biovolume of some major bacillariophycean taxa during exposure to Cu and also during recovery. All the species showed inhibitory effect of Cu on their total biovolume in a concentration-dependent fashion. All diatom species were also able to recover depending on the concentration of Cu that was used for their exposure. Higher the concentration of Cu during exposure, slower was the rate of recovery of the total biovolume of all the diatom species.
Figure 7 shows total biovolume of major green algal species during exposure to Zn and also during recovery. The total biovolume of S. quadricauda was always greater than the control at 1 µM Zn. However, it remained almost similar to that of the control at 5 µM Zn. S. bijugatus was slightly more sensitive to Cu than S. quadricauda; its population was greatly diminished at 10 µM Zn. S. dimorphus showed an interesting response after exposure to 1 and 10 µM Zn; its total biovolume remained considerably higher than that of the control. But, its total biovolume remained almost similar to that of the control at 5 µM Zn. S. opolinensis was tolerant to Zn also as a regular increase in total biovolume of S. opolinensis occurred with the passage of time. The recovery of the total biovolume of S. quadricauda and S. bijugatus from 10 and 5 µM Zn, respectively, occurred rather slowly, however, it was quicker in other cases.
A 10 µM Zn caused the disappearance of Chlorella vulgaris and Zygnema sp. (Fig. 8). Zygnema sp. disappeared at 5 µM Zn also. Ankistrodesmus falcatus was sensitive to Zn and its total biovolume decreased in a concentration-dependent manner. On the other hand, Cosmarium bioculatum showed a remarkable tolerance to all the tested concentrations of Zn. The figure also shows recovery of A. falcatus, C. vulgaris and Zygnema that depended on the concentration of Zn to which they were exposed (Fig. 8).
Figure 9 shows the total biovolume of major cyanophytes during exposure to Zn and also during recovery. Merismopedia sp. was unable to sustain at any tested concentration of Zn, and Microcystis aeruginosa and Chroococcus limneticus disappeared at 5 and 10 µM Zn. Aphanocapsa banarensis remained alive at 1 and 5 µM Zn. The biovolume of each species was considerably reduced by metal treatment. After the transfer of the assemblage to the fresh medium all the species did not show recovery.
Figure 10 shows the response of bacillariphycean members during exposure to Zn and also during recovery. The total biovolume of Navicula radiosa, Synedra sp., Nitzschia palea and Cyclotella sp. declined depending on the concentration of Zn. N. palea disappeared completely at 10 µM Zn. The biovolume of Cyclotella sp. exposed to 1 µM Zn was not different from that of the control. Biovolume of all the species of Bacillariophyta, which had earlier declined under Zn stress, started to recover when the algal assemblage was transferred to the fresh basal medium.