The presence of Chara exudates showed significant effects on the exponential growth rate of P‐sufficient C. minutissima in Exp. II, but not in Exp. III (the experi‐ ment with the younger Chara sprouts, Fig. 3.2). The lag phase of P‐sufficient C. minutissima in Exp. II was significantly shorter than the lag phase in Exp. III (p < 0.05; Fig. 3.3). Like for S. obliquus in Exp. I, the diffe‐ rence in length of the lag phase between the C. minutissima cultures in Exp. II and Exp. III can probably be explained by the initial inoculum size (Droop 1966, Eberly 1967, Table 3.3). The duration of the lag phase decreases with increasing inoculum size. Analysis of the estimated values for the duration of the lag phase (T) of P‐suf‐ ficient C. minutissima also revealed significant differences between the treatments (control and Chara). The extent of the allelopathic effect (elongated lag phase, T, in the presence of Chara water) was stronger in Exp. III than in Exp. II, as indicated by the significant interaction in the ANOVA (p < 0.05; Fig. 3.3). Multiple comparisons by means of Tukey testing (Tstatistics = 2.68), indicated a significant difference be‐
tween the control and Chara treatments in Exp. III (T = 7.09) but not for Exp. II (T = 0.62).
3.5
Discussion
As in the experiments of Blindow & Hootsmans (1991), not all the charophyte/ phytoplankton combinations showed convincing allelopathic inhibition. In the present study, a significant growth inhibition was found for Chlorella minutissima and Selenastrum capricornutum (Exp. II), while no significant effects were indicated for Scenedesmus obliquus (Exp. I). The exponential growth rate of C. minutissima was lowered by 3% (Exp. II), whereas it decreased with 7% in S. capricornutum (Exp. II). For the green alga C. minutissima, the duration of the lag phase was extended in the presence of water from a culture of young Chara sprouts (Exp. III). Comparing the present results with the hypothetical curves (Fig. 3.1), we conclude that each of the three suggested effects (no effect, extended lag phase, and decrea‐ sed growth rate) seem to be possible responses of phytoplankton to allelopathic substances excreted by the mixed culture of Chara globularis and C. contraria. More‐ over, the results agree with our hypothesis and indicate allelopathic interactions between charophytes and several planktonic algae. The exponential growth rate values in the controls estimated for the three different green algae were in accordance with those found in other investigations (Scenedes‐ mus: Reynolds 1984, Hälling‐Sørensen et al. 1996, Chlorella: Shi et al. 1999, 2000, and Selenastrum: Mayer et al. 1997, Grade et al. 2000). Whereas our study showed no effect of a mixed culture of two charophytes (Chara globularis var. globularis and C. contraria var. contraria) on the growth of S. obliquus, Blindow & Hootsmans (1991) found 10% inhibition in a similar combination: C. glo‐ bularis and Scenedesmus communis (Table 3.4) which was comparable with the 7% growth inhibition on S. capricornutum in our study. Other charophytes in the same study, however, did not show inhibitory effects. Howard‐Williams (1978) reported low epiphytic algal densities on C. globularis and Blindow (1987) observed high densities on C. tomentosa. Low epiphyton densities on C. globularis are in line with the hypothesized allelopathic activity of this aquatic macrophyte. The absence of an allelopathic effect on S. obliquus (Exp. I) might be explained by the composition of the charophyte culture. The mixing of C. globularis and C. con‐ traria may have diluted the allelopathic effect of C. globularis suggested by Blindow & Hootsmans (1991).
DIFFERENTIAL SENSITIVITY
89
Another possibility could be the differential sensitivity among Scenedesmus spe‐ cies. Blindow & Hootsmans (1991) used S. communis instead of S. obliquus as a test organism. Several other studies with different macrophytes (Table 3.4) demonstra‐ ted that Scenedesmus was not inhibited. This suggests that not all Scenedesmus spe‐ cies are sensitive to allelopathic substances from macrophytes. For the other two algae species used in the present study, growth inhibition as well as no allelopa‐ thic effects were found in other investigations (Table 3.4).
Additionally, there could be a variation over time in the allelopathic activity of macrophytes. In the study of Blindow & Hootsmans (1991), C. globularis did not al‐ ways show an effect on S. communis.
Table 3.4 Results from studies on the allelopathic effect of macrophytes on some green algae, similar to the ones used in the present study (+ = growth inhibition; - = no allelopathic inhibition).
Macrophyte Phytoplankton Effect Author(s)
Nitzschia frustulum* Chlorella vulgaris + Rice (1984) Emergent and submerged
macrophytes including Myriophyllum spicatum
Selenastrum capricornutum + Weaks (1988)
Chara globularis Scenedesmus communis + Blindow & Hootsmans 1991 Brasenia schreberi
(floating leaved macrophyte)
Chlorella pyrenoidosa
+
Elakovich & Wooten 1987 Ceratophyllum demersum Scenedesmus sp.
Chlorella sp. - Kogan et al. 1972 Chara tomentosa, C. hispida, & C. delicatula Ankistrodesmus bibraianus, Scenedesmus communis
- Blindow & Hootsmans 1991 Chara globularis Ankistrodesmus bibraianus - Blindow & Hootsmans 1991 C. demersum &
M. spicatum Scenedesmus sp., Chlorella sp. - Jasser 1995
The difference in allelopathic effects among strains of C. globularis could be a fourth possibility for the fact that our results did not show an allelopathic effect on S. obli‐ quus. Proctor (1971, 1975) studied the extent of reproductive isolation of C. globu‐ laris and concluded that there is a widespread genetic isolation between the diffe‐ rent populations: C. globularis is highly variable and consists of one or more biolo‐ gical species. The allelopathic activity among these species may, therefore, vary as well. The results from the present study also show that the duration of the lag phase of S. obliquus and P‐sufficient C. minutissima appeared to depend on the number of cells in the inoculum at the start of the experiment. The length of the lag phase varies inversely with inoculum size (Droop 1966, Eberly 1967).
Next, P‐limitation of C. minutissima did not increase the extent of the allelopathic effect on this species. This agrees with results from Fitzgerald (1969) who stated that allelopathic effects occurred when macrophytes and phytoplankton were grown on N‐limited medium, while they were absent under normal conditions or under P‐limitation. Moreover, comparison of the results from Exp. II and III suggests that there is an effect of the charophyte age upon the extent of the allelopathic effect of Chara. This correspondes with the findings of Hootsmans & Blindow (1991) who observed that the allelopathic activity of P. pectinatus diminished over the course of the season. In our study, the difference between the lag phase of C. minutissima in the control and Chara treatments (Exp. II and Exp. III) was significantly larger in the presence of young charophyte sprouts (Exp. III), while the effect of Chara water on the growth rate of phytoplankton was larger in the experiment with older charophytes (Exp. II). Unlike Exp. II, the effect of Chara water on the exponential growth rate of C. minutissima in Exp. III was not significant. Nevertheless, there seemed to be a trend towards a lower growth rate in the presence of Chara water. Young charophyte sprouts seem to enhance the duration of the lag phase, whereas older charophytes decrease the exponential growth rate of the algae. However, to elucidate the effect of charophyte age, more experiments should be carried out simultaneously with old and young sprouts.
DIFFERENTIAL SENSITIVITY
91
Our experimental evidence confirms the possible existence of allelopathic inter‐ actions between charophytes and planktonic green algae. Charophytes had a dif‐ ferential impact on the three green algae. This suggests that charophytes may in‐ fluence the phytoplankton composition by selective inhibition of some phytoplank‐ ton species. Full assessment of the strength of the interactions as a stabilizing feed‐ back requires field experiments and more laboratory experiments with other phy‐ toplankton species/groups and more charophyte species. As indicated by the pre‐ sent results, new experiments should also focus on the use of charophytes in dif‐ ferent growth stages. The presently established list of active Chara species and sensitive phytoplankton taxa is short, which calls for caution in statements on in situ relevance (Crawford 1977, 1979, Forsberg et al. 1990).