4.2. Comparisons of the allelopathic effects of C. australis and P.
crispus
The effect of live material and exudates of C. australis on A. variabilis
was greater than the effect of P. crispus. The allelopathic action
of
Potamogeton species on algal growth is known to be lower than
that
of Chara and some other macrophytes (Hilt and Gross, 2008).
The
nature of different allelochemicals plays an important part in
determining
their effects on algal growth. The lower inhibition of
algal
growth by P. crispus compared to C. australis may be due to
the
presence of labile compounds and their potential degradation
by
bacteria during the exudation period (He et al., 2008). In contrast,
during Exp. I, which took place under sterile conditions, the
allelopathic
action of the extracts of two macrophytes did not differ.
Moreover,
allelochemicals may be more concentrated in proximity
to
where they are first released (from the leaf boundary layer of the
macrophyte)
(Erhard and Gross, 2006). The effect diminishes away
from
the macrophyte because the chemicals become more dilutedin the waterbody. This may explain the stronger effect of P. crispus
in
live material tests compared with the exudate experiment and
suggests
that the inhibitory effect of P. crispus on algal growth may
be
very strong in the field when the plant covers large areas and
allelochemicals
are continually released.