used in the controls. The initial decline in cyanobacterial density
may
have been due to the high starting density of the inocula or
adjustment
to the new growth conditions. The introduction of
old
inocula is another possibility (inocula were not taken from an
exponentially
growing culture). After the initial decline in numbers,
the cyanobacteria in the controls grew strongly until day 6.
The
growth of A. variabilis in the controls (Exp. III) was followed by
a
discernible decline after day 4 (Fig. 4), which may have been due
to
the normal life cycle of the cyanobacteria (Fogg and Thake, 1987)
or
onset of resource limitation towards the end of the experiment.
The
presence of exudate resulted in an increased lag phase in A.
variabilis;
however, the effect was limited to days 4 and 6. This
may
be because we only used a single exudate application, when
repeated
additions may have been needed (Nakai et al., 1999). The
limited
effect of a single exudate application may be attributed to
the
degradation or metabolism of allelochemicals by the cyanobacteria
over time, suggesting these chemicals must be continuously
added
to
have
a prolonged algicidal activity (Gross et al., 1996;
Gross,
1999; Nakai et al., 1999). Since the plants were no longer
present,
additional
allelochemicals were not secreted once the
effect
began to lessen. This may also explain the lower impact of
the
exudates
on algal growth compared to the effect of extracts
and
live material of the two macrophytes. The exponential growth
rate
was
very low in the presence of P. crispus exudates suggesting
an
allelopathic effect (although not very strong) of this species
on
the cyanobacterium. The cyanobacterium never reached an
exponential
growthphaseinthepresenceofC.australisexudates.