| dc.description.abstract | The physical and, chemical factors which affect
sporulation, and the growth and survival of vegetative
populations of blue-green algae were examined in two
eutrophic kataglacial lakes of the Ellesmere (North Salop)
group of meres. The importance of individual factors were
examined under controlled conditions using natural
populations or uni-algal isolates from the meres.
The induction of akinete formation, which, in all the
cases examined, proceeds once the alga forms a surface bloom,
is thought to be due to the extreme conditions, particularly
of solar radiation, which prevail at the surface during the
summer. Winter blooms did not sporulate. Nutrient
deficiency, particularly of orthophosphate, is not
considered to be a critical factor. This is supported by
the finding that blooming algal material is not depleted of
phosphorus, that alkaline phosphate activity was low, and
that nutrient deficiency under controlled conditions did
not induce sporulation of the species predominant in the
meres.
Nitrogenase activity (acetylene reduction) and the
photosynthetic rate
(oxygen evolution) were observed to
fall in the period subsequent to bloom formation. The
reduction of measured net photosynthesis may be correlated
with the increase of bacterial numbers. The activity of
light-damaged material was lower than that of unaffected
populations. It appears that there must be a low cellular
carbon to nitrogen ratio before akinete differentiation
can begin, indicating that a reduction of photosynthetic
activity relative to that of nitrogon assimilation is
required.
Akinetes probably do not form a significant overwintering
mechanism in the lakes investigated. Numbers of
akinetes found during the winter were low. The bloom
populations probably develop from growth of the overwintering
planktonic vegetative populations during the
spring and early summer. Cornparisons of planktonic,
sedimenting and benthic algal material indicates that germination of akinetes shortly after their maturation
may give rise to the greater part of the overwintering
vegetative populations.
Physical rather than chemical facters appear to
control the population sizes and life cycles in the
environments studied. | en_US |
