The Ecology of Phytoplankton

216 GROWTH AND REPLICATION OF PHYTOPLANKTON
of cysts in nature (coastal waters, eutrophic
lakes) possibly occurs in response to cues that
anticipate ‘adverse’ conditions rather than the
actual onset of those adversities. The protoplasts
of newly formed cysts usually contain conspicuous
reserves of lipid and carbohydrate, accumulated
during stationary growth (Chapman et al.,
1980). The number of cysts produced by freshwater
Ceratium hirundinella in autumn has been
estimated from the sedimentary flux to account
for ≤35% of the maximum standing crop of vegetative
cells (Reynolds et al., 1983b). The success in
recruiting vegetative cells from excysting propagules
in the following spring is, in part, proportional
to the abundance of spores retained from
the previous year (Reynolds, 1978d; Heaneyet al.,
1981).
The excystment of vegetative cells from cysts
was first described by Huber and Nipkow (1922).
Much detail has been added from such landmark
micrographic investigations as those of
Wall and Dale (1968) andChapman et al. (1981). A
naked flagellate cell, or gymnoceratium, emerges
through an exit slit and soon acquires the distinctive
thecal plates of the vegetative cell. Heaney et
al. (1981) noted a sharp, late-winter recruitment
of new, vegetative cells of Ceratium to the plankton
of Esthwaite Water, UK, after the water temperature
exceeded 5 ◦ C, and coincident with an
abrupt increase in the proportion of the empty
cysts recoverable from the bottom sediments of
the lake.
Among the Volvocales, sexually produced
zygotes of (e.g.) Eudorina (Reynolds et al., 1982a)
and Volvox (Reynolds, 1983b) have the robust
appearance of resting cysts and, indeed, serve
as perennating propagules between population
maxima. Deteriorating environmental conditions
may trigger the onset of gametogenesis but formation
of the eventual resting stages cannot be
claimed certainly to have been consequential on
resource starvation. Among the Chrysophyceae,
there has evolved an opportunistic perennation
strategy, involving zygotic and asexual cysts that
are produced early in the growth cycle, when conditions
are supposedly good (Sandgren, 1988b).
This pattern of encystment apparently ensures
the production of resting stages during what
often turn out to be short phases of environmen-
tal adequacy but which are tenanted briefly by
vegetative populations.
In contrast, nostocalean Cyanobacteria produce
their asexual akinetes in rapid response
to the onset of physiological stress. Akinetes are
the well-known ‘resting stages’ of such genera as
Anabaena, Aphanizomenon and Gloeotrichia (Roelofs
and Oglesby, 1970; Wildman et al., 1975; Rother
and Fay, 1977; Cmiech et al., 1984). These, too,
have typically thickened external walls, within
which the protoplast remains viable for many
years. Livingstone and Jaworski (1980)germinated
akinetes of Anabaena from sediments confidently
dated to have been laid down 64 years previously.
On the other hand, rapid akinete production has
been stimulated in the laboratory by the sort of
carbon : nitrogen imbalance that occurs as a consequence
of surface blooming, and from which
conditions an effective means of escape is offered
(Rother and Fay, 1979). Moreover, substantial germination
can take place shortly (days rather than
months or years) after akinete formation, provided
the external conditions (temperature, light
and, possibly, nutrients) are suitable (Rother and
Fay, 1977). Reynolds (1972) observed that Anabaena
akinetes were regularly resuspended by wind
action in a shallow lake but failed to germinate
before a temperature or insolation threshold
had been surpassed. In other years, vegetative
filaments surviving the winter were sufficient to
explain the growth in the following season. These
thresholds could be important to the distributions
of individual species. The current spread
of Cylindrospermopsis raciborskii from the tropics
to continental lakes in the warm temperate belt
may be delimited by a germination threshold
temperature of 22 ◦ C(Padisák, 1997). The akinetes
of Gloeotrichia echinulata are able to take up phosphate
through their walls and colonies germinating
the following year can sustain substantial
growth even when limnetic supplies are small
(Istvánovics et al., 1993).
As suggested above, regenerative strategies are
not uniform among the phytoplankton, neither
is the production of spores and resting stages
exclusively brought on by ‘adverse conditions’.
However, the existence of resting propagules of
a given species are likely tolerant of more severe
conditions than vegetative cells and they do