The Ecology of Phytoplankton

increase the probability of survival through difficult
times and also perhaps raise the scale of the
infective inoculum when favourable conditions
return.
5.5 Growth of phytoplankton in
natural environments
The rates of cell replication and population
growth that are achieved in natural habitats have
long been regarded as being difficult to determine.
This is primarily due to the fact that
what is observable is, at best, a changing density
of population, expressed as species rate of
increase (rn, inEq.5.2) which falls short of the
rate of cell replication because of unquantified
dynamic losses of whole cells sustained simultaneously.
The net rate of change can be negative
(−rn) without necessarily signifying that true
growth has failed, merely that the magnitude of
rL, the rate of loss noted in Eq. (5.3), exceeds
that of replication, r ′ . The problem of patchiness
and advection (Section 2.7.2) provides the
further complication of compounded sampling
errors, in which even the observed rate of population
change (±rn) mayproveaninadequate base.
From the other direction, the true replication
rate cannot be estimated from measurable photosynthetic
or nutrient-uptake capacities, unless it
can be assumed with confidence that the actual
rate of growth is constrained by the capacity factor
concerned.
There are ways around these problems and
there are now several quite reliable, if somewhat
cumbersome, methods for estimating growth
rates in situ. Some of these approaches are highlighted
below, through the development of an
overview of dynamic trait selection in natural
habitats.
5.5.1 Estimating growth from observations
of natural populations
On the same basis that replication rates cannot
be sustained at a faster rate than cell division can
be resourced, it is clear that the observable rates
of population increase cannot exceed the rates of
recruitment through cell replication. The corol-
GROWTH OF PHYTOPLANKTON IN NATURAL ENVIRONMENTS 217
lary of this is that attestably rapid phases of population
increase, independent of recruitment by
importation from horizontally adjacent patches
or from germinating resting stages, are indicative
of yet higher simultaneous rates of cell replication.
Growth rates from episodic events
Generically, these accumulative phases fall into
two categories. One of these is the annually recurrent
and broadly reproducible event, such as the
spring increase of phytoplankton in temperate
waters, in response to strong seasonally varying
conditions of insolation (see Section 5.5.2). The
second is the stochastic event, when, perhaps, a
sharp change in the weather, resulting in the
fortuitous stagnation of a eutrophic water column,
or the relaxation from coastal upwelling,
or the deepening of a nutrient-depleted mixed
layer with the entrainment of nutrient-rich metalimnetic
water, or some abrupt consumer failure
through herbivore mortality, leads to the
realisation of potential respondent growth. In
this second category, the phases of increase
may be brief and sensing them, accurately and
with reasonable precision, requires the closeinterval
sampling of well-delimited populations.
The study of in-situ increase rates of phytoplankton
inBodensee (Lake of Constance), assembled
by Sommer (1981), was one that satisfied these
conditions. The research based on the large (1630
m 2 ), limnetic enclosures in Blelham Tarn, English
Lake District (variously also referred to as ‘Blelham
Tubes’, ‘Lund Tubes’ (Fig. 5.11), being isolated
water columns of ∼12–13.5 m in depth and
including the bottom sediment from the lake;
for more details, see Lund and Reynolds (1982),
carried out in the period 1970–84, has similarly
provided many insights into phytoplankton population
dynamics. Examples of specific increase
rates noted from either location are included in
Table 5.4.
The evident interspecific differences are
partly attributable to the time period of observation,
and the seasonal changes in water temperature
and in the insolation attributable to
seasonally shifting day length and vertical mixing.
In some instances, these environmental variations
are reflected in intraspecific variability in