Ecology of phytoplankton 2006
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Ecology of phytoplankton 2006

DisciplinaFitoplâncton12 materiais70 seguidores
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\u2013 from sampling through to count-
ing. Provided adequate steps were taken to sup-
press the errors of subsampling and counting
(Javornic\u2c7ky, 1958; Lund et al., 1958; Willén, 1976),
systematic differences in the numbers present in
the original samples could be detected at scales of
a few tens of metres but, on other occasions, not
for hundreds. Irish and Clarke (1984) analysed the
estimates of speci\ufb01c algal populations of algae in
similar samples collected from within the con-
\ufb01nes of a single Blelham enclosure (area 1641 m2,
diameter, 45.7 m) at locations nominated on a
strati\ufb01ed-random grid. They found that the coef\ufb01-
cients of variation varied among different species
of plankton, from about 5%, in the case of non-
motile, neutrally buoyant algae, to up to 22%
for some larger, buoyancy-regulating Cyanobacte-
ria. In another, unrelated study, Stephenson et al.
(1984), showed that spatial variability increased
with increasing enclosure size.
A general conclusion is that sampling designs
underpinning in-situ studies of phytoplankton
population dynamics must not fail to take notice
of the horizontal dimension. However, the size
of the basin under investigation is also impor-
tant. For instance, a coef\ufb01cient of variation of
even 22% is small compared with the outcome
of growth and cell division, where a popula-
tion doubling represents a variation of 100% per
Box 2.1 Langmuir circulations
Langmuir circulations are elongated, wind-induced convection cells that form at the
surface of lakes and of the sea, having characters first formalised by Langmuir (1938).
They take the form of parallel rotations, that spiral approximately in the direction of
the wind, in the general manner sketched in Fig. 2.26. Their structure is more clearly
understood than is their mechanics but it is plain that the cells arise through the
interaction of the horizontal drag currents and the gravitational resistance of deeper
water to entrainment. Thus, they provide the additional means of spatially confined
energy dissipation at the upper end of the eddy spectrum (Leibovich, 1983). In
this way, they represent a fairly aggressive mixing process at the mesoscale but
the ordered structure of the convection cells does lead naturally to a surprising
level of microstructural differentiation. Adjacent spirals have interfaces where both
are either upwelling simultaneously or downwelling simultaneously. In the former
case, there is a divergence at the surface; in the latter there is a convergence. This
gives rise to the striking formation of surface windrows or streaks that comprise
bubbles and such buoyant particles as seaweed fragments, leaves and plant remains,
insect exuviae and animal products as they are disentrained at the convergences
of downwelling water.
The dynamics and dimensions of Langmuir circulation cells are now fairly well
known. The circumstances of their formation never arise at all at low wind speeds
(U < 3\u20134 m s\u22121: Scott et al., 1969; Assaf et al., 1971). Spacing of streaks may be as
little as 3\u20136 m apart at these lower wind speeds, when there is an rough correlation
between the downwelling depth and the width of the cell (ratio 2.0\u20132.8). In the
open water of large lakes and the sea, where there is little impediment to Langmuir
circulation, the distance between the larger streaks (50\u2013100 m) maintains this
approximate dimensional proportionality, being comparable with that of the mixed
depth (Harris and Lott, 1973; Boyce, 1974). The velocity of downwelling (w >
2.5 × 10\u22122 m s\u22121) is said to be proportional to the wind speed (\u223c0.8 × 10\u22122
U): Scott et al., 1969; Faller, 1971), but the average velocities of the upwellings and
cross-currents are typically less.
Consequences for microalgae have been considered (notably by Smayda, 1970,
and George and Edwards, 1973) and are reviewed in the main text.
generation time (Reynolds, 1986b). Moreover, a
spatial difference within a closed area of water
only 45.7 m across is unlikely to persist, as the
forcing of the gradient is hardly likely to be
stable. A change in wind intensity and direc-
tion is likely to redistribute the same population
within the same limited space.
We may follow this progression of thinking
to the wider con\ufb01nes of an entire small lake,
or to the relatively uncon\ufb01ned areas of the open
sea. Before that, however, it is opportune to draw
attention to a relatively better-known horizon-
tal sorting of phytoplankton at the scale of a
few metres and, curiously perhaps, is dependent
upon signi\ufb01cant wind forcing on the lake surface.
The mechanism concerns the Langmuir circula-
tions, which are consequent upon a strong wind
acting on a shallow surface layer, when acceler-
ated dissipation from a spatially constrained vol-
ume generates ordered structures. These are man-
ifest as stripe-like \u2018windrows\u2019 of foam bubbles
on the water surface. Even now, the formation
of Langmuir cells is imperfectly understood but
their main properties are fairly well described
(see Box 2.1).
Although the characteristic current veloc-
ities prevalent within Langmuir circulations
(>10\u201320 mm s\u22121) would be well suf\ufb01cient to
entrain phytoplankton around the spiral trajec-
tories, the cells do have identi\ufb01able relative dead
Figure 2.26 Diagrammatic
section across wind-induced surface
flow to show Langmuir circulations.
Redrawn from Reynolds (1984a).
Figure 2.27 Schematic section
through Langmuir rotations to show
the likely distributions of
non-buoyant (\u2022), positively buoyant
(\ufffd) and neutrally buoyant, fully
entrained (\u2217) organisms. Based on
an original in George (1981) and
redrawn from Reynolds (1984a).
spots, towards the centre of the spiral, at the
base of the upwelling and, especially, at the top
of the convergent downwellings, marked by the
foamlines (see Fig. 2.26). Smayda (1970) predicted
the distributions of planktic algae, categorised by
their intrinsic settling velocities, within a cross-
section adjacent to Langmuir spirals. Indepen-
dent observations by George and Edwards (1973)
and Harris and Lott (1973) on the distributions
of real (Daphnia) and arti\ufb01cial (paper) markers in
the \ufb01eld lent support for Smayda\u2019s predictions.
Although mostly well-entrained, sinking particles
(\u3c1c > \u3c1w) take longer to clear the upwellings and
accumulate selectively there, buoyant particles
(\u3c1c < \u3c1w) will similarly take longer to clear the
downwellings and those entering the foamline
will tend to be retained. A schematic, based on
\ufb01gures in Smayda (1970) and George (1981), is
included as Fig. 2.27.
Such distributions of algae are not easy to ver-
ify by traditional sampling\u2013counting methods,
because the behaviour depends not only on the
match of the necessary physical conditions \u2013 the
circulating velocity, the width and penetration of
the rotations are all wind-in\ufb02uenced \u2013 but their
persistence (Evans and Taylor, 1980). Whereas it
may take some minutes to organise and generate
the circulation, a wind of \ufb02uctuating speed and
direction will be constantly initiating new pat-
terns and superimposing them on previous ones.
This behaviour does not suppress the fact that
larger, more motile plankters remain liable to
crude sorting, on the basis of their individual
buoyant properties, into a horizontal patchiness
at the relatively small scales of a few metres to a
few tens of metres.
Patchiness in small lake basins
With or without superimposed Langmuir spirals,
the horizontal drift is likely, at least in lakes, to
be interrupted by shallows, margins or islands,
where the \ufb02ow is subject to new constraints. Sup-
posing that little of the drifting water escapes
the basin, most is returned upwind in subsur-
face countercurrents (see Imberger and Spigel,
1987). In small basins, there is a clear horizon-
tal circulation, which George and