Ecology of phytoplankton 2006
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Ecology of phytoplankton 2006


DisciplinaFitoplâncton12 materiais69 seguidores
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limits and,
though they do \ufb02uctuate, the ratios with other
constituents do not vary by more than can be
reasonably explained in these terms.
For instance, carbon generally makes up
about half the dry organic mass of organic cells.
The normal content of phytoplankton strains
cultured under ideal laboratory conditions of
constant saturating illumination, constant tem-
perature and an adequate supply of all nutri-
ents, was found to be 51\u201356% of the ash-free
dry weight (Ketchum and Red\ufb01eld, 1949). A
slightly lower range (45\u201351%) was derived by
Round (1965) and Fogg (1975) from measure-
ments on freshwater phytoplankton. However,
the same sources of data showed extremes of
about 35%, in cells deprived of light or a sup-
ply of inorganic carbon, and 70%, if de\ufb01ciencies
of other elements impeded the opportunities for
growth.
The importance of carbon assimilation by
photoautotrophs to system dynamics has encour-
aged interest in being able to make direct esti-
mates of organismic carbon content as a function
of biovolume. It will be obvious, from the recog-
nition of the variability in the absolute contents
of carbon, its proportion of wet or dry biomass,
and the relative fractions of ash and vacuolar
space, that any general relationship must be sub-
ject to a generous margin of error. For instance,
Mullin et al. (1966) derived an order-of-magnitude
range of 0.012\u20130.26 pg C µm\u22123 for a selection
of 14 marine phytoplankters that included large
and small diatoms. Reynolds\u2019 (1984a) analysis of
data, pertaining exclusively to freshwater forms,
adopted simultaneous approaches to diatoms
and non-diatoms. The relatively low ash content
and absence of large vacuoles among the latter
permitted a much narrower relationship between
carbon and biovolume (averaging 0.21\u20130.24 pg
C µm\u22123). Supposing carbon makes up a lit-
tle under half of the ash-free dry mass and
that dry mass averages 0.47 pg µm\u22123 (Fig. 1.8),
this \ufb01gure is highly plausible. For diatoms,
there seemed little alternative but to calcu-
late carbon as a function of the silica-free dry
mass.
This approach does not satisfy the quest for
a volume-to-carbon conversion for mixed diatom-
dominated assemblages, which continues to tax
ecosystem ecologists. A recent re-exploration by
Gosselain et al. (2000) con\ufb01rms the wisdom of sep-
arating diatoms from other plankters. It provides
an evaluation of several of the available formu-
laic methods for estimating the carbon contents
of various diatoms.
Of the other elements comprising biomass,
nitrogen accounts for some 4\u20139% of the ash-free
dry mass of freshwater phytoplankters, depend-
ing on growth conditions (Ketchum and Red\ufb01eld,
THE CONSTRUCTION AND COMPOSITION OF FRESHWATER PHYTOPLANKTON 31
Table 1.5 The silicon content of some planktic diatoms relative to cell volume and surface area
Species Si
(pg cell\u22121)
v
(µm3)
s
(µm2)
Si
(pg µm\u22123)
Si
(pg µm\u22122)
Referencesa
Asterionella
formosa
61.0 630 860 0.097 0.071 Reynolds and Wiseman
(1982)
Fragilaria
crotonensis
117.8 780 1080 0.151 0.109 Reynolds and Wiseman
(1982)
Stephanodiscus
rotula
1075 8600 2574 0.125 0.418 Gibson et al. (1971)
1942 15980 4390 0.122 0.442 Thitherto unpublished records
cited in Reynolds (1984a),
calculated indirectly from
SiO2 uptake
978 8300 2580 0.118 0.379 Thitherto unpublished records
cited in Reynolds (1984a),
calculated indirectly from
SiO2 uptake
751 5930 1980 0.127 0.379 Thitherto unpublished records
cited in Reynolds (1984a),
calculated indirectly from
SiO2 uptake
Stephanodiscus
hantzschii
16.4 600 404 0.027 0.041 Thitherto unpublished records
cited in Reynolds (1984a),
calculated indirectly from
SiO2 uptake
a All citations converted from the original published data quoted content in terms of SiO2, by
multiplying by 0.4693.
1949; Lund, 1965, 1970; Round, 1965). Maxi-
mum growth rates are sustained by cells con-
taining nitrogen equivalent to some 7\u20138.5% of
ash-free dry mass. Among freshwater algae, at
least, the phosphorus content is yet more vari-
able, although again, maximum growth rate is
attained in cells containing phosphorus equiva-
lent to around 1\u20131.2% of ash-free dry mass (Lund,
1965; Round, 1965). Growth is undoubtedly pos-
sible at rather lower cell concentrations than
this but further cell divisions cannot be sus-
tained when the internal phosphorus content is
too small to divide among daughters and can-
not be replaced by uptake. This concept of a mini-
mum cell quota (Droop, 1973) has been much used
in the understanding the dynamics of nutrient
limitation and algal growth: for phytoplankton,
the threshold minimum seems to fall within the
range 0.2\u20130.4% of ash-free dry mass. The inves-
tigation of Mackereth (1953) of the phosphorus
contents of the diatom Asterionella formosa, which
reported a range of 0.06 to 1.42 pg P per cell, is
much cited to illustrate how low the cell quota
may fall. The lower value, which is, incidentally,
corroborated by data in earlier works (Rodhe,
1948; Lund, 1950), corresponds to \u223c0.03% of ash-
free dry mass. On the other hand, cell phos-
phorus quotas may be considerably higher than
the minimum (certainly up to 3% of ash-free
dry mass is possible: Reynolds, 1992a), especially
when uptake rates exceed those of deployment
and cells retain more than their immediate needs
(so-called luxury uptake). Uptake and retention of
phosphorus when carbon or nitrogen supplies
are limiting uptake (cell C or N quotas low) may
also result in high quotas of cell phosphorus.
32 PHYTOPLANKTON
Analogous arguments apply to the minimal
quota of all the other cell components. How-
ever, it is the variability in the carbon, nitrogen
and phosphorus contents that is most used by
plankton ecologists to determine the physiologi-
cal state of phytoplankton. Taking the ideal quo-
tas relative to the ash-free dry mass of healthy,
growing cells as being 50% carbon, 8.5% nitro-
gen and 1.2% phosphorus, these elements occur
in the approximate mutual relation 41C : 7N : 1P
(note, C : N \u223c6). Division by the respective atomic
weights of the elements (\u223c12, 14, 31) and normal-
ising to phosphorus yields a de\ufb01ning molecular
ratio for healthy biomass, 106C : 16 N : 1P.
This ratio set is well known and is generally
referred to as the Redfield ratio. As a young marine
scientist, A. C. Red\ufb01eld had noted that the com-
position of particulate matter in the sea was sta-
ble and uniform in a statistical sense (Red\ufb01eld,
1934) and, as he later made clear, \u2018re\ufb02ected . . .
the chemistry of the water from which materials
are withdrawn and to which they are returned\u2019
(Red\ufb01eld, 1958). The notion of a constant chem-
ical condition was clearly intended to apply on
a geochemical scale but the less-quoted investi-
gations of Fleming (1940) and Corner and Davies
(1971) con\ufb01rm the generality of the ratio to living
plankton.
It is, of course, very close to the approxi-
mate ratio in which the same elements occur
in the protoplasm of growing bacteria, higher
plants and animals (Margalef, 1997). Stumm and
Morgan (1981; see also 1996) extended the ideal
stoichiometric representation of protoplasmic
composition to the other major components
(those comprising >1% of ash-free dry mass \u2013
hydrogen, oxygen and sulphur) or some of those
that frequently limit phytoplankton growth in
nature (silicon, iron). The top row of Table 1.6
shows the information by atoms and the sec-
ond by mass, both relative to P. The third line is
recalculated from the second but related to sul-
phur. Unlike carbon, nitrogen or phosphorus, sul-
phur is usually superabundant relative to phyto-
plankton requirements and plankters have no
special sulphur-storage facility. Following Cuhel
and Lean (1987a, b), sulphur is a far more stable
base reference and deserving of wider use than it
receives. Unfortunately, few studies have adopted
the