The Ecology of Phytoplankton

30 PHYTOPLANKTON
Figure 1.9 The silicon content of selected diatoms from
the freshwater ( ) ormarine () phytoplankton or other
aquatic habitat (), plotted on log/log scales against (a) cell
volume and (b) surface area. Ast refers to Asterionella, Fra to
Fragilaria and Ste to species of Stephanodiscus; Bac refers to
Bacillaria, Dit to Ditylum, Nit to Nitzschia, SketoSkeletonema
and Tha to Thalassosira. The equations of the least-squares
regression fitted to the data in (a) is log [Si] = 0.707 log v –
0.263 (r = 0.85); that for (b) is log [Si] = 1.197 log s – 1.634
(r = 0.83). Redrawn, with permission, from Reynolds (1986a).
collectively that the quantities of the components
vary within generally consistent limits and,
though they do fluctuate, 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 temperature
and an adequate supply of all nutrients,
was found to be 51–56% of the ash-free
dry weight (Ketchum and Redfield, 1949). A
slightly lower range (45–51%) was derived by
Round (1965) and Fogg (1975) from measurements
on freshwater phytoplankton. However,
the same sources of data showed extremes of
about 35%, in cells deprived of light or a supply
of inorganic carbon, and 70%, if deficiencies
of other elements impeded the opportunities for
growth.
The importance of carbon assimilation by
photoautotrophs to system dynamics has encouraged
interest in being able to make direct estimates
of organismic carbon content as a function
of biovolume. It will be obvious, from the recognition
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 subject
to a generous margin of error. For instance,
Mullin et al. (1966) derived an order-of-magnitude
range of 0.012–0.26 pg C µm −3 for aselection
of 14 marine phytoplankters that included large
and small diatoms. Reynolds’ (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 0.21–0.24 pg
C µm −3 ). Supposing carbon makes up a little
under half of the ash-free dry mass and
that dry mass averages 0.47 pg µm −3 (Fig. 1.8),
this figure is highly plausible. For diatoms,
there seemed little alternative but to calculate
carbon as a function of the silica-free dry
mass.
This approach does not satisfy the quest for
avolume-to-carbon conversion for mixed diatomdominated
assemblages, which continues to tax
ecosystem ecologists. A recent re-exploration by
Gosselain et al.(2000) confirms the wisdom of separating
diatoms from other plankters. It provides
an evaluation of several of the available formulaic
methods for estimating the carbon contents
of various diatoms.
Of the other elements comprising biomass,
nitrogen accounts for some 4–9% of the ash-free
dry mass of freshwater phytoplankters, depending
on growth conditions (Ketchum and Redfield,