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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). Maximum growth rates are sustained by cells containing nitrogen equivalent to some 7-8.5% of ash-free dry mass. Among freshwater algae, at least, the phosphorus content is yet more variable, although again, maximum growth rate is attained in cells containing phosphorus equivalent to around 1-1.2% of ash-free dry mass (Lund, 1965; Round, 1965). Growth is undoubtedly possible at rather lower cell concentrations than this but further cell divisions cannot be sustained when the internal phosphorus content is too small to divide among daughters and cannot be replaced by uptake. This concept of a minimum 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 0.2-0.4% of ash-free dry mass. The investigation 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 ~0.03% of ash-free dry mass. On the other hand, cell phosphorus 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.

Analogous arguments apply to the minimal quota of all the other cell components. However, it is the variability in the carbon, nitrogen and phosphorus contents that is most used by plankton ecologists to determine the physiological state of phytoplankton. Taking the ideal quotas relative to the ash-free dry mass of healthy, growing cells as being 50% carbon, 8.5% nitrogen and 1.2% phosphorus, these elements occur in the approximate mutual relation 41C: 7N: 1P (note, C :N ~6). Division by the respective atomic weights of the elements (~12,14, 31) and normalising to phosphorus yields a defining 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. Redfield had noted that the composition of particulate matter in the sea was stable and uniform in a statistical sense (Redfield, 1934) and, as he later made clear, 'reflected . . . the chemistry of the water from which materials are withdrawn and to which they are returned' (Redfield, 1958). The notion of a constant chemical condition was clearly intended to apply on a geochemical scale but the less-quoted investigations of Fleming (1940) and Corner and Davies (1971) confirm the generality of the ratio to living plankton.

It is, of course, very close to the approximate 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 -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 second by mass, both relative to P. The third line is recalculated from the second but related to sulphur. Unlike carbon, nitrogen or phosphorus, sulphur 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 recommendation. The fourth line relates the Redfield ratio to the base of carbon (= 100, for convenience), while further entries give elemental ratios for specific algae, reported in the literature but cast relative to carbon. Chlorella is a freshwater chlorophyte and Asterionella (formosa) is a freshwater diatom, having a siliceous frustule. The 'peridinians' are marine. Approximations of the order of typical elemental concentrations in lake water are included for reference. They are sufficiently coarse to pass as being applicable to the seas as well. The important point is that plankters are faced with the problem of gathering some of these essential components from extremely dilute and often vulnerable sources.

As applied to phytoplankton, the Redfield ratio is not diagnostic but an approximation to a normal ideal. However, departures are real enough and they give a strong indication that the cell is deficient in one of the three components. Extreme molecular ratios of 1300 C:P and 115 N:P in cells of the marine haptophyte, Pavlova lutheri, cultured to phosphorus exhaustion, and of 35 C : P and 5 N : P in nitrogen-deficient strains of the chlorophyte Dunaliella (from Goldman et al.,

1979), illustrate the range and sensitivity of the C :N:P relationship to nutrient limitation.

Because the normal (Redfield) ratio is indicative of the health and vigour that underpin rapid cell growth and replication, and given that departures from the normal ratio result from the exacting conditions of specific nutrient deficiencies, it is tempting to suppose that cells to which the normal ratios apply are not so constrained and must therefore be growing rapidly (Goldman,

1980). It would follow that, given the stability of the ratio in the sea, natural populations having close-to-Redfield composition are not only not nutrient-limited but may be growing at maximal rates. This may be sometimes true but there is a possibility that biomass production in oceanic phytoplankton is less constrained by N or P than was once thought (see Chapter 4). However, there are other constraints on growth rate and upon nutrient assimilation into new biomass, which may tend to uncouple growth rate from nutrient uptake rate (see Chapter 5). Tett et al. (1985) provided examples - of phytoplankton in continuous culture, of natural populations of Cyanobacteria

Table 1.6 Ideal chemical composition of phytoplankton tissue and relative abundance of major components by mass

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