Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 319

TABLE 16.2 Elemental Composition of Algae and Plants Compared to
Availability in Freshwater (World Rivers) a

Average

Element Plants and algae World rivers demand/supply

H 13,400,000 3,520,000,000 1

O 5,880,000 1,780,000,000 1

C 2,750,000 31,900 86

N 689,000 525 1312

Si 163,000 7,390 22

K 34,900 1,880 18

P 24,400 10 2440

Ca 23,300 12,000 2

Na 17,900 8,340 2

Mg 17,100 5,260 3

S 13,700 3,980 3

Fe 7,240 401 18

Zn 314 5 63

B 264 296 1

Cu 102 5 20

Mn 82 9 9

Mo 1 1 1

a All composition data are in moles or atoms relative to molybdenum. Data on algae from
Healy and Stewart (1973) and on plants and algae and rivers from Vallentyne (1974).
Average demand/supply is algae and plants divided by rivers. Diatoms have the listed
silicon requirement; most other plants and algae do not require this much.

state or taxonomy. For example, diatoms require much more silicon than
other organisms. The plasticity of nutrient contents varies among individ-
ual organisms. Many primary producers can build up significant amounts
of starch, lipids, or cellulose to store carbon and alter their C:N:P accord-
ingly. Thus, stoichiometry can be very useful for determining and under-
standing nutrient limitation.
Some nutrients can be acquired relatively easily by most cells. Oxygen
and hydrogen are easily available to photosynthetic organisms able to split
water. Carbon is generally available to autotrophs in the form of CO 2 . Ul-
timately, most of the other nutrients must come from weathering of the
earth's crust. The relative abundance of materials dissolved in rivers pro-
vides an approximate guide to the comparative supplies of nutrients avail-
able to aquatic organisms (Table 16.2). In general, carbon, nitrogen, and
phosphorus are difficult to obtain, given relative concentrations in the
world's rivers. However, CO 2 readily enters water from the earth's atmos-
phere and thus can be replenished quickly in most surface waters. This
means that nitrogen and phosphorus are most likely to limit growth of al-
gae and aquatic macrophytes.
Factors such as chemical characteristics, geology, and human land use
alter the availability of nutrients in aquatic systems. For instance, phos-
phate tends to bind to clays, so it is transported slowly into aquatic sys-
tems. Nitrogen can be lost from anoxic systems by denitrification but can
be gained by nitrogen fixation, so the relative supply of nitrogen in indi-
vidual systems may be difficult to predict. Geology and land use in partic-