Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 318

5

A

4

3

2

1

1.5

C

0

0 200 400 600 800 1000 1.3

mol quanta m -2 s -1 ) Light ( μ

1.0

0.8

0.10

0.5

B

0.08

0.3

0.06

0.0

0123456
μΜ ΝΗ 4 +

0.04

0.02

0.00

0 200 400 600 800 1000
Light ( μ mol quanta m -2 s -1 )

FIGURE 16.4 Uptakes of nitrate (A) and ammonium (B) as a function of light (reproduced
with permission from Dodds and Priscu, 1989) and nitrate (C) as a function of ammonium
(reproduced from Dodds et al., 1991, by permission of Oxford University Press) for epilim-
netic plankton in Flathead Lake Montana, July 1987.

sediments compared to supply from the water column has been debated
for many years (Sculthorpe, 1967). Stream macrophytes may rely more
heavily on nutrients in the water column than lake macrophytes (Pelton
et al., 1998). Emergent vegetation in wetlands probably acquires the
majority of its nutrients from the sediments.

NUTRIENT LIMITATION AND RELATIVE AVAILABILITY

Relative Availability of Nutrients
The next step is to explore nutrient requirements of cells and how
these requirements translate into nutrient limitation of growth. Nutrient
limitation is the control of growth or production by a nutrient or nutrients
(in contrast to limitation by other factors, such as light, predation, or tem-
perature). The concept of nutrient limitation is central to aquatic ecology
because it allows determination of which nutrient or nutrients control pri-
mary production or heterotrophic activity of the ecosystem.
All organisms have approximately the same nutrient requirements be-
cause they are all built of the same major types of molecules. The typical
composition of algal cells with balanced growth (Table 16.2) reveals that
various nutrients are required in different amounts. The ratio of these nu-
trients to each other is called the stoichiometry . The stoichiometry of car-
bon, nitrogen, and phosphorus at balanced growth is generally 106:16:1
(C:N:P by atoms or moles) and is referred to as the Redfield ratio (Red-
field, 1958). Variations in stoichiometry can be related to physiological