Environmental Engineering Reference
In-Depth Information
8
P
bmax
6
4
α
2
0
0
200
400 600
Light (
µ
Einst m
-2
s
-1
)
800
1000
1200
FIGURE 2.3
Relationship of net primary production to light for the Hudson River phytoplankton. Parameters
are defined in the text.
(Redrawn from
Cole and Caraco 2006
using additional data from the authors.)
increase (
) and the maximum rate of photosynthesis (
P
bmax
). A light-production rela-
tionship combined with detailed data on light intensity for an ecosystem provides a
means to estimate net primary production over time and space. Thus, light is a key
variable used to model primary production within and among ecosystems. However,
total light input to an ecosystem is not the only factor determining primary production.
Other factors are important in limiting primary production, particularly temperature,
nutrients, and water.
Primary production is nutrient-limited in most ecosystems. In 1840 Justus Leibig, a
German agronomist and chemist, first proposed the idea that a single factor limits produc-
tion. In terrestrial ecosystems water and/or specific nutrients such as potassium, calcium,
nitrogen, or phosphorus are often in shortest supply. In aquatic ecosystems, the nutrients
nitrogen and phosphorus frequently are most limiting. Single-factor limitation is a useful
but simplistic view for two reasons. First, organisms can use one resource to obtain
another. For instance, a tree with sufficient light but insufficient nitrogen can grow more
roots to obtain more nitrogen, so in effect both light and nitrogen limit the growth of a
tree. Second, within an ecosystem, there may be variation across space, time, or species in
which the factor is limiting production. For example, in aquatic ecosystems diatoms are
often limited by silica while other phytoplankton are limited by other elements such as
phosphorus, nitrogen, or iron. Diatoms flourish in many ecosystems until silica is
exhausted and then are replaced by other forms. Similarly, the NPP of a grassland may be
limited by nitrogen in wetter areas and by water in drier areas, so the NPP of the entire eco-
system could be increased by either fertilization or irrigation. Thus, in a mechanistic sense,
single-resource limitation does not completely account for the processes that regulate
primary production of ecosystems. Nevertheless, the limiting resource concept is very use-
ful for describing regulation of primary production, interpreting relative availability among
nutrients (stoichiometry), and developing models about how ecosystems respond to
changes in inputs.
α
