Environmental Engineering Reference
In-Depth Information
algebra, we find that a consumer will be
nutrient-limited if
respiration as consisting of two parts, a
fixed maintenance respiration that occurs
even when the consumer is not growing,
and a variable respiration that depends on
the production of the consumer. We can
then write net growth efficiency as
c
ε
gc
. ð
Q
i
=
Q
g
Þ
ð
3
:
B7
Þ
where
c
is the ratio of the assimilation effi-
ciencies for carbon and for the nutrient
(
ε
ac
/
ε
gc
is the net growth efficiency for
carbon, and
Q
i
is the nutrient : carbon ratio
of the diet. The consumer will be nutrient-
limited if the left side of the equation is large
and the right side is small. Thus, nutrient
limitation will occur if the consumer is more
efficient at extracting carbon than nutrients
from its food, if it has a high net growth effi-
ciency (for carbon), and if the consumer's
body is much more nutrient-rich than its
food. The opposite conditions will lead to
energy limitation. This means that (1) home-
otherms, with their very low growth effi-
ciencies (
Table 3.1
), are more likely to be
energy-limited than poikilotherms; (2) fast-
growing consumers are more likely to be
phosphorus-limited than slow-growing con-
sumers, because fast-growing consumers
have a high phosphorus : carbon content
(because fast growth requires a lot of
RNA, which is rich in phosphorus;
Sterner
and Elser 2002
); and (3) consumers eating
diets with high carbon : nutrient contents
(e.g., terrestrial plants) are likely to be
nutrient-limited (
Sterner and Elser 2002
). It is
also worth noting that the energetic demands
of respiration mean that the carbon : nutrient
content of a consumer's ideal diet should
always be much higher than the carbon:nutri-
ent content of the consumer itself, and that
cannibalism does not provide the ideal diet,
from a stoichiometric point of view (to say
nothing of its social shortcomings!).
We can squeeze a little more insight
from
Eq. (3.B7)
. When conditions are really
poor, a consumer may spend all of its
energy on respiration, and not grow at all.
It may therefore be useful
ε
an
),
P
c
ε
gc
5
ð
3
:
B8
Þ
ð
vP
c
1
R
maint
Þ
where the two terms in the denominator are
the variable and fixed parts of respiration.
Substituting this new definition of net
growth efficiency into
Eq. (3.B7)
, we find that
P
c
c
. ð
Q
i
=
Q
g
Þ
ð
3
:
B9
Þ
ð
vP
c
1
R
maint
Þ
which allows us to make one final observa-
tion about when nutrient limitation of consu-
mers is most likely to occur: When growth is
slow, so that net growth efficiency
approaches zero (
del Giorgio and Cole 1998
),
consumers are less likely to experience nutri-
ent limitation than when growth is fast.
There are several ways by which consu-
mers can balance simultaneous demands
for energy and multiple nutrients, or even
specific biochemical compounds (
Brett
1993; Sterner and Elser 2002
). The con-
sumer may adjust its diet to better meet its
needs; microbial consumers may adjust the
exoenzymes they produce to allow for eas-
ier assimilation of the compounds they
need. Consumers may produce or draw on
internal stores of critical compounds—the
most familiar is energy storage as fat, but
calcium may be deposited and withdrawn
from bones. Finally, consumers may adjust
the efficiency with which they extract and
retain nutrients. Thus, assimilation, respi-
ration, and recycling of elements may
change as a consumer's diet and needs
change.
to consider
