Environmental Engineering Reference
In-Depth Information
UNITS USED IN STUDIES OF
ECOLOGICAL ENERGETICS
If you've taken a physics class recently, you know that the proper units of energy
content and flow are joules (kg-m
2
/s
2
) and watts (joules/s), respectively. It may seem
confusing, then, that ecologists studying energy flow almost never express their results
in terms of joules or watts. Rather, most ecologists implicitly equate energy with biomass,
because biomass is the carrier of energy in organisms, and is easier to measure than
energy content. Biomass thus implies energy content, and the production or destruction
of biomass implies energy flow. Consequently, ecologists usually express energy in units
of biomass (i.e., grams of live mass, dry mass, ash-free dry mass, or organic carbon). Other
units sometimes used in ecological energetics are the mass of oxygen produced or con-
sumed by photosynthesis or respiration, or calories (an obsolete unit of energy content).
Table 1
shows conversions between units commonly used in ecological energetics.
TABLE 1
Approximate conversion factors between energetic units used in ecological studies. Except for the
conversion between joules and calories (which is exact), the conversion factors are approximate and can vary
substantially among organisms and among tissues in an individual organism. Both the photosynthetic quotient
and the respiratory quotient are assumed to equal 1.
Units Converted From
Units Converted To
Joules
Calories Carbon (g) Oxygen (g) Dry Mass Wet Mass Ash-Free Dry Mass
10
2
5
10
2
5
10
2
5
10
2
4
10
2
5
Joules
1
0.239
2
6
5
2.5
4.3
3
3
3
3
3
10
2
5
10
2
4
10
2
4
10
2
3
10
2
4
Calories
4.18
1
9
2.5
2
1
1.8
3
3
3
3
3
10
4
10
4
Carbon (g)
4.5
3
1
3
1
2.7
2.2
11
1.9
10
4
10
3
Oxygen (g)
1.7
3
4
3
0.375
1
0.8
4
0.7
10
4
10
3
Dry mass
2
3
5
3
0.45
1.2
1
5
0.9
10
3
10
3
Wet mass
4
3
1
3
0.09
0.24
0.2
1
0.17
10
4
10
3
Ash-free dry mass
2.3
3
6
3
0.5
1.4
1.2
6
1
Modified from
Cummins and Wuycheck (1971)
,
Peters (1983)
,
Benke (1993)
,
Cattaneo and Mousseau (1995)
, and other sources.
References
Benke, A.C., 1993. Concepts and patterns of invertebrate production in running waters. Verh. Int. Ver. Theor.
Angew. Limnol. 25, 15
38.
Cattaneo, A., Mousseau, B., 1995. Empirical analysis of the removal rate of periphyton by grazers. Oecologia 103,
249
254.
Cummins, K.W., Wuycheck, J.C., 1971. Caloric equivalents for investigations in ecological energetics. Mitt. Int.
Ver. Theor. Angew. Limnol. 18, 1
158.
Libralato, S., Coll, M., Tudela, S., Palomera, I., Pranovi, F., 2008. Novel index for quantification of ecosystem
effects of fishing as removal of secondary production. Mar. Ecol. Prog. Ser. 355, 107
129.
Peters, R.H., 1983. The ecological implications of body size. Cambridge University Press, New York.
