Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 450

A

Top carnivores

Carnivores

Herbivores

Producers

Decomposers

0 200 400 600 800 1000

Biomass (g m -2 )

Growth
Respiration

B

Top carnivores

Carnivores

Herbivores

Producers

Decomposers

0 5000 10000 15000 20000 25000

Production (kcal m -2 y -1 )

C

Top carnivores

Carnivores

Herbivores

Producers

Decomposers

0 200 400 600 800 1000 1200

Production : Biomass (kcal g -1 y -1 )

FIGURE 22.2 Biomass (A), production (B), and production per unit biomass (C) of Silver
Springs, Florida. Note that production is broken into respiration and growth (data from
Odum and Odum, 1959).

Although energy flux certainly makes sense as a primary unit when
discussing production of food for secondary consumers and above, it may
not be as important for primary producers, herbivores, and microbes,
which can be limited by nutrients other than carbon. Some investigators
have assumed that most heterotrophic organisms are limited by carbon
supply in aquatic ecosystems. However, rates of fungal and bacterial ac-
tivity and growth in streams can be limited by nitrogen or phosporus
(Suberkropp, 1995; Tank and Webster, 1998). Also, pelagic bacteria are
important consumers of phosphorus and compete well for it with
phytoplankton (Currie and Kalff, 1984). Thus, consideration of energy
limitation alone does not always provide an accurate description of func-
tional ecosystem relationships. Therefore, another branch of ecosystem
science is concerned with nutrient budgets, which quantify fluxes in
nutrient cycles.
I present two examples of nutrient budgets, a flux diagram and a table
that tallies total inputs and outputs. A flux diagram allows representation
of fluxes that occur within a system. Often, input and output budgets treat