Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 461

TABLE 22.3 Generalized Ecosystem Characteristics of Temperate Lakes of Different
Trophic Levels

Anoxic Hypolimnetic O 2 Relative rates

Productivity hypolimnion depletion rate Factors of denitrification

(mg C m 2 (mg m 2 (in sufficiently limiting and nitrogen

day 1 ) day 1 ) Type deep lakes) production fixation

Oligotrophic 300 No 250 N and P Low

Mesotrophic 300-600 Maybe 250-400 N, P, and Medium

grazing

Eutrophic 600 Yes 400 N or light High

are dominated by biomass produced by phytoplankton photosynthesis that is
consumed by zooplankton, and zooplankton are consumed by fishes. This
view is useful because it allows simple models of lake ecosystems to be con-
structed. A model of this type is presented in Fig. 22.8. The idea that all fluxes
can be accounted for in the closed basin is of particular predictive value; mod-
els of lake eutrophication (see Chapter 17) can represent material balances
and planktonic algal biomass of lakes reasonably well. Of course, as with all
ecological constructs, there are exceptions to the simplification.
Many ecological studies have assumed that benthic primary produc-
tion is not important. In most large, deep lakes, this approximation is
probably reasonable. However, there are more small lakes than large lakes,
and reservoirs tend to be shallow (see Chapter 6). Thus, benthic primary
production may play a significant role in lake ecosystems (i.e., half or more
of the production may be attributed to littoral algae or macrophytes in
shallow lakes; Wetzel, 1983; Figs. 22.1 and 22.9).

Dry and wet
deposition

N 2 CH 4 , N 2 O,

H 2 S

CO 2

Nutrient
inflow

Nutrient
outflow

Fish

Zooplankton

Phytoplankton

Detritus

Dissolved nutrients

Benthos

Burial

FIGURE 22.8 A simple diagram of nutrient flux through a lake ecosystem. The system is
represented as a two-compartment bioreactor with a pelagic zone and the benthos (modified
from Covich et al., 1999).