Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 462

Phytoplankton
and bacteria

Zooplankton

Stream output (3)

Release (15)

Littoral
producers

Dissolved
organic C

Stream output (36)

Release (3)

Photosynthesis
(128)

Allochthonous influx
(14)

Dissolved
inorganic C

Atmospheric influx
(42)

Benthic detritus

Burial (15)

FIGURE 22.9 Diagram of carbon flux in Lawrence Lake, Michigan (data from Wetzel, 1983).

A carbon budget for Lawrence Lake, Michigan, illustrates some of the
primary carbon flux pathways in lakes (Fig. 22.9). This lake has signifi-
cantly greater rates of autochthonous production than allochthonous in-
puts. Thus, primary producers in the system dominate carbon cycling.
Macrophytes and associated algae were responsible for about two-thirds
of the primary production. A large portion of the macrophyte production
ended up as benthic detrital carbon, whereas less than one-third of the phy-
toplankton production ended up in the sediments. Rates of carbon burial
were about half of export via streams, and the lake was a net source of or-
ganic carbon to the watershed.
Rates of heterotrophy exceed photosynthetic rates when a broad num-
ber of lakes are considered. This is based on analysis of the degree of sat-
uration of CO 2 in 4665 lakes throughout the world (Cole et al., 1994).
The data showed that 87% of the lakes were supersaturated with CO 2 , in-
dicating that respiration rates exceed carbon sequestration and export.
These results can be explained most easily if externally derived carbon (al-
lochthonous sources) exceeds washout plus burial in the sediments. Thus,
the CO 2 data indicate that lakes are generally heterotrophic. Of course,
this generalization covers a range of lake types. High rates of respiration
relative to photosynthesis may be common in more oligotrophic aquatic
ecosystems (Duarte and Agustí, 1998). Trophic cascades may alter the rel-
ative importance of heterotrophy in lakes (Carpenter et al., 2001). In some
large lakes, such as the Great Lakes of North America, photosynthesis is
likely high relative to allochthonous organic carbon input. Other systems
may be driven by external carbon inputs (in fine and dissolved organic ma-
terial) that are consumed by bacteria, and bacteria are consumed by zoo-
plankton. Lakes with high concentrations of nonliving suspended particles
can support a productive fish community despite very low algal biomass
and productivity, and small humic lakes may have high production of bac-
teria that consume humic substances (Münster et al., 1999).
The idea that benthic primary production and allochthonous carbon
provide considerable energy input into the food web complicates the view