Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 463

of energetics of lakes compared to that of a simple model considering phy-
toplankton-zooplankton-fish linkages. The actual importance of each path
of energy flux is context-dependent. If a lake is shallow and clear, macro-
phytes may dominate, whereas a large, deep, clear lake will be dominated
by phytoplankton. A lake with high throughput and an extensive littoral
zone may function more similarly to a stream and be dominated by al-
lochthonous carbon sources.
In Chapter 19, I discussed food webs in lakes and the trophic cascade
systems of interacting populations of organisms, but not from the per-
spective of ecosystem energy flux. An interesting aspect of ecosystem en-
ergy flux is related to the fact that primary producers are usually limited
by nutrients, but consumers are limited by energy. Where the switch from
nutrient to energy limitation occurs depends on the stoichiometry of the
system. The stoichiometry of grazers can feed back and intensify or relieve
nutrient limitation (Elser and Urabe, 1999). Thus, predicting ecosystem en-
ergy flux may require knowledge of community structure. For example,
large Daphnia lower phytoplankton by grazing and intensify phosphorus
limitation because of their high phosphorus demand (Elser and Hassett,
1994). Changes in trophic structure that alter Daphnia populations can
thus affect factors that limit primary production.
Viewing lakes from a regional or landscape perspective can yield im-
portant information (Magnuson and Kratz, 1999; Kratz and Frost, 2000).
One of the major aspects of groups of lakes is the coherence of lake prop-
erties with time (Magnuson et al., 1990). Documenting this coherence al-
lows estimation of how well research results from one lake in an area can
be extrapolated to another. For example, lakes tend to have more similar
chemical and biological properties across a landscape when hydrological
throughput is high (Soranno et al., 1999). Lakes have also been classified
by how well they are linked to other lakes by hydrology and by how far
down in the drainage they are (similar to stream ordering). This classifica-
tion correlates with patterns of species richness, chlorophyll concentra-
tions, and major ion concentration (Riera et al., 2000).
WETLANDS
Wetland ecosystems can be classified along a hydrological continuum
from those that have very little hydrological throughput to those that are
closely coupled to rivers, estuaries, or lakes. In Chapter 4, I introduced the
idea that some wetlands can have high hydrological throughput
(minerotrophic), whereas others are fed mainly by precipitation and have
low hydrological throughput (ombrotrophic). This variation in hydrology
has implications for ecosystem function. Minerotrophic wetlands utilize
nutrients from outside and nutrients can be washed from them easily. Om-
brotrophic wetlands must rely more heavily on nutrient input from pre-
cipitation and internal nutrient cycling.
Autochthonous production in wetlands is usually extremely high; wet-
lands are characterized by some of the highest rates of primary productiv-
ity of any habitats on Earth (Fig. 22.10). Production of even temporary
wetlands may be important across dry landscapes because the majority of