Environmental Engineering Reference
In-Depth Information
18.1 Trophic Organization of Aquatic Ecosystems
From the point of view of functional organization, all ecosystems are characterized
by fluxes in matter and energy. They have structural components, trophic levels,
which enable and control the transport of these fluxes through the ecosystem. When
we speak of a trophic level we mean all biota in a food chain or food web of an
ecosystem that are the same number of links away from the ultimate source of
energy, which is usually solar radiation. In the lowest trophic level (level 1) are the
primary producers, which are normally green autotroph plants, and which support a
chain or web of consumers. Depending on their feeding behaviour, the consumers
are separated into the group of herbivores (trophic level 2), which feed on the
producers, first-order carnivores (trophic level 3), which feed on herbivores, and,
often, second-order carnivores (trophic level 4) that feed on first-order carnivores.
Whether there are still higher level carnivores is ecosystem-dependent. Decom-
posers, which enable the recycling of nutrients by feeding on the organic litter, are
often classed into the bottom of the food web. In 1941 the ecologist Raymond
Lindeman (1941) published a fundamental study on the trophic interrelations of the
Cedar Bog Lake, Minnesota, USA, a late stage eutrophic lake. Due to the theoreti-
cal implications and the high number of successive studies that were motivated by
the work of Lindeman, this study can be regarded as one of the most important
studies for modern aquatic community ecology.
Most importantly, Lindeman's work focused on the flow of energy up the food
chain from the autotrophs to the top carnivores, and thus on the dependence of each
trophic level on the one below it. Biomass usually decreases with each higher
trophic level, which is referred to as trophic pyramid. Lindeman explained this
phenomenon as a consequence of respiration within each trophic level and of the
high losses of energy during transfer from one trophic level to the next, due, in part,
to The Second Law of Thermodynamics. Also, he assumed that control of these
flows went primarily in one direction, from the autotrophs up to higher trophic
levels. In the sense that any given trophic level depends on the conditions of the
level below it, Lindeman expressed here the idea of bottom-up control. Figure
18.1
shows such a typical example of pure bottom-up control in a four-level food chain
in an ecosystem.
Hairston, Smith, and Slobodkin (1960) reviewed a number of different food chain
studies derived from different habitats. They concluded that interspecific competi-
tion exists within each of the trophic levels of the producers, the carnivores and the
decomposers. They also inferred that the herbivore trophic level is rarely food-
limited; instead it appears to be predator-limited. Hence, Hairston et al. introduced
the idea that in food chains the controlling effects can also go 'down' the food chain,
which is a mechanism that is termed top-down control. These two concepts of
bottom-up and top-down control in trophic structures of ecosystems have been and
are still widely investigated and seem to be of extraordinary importance for aquatic
systems, in particular (DeAngelis et al. 1996). The concepts have been extended in
such a way that ecologists encompass not only the impacts of lower on higher
