Biota of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 411

15

Biota of Lakes and Reservoirs

15.1 CLASSIFICATION
The biota of lakes and reservoirs are very taxonomically diverse. However, many of these organ-
isms share common characteristics that are useful in their management, based on where they live,
both physically and in their relationship to other organisms. Two of the commonly used classiica-
tion schemes, as discussed in the following sections, are based on trophic levels and zonation.

15.1.1 t ropHIc L eVeL
The biological communities in lakes and reservoirs are often classiied into trophic levels according
to their source of energy or organic materials, where trophic derives from the Greek trophē refer-
ring to food or feeding. One of the systems used to characterize the condition of lakes is based on
productivity and is referred to as the trophic condition (e.g., eutrophic and oligotrophic). The organ-
isms that make up that productivity may be divided into trophic levels consisting of primary and
secondary consumers and decomposers.
Primary producers are those organisms that synthesize organic material from inorganic mate-
rials, such as elemental nutrients and carbon dioxide. These organisms are also the autotrophs,
or “self-feeders.” Autotrophs require an energy source, which may include light (phototrophs) or
chemical reactions (chemoautotrophs). Phototrophs are restricted to the upper levels of lakes and
reservoirs and are limited by the availability of light and nutrients. Phototrophs include small loat-
ing plants, phytoplankton, the larger macrophytes and their attached periphyton. Chemoautotrophs
more commonly live in the harsher anaerobic portions of lakes such as an anoxic hypolimnion and
sediments. These include bacteria and archaea that use inorganic energy sources, such as nitrates,
oxides of manganese and iron, and sulfates.
Consumers are those organisms that feed on primary producers or other consumers. That is, they
cannot ix carbon so they must rely on consuming those organisms that can ix carbon. Primary
consumers are those organisms that feed directly on the producers (living or detrital), such as zoo-
plankton, and are the herbivores. Organisms that feed on the primary consumers are the secondary
consumers, such as some invertebrates and planktivorous ish (planktivores). And so on, until the
tertiary consumers that feed on the piscivorous ish, known as the piscivores, which are at the “top
of the food chain.”
Decomposers are those organisms that convert organic material back into nutrients, for example,
mineralization. Decomposers may be found throughout lakes, but they dominate in the aphotic zone,
where, in the absence of light, they feed on the lux of organic material falling from the photic zone.
One consequence of the trophic levels is that since no transfer of energy is 100% eficient, the
higher the order is on the food chain, the less biomass that can be supported. This is often depicted
as the ecological pyramid (Figure 15.1). Also, note that only the primary producers are limited by
the availability of light. The consumers are not restricted to the photic zone.
There are exceptions to the foregoing classiication, as with all classiications of biotic assem-
blages. For example, there are organisms that are facultative heterotrophic phototrophs, or faculta-
tive phototrophs. That is, they are generally primary producers, but can also become consumers.
They are not “obligate” phototrophs but “facultative” phototrophs, so that primary production is not
essential for their growth, allowing them to grow in both the light and the dark.
The relative importance and classiication of some organisms are based on what they produce
and what they consume. Primary producers consume nutrients during growth and that growth is