Biota of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 413

Aerobic

Nitrification

NH + NO - NO -

Denitrification

NO 3 NO -

N 2

N 2 O NO

NH +

Anaerobic

FIGURE 15.2 Nitrogen transformation processes.

One consequence of anaerobic reduction versus aerobic oxidation is that there is a reduction in
energy yield. That is, organisms obtain less energy from a chemical reduction in the absence of free
oxygen. Under anaerobic conditions, organisms that can reduce nitrate nitrogen get the most “bang
for the buck,” followed by sulfate reduction and inally methanogenesis. Organisms that reduce
sulfate get less than 30% of the energy yield of aerobic respiration, while methanogenesis generates
only about 1% of the energy yield of aerobic respiration (Figure 15.3). Therefore, as systems become
anaerobic, there is usually a population shift, with populations initially dominated by the nitriiers,
and then the sulfate reducers, and inally the methanogens.

15.1.2 z onatIon
A second classiication system is based on where the organisms live. The zones included are

• Psammolittoral zone

• Littoral zone

• Limnetic or pelagic zone

• Benthic zone

Aerobic respiration

Nitrate reduction

Sulfate reduction

Methanogenesis

FIGURE 15.3 Relative energy yield (energy produced per mole oxidant) associated with oxidation and
reduction processes.