Biota of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 412

e ecological pyramid

Piscivores

Energy
flow

Nutrient
cycling

Planktivores

Herbivores

Primary producers Sunlight

Bacteria and benthic detritivores
Organic matter decomposition

FIGURE 15.1 Food pyramid. (From Water on the Web, Monitoring Minnesota lakes on the Internet and
training water science technicians for the future—A national on-line curriculum using advanced tech-
nologies and real-time data, University of Minnesota-Duluth, Duluth, MN, 2004, Available at http://
Water OntheWeb.org.)

limited by the availability of nutrients, principally phosphorus and nitrogen. Those nutrients are
subsequently recycled by plant death and excretion. Thus, controlling plant growth is often directed
toward controlling nutrients, such as in developing nutrient criteria.
One important example where products are important is the methanogens that produce methane
as a metabolic by-product under anaerobic conditions. Another example where both products and
uptake are important is the processing of nitrogen. Common forms of nitrogen include that in living
organisms, nonliving organic nitrogen, inorganic forms (ammonia, NH 4 ; nitrite, NO 2 ; nitrate, NO 3 ),
and dissolved nitrogen gas (N 2 ). Organic nitrogen typically refers to the nonliving organic material
that was once a component of living organisms (e.g., both autotrophs and heterotrophs; detrital or
excreted materials). The bacterial breakdown of these organic materials produces inorganic ammonia.
In the nitriication process, under aerobic conditions, ammonia is converted to nitrite and ultimately
to nitrate (Figure 15.2). This process is important in lake environments since inorganic nitrogen is an
essential nutrient for primary production (for the autotrophs). Dissolved nitrogen (N 2 ) is not biologi-
cally available to most autotrophs, but some do have the ability to ix nitrogen, converting N 2 to ammo-
nia (NH 4 ). Another important group of organisms are those that transform inorganic N to gaseous
forms (e.g., ultimately N 2 ). This transformation is a pivotal sink of inorganic nitrogen in many aquatic
environments, including lakes and reservoirs. The processes include:

• Denitriication: The microbial transformation of NO 3 to gaseous nitrogen, which is usu-

ally accomplished by facultative heterotrophic chemotrophs. In this case, the bacteria can
obtain oxygen from either that dissolved under aerobic conditions or from the nitrate mol-
ecule under aerobic conditions. Denitriication is a primary mechanism for the removal of
nitrogen from lakes (Saunders and Kalff 2001).

• Anaerobic ammonium oxidation (anammox): In this case, bacteria combine ammonium

and nitrite or nitrate to ultimately form nitrogen gas. This process, more recently discov-
ered, has been found to be of particular importance in marine environments, accounting
for up to 50% of the total nitrogen turnover (Kuypers et al. 2003, 2005; Dalsgaard et al.
2005). However, the importance of anammox in freshwater lacustrine nitrogen cycling
remains to be determined (Rissaned et al. 2011).