Nitrogen, Sulfur, Phosphorus, and Other Nutrients - Freshwater Ecology: Concepts and Environmental Applications - page 251

Nitrate is generally the predominant form of dissolved inorganic ni-
trogen in oxidized waters, and ammonium is the predominant form in
anoxic waters. The absolute amounts of each are highly variable in many
freshwaters (see Fig. 11.3; nitrate). The biotic processes that lead to re-
duced and oxidized forms of ammonium (e.g., nitrification, remineraliza-
tion, and denitrification) can vary over time and space in freshwater habi-
tats. Also, soils have a greater affinity for nitrate than ammonium, but
different soils have varying degrees of affinity. Rainwater generally has
more nitrate than ammonium and nitrogen concentrations are geographi-
cally patchy, dependent in part on human activities. Thus, absolute con-
centrations of dissolved inorganic nitrogen can be highly variable.
In a lake with an anoxic hypolimnion, ammonium dominates the hy-
polimnion, and nitrate is mainly confined to the epilimnion during strati-
fication (Fig. 13.4). Two processes drive this spatial pattern of nitrate and
ammonium concentrations: Nitrification transforms ammonium to nitrate
in the presence of O 2 , and nitrate is removed by denitrification under
anoxic conditions. A similar pattern is observed in oligotrophic lakes and
wetlands, except high ammonium is generally confined to the anoxic zone
within the sediments.
High ammonium in streams is often associated with input of anoxic
groundwaters or pollution (Duff and Triska, 2000). The temporal varia-

0

A

1

2

3

4

5

Jan Mar May Jul Sep Nov

0

B

1

2

3

4

5

Jan Mar May Jul Sep Nov

Month

FIGURE 13.4 Distribution of nitrate (A) and ammonium (B) in hypereutrophic Wintergreen
Lake, Michigan, as a function of depth and time. Ice cover occurred from January to March.
Darker colors represent higher concentrations. Contours are reported in g liter 1

(repro-

duced with permission from Wetzel, 1983).