Environmental Engineering Reference
In-Depth Information
Silicon can become depleted in the epilimnion of both eutrophic and
oligotrophic lakes during summer. Depletion occurs because of the relatively
slow cycling of silicon, its incorporation into diatom frustules, and subse-
quent sinking of frustules before silicon can be redissolved. Dissolved sili-
con concentrations build near the sediments during summer and winter
stratification (Fig. 13.10) because diatom frustules sink to the sediments and
slowly dissolve. The temporal and spatial variation in silicon availability
may be important in the successional processes of phytoplankton, as di-
atoms deplete silicon seasonally (Fig. 13.11). In contrast, concentrations of
silicon in rivers are remarkably constant over time and across continents
(Wetzel, 2001); thus, periphytic diatom communities in flowing waters are
less likely to be influenced by seasonal changes in silicon concentrations.
Iron
Iron is a key element in many biological molecules, including electron
transport proteins, hemoglobins, enzymes used in synthetic pathways for
chlorophyll and proteins, and enzymes used for nitrate assimilation, pho-
tosynthesis, and other essential metabolic processes. The total demand for
iron is not high relative to nitrogen, carbon, and phosphorus because iron
serves as a cofactor in these reactions (one or a few iron atoms for each
Ice
0
A
1
2
3
0.5
4
0.5
5
Jun
Aug
Oct
Dec
Feb
Apr
Jun
Ice
0
B
2
4
6
8
10
12
Jan
Mar
May
Jul
Sep
Nov
Month
FIGURE 13.10
Concentration of silica as a function of depth and time in hypereutrophic
Wintergreen Lake, Michigan (A), and oligotrophic Lawrence Lake, Michigan (B). Concen-
trations are given in mg liter
1
, with darker contour fills corresponding to greater concen-
trations (reproduced with permission from Wetzel, 1983).
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