Nitrogen, Sulfur, Phosphorus, and Other Nutrients - Freshwater Ecology: Concepts and Environmental Applications - page 262

Oxic

Fe(OH) 3 and
FePO 4 precipitation and
deposition

Biological and abiological
oxidation

Assimilation Assimilation

Organic Fe Fe 2+ Fe 3+

Heterotrophy Remineralization Biological and abiological

reduction

Anoxic

FeS deposition Fe(OH) 3 and

FePO 4 dissolution

FIGURE 13.12 A conceptual diagram of the iron cycle.

growth can eventually plug the well intake screens and the porous materi-
als surrounding the well intake. Removing the bacterial growth can be dif-
ficult, and measures such as back-flushing the well provide only temporary
help.
Iron cycling (Fig. 13.12) is similar to nitrogen, carbon, and sulfur cy-
cling because the concepts of redox and potential energy can be used to or-
der and understand the processes. The iron cycle was one of the first nu-
trient cycles to receive extensive study (Mortimer, 1941).
Manganese cycling is similar to iron cycling in many ways. It will not
be discussed in detail here; the interested reader is referred to Wetzel
(2001). There are some differences between the two cycles. For example,
iron oxidation is spontaneous in the oxic waters of lakes, but manganese
oxidation requires the high pH and redox associated with photosynthesiz-
ing organisms (Richardson et al., 1988). Manganese nodules are occasion-
ally found in freshwaters. These nodules are formed when bacteria oxidize
reduced iron and manganese (Atlas and Bartha, 1998; Chapnick et al.,
1982).
Minor nutrient cycles, such as molybdenum, selenium, and copper, are
also much less studied probably because even though organisms require
these micronutrients, the requirement is small and the chances of these el-
ements limiting system production are relatively low (but see Goldman,
1972). An exception to the idea that minor nutrients limit productivity is
the case of molybdenum limitation in Castle Lake, California. In this case,
molybdenum is used for nitrogen assimilation. Alder trees in the watershed
scavenge the molybdenum, creating apparent molybdenum limitation in
the lake (Goldman, 1960).

GRADIENTS OF REDOX AND NUTRIENT CYCLES AND INTERACTIONS
AMONG THE CYCLES

Redox is a major factor controlling biological and abiological nutrient
transformations as stressed in previous discussions of biogeochemical cy-
cling. Metabolic activities that predominate in a specific environment can be
predicted in part by the redox potential of that system. Redox decreases with