Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 322

for such tests, but use of other permeable surfaces is increasing (Winter-
bourn, 1990). Factors that can interfere with these tests include grazers
that crop algae as it grows and the inability to duplicate natural surfaces
(e.g., nutrients rarely diffuse out of a solid surface in nature).
Another alternative is to take a water sample, return it to the laboratory,
and test how well it stimulates production of laboratory cultures of algae
(Eaton et al., 1995). In this case, different nutrients can be added to incu-
bations to assess which nutrient is limiting algal growth. The drawbacks to
this method are that it does not simulate natural conditions and species of
phytoplankton used in the test may not be found in the system of interest.
Duckweed (Lemna minor) also has been suggested as a test organism in the
laboratory (Eaton et al., 1995) because its wide distribution in nature and
small size make it easy to obtain and it is simple to grow in specific media.
Finally, a variety of short-term physiological bioassays have been pro-
posed (Beardall et al., 2001). Of these, the most successful has been use of
Redfield ratios for indication of nutrient limitation (Example 16.2). Other
physiological methods are generally less reliable although quicker than
growth-based bioassays (Dodds and Priscu, 1990).
Stoermer, 1972). In some cases, micronutrients such as molybdenum may
limit algal growth (Goldman, 1960, 1972; Howarth and Cole, 1985). Early
recognition of this effect is one of the many contributions by Dr. Charles
Goldman (Biography 16.1). Not all lakes are nutrient limited (Tilzer et al.,
1991).
Assays on wetlands indicate that N or P limit plant production in
many peat mires, but K may also limit plant growth (Verhoeven, 1986). A
literature search on the N and P stoichiometry of wetland plants and soils
suggests that P limitation, or N and P colimitation, is common in most wet-
land types (Bedford et al., 1999). Wetlands with high hydraulic through-
put (drained wetlands) may be more prone to K limitation (Van Duren et
al., 1997). Wetland rice production may also be limited by zinc concen-
trations (Neue et al., 1998). As with lakes and streams, some cases of mul-
tiple nutrient limitations occur in wetlands (Fig. 16.6B).
Streams can also be limited by N, P, N and P, or neither (Fig. 16.6C).
This indicates that it is unwise to assume that any individual nutrient lim-
its primary production in streams. It has been suggested that CO 2 also lim-
its some primary producers in streams (Dodds, 1989; Raven, 1992) and
that silicon can limit epiphytic diatoms at times (Zimba, 1998). Cases of
no nutrient limiting growth are expected to be more common in streams
than lakes or wetlands because of the greater influence of riparian canopy
cover on light in streams and scouring floods that remove algal biomass.
The occurrence of limitation of primary producers by more than one
nutrient in wetlands, streams, and lakes raises the question of explaining
colimitation in light of application of Leibig's law (Dodds et al., 1989).
Given that streams are commonly far from equilibrium, the existence of