Environmental Engineering Reference
In-Depth Information
lead to a shift to stronger P limitation in phytoplankton (Sterner, 1990;
Sterner et al., 1992) because of preferential assimilation of P relative to N and
relatively high ratios of N:P in nutrients remineralized by Daphnia (Sterner
and Hessen, 1994). Thus, the concepts of stoichiometry and nutrient limita-
tion have implications for food webs and ecosystem function.
SUMMARY
1. Nutrient uptake can be described by the Michaelis-Menten uptake
equation. After nutrients are taken up, they must be assimilated or
converted to the chemical compounds that make up cells. The
equations that describe this process are the Droop equation, which
links intracellular nutrient concentrations with growth, and the
Monod equation, which describes the relationship of external
dissolved nutrients to growth rate.
2. Other factors that influence uptake and assimilation of nutrients
include the ability to acquire and store nutrients for later use (luxury
consumption), temperature, and light.
3. Nutrients are required in known ratios (stoichiometry) for growth. The
ratio of C:N:P required for algal growth is approximately 106:16:1 by
moles and is known as the Redfield ratio. If the relative availability of
a nutrient is lower than its requirement, it can be limiting.
4. Some aquatic scientists maintain that only one nutrient can be
limiting at a time, but others argue that more than one nutrient can
limit primary producer assemblages at a time. Empirical evidence
suggests that N, P, or both are usually the limiting nutrients in lakes,
streams, and wetlands.
5. Nutrients can be supplied from outside (new nutrients) or inside the
system by the process of remineralization (regeneration). Nutrient
remineralization provides the primary source of the nutrients available
to primary producers in aquatic ecosystems. Sources of these
remineralized nutrients include organic material excreted or lost by
producers and processed by heterotrophs and excretion by predators
or consumers.
6. The balance of uptake and remineralization often controls dissolved
inorganic nutrient concentrations in aquatic ecosystems.
7. Microbes are responsible for the bulk of remineralization in most
ecosystems, but in certain cases larger organisms can be important.
8. Nutrient pulses created by larger organisms can persist in the
environment, but those produced by smaller microbes likely disperse
quickly by diffusion.
9. Stoichiometry of heterotrophs feeds back to alter nutrient limitation
of primary producers.
QUESTIONS FOR THOUGHT
1. Why might dissolved nutrient levels be more variable in streams than
in large lakes?
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