Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 326

EXAMPLE 16.2.
Using Redfield Ratios of Primary Producers to Indicate Nutrient Deficiency

Molecular ratios of C, N, and P are listed for phytoplankton assemblages
from three separate lakes. Use the ratios to predict nutrient limitations.

Phytoplankton stoichiometry C:N:P (molar) C:N (molar)

Balanced growth 106:16:1 6.6:1

Mirror Lake 212:32:1 6.6:1

Deep Lake 106:3:1 35:1

Clear Lake 400:16:1 25:1

The comparison must be made to balanced growth (the Redfield ratio).
The C:N ratio is presented to clarify the example. In Mirror Lake, the N:P
ratio is greater than 16, so P is limiting relative to N. The C:N ratio in Mir-
ror Lake is the same as balanced growth, so N likely is not limiting but P is
limited. In Deep Lake, the N:P ratio is lower than balanced growth, the C:P
ratio is the same as balanced growth, and the C:N ratio is greater than bal-
anced growth; thus, N is limiting in this lake. Finally, in Clear Lake the N:P
ratio is the same as balanced growth, but the C:P and C:N ratios exceed
those at balanced growth so both N and P are limiting.

lease of inorganic nutrients by organisms. Research has shown that regen-
eration is an important ecosystem driver, particularly in the epilimnia of
large lakes (Fee et al., 1994). Thus, the productivity of a system is a func-
tion of both regeneration and supplies of new nutrients. Here, I describe
how the remineralization occurs, what organisms are responsible, and the
dynamics of nutrient remineralization.

What Short-Term Processes Control the Levels of Dissolved Inorganic
Nutrients Such as Ammonium and Phosphate?
If the water chemistry of a lake, a stream at base flow, wetland pool,
or groundwater source is sampled each day for several days, the concen-
trations of ammonium and phosphate generally vary little. This lack of
variability over short periods of time occurs even though uptake rates are
often sufficient to completely remove dissolved inorganic nutrients in con-
siderably less than 1 day.
The balance between uptake and remineralization is the reason that
nutrient concentrations are moderately stable in the short term (Dodds,
1993). Uptake removes nutrients at a variable rate, and remineralization
replenishes them at an approximately constant rate (Fig. 16.8). This re-
plenishment results in a dissolved nutrient concentration that is moderately
resistant to perturbation over hours to days. This balance of uptake and
remineralization is common in many planktonic systems (Fig. 16.9). Given