Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 325

same nutrients, the competitive exclusion principle should limit the number of
species that are present at any time. The paradox he noted is that a typical
mesotrophic or oligotrophic lake has many species (typically 10-100) of phy-
toplankton present at any one time. Explanations for plankton diversity in-
clude the following: (i) Predation by zooplankton and viruses removes domi-
nant competitors (Suttle et al., 1990), (ii) pulses and micropatches of nutrients
from uneven mixing and excretion lead to nonequilibrium conditions (dis-
cussed later), (iii) mutualistic or beneficial interactions promote otherwise in-
ferior competitors, (iv) many lakes are not at equilibrium conditions over
timescales greater than 1 month and the time required for dominant phyto-
plankton species to outcompete inferior competitors is more than 1 month
(Harris, 1986), (v) different competitive abilities lead to different nutrients lim-
iting different species, and (vi) chaos (in the mathematical sense) arises when
species compete for three or more resources (Huisman and Weissing, 1999).
The idea of different competitive abilities led to the resource ratio theory and
the determination of how the Redfield ratio is linked to nutrient limitation.
RESOURCE RATIOS AND STOICHIOMETRY OF PRIMARY PRODUCERS
Primary producers can alter the relative proportions of elements that
comprise their cells (their stoichiometry). This adaptation allows produc-
ers to acquire and store cellular components when resources are not limit-
ing and utilize them during times when they are limiting. Luxury con-
sumption (discussed previously) alters the stoichiometry of the primary
producers. When nutrients are limiting, photosynthesis still occurs and
leads to accumulation of carbon in lipids or starch (i.e., cellular stoi-
chiometry is shifted toward relatively more carbon). This relationship be-
tween nutrient supply and cell stoichiometry has led to the extensive use
of deviations from the Redfield ratio to indicate nutrient limitations.
The Redfield ratio is derived from nutrient contents of phytoplankton
grown with excess concentrations of all nutrients at conditions optimal for
maximum growth. Deviations from these ratios indicate limitation by nu-
trients (Example 16.2). A similar approach may be useful in determining
nutrient limitation of wetland plants (Boeye et al., 1997; Bedford et al.,
1999).
NUTRIENT REMINERALIZATION
Uptake rates of nutrients in aquatic habitats are high enough that the
dissolved pools of nutrients will be depleted rapidly if they are not replen-
ished (Axler et al., 1981; Kilham and Kilham, 1990). Turnover may take
hours or days for the nitrate pool or only seconds for the phosphate pool,
but without supply of nutrients from some source uptake cannot continue
at rates measured in the environment. In general, external sources of nu-
trients (“new nutrients”), such as river and groundwater inflow to lakes
and wetlands or atmospheric deposition, cannot supply nutrients at mea-
sured rates of uptake. Thus, the predominant short-term source of nutri-
ents is from remineralization (also known as regeneration ), which is the re-