Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 331

The importance of pulses from zooplankton excretion is uncertain. Ar-
tificial nutrient pulses change phytoplankton community composition in al-
gal cultures (Scavia and Fahnenstiel, 1984). If it is true that most nutrient
remineralization in planktonic communities is dominated by microorgan-
isms smaller than copepods and cladocerans, then such pulses are less
likely to have overall importance. Very small cells at most leave small
pulses of remineralized nutrients. Given Fick's law (see Chapter 3 on prop-
erties of water), we know that diffusion rates are greater at smaller scales.
Small pulses probably disperse quickly and are unlikely to be present for
long enough to stimulate large planktonic algae. Other factors such as
grazer resistance may select for larger cells, and nutrient pulses may not be
necessary to explain their presence (see Sidebar 3.1 on factors selecting for
morphology of plankton).
STOICHIOMETRY OF HETEROTROPHS, THEIR FOOD, AND
NUTRIENT REMINERALIZATION
Heterotrophs remineralize nutrients when they are in excess of re-
quirements. The stoichiometry of many heterotrophs is similar to that of
the Redfield ratio, and they are generally much less flexible than primary
producers at altering these ratios. Because heterotrophic organisms need to
meet both their energy and carbon demands for growth from the organic
material they consume, the nutrients in the food they eat can frequently ex-
ceed the amount needed.
As an example of the stoichiometric effects of the carbon requirement
for both growth and respiration, consider a fish that is able to convert only
10% of the carbon it consumes into biomass. The remaining 90% of the
carbon must be used to create energy for metabolism. If food is consumed
that has the Redfield ratio of 106:16:1 mol of C:N:P, only 1/10th of the C,
N, and P can be used for growth. The excess N and P will be excreted.
Food for heterotrophs is not always at the Redfield ratio, and require-
ments of all heterotrophs are not the same as the Redfield ratio. Consid-
eration of stoichiometry has led to much study of the requirements for ra-
tios of nutrients, the stoichiometry of heterotrophs, and the composition
of their food.
Most bacterial heterotrophs rely on dissolved organic material for car-
bon, nitrogen, and phosphorus requirements. This material ultimately
comes from primary producers (either phytoplankton in lakes or benthic
algae and terrestrial vegetation in wetlands and streams) and can vary con-
siderably in stoichiometry, as discussed previously. Bacteria can retain N
increasingly as the C:N ratio of the dissolved, organic material consumed
decreases; thus, net remineralization is high at low C:N ratios (Fig. 16.10).
The dissolved organic carbon available to bacteria may be poor in N
and P, and they may need to meet their requirements for these materials by
incorporating (also referred to as immobilizing or assimilating) inorganic
forms, such as nitrate, ammonium, and phosphate (Tezuka, 1990). Thus,
a significant portion of inorganic nutrient uptake in some lakes can be at-
tributed to bacteria (Currie and Kalff, 1984; Dodds et al., 1991).