Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 328

take and remineralization, not standing stocks, of nutrients are the impor-
tant parameters. For example, the algae in a highly eutrophic lake can have
a very high nutrient demand and thus keep phosphate and ammonium lev-
els near or below detection. Conversely, if the algal bloom in the same lake
is declining and dead cells are decomposing, very high levels of ammonium
and phosphate can occur. High values for dissolved inorganic nutrients are
not confined to eutrophic systems; in a very carbon-limited aquifer, dis-
solved inorganic nutrients may be high even though the system has very
low levels of microbial activity (e.g., it is relatively “oligotrophic” for a
groundwater aquifer). Consequently, understanding the processes leading
to supply and consumption of nutrients may be more important than just
knowing the concentrations of the inorganic nutrients.
Processes Leading to Remineralization
Several processes can lead to nutrient remineralization; some reminer-
alized nutrients originate from decomposition of dissolved and particulate
organic material and others from living organisms.
Dissolved nutrients in organic form are common in aquatic environ-
ments because organisms release them with normal metabolic activity or
when cells break and release their contents. Up to half of photosynthetic
carbon fixation by algae can be released directly into the dissolved form,
even by healthy cells (Zlotnik and Dubinsky, 1989). These dissolved or-
ganic molecules often contain nitrogen and phosphorus. Part of this leak-
age may be associated with release of extracellular enzymes such as phos-
phatase, and part may be unavoidable losses. In addition, a certain
proportion of cells die and break from viral infection or other causes.
These cells release dissolved organic nutrients into solution.
Additional organic molecules containing nutrients are released as part
of the activities of heterotrophic organisms. These predatory organisms
release organic molecules as excreta or as a result of sloppy feeding. As
these organic molecules are released, they become available to heterotrophic
organisms for consumption. In the process of being broken down, they re-
lease their associated nutrients in inorganic form.
Heterotrophic organisms that engulf living and nonliving organic ma-
terial often acquire nutrients such as nitrogen and phosphorus in excess of
their requirements. The excess nutrients are excreted into the aquatic en-
vironment. Processes controlling excretion rate are discussed later.
Given the variety of processes that can lead to remineralization, which
are the most important? Few studies have partitioned out the relative con-
tributions of different organisms to remineralization. The most common
way to partition remineralization into functional groups is to size frac-
tionate (separate into different size classes by filtration). For the most part,
these experiments indicate that very small organisms dominate nutrient
remineralization in many planktonic systems (Table 16.3). Similar patterns
are observed for size fractionation of uptake (data not shown). This route
for nutrient remineralization is often called the microbial loop, where
small, unicellular algae, bacteria, viruses, protozoa (particularly very small
flagellates), and rotifers rapidly recycle carbon and nutrients. The micro-
bial loop dominates nutrient cycling in many groundwater aquifers and