Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 330

sediments in which larger organisms do not occur. The microbial loop will
be discussed in greater detail in Chapter 18.
Cases exist in which nutrient supply associated with larger organisms
is very important. These cases often involve making nutrients available
from outside the system. Examples of external sources in streams include
nutrient input from rotting carcasses of salmon after they have spawned
and died (Kline et al., 1990; Bilby et al., 1996); nutrient input to streams
from vegetation that has fallen from riparian plants and has been processed
by invertebrates; and activities of beavers bringing terrestrial nutrients into
streams (Fig. 22.3; Naiman et al., 1988, 1994). In lakes, vertical migration
of zooplankton from the hypolimnion to the epilimnion and subsequent
movement of nutrients from the hypolimnion can occur. Also, movement
of benthic organisms, such as the amphipod Gammarus, from sediments
into the water column can bring as much as 33% of the phosphorus into
the water column (Wilhelm et al., 1999). Salmon carcasses left after re-
production also bring considerable amounts of marine-derived nitrogen
into lakes. In coastal Alaskan lakes this input is enough to cause substan-
tial effects on ecosystem production and phytoplankton and zooplankton
community structure (Finney et al., 2000). Excretion into ponds and wet-
lands by flocks of ducks and movement of hippopotami out to graze ter-
restrial vegetation and excreting the material into rivers or wetlands are
other examples of such nutrient supply by larger organisms.
Remineralization as a Source of Nutrient Pulses in Lentic Systems
Large algal cells are poor competitors for nutrients at low concentra-
tions relative to smaller cells (Suttle et al., 1988). Larger cells have a low
ratio of surface area to volume and cannot assimilate nutrients as well.
Nonetheless, many large algal cells can be found in nutrient-limited waters.
These cells may be able to use high concentrations of nutrients associated
with pulses and, thus, maintain competitive ability by storing nutrients
for use between pulses. The sources of such nutrient pulses have received
some study.
Pulses of nutrients provided by zooplankton excretion have been sug-
gested as important sources of patches of elevated nutrients in planktonic
environments. In an ingenious study, Lehman and Scavia (1982) proved that
zooplankton could produce pulses that remained stable for long enough to
give phytoplankton cells in their vicinity a possible competitive advantage.
In their study, a culture of the cladoceran Daphnia was fed with algal cells
that had been labeled with radioactive phosphorus ( 32 P). These radioactive
Daphnia were transferred into bottles containing unlabeled planktonic al-
gae. After a short time, the algal cells were harvested and some were placed
on microscope slides. The slides were coated with a photographic emulsion
sensitive to the radioactive 32 P. After the slides were exposed and developed,
microscopic examination determined if each cell had taken up radioactive
phosphorus and how much. Analysis of the distribution of the label in the
algal cells revealed that some cells had significantly more label in them than
would be expected if the phosphorus excreted by the Daphnia was dis-
persed completely into the bottle. Thus, the experiment demonstrated that
nutrient pulses could exist in planktonic communities.