Environmental Engineering Reference
In-Depth Information
1. Nutrients build in the euphotic zone of lakes during the winter
because nutrients mix from the hypolimnion during fall mixing, light is
limiting, and there is little demand for nutrients by phytoplankton. Light
becomes limiting because of less solar irradiance and phytoplankton may
be mixed deeply below the compensation point, or ice cover severely
limits light input.
2. During the spring, populations of small, rapidly growing
phytoplankton species peak because nutrients and light are high and
zooplankton grazing is low.
3. In late spring, reproduction allows zooplankton populations to
increase, and their grazing causes a clear water phase that lasts until
grazer-resistant species of phytoplankton develop during the summer.
4. Zooplankton populations decrease because of declining food and
increased fish predation.
5. A later bloom of cyanobacteria may occur with higher lake
temperatures and lower N availability.
6. Fall mixing can stimulate a second bloom of edible phytoplankton
and more large zooplankton.
This hypothetical successional cycle is diagramed in Fig. 20.5.
Fishes can have life histories that allow different coexisting species to
hatch at distinct and predictable times in a seasonal sequence. Amundrud
et al.
(1974) demonstrated an order of appearance of larvae in a small eu-
trophic lake (Fig. 20.6). Yellow perch
(Perca flavescens)
larvae appeared
first, followed by log perch
(Percina caprodes),
black crappies
(Pomoxis ni-
gromaculatus),
and finally pumpkinseeds and bluegills (
Lepomis
spp.).
This suggests that competition has led these fishes to habitat partitioning,
or that they specialize on prey that occur at different times during the sea-
sonal successional cycle.
On a longer time frame, succession can occur in newly created lakes
or existing lakes that are disturbed heavily. Creation of new lentic habitat
is not a rare occurrence in our modern world due to the establishment of
new reservoirs. The general view is that an early eutrophication phase is
common (Donar
et al.,
1996). Sequential colonization of the reservoir by
invertebrates also occurs, related to formation of a sedimented bottom and
establishment of macrophytes (Voshell and Simmons, 1984; Bass, 1992).
The sequence of events that occur following installation of a new reser-
voir can vary depending on the area being dammed, the morphology of the
reservoir, and a variety of other factors. For example, community formation
was contrasted in two reservoirs in south Saskatchewan (Hall
et al.,
1999).
Paleolimnological techniques were used to analyze algal and chironomid
midge communities in the two reservoirs over time. One reservoir was
formed by damming a river (Lake Diefenbaker) and the other by raising the
level of an existing lake (Buffalo Pound Lake). The river reservoir was char-
acterized by fluctuations in water level of 6 m per year and the flooded lake
by fluctuations of 1-3 m per year. Lake Diefenbaker (500 km
2
) exhibited a
typical sequence of succession found in newly formed reservoirs; an initial
period (4 years) of eutrophic conditions occurred, followed by a decade of
mesotrophy and a recent shift back to eutrophic conditions. Buffalo Pound
Lake (50 km
2
) had lower phytoplankton biomass after flooding, but midge
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