Ecologists have studied the responses of pelagic ecosystems
to varying levels of nutrient loading and fish predation on many
different scales ranging from small experimental enclosures to
whole lake systems. There are recurring patterns of response to
these two variables, which are generally predictable on the basis
of biogeographic, behavioral, morphological, and physioenvironmental
ecological principles applied to the life histories
stategies of individual species. This study focused on the changes
in pelagic trophic structure in response to varying levels of
productivity (regulated by nutrient loading rates) and fish
densities (determined by stocking densities). Zooplankton life
history traits used to explain the community responses to these
two variables included: body size at sexual maturity, generation
time, reproductive potential, temperature dependant growth
rates, morphological adaptions to vertebrate and invertebrate
predation, and modes of food acquisition. A small naturally
fishless lake in southeastern Alaska was the observational unit
of this study conducted over a seven year period from 1980
through 1986. The objectives of this study were to 1) test
whether additions of inorganic fertilizer to the lake would
increase its capacity to produce coho (Oncorhyncus kisutch)
smolts, and 2) describe the structural and functional changes in
the pelagic ecosystem in response to varying rates of nutrient
loading and zooplanktivore densities. The study design consisted
of no treatments during four of the study years, and one year of
each of the following treatments: 1) fertilization only, 2) no
fertilization and low stocking densities (2,000 fry/ha), and 3)
fertilization and high stocking densities (4,000 fry/ha). During
nonfertilized years, rainfall was the driving force behind
nutrient loading rates. Nutrient loading rates (exclusive of
fertilizer additions) displayed a two to three fold magnitude in
annual variation over the seven year study period. Changes in
nutrient loading during nonfertilized years appeared to regulate
pelagic productivity, but in a complex and counter intuitive
manner. In contrast to the widely documented positive
correlation between chlorophyll concentrations and nutrient
loading rates, grazing pressure by zooplankton appeared to be
the factor regulating standing crops of phytoplankton. During
nonfertilized years, mean summer total chlorophyll was
inversely correlated to total zooplankton biomass, and displayed
a 2.5 fold magnitude in annual variation. Diaptomus kenia
biomass (the primary component of total zooplankton biomass)
was low in 1980 following a year with a low amount of summer
rainfall (1979), then increased after each year (1980 and 1981)
with relatively high amounts of summer rainfall. The positive
response of D. kenia to conditions stimulated by high summer
rainfall (and high nutrient loading rates) was delayed by their
long generation time (one reproductive period per year). D. kenia
and total zooplankton biomass peaked in 1982, the year that the
least amount of summer rainfall was observed. In contrast to the
intuitive perception that nutrient poor conditions are less
productive, the highest zooplankton production occurred when
nutrient loading rates and phytoplankton concentrations were at
their lowest levels. Accelerated nutrient regeneration rates and
phytoplankton turnover times were the most probable factors
which allowed the high zooplankton production to be maintained
under these nutrient poor conditions. Fertilizer treatments
increased the mean summer total chlorophyll concentrations four
to six fold over the maximum observation (1.03 μg/1) for
nonfertilized years. At high nutrient loading rates, the
relationship between total chlorophyll and zooplankton biomass
switched to a positive correlation. Rapid egg production and
short maturation times allowed the cladocerans to most
efficiently exploit the food resources stimulated by fertilization.
B. longirostris dominated zooplankton biomass during the
fertilized stocking period, but was only preyed upon by a small
percentage of the fish when larger zooplankters (Holopedium
gibberum and Epischura nevadensis) were abundant.
Fertilization treatments increased the the coho smolt producing
capacity of the lake by about 60 %. / Graduation date: 1991
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/38076 |
| Date | 20 August 1990 |
| Creators | Cameron, William A. |
| Contributors | Larson, Gary L. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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