Responses to fertilization and fish stocking in the pelagic ecosystem of a naturally fishless lake Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ws859j634

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  • 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 %.
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