This thesis examines broad-scale patterns in the food web structure of lake ecosystems. The unifying framework for this analysis is the concept of trophic position, which represents the energy-weighted trophic path length leading to a consumer or population. Trophic position was estimated for over 550 populations of fish (16 species) using quantitative dietary data. A method to estimate trophic position of aquatic consumers using stable isotope ratios was developed; this involved correcting for within and among-lake variation in delta15N of primary consumers (organisms used to represent the base of the food web). We report an overall correspondence between dietary and isotopic estimates of trophic position; for each species, trophic position generally ranged the equivalent of one trophic level unit among-populations. The concept of discrete trophic levels provided only a qualitative description of energy flow pathways in aquatic food webs. Among-population variation was much greater than within-population variation in trophic position of lake trout. Long-standing hypotheses about the determinants of food chain length were examined; food chain length was most closely correlated with species richness and lake area. / The trophic position approach was used to examine two separate environmental problems relevant to lakes: the bioaccumulation of persistent contaminants in food chains and the impacts of invasive species. Trophic position was the major determinant of PCB levels in lake trout; these relationships were used to characterize biomagnification factors (BMFs) for this and a number of other contaminants. Furthermore, the introduction of smelt into lakes was linked to increased levels of PCB and Hg contamination in lake trout. Stable isotopes were used to quantify the impacts of smallmouth bass and rock bass invasions on food webs leading to lake trout. Lake trout from invaded lakes exhibited reduced consumption on littoral prey fish; a food web shift that is likely to have detrimental impacts on native lake trout populations.

Filter-feeding macrozooplankton were collected from 24 lakes in south-central Ontario to examine relationships between environmental factors and methyl mercury accumulation. Zooplankton methyl mercury levels ranged from 19 to 448 ng$ rm cdot g sp{-1}$ dry weight in the study lakes and were highest in zooplankton from acidic brownwater lakes. Water color and lake water pH were the best predictors of methyl mercury levels in zooplankton explaining 73% of the variation. Methyl mercury concentrations were positively correlated with water color and inversely correlated with lake water pH. Water color explained a greater portion of the overall variance in methyl mercury levels, indicating that the supply of mercury from the drainage basin plays a key role in determining methyl mercury concentrations in the lacustrine biota. Zooplankton methyl mercury levels were well correlated with mercury concentrations in smallmouth bass (Micropterus dolomieui) and largemouth bass (Micropterus salmoides) from 11 of the study lakes showing zooplankton to be good indicators of the relative bioavailability of mercury at the base of the food chain.

No abstract available. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1980.

Thesis (M.S.)--Youngstown State University, 1996. / Includes bibliographical references (leave 71-73).

Thesis (doctoral)--Wageningen Agricultural University, 1998. / Summary and conclusion also in Dutch. Includes bibliographical references.

Thesis (M.S.)--Oregon State University, 1991. / Typescript (photocopy). Includes bibliographical references (leaves 78-82). Also available on the World Wide Web.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1971. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 1986. / Vita. Includes bibliographical references (leaves [124]-137).

No description available.

Two biomass indicators (direct counts and ATP analysis) and two activity estimators (glucose uptake and dehydrogenase activity) were used to study the bacteria at 1 m water depth in Marlon Lake sediments. Direct count-biomass estimates for bacteria averaged 0.6l gC/m², were high in summer, declined rapidly in fall, then increased during the winter. Microorganisms less than approximately 30 μm diameter had a mean biomass of 1.28 gC/m² as measured by ATP analysis. Seasonal variations in this figure paralleled changes in the algal population, although algal contributions to the microbial biomass were less than 50 per cent. ATP analysis was also used to estimate the biomass of the sediment community, excluding animals greater than approximately 5 mm in length. The mean community biomass was 4.69 gC/m². Comparison of ATP data with enumeration data obtained by others, suggests that ATP is a good biomass indicator, except when cellular ATP levels are changed in reaction to biotic or abiotic environmental factors. It is proposed that, in situations such as intense grazing or rapid Increases or decreases in temperature, ATP measurements reflect not only biomass but also activity. At such times ATP-biomass data may Indicate biomass potential, or the capacity of the population to maintain its biomass under abnormally high rates of loss. Dehydrogenase activity, or respiratory potential, of the sediment bacteria was assayed using trlphenyl tetrazolium chloride. The estimate for annual rate of carbon loss as CO2 from the bacterial population (19.3 g C/m²•year), was almost identical to previously reported data obtained by measuring oxygen consumption in normal and antibiotic-treated sediment cores. Uptake of ¹⁴C-glucose was determined in mixed, diluted sediments. The maximum uptake rate (9.6 g C/m².year), natural uptake rate (5.3 S C/m².year) and the natural turnover time (0.31 h) were similar to previous data for these sediments. This suggests that, at least in terms of function, the bacterial population is quite stable from year to year. Biomass and activity of the Marion Lake sediment bacteria were found to be in phase in late spring through early fall, but activity remained low in winter despite Increases in the microbial biomass, and increased much more rapidly than biomass in early spring. The dynamics of the bacterial population are discussed in terms of these relationships. The size of the algal and bacterial populations and organic matter reservoirs, and the flux rates for carbon between these compartments are summarized. / Science, Faculty of / Zoology, Department of / Graduate