Ecological stoichiometry is the study of the balance of chemical substances in ecosystems. In freshwaters, research has focused on how the ratios of carbon, nitrogen and phosphorus in organisms and their environment affect ecosystem processes. Because autotrophs have variable stoichiometry, particulate C:N:P ratios are used to assess nutrient availability in lakes. Zooplankton have relatively fixed stoichiometry and so differences between their body stoichiometry and the stoichiometry of their food can constrain their growth. Ecological stoichiometry predicts that zooplankton with low C:P body ratios (e.g., Daphnia) will be limited by the P content of their food in lakes where seston C:P is high. The stoichiometric theory of consumer-driven nutrient recycling (CNR) predicts that the stoichiometry of a consumer will influence the stoichiometry of the nutrients they regenerate through such processes as egestion and excretion. In lakes, zooplankton with a low body N:P are expected to regenerate nutrients in a high N:P ratio, potentially shifting nutrient limitation of the food web from N to P limitation. I used data from 99 Canadian lakes to test the following: a. Are particulate C:P and N:P ratios consistent with other P deficiency indicators? b. Do seston C:P and N:P ratios affect zooplankton community composition? c. Does zooplankton community composition affect plankton P limitation as predicted by CNR? Particulate C:P and N:P ratios generally agreed with other P deficiency indicators, except dissolved phosphate turnover times (TTPO4). C:P and N:P suggested P sufficiency more often than TTPO4, possibly because these two indicators respond to P deficiency over different time scales. Most zooplankton biomass parameters were negatively related to seston C:P ratios consistent with improved food quality at lower seston C:P. There was, however, no evidence that Daphnia were more strongly affected than any other zooplankton. Turnover times of particulate P in the whole plankton assemblage were not related to zooplankton community structure parameters. However, particulate P turnover in the >200 µm size fraction increased with increasing zooplankton biomass. There was no evidence for a particular effect of Daphnia on particulate P turnover. Phosphorus deficiency indicators showed a trend of relaxing P deficiency as zooplankton biomass and the proportion of Daphnia increased. This contradicts the predictions of CNR which suggest that Daphnia should cause greater P deficiency in lakes.

Aerobic methanotrophs (MOB) are a functional group of proteobacteria that use methane as their only energy and carbon source. Phylogenetically, such methanotrophs are affiliated with present alpha and gamma proteobacteria. Methanotrophic microorganisms play an essential role in the methane cycle, since they to a great extent reduce potential methane emissions to the atmosphere. In stratified lakes the aerobic methanotrophs are commonly present at the oxic-anoxic interfaces. This study aims to identify the environmental factors that could regulate the distribution of aerobic methanotrophs in the water column of freshwater lakes as well to assess whether a vertical community structure exists there. Lakes Erken and Tämnaren located in central Swedenwere studied using molecular analysis and environmental data. Aerobic methanotrophs were detected throughout the water column of Lake Erken. Two types of methanotroph communities were identified, with distribution patterns that appeared to be correlated to levels of methane in the water. Furthermore one of these communities feature changes in the relative abundance of its member populations along a depth gradient suggesting further differentiation along chemical gradients in the hypolimnion. Sequencing of pmoA amplicons revealed only members of the gamma proteobacteria methanotrophs. Also two novel operational taxonomic units were identified. Methanotrophs communities between Tämnaren and Erken also differ. Together these results indicate the existence of distribution patterns in methanotroph freshwater communities.

Thesis (M.S.) in Resource Economics and Policy--University of Maine, 2009. / Includes vita. Includes bibliographical references (leaves 71-80).

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. William C. Burnett, Florida State University, College of Arts and Sciences, Department of Oceanography. Title and description from dissertation home page (viewed June 16, 2004). Includes bibliographical references.

In this thesis we develop a model for the long-time horizontal circulation in a shallow lake. The goal is to have a model that can capture the large-scale features of the circulation yet can be run quickly and cheaply. We start with shallow water models and add relevant physical terms: Coriolis force, wind shear, bottom drag, viscosity, and nonhomogeneous boundary conditions. The resulting equations are similar to the two-dimensional Navier Stokes equations and can be analyzed with similar methods. We pose the equations in a weak form and show that they are well-posed. We then discretize the equations. We use the finite element method for the spatial discretization and show that our choice of elements satisfies stability criteria. Next we test our model. We first consider problems with analytically tractable behavior and verify that our model produces correct results. Then we model Lake Erie, both with no wind and with a steady wind. We compare the results of our model to experimentally obtained measurements of the currents. Our results compare well under conditions of no wind or of steady wind, but not as well when the wind is variable.

Recently, the growth and condition of Lake Whitefish (Coregonus clupeaformis), an important commercial fishery commodity, has declined in some areas of the Great Lakes. As Lake Whitefish are benthic feeders, historically the bulk of their diet was made up of the energy rich Diporeia hoyi, an amphipod whose abundances declined concurrently with reductions in Lake Whitefish growth and condition. Lake Whitefish populations from lakes Michigan, Erie and Superior have been used to determine whether there is a plausible link between the declines in Lake Whitefish condition and Diporeia abundance as indicated by stable isotope analysis, dietary and condition indices for Lake Whitefish. The first study was Lake Michigan specific and tested the hypothesis that condition, in terms of relative weight, percent lipid and docosahexaenoic acid (DHA), improved as the proportion of high quality prey (e.g., Diporeia) in the diet increased. Samples of spawning whitefish from four regions (northwest, Naubinway, Elk Rapids and southeast) around Lake Michigan had distinct mean carbon and nitrogen isotope signatures. The signatures indicated Lake Whitefish may be using a variety of prey items, especially the Naubinway population where fish occupy the largest isotopic niche space. Relative weight was significantly higher in the southeast and lower for all northern regions. The mean measured lipid from Lake Whitefish dorsal skinless muscle biopsies was highest for northwest region fish. DHA was significantly different among studied regions, with higher mean values in Elk Rapids and the northwest. No linear relationships between stable isotope measures and condition metrics were found. These results suggest that Lake Whitefish are coping with declining Diporeia abundances by feeding on alternate prey. Results do not substantiate the hypothesis of a relationship between condition and prey use, although Lake Whitefish from Elk Rapids and the northwest had high quality prey and good condition. The second study incorporated eight spawning populations around lakes Michigan, Erie and Superior to determine if there were differences in energy available to female Lake Whitefish. The hypotheses tested were three fold: (1) observed differences in growth and reproduction among populations were driven by energy availability, (2) populations with low energy reserves exhibit reproductive trade-offs and (3) high energy reserves are related to Diporeia consumption. Lake Whitefish from lakes Erie and Superior both displayed high growth and no trade-offs between egg size and number. Populations from Lake Michigan all had low investment in growth, significantly lower gonadosomatic index (GSI) and four of these populations displayed significant trade-offs between egg size and number. Growth and GSI tended to increase with mean total lipid stored in muscle and populations displaying reproductive trade-offs had significantly lower muscle lipid, however, these trends were not evident when using DHA as a measure of good nutrition. No relationship was evident between Diporeia consumption and populations with high muscle lipid stores. Overall, my results suggest that while changes in the availability of prey resources has had an effect on Lake Whitefish populations, the impact of declining abundances of Diporeia alone cannot explain the systemic changes in Lake Whitefish condition across the Great Lakes basin.

[Truncated abstract] Coastal lagoons play an important role in the transport of materials between the coastal zone and the ocean. Understanding the dynamics associated with the movement of waters between and within these systems is therefore significant in defining the ecological health of the system. An important sub category of lagoons is Intermittently Closing and Opening Lakes and Lagoons (ICOLLs). These systems lack any significant river inflow; have a restricted sill type inlet and experience intermittent exchange with the ocean, making them susceptible to the retention of nutrients and pollutants from the catchment. The duration and frequency of an opening event may vary from weeks to months between each ICOLL, and inter and intra annually, respectively, and during an opening event, there are appreciable fluctuations in water level (1-3m range) accompanied by large changes in salinity (7 30ppt) within a short timeframe (hours). . . Regardless of the processes complete oceanic flushing was still predicted for each system, by the end of their respective opening events. The modelling work successfully reproduced the spring tidal setup in water level and exchange, using real bathymetry and meteorological forcing and defined the spring tidal set-up as the key predictable process in the exchange of water and salt between the larger ICOLL and the ocean. It was also shown that strong winds had the capacity to influence the magnitude of the exchange. The overall outcomes of this research therefore include the identification of key physical processes associated with the variability of the hydrodynamics within and between ICOLLs, which will aid in the future management of these highly dynamic systems.

Thesis (M.Sc.)--York University, 2006. Graduate Programme in Biology. / Typescript. Includes bibliographical references (leaves 88-108). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR19705

Diss. (sammanfattning) Uppsala : Uppsala universitet, 2005. / Härtill 5 uppsatser. Med sammanfattningar på svenska och tyska.

Thesis (M.S.)--Bowling Green State University, 2008. / Document formatted into pages; contains vi, 52 p. : ill., maps. Includes bibliographical references.