Fatty acid (FA) analysis is a powerful ecological tool for examining trophic relationships among marine organisms. Its application in benthic food webs may be limited because many benthic organisms consume a highly mixed diet, and FA metabolism of benthic invertebrates may obscure dietary markers. This thesis examines the use of FA as dietary tracers for studying the diets of benthic invertebrates and the fate of primary production in rocky subtidal food webs. In Chapter 2, I review the use of FA for studying benthic marine food webs, and suggest that field studies using FA analysis should also include data from controlled feeding experiments, gut contents, or stable isotope analysis to provide more reliable results. In Chapter 3, I compare FA composition among sea urchins fed four natural algal diets in a controlled feeding study. These sea urchins substantially modified their dietary FA but differed in their overall FA composition according to diet. In Chapter 4, I use FA to trace the invasive alga Codium fragile ssp. fragile and the native kelp Saccharina longicruris through two trophic transfers in an experimental food web. Substantial signal attenuation occurred with each trophic transfer, suggesting that FA analysis may be of limited use for tracing benthic primary producers in field studies. In Chapter 5, I use FA in conjunction with stable isotope analysis and gut contents analysis to investigate the contribution of detrital kelp to the diet of sea urchins in habitats adjacent to kelp beds. FA analysis was unable to distinguish among sea urchins at different distances from the kelp bed, but the results of all analyses indicated that the availability of kelp detritus declines with distance from the kelp bed, and that sea urchins in low-productivity habitats rely on both kelp detritus and benthic diatoms. In Chapter 6, I summarize the overall findings and suggest experimental and statistical methods to address some of the problems associated with using FA analysis to study trophic relationships in benthic food webs.

Biodiversity can be described as the total variation of life forms, where diversity ranges from gene level up to the ecosystem level. The diversity can be calculated in a number of ways, and this study use one of these methods. In this study empirical food webs have been used and analyzed, where eleven characters are defined and used as parameters to calculate the Euclidean distances between food webs that describe the variation that may exist within classes of terrestrial, marine and freshwater habitats. The class who stood out and showed the greatest diversity at the ecosystem level was the marine food webs, which showed a high value of the average euclidean distance. The other networks were not as distinctive and the average of the euclidean distance in these classes was comparatively low.

Thesis (M. Ag. Sc.)--University of Adelaide, Dept. of Entomology, 1985. / Mounted illus. Includes bibliographical references (leaves 141-153).

Thesis (Ph. D.)--University of Wyoming, 2006. / Title from PDF title page (viewed on June 26, 2008). Includes bibliographical references.

No description available.

Stream food webs are supported by carbon produced within the stream (autochthonous) and from terrestrial environments (allochthonous). Allochthonous carbon (C) inputs are assumed to be the dominant C source supporting food webs within small streams, but few direct estimates of resource use in small streams have been made, especially in boreal streams. The objective of this study was to determine the relative dependence on allochthonous and autochthonous C by consumers in relation to C pools within streams with high terrestrial inputs. Furthermore, this study aimed to investigate if the relative resource use of allochthonous and autochthonous C by consumers differed among seasons (summer and fall), between streams of different sizes, and locations within the catchment. To estimate consumer resource use, δ2H signatures for organic C sources were compared to those of six key consumers in five streams of varying catchment sizes in northern Sweden. Macroinvertebrate biomass was quantified to calculate a taxa-specific biomass-weighted allochthony, and compared with the mass of different C pools potentially available for consumers. The biomass-weighted mean allochthony for all samplings ranged between 43.5-61.5%; there was thus high autochthonous support despite low algal density and high terrestrial C pools within the streams. No significant trend in allochthony was observed over season (linear regression, p-value >0.05). Allochthony differed by invertebrate taxa and was not related to stream size or location in catchment. These results suggest that autochthonous C is far more important for consumers in boreal streams than previously recognized.

Generalist predators can contribute to vital ecosystem services by potentially inducing trophic cascades as natural enemies of pests in agroecosystems. As the human population of the world gets larger, we need to produce more food on ever-smaller swaths of available land relying on ecosystem services, in the form of pest control, that may contribute to agricultural sustainability. Teasing apart the exact trophic linkages between predators and prey is a vital first step and essential to uncovering which predators are inducing trophic cascades and should be enhanced through conservation biological control. Combined with ecological experimentation, the main tool used throughout my research to identify trophic linkages is molecular gut-content analysis. I began by investigating mass sampling techniques and found they do not cause contamination in gut-content analysis and may be a simple method for collecting large numbers of cryptic predators for use in determining trophic linkages. Additionally, my research uncovered trophic interactions between stink bugs and generalist predators at multiple scales. Overall, I successfully designed molecular methods to investigate relationships between agricultural pests and generalist predators. A multi-year field study uncovered low predation on stink bug pests in contrast to previous research suggesting that generalist predators were contributing highly to biological control. This research highlights the need for replicated studies before making broad conservation biological control decisions. Although generalist predators were not consuming stink bugs in large numbers, my field cage study showed evidence of superfluous killing by spiders on adult stink bugs, highlighting the need to combine ecological studies with molecular methods to understand consumptive and non-consumptive effects on prey items. Gut-content analysis showed no evidence of consumption, but the field cage study allowed me to uncover the complicated relationships between spiders and stink bugs. In addition, I showed an invasive species can be detected in new areas through molecular gut-content analysis of predators before other sampling methods.

Biodiversity loss is one of the major threats to humanity. This has led to an increasing amount of research on biodiversity on genetic and species levels. Studies of diversity at the ecosystem level has however been neglected. An important aspect of ecosystems is food webs that describe the predation-prey interactions between species. Properties explaining the topological structure of food webs can be used to compare and highlight differences between ecosystems. In the present study, topological network properties are used to compare the diversity of network structures between groups of empirical food webs. Differences between 45 aquatic and 45 terrestrial food webs are compared as well as the effects of species richness on lake network structure diversity. Network structure diversity is measured as the average Euclidean distance from food webs to their group centroid in a multidimensional space of network properties. While the average network structure differs between aquatic and terrestrial food webs, no significant difference in variation is found. For 128 Swedish and 48 North American lake food webs, increasing species richness is shown to decrease network structure diversity. A higher diversity of network structures could potentially indicate a more ways to cope with disturbances or provisions of a higher variety of ecosystem services. Preliminary tests of ecosystem diversity effects on stability were conducted but proved inconclusive.

Given the rapidly increasing visitation levels to Fraser Island, there is increasing concern that tourist activities may threaten the long-term ecological health of the region's unique dune lakes. This project aimed to investigate the consequences of tourist use of Fraser Island's dune lakes and to develop appropriate monitoring tools and management objectives in light of the projected increases in visitation levels in the foreseeable future. The initial phase of this research aimed to identify the relative importance of some of the most popular dune lakes on the island as key destinations for tourists. Tourist surveys, in conjunction with the development of a Tourist Pressure Index (TPI), which quantifies logistic, social and natural variables, identified Lakes McKenzie, Allom and Birrabeen as the lakes most at risk from excessive tourist use. In addition, analyses of water quality in 15 lakes on Fraser Island aimed to determine the current trophic status of dune lakes on Fraser Island and the ecological implications of tourist use of these systems. Detailed comparisons of nutrient and chlorophyll a concentrations in five popular dune lakes in February 1990 (data from Arthington et al. 1990) and February 1999 suggested that productivity has increased significantly in the past decade. More detailed examinations of nutrient and algal variables in five popular perched dune lakes revealed that while ambient nutrient and phytoplankton chlorophyll a concentrations remained relatively stable, periphyton chlorophyll a concentrations increased over the course of the 1999-2000 summer in most lakes. Significantly, these increases were found only in heavily visited (disturbed) sites in the clear lakes examined (McKenzie and Birrabeen). In these lakes, where algal growth is likely to be only limited by nutrient availability, tourist nutrient additions may stimulate excessive periphyton production. Experimental algal bioassays identified that phytoplankton and to a lesser degree periphyton growth was stimulated by nutrient additions in all five perched dune lakes. However, the degree to which growth was stimulated was both lake and nutrient (nitrogen versus phosphorus versus nitrogen + phosphorus) dependent, highlighting the variable nature of systems within a relatively small geographic range. Since periphyton biomass was higher in heavily visited areas of lakes and was likely to be stimulated by nutrient additions by tourists, stable isotope analyses of littoral zone food webs were conducted to quantify the percent contribution of periphyton to consumer diets. There was a trend towards higher periphyton contributions in systems identified as key tourist locations (on the basis of their TPI scores) and this indicates that increasing visitation may increase the contribution of periphyton to littoral zone food webs, both via increases in the quantity and quality of periphyton as a food resource. To further explore the contribution of periphyton in littoral zone food webs of heavily visited lakes, a 15N-tracer addition experiment was conducted to establish the fate of nutrient additions within the littoral zone. Nutrients were added in quantities that mimicked those likely from tourists, to enable a realistic appraisal of the fate of tourist additions. As expected, periphyton rapidly assimilated the added 15N-tracer and was found to be the first and most significant sink for nutrients entering the littoral zone. Finally, the results from this research were used to develop a conceptual model of nutrient enrichment for perched dune lakes on Fraser Island. The model indicates that although nutrient additions from tourists may lead to undesirable increases in periphyton biomass, the degree to which this is deemed to be a detrimental ecological outcome is likely to be mediated by water level fluctuations and the consumptive capacity of grazers. Given that excessive periphyton growth is likely to be seen as negative impact of tourism, regular periphyton monitoring (biomass and percent contribution to littoral zone food webs) should be built into an updated monitoring program for this series of dune lakes. Whilst the implementation of periphyton monitoring is likely to enable the early detection of deleterious impacts of excessive tourist use, it is likely that the long-term conservation of the region will, in the future, require the implementation of strict visitation level guidelines, to ensure that the irreversible consequences of long term additions of nutrients are ameliorated.

The major aim of this thesis project was to determine the variables that most explain the elevated mercury concentrations in lake trout (<i>Salvelinus namaycush</i>), a predatory aquatic fish species in some lakes in northwestern Canada. High mercury concentrations in lake trout in other regions have been associated with the biological features of the fish and various chemical and physical aspects of their aquatic ecosystems. Data including lake trout age, length, weight, and stable isotope values, water chemistry, latitude, and lake and watershed area were collected, compiled and then included in statistical analyses of the factors affecting mercury concentration in the muscle of lake trout from a series of lakes from the Mackenzie River Basin (MRB) in the Northwest Territories (NT), Canada. These results are reported in Chapter 2. Fish age and lake surface area were the most important variables affecting mercury concentrations. However mercury concentration in muscle also was significantly (p < 0.05) related to: fish length, weight, and δ13C; watershed area to lake area ratio; and to total mercury concentration in zooplankton and water. These variables were run through best subsets analyses and multiple regressions in order to determine the regression equation most efficiently capable of predicting mercury concentration in lake trout in unstudied lakes in the MRB region. The resulting equation was: log Hg = 0.698 (0.0156 × latitude) + (0.0031 × age) + (0.000535 × length) (0.245 × log lake area) + (0.00675 × watershed area/lake area ratio), r2 = 0.73<p>Small lakes located in the southern NT and dominated by large and/or old lake trout are most likely to have lake trout whose mean mercury concentrations exceed 0.5 μg/g; the guideline for the commercial sale of fish. Latitude may be linked to mean annual temperature (and variables such as duration of ice cover, summer water temperature) while fish age and length may be related in part to fishing pressures and growth rates on these lake populations. In chapter 3, a more in-depth study was undertaken to investigate of role of feeding and relative tropic level in the bioaccumulation of mercury in lake trout. This was accomplished by comparing MRB lake trout population characteristics with those from a series of lakes in northern Alberta and Saskatchewan (NAS). The two population groups were compared with respect to size, age, growth rates, and mercury concentrations. In addition, trophic and mercury biomagnification relationships, as inferred from stable carbon and nitrogen isotope analyses, for the two lake trout populations were compared. Lake trout from the NT exhibited significantly higher mercury concentrations than those from the NAS lakes (p < 0.001). Mercury concentrations in biota (including lake trout, forage fish, benthic invertebrates and zooplankton) were positively and significantly correlated to δ15N values in all lakes in both of the study areas (p < 0.001). Mercury biomagnification in the NT lakes, as estimated from the slope of δ15N versus mercury concentration, was lower than in the NAS lakes. Thus, mercury biomagnifies more slowly in NT lake trout, but because of their greater mean age, reaches higher values than in NAS lakes. Northwest Territory lake trout generally exhibited more negative δ13C values, indicating more pelagic feeding habits than in NAS lakes: higher mercury concentrations previously have been associated with more pelagic feeding. <p>Finally, the relationship between mercury levels and growth rates in lake trout was investigated by comparing NAS and NT lake trout populations. These results are reported in chapter 4. Lake trout from the NT lakes grew at a slower rate (10.4 mm per year) than those from the NAS lakes (35.1 mm per year). Log mercury concentration was inversely correlated (p < 0.001) with growth rate for both lake trout populations; however, growth rate explained more of the variation in mercury level in the NT lakes than in the NAS lakes (NT, r2 = 0.11, p < 0.001; NAS, r2 = 0.03, p = 0.024). However, the correlation between mercury concentration and growth rate in the NAS study area improved when Reindeer Lake, possibly affected by anthropogenic inputs, was removed from the analyses (r = 0.13, p = 0.001). Therefore, lower mercury levels in lake trout are associated with higher growth rates through growth dilution. The higher mercury concentrations in NT lake trout are due not only to the old age of the fish, but to slower growth rates as well.