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Large rivers in North America have been subjected to a variety of hydrologic alterations that have negatively impacted aquatic fauna. These impacts have triggered restoration efforts, including management of flows, to restore or maintain ecological integrity. Ecological data relevant to flow management and habitat restoration is scarce, and conceptual models of ecosystem function have been widely applied to large rivers despite a lack of quantitative evaluation of these models. Here, I examine demographic and trophic dynamics of fishes with divergent life histories and trophic guilds in relation to habitat heterogeneity and flow variability in a relatively unaltered floodplain system: the Brazos River, Texas. Reproductive activity of fishes with three divergent life history strategies was positively associated with long-term river hydrology, although species with alternate strategies exploited different portions of the hydrograph (peak flow versus increasing flow). Despite the positive association with hydrology, low-flow periods were favorable for recruitment, and food resources for larvae and juveniles were denser during these periods. Some species used both oxbow and channel habitats during some point in their life cycle, whereas other species appeared to be almost entirely restricted to one habitat type. Terrestrial C3 macrophytes accounted for a significant fraction of the biomass of most consumer species examined. Small-bodied species in oxbow lakes assimilated large fractions of biomass from benthic algae, whereas this pattern was not observed in the river channel. Frequent flow variations in the river channel may reduce algal standing stocks, and significant contributions from autochthonous algal sources may only occur during low-flow periods. Trophic positions of detritivores indicated that terrestrial carbon sources were assimilated, for the most part, via invertebrates rather than by direct consumption. My results indicate that current conceptual models are too vague to provide accurate predictions for restoration strategies in rivers with variable flow regimes. Flow and habitat management strategies that focus on reproducing key features of historical fluvial dynamics are likely to be more successful than strategies that focus on single indicator species or flow dynamics that differ from the historical hydrograph.

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Freshwater ecosystems reflect the condition of their surrounding landscape, and thus are particularly vulnerable to anthropogenic stressors associated with human land-use. One of the most prevalent stressors on stream ecosystems in agricultural regions, such as the Canterbury Plains of New Zealand, is eutrophication, or increased primary productivity. The aim of this thesis was to investigate effects of eutrophication on stream communities, specifically food web structure and ecosystem function. From a food web perspective, eutrophication is a shift in the form and amount of available energy from externally-produced (allochthonous) to internal (autochthonous) basal resources. Such shifts are frequently associated with land-use intensification, due to riparian vegetation removal and increased nutrient inputs, both of which enhance autochthonous production. A field survey across a gradient of eutrophication showed that eutrophic stream food webs are largely autochthonously-based and often contain large numbers of defended primary consumers, which form trophic bottlenecks and prevent energy from reaching higher trophic levels. Consequently, while there is more total energy available, less of that energy is in a usable form for stream food webs. Moreover, I found that eutrophic streams are largely composed of generalist consumers, which shift their diets to refocus on autochthonous resources with increasing productivity. Given that eutrophication causes food web resources to become more homogenous and was a primary driver of food web change, I tested whether reintroducing allochthonous subsidies would alter or reverse the negative effects of eutrophication. To do this I conducted a short-term community assembly experiment and a year-long population biomass accrual study. I found that the simplified, generalist-dominated communities in eutrophic streams did not respond to changes in resource diversity as predicted by food web theories, which are based on more diverse food webs. After restoration of allochthonous subsidies, defended generalist taxa continued to dominate the invertebrate communities. However, while restoring allochthonous subsidies did not mitigate the numerical dominance of defended consumers, the biomass accrual of other, previously excluded desirable taxa, such as mayflies and predatory invertebrates, increased following resource additions. This indicates that more energy reached the top of the food web, suggesting that resource additions alleviated trophic bottlenecks. Overall, my findings have advanced current knowledge about key mechanisms driving food web responses to both anthropogenic stress and to restoration efforts, which can be applied to improve management and restoration of stream ecosystems.

I investigated, water, invertebrates and fishes from lakes in Banff National Park and Kananaskis Country, Alberta for selenium (Se), an element known to be toxic to vertebrates. At some depths, Se concentrations in sediment exceeded recognized thresholds for bird and fish reproductive impairment. Se concentrations in water were over USEPA guidelines after spring melt runoff. In aquatic invertebrates, Se concentrations exceeded values known to cause reproductive impairment in fish and bird predators. Se concentrations in all fish species exceeded known thresholds for reproductive impairment in avian consumers and the majority surpassed concentrations that would negatively affect wildlife and human consumers. Se concentrations in some fish species have significantly increased over the past 6-16 years. The strongest predictors of fish Se concentrations were growth rate, condition factor, age, weight, trophic position (within lakes) and vegetation type (among lakes). These results suggest that consumption advisories are desirable for several lakes in the Banff and Kananaskis area, and that Se concentrations in fish from other area lakes should be investigated. / Ecology

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Numerous anthropogenic disturbances have occurred in large lakes over recent decades. These may alter concentrations of the biomagnifying contaminant mercury (Hg) in fish, but long-term impacts of disturbances on Hg trophodynamics are poorly understood. Elemental analyses of archived museum ichthyology specimens could be used to study historical, pre-disturbance fish food webs, but there is uncertainty about effects of chemical preservatives on the results of such analyses. In this thesis, long-term preservation effects were studied, and archived fish were used to reconstruct historical food webs and Hg trophodynamic patterns in three large North American lakes, Nipigon, Simcoe and Champlain. After 24 months of formalin/ethanol preservation, fish muscle delta-15N and delta-13C had average changes of +0.4 ‰ and -0.9 ‰, respectively. Shifts in mean Hg concentration was +5 % after 12 months. A suite of 26 other elements analyzed over 24 months showed consistent responses to preservation, usually involving an increase in concentration immediately following preservation. In the second phase of the thesis, stable isotope and Hg analyses were performed on archived and modern fish from the study lakes, dating to the 1920s-60s and 2006-7, respectively. Trophic relationships were often relatively stable over time, but stable isotope metrics revealed a decrease in Lake Nipigon delta-15N range and less pelagic feeding among Lake Simcoe pumpkinseed and yellow perch. In Lake Champlain, the re-introduction of lake trout in recent decades did not have a major effect on overall food web dimensions. Significant Hg biomagnification factors were found in 1920s and 2006-7 Lake Nipigon (which were not statistically distinguishable from each other) and 2006 Lake Champlain. These biomagnification factors ranged from 0.09 to 0.17, which is within the range found in other studies globally. Archived fish and government monitoring records indicated that fish Hg concentrations decreased in Lakes Simcoe and Champlain since historical periods, but remained similar or increased in Lake Nipigon. This thesis confirms the utility of archived fish for elemental analyses. It highlights the risks of Hg contamination and food web change that may be faced by remote lakes, and it provides evidence for relatively stable Hg biomagnification rates in large lakes. / Thesis (Ph.D, Biology) -- Queen's University, 2013-01-30 15:43:33.438