Freshwater ecosystems are some of the most threatened on the planet. Efforts to conserve, restore, or otherwise manage large rivers and the services they provide are hindered by limited understanding of the functional dynamics of these systems. This shortcoming is especially evident with regard to trophic structure and energy flow. In this study I use natural abundances of carbon and nitrogen isotopes to examine patterns of energy flow and food-chain length of large-river food webs characterized by different landscape-scale hydrologic features. Ten locations along an approximately 500 km stretch of the Upper Paraná River Basin, Brazil, provided the setting for this work. Carbon derived from C3 plants and phytoplankton were the dominant energy sources across all webs, but relative contributions differed among landscape types (low-gradient river, high-gradient river, river stretches downstream of reservoirs, and reservoirs). Increases in food chain length corresponded with higher relative importance of phytoplankton derived carbon, likely due to size-structured effects of the phytoplankton-zooplankton-secondary consumer trophic link. River impoundment corresponded with decreased ecological and economic efficiency of fisheries production, an important ecosystem service provided by many tropical rivers.

Sandy beach ecosystems have been studied worldwide; however, ecological data are sparse for the extensive barrier islands of Florida. Accordingly, I investigated the feeding patterns of the ghost crab (Ocypode quadrata), a dominant omnivore inhabiting beaches along the Floridian coast. Density data was collected for ghost crabs and swash macroinfaunal prey. In addition, I utilized stable isotopes in conjunction with the mixing models IsoSource and SIAR to characterize diets of ghost crabs across three barrier islands in spring and summer 2011. Results showed that ghost crabs at Cayo Costa feed primarily on swash macroinfauna, while those from Anclote Key shifted their diet to one comprised primarily of semi-terrestrial amphipods. However, at Honeymoon Island, ghost crab isotopic signatures were best explained by a mixed diet of both macroinfauna and wrack-associated prey. The unique consumption of wrack fauna at Anclote Key co-occurs with comparatively low infaunal densities and biomass, and modified ghost crab behavior due to trait-mediated effects. My results are novel because they suggest that wrack-associated fauna may be an important food source for ghost crabs in certain beach regimes.

The Chukchi and Beaufort Sea shelves host diverse and productive seafloor ecosystems important for carbon and nitrogen cycling for the Arctic Ocean. The benthic food web transfers energy from primary producers to high trophic level organisms (e.g., birds, fish, and mammals), which are important for cultural practices and subsistence hunting by Native Alaskans. This work focuses on the trophic ecology of arctic food webs through use of several different approaches. First, variation in the natural abundance of stable carbon and nitrogen isotopes facilitated the identification of trophic pathways and, subsequently, allowed the comparison of trophic guilds and food webs from the Chukchi and Beaufort Seas. Compared to water column and sedimentary organic matter end-members, second trophic level grazers and suspension feeders were conspicuously ¹³C-enriched throughout the Chukchi Sea, which supports the hypothesis that microbial degradation of organic matter occurred prior to metazoan assimilation. Second, food web recovery from disturbances caused by exploratory oil drilling at the seafloor that had occurred approximately 20 years prior were assessed in both the Chukchi and Beaufort Seas. Based on isotopic trophic niche overlap between organisms common to drilled and reference sites in the Chukchi and Beaufort Seas, the oil drilling sites had similar food web structure, indicating recovery from the activity associated with the drilling process. Third, photosynthetic pigment biomarkers were used to better understand the diagenetic process, specifically focusing on how both microbial and metazoan grazing pathways degrade organic matter in relation to seasonal sea ice retreat in the Chukchi Sea. The benthic macrofaunal and microbial food web caused rapid degradation of organic matter upon the initial pulse of microalgal food sources to the seafloor. These diagenetic pathways are linked to the ¹³C-enrichment of residual organic matter, which corresponds to the stable isotope values measured in the benthic macrofauna. Lastly, high-precision liquid chromatography and spectrophotometry were compared for estimating sedimentary pigments in the marine environment. Substantial differences in pheopigment (chlorophyll degradation products) concentrations were observed between the two techniques, suggesting the need for revisions to the monochromatic spectrophotometric equation that relates absorbance to pigment concentrations. One pheopigment, pheophorbide, was found to interfere with the accuracy of the spectrophotometric equation and caused the overestimation of pheopigments. / text

Analysing ecological communities as complex networks of interactions has become an important tool for ecologists. Understanding how these networks change through time, over landscapes, or in response to disturbances is a primary goal of community ecology. The number of interactions and the way in which those interactions organise themselves as individuals, small groups, and the whole community can play an important role in predicting how ecological communities will respond to disturbances. In this thesis, we investigated variation in network structure at several scales both empirically and in a theoretical context. Our first hypothesis was that the structural role of species in a variable system would show little variation, despite high levels of species turnover and a fragmented landscape. In a collaboration with Riikkaa Kaartinen and Tomas Roslin, we studied the distribution of species’ roles at three scales in host-parasitoid networks collected from a fragmented forest in Finland. We found that species’ roles were remarkably consistent through time and in the presence of species turnover. These results suggest that species’ roles may be an intrinsic property of species and may be predictable over spatial and temporal scales. Our second study investigated the structural variation of simulated ecological networks and the relationship between structural variation and whole-network measures of network organization, such as connectance, nestedness, and modularity. We quantified structural variation of networks at three scales, macro-scale, motif-scale, and participation scale. These scales represent whole-network measures (macro-scale), sub-network measures (motifs – small groups of interacting species), and individual measures (motif participation). We compared the variation in these structures to connectance, nestedness, and modularity. We found that at fixed levels of connectance, nestedness, and modularity, the motif profiles of networks and the distribution of species across those profiles showed remarkable dissimilarity. This result suggests that networks displaying similar macro-scale structural measures can be composed of vastly different motif- and participation-scale structures. Together, the work that makes up this thesis suggests that we should give more attention to the meso-scale structures of ecological networks. As the more detailed perspective of motifs can capture additional detail about the structure of empirical networks, and as a result, provide a clearer picture of ecological communities. In addition, we found that the particular species themselves can have a significant impact on the meso-scale structure and, in some cases, may impose strict limitations on what interactions can occur within a community. This has important implications for our understanding of how ecological networks are built and maintained, and thereby for our understanding of the stability and resilience of ecological communities.

No description available.

Cumulative effects, often minor individually but collectively significant, are continually being grappled with by researchers, policy makers and practitioners. Despite this the Canadian approach to cumulative effects assessment is thought by many to be ineffective. In this thesis I investigate the literature that surrounds cumulative effects and uncovered three distinct themes that occur chronologically: genesis, project-based approach and integration. During the genesis phase cumulative effects nomenclature, ideas and frameworks was created. The main theme of this era was to asses these effects at a large scale and to have a strong understanding of a systems ecology prior to the assessment. This approach was found to be too complex and so a more narrow project-based approach was implemented and still remains today. This approach is heavily criticized and as such researchers are now trying to find an approach that integrates these two divergent themes into a regional level assessment. I have found there to be several frameworks but an absence of effective regional methodologies. There is a need for regional metrics if this approach is to ever be institutionally supported. Food web structure can be evaluated at multiple scales and has been shown to be responsive to environmental variation; thus, it has potential for application as a metric for cumulative effects. Here, using stable isotope analysis, I field test integrative measures of food web structure (food chain length, habitat coupling, trophic omnivory) at sites of varying degrees of anthropogenic stressors in Georgian Bay, Ontario to evaluate the use of food webs as a metric for cumulative effects assessment. I found that food web structure varied significantly among sites. Sites with high levels of stress displayed structural characteristics reflective of human activities such as shorter food chain lengths, increased trophic omnivory, and reduced habitat coupling relative to the non-stressed sites. These results indicate that food web structure as an ecosystem level metric may provide insight into anthropogenic activities, and may be applied routinely as a metric for doing Cumulative Effects Assessment. / University of Guelph, Saugeen Ojibway Nation, Georgian Bay Forever

The excessive deposition of fine inorganic sediment (<2 mm) is a major pathway by which agricultural land uses exert pressure on stream ecosystems. However, less well understood are the underlying mechanisms driving threshold biotic responses and the ecological consequences of community changes to sedimentation. Reviewing the literature, I found that sedimentation can affect algal and detrital pathways, and invertebrate community composition may show abrupt shifts with increased sediment. Moreover, functional changes to communities potentially leads to simpler food webs, with altered interactions and decreased ecosystem function. After identifying these knowledge gaps, I conducted survey and experimental research using agricultural streams on the Canterbury Plains of New Zealand’s South Island. Results from my survey of 30 streams along a sedimentation gradient showed that pollution-sensitive invertebrates (% EPT; Ephemeroptera, Plecoptera, Trichoptera) demonstrated threshold responses to sediment that varied with spatial scale, and change-point analysis indicated marked declines beyond 20% fine sediment covering streambed reaches. Structural equation modeling indicated that decreased habitat availability was a key mechanism contributing to these changes. To better understand the functional consequences of altered community structure, I investigated food webs in 12 streams along the gradient. The results showed a compression of community trophic niche space, suggesting that in particular, primary consumers became trophically more equivalent. The simplification of stream food webs with increasing sediment appeared to be the result of functional changes to invertebrate communities, with fewer specialised consumers, and shifts in the availability of basal resources. Using field and laboratory experiments investigating litter breakdown and invertebrate feeding, I found that the net consequence of functionally less diverse stream communities with increased sediment was impaired ecosystem function, demonstrated by a reduction in litter breakdown rates. The reduction of detrital resource availability through burial by sediment in laboratory mesocosm experiments strongly influenced detrital consumption rates, thus leading to reduced growth and survival of detritivorous caddisflies. The survey and experimental results support my postulate that sediment deposition causes environmental stress by degrading benthic habitat and making associated food resources (e.g., periphyton and leaves) less available. Overall, my results have provided new insights into sediment impacts on stream communities and have furthered our understanding of how these changes affect the structure and functioning of stream ecosystems.

Mercury (Hg) is a global pollutant, yet Hg biomagnification, the increase in Hg with trophic level, remains poorly characterized in many regions, especially at subtropical latitudes. The present study assessed subtropical reservoirs of eastern China, which provided an opportunity to quantify Hg biomagnification under highly altered conditions that included high atmospheric Hg deposition, use of reservoirs for fisheries, manipulation of food webs through stocking and high fishing pressure, and increasing eutrophication. Despite China’s Hg emission and deposition rates that are among the highest worldwide, low fish Hg concentrations and Hg biomagnification rates were found; this was explained by food web structure and fish species characteristics. Stocked species occupied lower trophic levels and had significantly lower Hg concentrations relative to wild fishes. Evidence of decreased Hg concentrations with eutrophication (as indicated by chlorophyll-a) was observed, suggesting algal biodilution and/or somatic growth dilution. Relative to temperate lakes, zooplankton density dilution may also be causing reduced Hg concentrations in subtropical biota. Hydrogeomorphic features, such as water retention time and percent crop cover, explained Hg bioaccumulation factors and Hg concentrations at the base of the food web. Eutrophication and hydrogeomorphic features also influenced the bioavailability of selenium, which can protect against the toxicity of Hg at adequate concentrations, and the concentration of eicosapentaenoic acid, a beneficial fatty acid, in the planktivorous Bighead Carp (Hypophthalmichthys nobilis). This may indicate that the risk of exposure to the neurotoxicant methylmercury relative to benefits of fish consumption may increase with eutrophication in some fish species. Overall, the findings of this study suggest food web structure, eutrophication, and hydrogeomorphic features together explain low Hg concentrations in anthropogenically modified subtropical reservoirs in eastern China. / Thesis (Ph.D, Biology) -- Queen's University, 2014-07-01 11:35:12.637

Surface streams in karst landscapes are often characterized by high nutrient levels due to incomplete filtration through series of innumerable, below-ground conduits. Seasonal growth of the filamentous alga, Cladophora, is typically associated with nutrient-rich waters. This research compared macroinvertebrate food web structure between riverine reaches with contrasting underlying karst topography, nutrient levels, and Cladophora cover during summer 2012 and autumn 2013. Recent work in these reaches found a high correlation between Cladophora cover and nutrient content, particularly nitrate. Four questions were addressed during this study: 1. Do longitudinal trends in algal and consumer δ13C values relate to decreased DIC availability in larger watersheds? 2. Are trophic niche breadths narrower in more karstified reaches than in less karstified reaches due to longitudinal differences in Cladophora standing stocks? 3. Do differences in trophic-dynamic relationships between primary consumers and their food resources reflect the marked distinction in Cladophora standing stocks in two sections of the upper Green River that flow through differing levels of karstification? 4. Are consumers assimilating primarily autochthonous or allochthonous food resources? Consumers and algae became more 13C-depleted in downstream reaches, which is opposite to published data in other streams. Underlying causes for this pattern are uncertain, but one plausible cause is an increase in DIC availability downstream. Karstrelated hydrology may potentially alter or even reverse normal longitudinal gradients within in-stream producer and subsequently, consumer δ13C values. Since consumers were sampled during low-Cladophora conditions during 2013 and within a few weeks of the onset of the Cladophora bloom in 2012, stable isotopic results may be more representative of primary consumer diets during pre- Cladophora bloom periods. Although Cladophora cover was significantly higher in downstream reaches during both years, food-web structure was similar in all reaches. Consumer niche breadth was similar across reaches, and mixing model analyses suggested that primary consumers in all reaches assimilated similar amounts of Cladophora. The contribution of both autochthonous and allochthonous food resources to the assimilated diet of primary consumers appeared to be similarly important. These results suggest that allochthonous resources may be important in some midreach food webs, especially during periods of low algal growth.

Freshwater mussels are the most imperiled group of freshwater invertebrates globally. Recent research suggests a better understanding of mussel feeding ecology may facilitate and improve conservation efforts. The use of stable isotopes is becoming an increasingly common method to study aquatic food webs. Carbon (C) and nitrogen (N) are two of the most frequently employed elements in food web studies. Differences in natural abundance of 13C/12C can indicate which food sources are the basal sources of carbon incorporated into a consumer’s tissue, while the ratio of 15N /14N provides a method of assessing trophic position within a food web. Attached macroalgae, including the genus Cladophora, may be the dominant primary producers in running water systems. Cladophora, however, has not yet been indicated as a prominent assimilated food source for freshwater mussels. The overall purpose of this study was to assess if the diet of two common Green River mussel species, Amblema plicata (Say) and Elliptio dilatata (Rafinesque) were influenced by the seasonal change in availability of Cladophora during a summer-autumn rapid growth period. Two specific questions were asked: 1) Are the assimilated diets different between control and treatment areas, and 2) are the assimilated diets influenced by differing Cladophora levels across the study period? A mesocosm approach was employed in order to manipulate Cladophora levels within a treatment area. Seventy-two mussels, 36 each species, were sampled across four months,twice between control (= reach-scale, heavy Cladophora cover) and treatment (= localscale removal of Cladophora) areas. The freeware program, IsoSource, a concentration weighted linear mixing model, was used to determine the potential contribution of potential food sources to the diet of both mussel species. IsoSource revealed that Cladophora was the primary assimilated food source for both species across the study period. Although assimilated diets were not different between control and treatment areas, diets were, however, influenced by Cladophora availability across time. The results of this study indicate that, during bloom conditions, Cladophora is the primary carbon source for both A. plicata and E. dilatata and may form the base of food webs in the upper Green River.