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The tundra ecosystem, with its frost-molded landscape and large peat reserves, is vulnerable to climate change. Thus, any increase in temperature due to global warming will cause changes in above-and belowground biota. Understanding the linkage between these biotas will help make prediction of the biodiversity and ecosystem functioning when global change phenomena occur, and consequently aid in making management strategies. The role of nematodes in nutrient cycling and decomposition, among many other attributes, make them useful organisms to study soil processes. Associations between plant and nematode communities, from six sites (Berm Face, Berm Crest, Tundra Heath, Polygon, Hummock and Forest) and from within a young, visually homogeneous tundra heath field site, were examined in the subarctic environment of Churchill, Manitoba. The study also provided nematofaunal information, which is very limited in this region. Multivariate analyses of nematode taxa abundance revealed four distinct groupings: Berm Face, Berm Crest, heaths (Tundra Heath, Polygon and Hummock) and Forest. The result showed a parallel relationship between nematode and vegetation assemblages, and thus, a seeming interdependency between above-ground and below-ground biota. Conversely, association between nematode and plant assemblages within a visually homogeneous tundra site was not as obvious. At this fine scale, the heterogeneous nature of edaphic factors and not plant assemblages is hypothesized to influence within-site nematode communities. The thesis also provided results to improve nematofaunal analysis to enhance their utility as bioindicators of soil food webs.

Acid mine drainage (AMD) is a significant environmental issue worldwide, which often causes severe contamination and marked species losses in receiving streams. However, little is known about how this stress alters food webs and ecosystem function. I conducted a literature review, which revealed that AMD-impacted streams generally had depauperate benthic communities dominated by a few tolerant species and impaired ecosystem processes. Next, using survey and experimental-based approaches, I investigated food web structure and energy flow in these highly stressed streams, which typically have low pH (< 3), high concentrations of dissolved metals (Al, Fe), and substrata coated with metal hydroxide precipitates, on the South Island, New Zealand. Inputs of AMD caused substantial loss of consumers and reduced the overall number of links between species generating small and simplified food webs, with few invertebrates and no fish. Comparative analysis of food webs from a survey of 20 streams with either anthropogenic or natural sources of acidity and metals, indicated that anthropogenic sources had a stronger negative effect on food web properties (size, food chain length, number of links); an effect driven primarily by differences in consumer diversity and diet. However, the presence of fewer trophic levels and reduced trophic diversity (detected using isotopic metrics), were common structural attributes in AMD-impacted webs along a pH gradient, regardless of impact level. Furthermore, complementary dietary analyses of consumer gut contents and stable isotope signatures (δ13C and 15N) confirmed that primary consumers fed generally on basal resources and that there were few predatory interactions, which reflected low densities of small-bodied chironomids. This suggests that food quantity was unlikely to limit primary consumers but that reduced prey availability may be an additional stressor for predators. In these radically re-structured food webs, trophic bottlenecks were generated at the primary consumer level and energy flow to higher consumers was disrupted. However, streams still retained some limited function, including slow leaf litter breakdown, which provided detrital resources and supported the small food webs. Overall, my findings have furthered our understanding of these highly stressed stream ecosystems by providing new insights into interactions among species and trophic levels that structure food webs and enable function.

Global warming and increasing atmospheric nitrogen deposition are ranked as second and third most important global drivers of biodiversity loss. Widespread species losses have deep implications for the functioning of ecosystems, the delivery of essential ecosystem services and their resilience to future environmental perturbations. There is growing recognition that interactions between species play a crucial role in determining the response of ecosystems to global environmental changes. Moreover, evidence of synergistic effects between global change drivers has prompted numerous calls to integrate multiple drivers in ecological research. Nevertheless, empirical studies assessing the impacts of temperature and nitrogen on communities at multiple trophic levels are largely absent. This thesis explores the effects of temperature and nitrogen on a tri-trophic system comprising plants, herbivores and natural enemies. The first chapter shows impacts of the drivers on the composition and phenology of an herbivore community. The second chapter highlights changes in biomass under the treatments at three trophic levels. The third chapter explores, for the first time, the impacts of temperature and nitrogen on quantitative food webs. Finally, the last data chapter uses body size as an important species trait to gain insights on the mechanisms causing shifts in food web structure. The key findings of this thesis were i) trophic interactions largely mediated the effects of both global change drivers ii) In particular, strong bottom-up effects determined the system response, with herbivores responding positively and consistently more so than plants and parasitoids in particular. However, iii) this contrasting response was not explained by a phenological mismatch. iv) Food-web structure responded to the changes in composition of herbivores and parasitoids, but shifts in interaction structure did not affect the resilience of the food. However, temperature and nitrogen impacted host-parasitoid food-web structure by altering the response of parasitoid species to host density and size structuring, which is likely to bear consequences on host-parasitoid co-evolution and future food-web architecture and stability. Finally, v) we found frequent, non-additive interactions between the global change drivers. We conclude that co-occurring temperature and nitrogen are likely to alter food-web structure and overall ecosystem balance, with increasing herbivore dominance likely to have important implications for ecosystem functioning and food-web persistence.

Environmental stress and disturbance can affect the structure and functioning of marine ecosystems by altering their physical, chemical and biological features. In estuaries, benthic invertebrate communities play important roles in structuring sediments, influencing primary production and biogeochemical flux, and occupying key food web positions. Stress and disturbance can reduce species diversity, richness and abundance, with ecological theory predicting that biodiversity will be at its lowest soon after a disturbance with assemblages dominated by opportunistic species. The Avon-Heathcote Estuary in Christchurch New Zealand has provided a novel opportunity to examine the effects of stress, in the form of eutrophication, and disturbance, in the form of cataclysmic earthquake events, on the structure and functioning of an estuarine ecosystem. For more than 50 years, large quantities (up to 500,000m3/day) of treated wastewater were released into this estuary but in March 2010 this was diverted to an ocean outfall, thereby reducing the nutrient loading by around 90% to the estuary. This study was therefore initially focussed on the reversal of eutrophication and consequent effects on food web structure in the estuary as it responded to lower nutrients. In 2011, however, Christchurch was struck with a series of large earthquakes that greatly changed the estuary. Massive amounts of liquefied sediments, covering up to 65% of the estuary floor, were forced up from deep below the estuary, the estuary was tilted by up to a 50cm rise on one side and a corresponding drop on the other, and large quantities of raw sewage from broken wastewater infrastructure entered the estuary for up to nine months. This study was therefore a test of the potentially synergistic effects of nutrient reduction and earthquake disturbance on invertebrate communities, associated habitats and food web dynamics. Because there was considerable site-to-site heterogeneity in the estuary, the sites in this study were selected to represent a eutrophication gradient from relatively “clean” (where the influence of tidal flows was high) to highly impacted (near the historical discharge site). The study was structured around these sites, with components before the wastewater diversion, after the diversion but before the earthquakes, and after the earthquakes. The eutrophication gradient was reflected in the composition and isotopic chemistry of primary producer and invertebrate communities and the characteristics of sediments across the sample sites. Sites closest to the former wastewater discharge pipe were the most eutrophic and had cohesive organic -rich, fine sediments and relatively depauperate communities dominated by the opportunistic taxa Capitellidae. The less-impacted sites had coarser, sandier sediments with fewer pollutants and far less organic matter than at the eutrophic sites, relatively high diversity and lower abundances of micro- and macro-algae. Sewage-derived nitrogen had became incorporated into the estuarine food web at the eutrophic sites, starting at the base of the food chain with benthic microalgae (BMA), which were found to use mostly sediment-derived nitrogen. Stable isotopic analysis showed that δ13C and δ15N values of most food sources and consumers varied spatially, temporally and in relation to the diversion of wastewater, whereas the earthquakes did not appear to affect the overall estuarine food web structure. This was seen particularly at the most eutrophic site, where isotopic signatures became more similar to the cleaner sites over two-and-a-half years after the diversion. New sediments (liquefaction) produced by the earthquakes were found to be coarser, have lower concentrations of heavy metals and less organic matter than old (existing) sediments. They also had fewer macroinvertebrate inhabitants initially after the earthquakes but most areas recovered to pre-earthquake abundance and diversity within two years. Field experiments showed that there were higher amounts of primary production and lower amounts of nutrient efflux from new sediments at the eutrophic sites after the earthquakes. Primary production was highest in new sediments due to the increased photosynthetic efficiency of BMA resulting from the increased permeability of new sediments allowing increased light penetration, enhanced vertical migration of BMA and the enhanced transport of oxygen and nutrients. The reduced efflux of NH4-N in new sediments indicated that the capping of a large portion of eutrophic old sediments with new sediments had reduced the release of legacy nutrients (originating from the historical discharge) from the sediments to the overlying water. Laboratory experiments using an array of species and old and new sediments showed that invertebrates altered levels of primary production and nutrient flux but effects varied among species. The mud snail Amphibola crenata and mud crab Austrohelice crassa were found to reduce primary production and BMA biomass through the consumption of BMA (both species) and its burial from bioturbation and the construction of burrows (Austrohelice). In contrast, the cockle Austrovenus stutchburyi did not significantly affect primary production and BMA biomass. These results show that changes in the structure of invertebrate communities resulting from disturbances can also have consequences for the functioning of the system. The major conclusions of this study were that the wastewater diversion had a major effect on food web dynamics and that the large quantities of clean and unpolluted new sediments introduced to the estuary during the earthquakes altered the recovery trajectory of the estuary, accelerating it at least throughout the duration of this study. This was largely through the ‘capping’ effect of the new liquefied, coarser-grained sediments as they dissipated across the estuary and covered much of the old organic-rich eutrophic sediments. For all aspects of this study, the largest changes occurred at the most eutrophic sites; however, the surrounding habitats were important as they provided the context for recovery of the estuary, particularly because of the very strong influence of sediments, their biogeochemistry, microalgal and macroalgal dynamics. There have been few studies documenting system level responses to eutrophication amelioration and to the best on my knowledge there are no other published studies examining the impacts of large earthquakes on benthic communities in an estuarine ecosystem. This research gives valuable insight and advancements in the scientific understanding of the effects that eutrophication recovery and large-scale disturbances can have on the ecology of a soft-sediment ecosystem.

This dissertation combines laboratory and field experiments to investigate the mechanisms of atropisomer enrichment for chiral polychlorinated biphenyls (PCBs) and their metabolites in organisms. Stereoselective biotransformation and bioaccumulation were identified as two major reasons for the different environmental fate of PCB atropisomers. Other affecting factors, such as presence of nanoparticles and changes in feeding ecology of organisms, also affect the fate of chiral contaminants. In vitro incubations of rat cytochrome P-450 2B1 (CYP2B1) isozyme with chiral PCBs indicated that different biotransformation kinetics and competition among PCB congeners or between atropisomers were two main factors affecting atropisomer enrichment. Different interactions between chiral PCB congeners or atropisomers with rat CYP2B1 may occur at the molecular level. Non-racemic meta-hydroxylated-PCBs (5-OH-PCBs) were the major metabolites. CYP-mediated stereoselective formation of dihydroxylated PCBs from OH-PCBs was observed. Gold nanoparticles affected biotransformation activity of rat CYP2B1 and changed PCB atropisomeric composition, directly by electrostatic interaction, or indirectly by changes to the surrounding ionic strength. Thus, stereoselective metabolism of chiral PCBs and OH-PCBs by CYPs is a major mechanism for atropisomer enrichment of PCBs and their metabolites in the environment, with the degree of enrichment dependent, at least in part, on charged nanoparticles and stereoselective interference of atropisomers with each other at the enzyme level. The atropisomer compositions of chiral PCBs were measured in the marine biota of Cumberland Sound (Canada) and Svalbard (Norway). High trophic level organisms, including harp seal, beluga, and narwhal reported for the first time, had species-specific atropisomer signatures, likely due to a combination of in vivo biotransformation and trophic transfer. PCB chiral signatures in Greenland sharks supported the hypothesis that some of these PCB atropisomer compositions shifted over time and space, possibly due to a change in feeding ecology. To our knowledge, this is the first report to investigate temporal trends of PCB atropisomer signatures in Arctic biota.

Arctic food webs are characterized by multi-year predator-prey cycles. Arctic foxes (Vulpes lagopus) feed primarily on rodents, but also on avian and marine prey when rodents are scarce. I examined temporal variation in the arctic fox diet related to food availability and stress hormones (i.e. cortisol). Lemmings (Dicrostonyx richardsoni), goslings, and goose eggs were important components of the fall and winter diet. Goslings were important in fall, even when rodents were abundant. Lemmings were most important in winter, even when lemming densities were low. Consuming stored eggs did not reduce cortisol concentrations, suggesting that arctic foxes do not prefer stored eggs to lemmings. I also found that prey hormones increased fecal hormone concentrations of captive arctic foxes, introducing an additional caution for hormone studies involving predators. Nonetheless, relationships between stress hormone concentrations and changes in food availability can provide insight regarding the importance of food sources to consumer populations.

Periodic monitoring of contaminant levels in fish from the Yukon Territory indicated that organochlorine (OC) contaminants had rapidly declined since the early 1990s. This study examined OC concentrations, including chlordane (sigma-CHL), sigma-DDT, hexachlorocyclohexane (sigma-HCH), toxaphene (sigma-CHB), sigma-PCB and chlorinated benzenes (sigma-CBz) in sentinel fish (species of consistent annual observation and collection) from two Yukon lakes (Kusawa, Quiet), and from the aquatic food web of a focus lake (Lake Laberge) across several temporal points between 1993 and 2003. OC analysis and phytoplankton counts from dated sediment cores as well as climate data were also collected. Population, morphological (length, weight, age), biochemical (lipid content, delta-13C, delta-15N) and OC contaminant data for fish and invertebrates (zooplankton, snails, clams) were reviewed to elucidate the primary causes for these OC declines. Although some spatial differences in contaminant levels exist between the Yukon lakes, OC concentrations were declining for lake trout in all three lakes, with declines also noted for burbot from Lake Laberge. Several other fish species as well as zooplankton from Lake Laberge exhibited decreases in contaminant levels except northern pike, which registered consistently higher levels from 1993 to 2001. There was no evidence to support the hypotheses of changes in fish trophic levels or food sources with the exception of burbot, which marginally decreased, and northern pike, which climbed a half trophic level. Through OC flux analysis in dated sediments, the hypothesis that declines in abiotic deposition affected the contaminant levels in the food web was also negated. The closure of the Lake Laberge commercial fishery resulted in faster fish growth and larger fish populations, which are contributing to biomass dilution of OC concentrations, higher OC biomagnification factors for some species and likely changes in predator-prey interactions as resource competition increases. The large ratio of OC decreases in the lower vs. higher trophic levels of Lake Laberge have increased food web magnification factors (FWMF) for all six OC groups. It is also suspected that above-average temperatures and below-average precipitation in the lower Yukon region over the 1990s may have contributed towards an increase in lake primary production resulting in biomass dilution of contaminants in zooplankton for all three study lakes. Concurrently, shifts in the Lake Laberge zooplankton community, from climate fluctuations or increased fish predation, have gone from an abundance of Cyclops scutifer in 1993 to dominance by Diaptomus pribilofensis in 2001, although sample sites were limited. Characteristics specific to each species (e.g. body size, composition and metabolism) likely play a role in the significant OC declines measured in zooplankton. Fluctuations in population dynamics, species characteristics and OC contaminant concentrations in the Lake Laberge ecosystem may continue for several years to come. Sentinel species such as lake trout, burbot, whitefish, cisco and plankton should continue to be monitored in all three Yukon lakes for future temporal correlations with contaminants or climate change.

The tundra ecosystem, with its frost-molded landscape and large peat reserves, is vulnerable to climate change. Thus, any increase in temperature due to global warming will cause changes in above-and belowground biota. Understanding the linkage between these biotas will help make prediction of the biodiversity and ecosystem functioning when global change phenomena occur, and consequently aid in making management strategies. The role of nematodes in nutrient cycling and decomposition, among many other attributes, make them useful organisms to study soil processes. Associations between plant and nematode communities, from six sites (Berm Face, Berm Crest, Tundra Heath, Polygon, Hummock and Forest) and from within a young, visually homogeneous tundra heath field site, were examined in the subarctic environment of Churchill, Manitoba. The study also provided nematofaunal information, which is very limited in this region. Multivariate analyses of nematode taxa abundance revealed four distinct groupings: Berm Face, Berm Crest, heaths (Tundra Heath, Polygon and Hummock) and Forest. The result showed a parallel relationship between nematode and vegetation assemblages, and thus, a seeming interdependency between above-ground and below-ground biota. Conversely, association between nematode and plant assemblages within a visually homogeneous tundra site was not as obvious. At this fine scale, the heterogeneous nature of edaphic factors and not plant assemblages is hypothesized to influence within-site nematode communities. The thesis also provided results to improve nematofaunal analysis to enhance their utility as bioindicators of soil food webs.

Phytoplankton produce allelochemicals; excreted chemical substances that are affecting other microorganisms in their direct environment. In my thesis, I investigated strain specific variability in the expression of allelochemicals of the harmful flagellate Prymnesium parvum, that is euryhaline but mainly bloom forming in brackish water. I found a large variation among strains, but further showed that all strains of P. parvum were more allelopathic in brackish water compared to marine water. In a marine microbial community, allelochemicals can affect prey, competitors and grazers both, directly and indirectly. For instance, in a food web where grazing controls prey abundance, the negative direct effect of allelochemicals on grazers will positive affect their prey. During my thesis, I investigated how marine microbial communities respond to the addition of allelochemicals. I performed field experiments with microbial communities from seawater collected from different places over Europe, and tested how this communities respond to the addition of allelochemicals from the dinoflagellate Alexandrium tamarense. Before I incubated the microbial communities for several days with A. tamarense algal filtrate, I evaluated the allelopathic efficiency of the algal filtrates with an algal monoculture of Rhodomonas spp. This allowed me to compare the effect of A. tamarense filtrate between the different microbial communities. In general, bacteria reached higher abundances when allelochemicals were present. As allelochemicals also inhibited nanoflagellates and ciliates, we concluded, that allelochemicals indirectly benefit bacteria by reducing grazing pressure. In microbial food webs with many heterotrophic grazers, allelochemicals further benefitted other phytoplankton by inhibiting grazers. It was also shown that bioavailable DOM is released from a microbial community when allelochemicals are present. As most DOM was released from the seawater fraction &gt; 60 μm, we concluded, that larger microorganisms are more affected by allelochemicals than smaller microorganisms. The results can be explained by the surface to volume ratio of microorganisms: Larger organisms provide more contact surface for allelochemicals, and therefore, are probably more vulnerable towards allelochemicals. In conclusion, the effect of allelochemicals on a microbial community depends among others on the structure of the microbial food web, the amount of available DOM, the particle density in the seawater and the composition of the phytoplankton community.