The current study developed a paleolimnological approach to assess changes in diatom assemblages (class Bacillariophyceae) from present-day lake sediments in comparison to those deposited before significant human impact (ca. pre-1850) from 51 Halifax (Nova Scotia, Canada) region lakes in conjunction with a regional diatom-based transfer functions for pH and total phosphorus. All 51 lakes showed changes in diatom assemblages between the present-day and pre-industrial assemblages that was greater than would be expected (i.e. similarity between ‘top’ and ‘bottom’ samples was much less than the similarities within triplicate ‘top’ and ‘bottom’ samples). To help identify the most important environmental stressors impacting diatom assemblages in these lakes, diatom-based reconstructions of inferred changes in pH (DI-pH) and total phosphorus (DI-TP) that were greater than the Root Mean Squared Error (RMSE) of the respective inference models were reconstructed for each of the 51 lakes. For example, a decrease in DI-pH greater than the RMSE of the pH model would be a strong indication of recent acidification, whereas an increase in DI-TP greater than the RMSE of the model indicates nutrient enrichment. Based on this approach, 4% of the lakes are showing acidification-related trends occurring in lakes with low pre-industrial pH values and relatively undisturbed watersheds. Almost 14% of the study lakes have been impacted by nutrients and characterized by watershed development and high concentrations of TP. Approximately 4% of lakes showed oligotrophication and acidification. Diatom assemblages from almost 20% of the study lakes that were relatively unimpacted by the afore-listed environmental stressors show trends consistent with climate warming. These lakes show an increase in DI-pH greater than the RMSE of the inference model, and floristic changes typically showed a decrease in the relative abundance of Aulacoseira distans paralleled with increase in Cyclotella stelligera and other planktonic diatoms in the modern sediments. In addition, Diatoma tenue and Diploneis parma, diatoms tolerant of high conductivity, increased in 45% of the study lakes suggesting road salt as an additional stressor. This thesis provides a rapid paleolimnological-based technique to assess regional water-quality changes, and further demonstrates the complexity of ecological changes within freshwater resources. / Thesis (Master, Biology) -- Queen's University, 2009-09-10 15:18:07.01

Interactions between multiple anthropogenic stressors can have unexpected synergistic or antagonistic effects, making it difficult to predict their combined effect using single stressor studies. The interaction between invasive consumers and nutrient enrichment is particularly important as both of these stressors frequently co-occur and their respective bottom-up and top-down effects have the potential to interact across multiple trophic levels. We conducted a mesocosm experiment that crossed an increasing nutrient addition gradient against an increasing zebra mussel invasion gradient. Native zooplankton communities were added to the mesocosms, and after three months we examined how the single stressor effects on available resources and the zooplankton community were altered by their multiple stressor interaction. Added nutrients had no effect on primary producer abundance, but increased the abundance and dominance of the top consumer, which likely increased predation pressure on the producers and so prevented their response to increased nutrients. Zebra mussels reduced total phytoplankton abundance by ~75%, rotifer abundance by ~80%, and shifted communities towards dominance of cladocerans and adult/juvenile copepods. When combined, the top-down control exerted by the mussels interacted antagonistically to prevent any bottom-up influence of nutrient enrichment on the zooplankton community. These results provide insight into the potential outcomes of nutrient and invasive consumer stressor interactions, and illustrate the need for researchers to consider single stressor problems in a multiple stressor context. / Thesis (Master, Biology) -- Queen's University, 2014-01-31 15:20:15.387

A Holocene lake sediment record spanning 7300 years from Wishart Lake, in the Turkey Lakes Watershed central Ontario, was produced to determine the main drivers of long-term change in the lake ecosystem. The mid-Holocene sediments are dominated by benthic diatoms, around 4000 yrs BP, diatoms in the genera Cyclotella and Tabellaria increase. Comparisons with nearby pollen records show that changes in local vegetation and diatom assemblages occurred synchronously, suggesting a response to nutrient changes caused by vegetation succession. The most recent sediments, dated using activity of 210Pb and the Ambrosia pollen rise, show increases in planktonic Cyclotella spp., particularly around the Ambrosia rise (1880 AD). Thus, the diatom changes in the recent sediments suggest a response to multiple stressors. However, due to some buffering capacity in watershed soil, recent acidification of the lake has not taken place despite nearby industrial air pollution, tracked since 1980 by an on-site monitoring program.

A Holocene lake sediment record spanning 7300 years from Wishart Lake, in the Turkey Lakes Watershed central Ontario, was produced to determine the main drivers of long-term change in the lake ecosystem. The mid-Holocene sediments are dominated by benthic diatoms, around 4000 yrs BP, diatoms in the genera Cyclotella and Tabellaria increase. Comparisons with nearby pollen records show that changes in local vegetation and diatom assemblages occurred synchronously, suggesting a response to nutrient changes caused by vegetation succession. The most recent sediments, dated using activity of 210Pb and the Ambrosia pollen rise, show increases in planktonic Cyclotella spp., particularly around the Ambrosia rise (1880 AD). Thus, the diatom changes in the recent sediments suggest a response to multiple stressors. However, due to some buffering capacity in watershed soil, recent acidification of the lake has not taken place despite nearby industrial air pollution, tracked since 1980 by an on-site monitoring program.

Estuarine systems are of crucial importance to the provision of goods and services on a global scale. High human population densities in coastal systems have caused an increasing input of pollutants, of which nutrient pollution is of major concern. Increasingly, these areas are also impacted by physical disturbance, which can originate from anthropogenic sources (e.g. bait digging, shipping) or climate change causing increasingly frequent and intense storms. The individual impacts of such stressors on ecosystems have been investigated however their combined impacts have received less attention. Cumulative impacts of multiple stressors are unpredictable and will likely result in non-additive effects. Further, the effect of local environmental context on multiple stressors is a relatively understudied topic. Work in this thesis compared the combined impact of nutrient enrichment and physical disturbance in Scotland and Australia, using a series of manipulative field experiments. Results demonstrate that response to stressors is highly context dependent, varying between and within geographic locations. While the background levels of stress may vary, by comparing these two locations it is possible to comment on the adaptations and response that communities within different parts of the world display when subjected to additional stress. This study demonstrates that environmental context must be considered when implementing future management practices. Further work demonstrated that the impact of multiple stressors varies depending on how the stress is applied –whether stressors are applied simultaneously or whether there is a delay between two stressors. This study was among the first of its kind, assessing the implications of how multiple stressors react with each other given the order and intensity in which stressors were applied. Results demonstrated that systems can become sensitised to stress making them increasingly vulnerable to additional stress. Future research should be focussed on incorporating ecologically relevant scenarios of how stressors will impact estuaries while considering how environmental context will mediate impacts.

Hermatypic, or reef-building, corals (Order Scleractinia) are the foundation of coral reefs, providing a diversity of structurally complex habitats for the myriad species in these biologically diverse ecosystems. However, both local and global anthropogenic stressors threaten the persistence of these corals. For this thesis, thirty 16m2 permanent photoquadrats at 10 shallow forereef sites around Kiritimati (Christmas Island, Republic of Kiribati) were monitored across a four-year study encompassing the 2015-2016 El Niño derived marine heatwave, and subsequent mass coral mortality event. Sites were exposed to either low, medium, or high levels of local anthropogenic disturbance. My objective herein was to examine the effects of a mass coral mortality event on reef structural complexity, from the end of the event to three years afterwards. To do so, I digitally quantified six metrics of structural complexity for each photoquadrat sampled across three resolution scales for each of the five expeditions. Plots from 2015, 2017, and 2019 were later annotated based on the morphological structure present. I found that while significant declines in multiple of habitat metrics occurred by the end of the heatwave, no further significant declines occurred thereafter. However, this trend was lost as resolution scale increased, indicating the trends seen in the habitat metrics at 1.0 cm were likely documenting the shift from live coral towards abiotic dominated reefs. Anthropogenic disturbance compounded the El Niño’s effect, ensuring high disturbance sites had the lowest structural complexity values throughout the study. Lastly, live branching, tabulate, foliose, and submassive coral morphologies were found to be most closely associated with the different habitat complexity metrics. These results highlight the importance live coral structure has on reef structural complexity, illustrate the importance of model resolution when quantifying trends in structural complexity, pinpoint coral morphologies creating reef structural complexity, and further emphasize the need to limit the effects of local anthropogenic disturbance on coral reefs. / Graduate / 2023-04-15

The frequency, duration, and co-occurrence of several environmental stressors are increasing globally. Multiple stressors may have compounding or interactive effects on animals, resulting in either additive or non-additive costs, but animals may mitigate these costs through various strategies of resource conservation or shifts in resource allocation. Thus, through two related factorial experiments, I measured a range of traits—from those related to life history and behavior to underlying physiology— to investigate the nature of costs (additive, non-additive, or neither additive nor non-additive), cost-mitigating strategies (resource conservation or allocation), and life-history strategy related to multiple stressors. First, I leveraged life-history strategy differences in the sand field cricket, Gryllus firmus, to investigate the individual and interactive effects of food and water limitation on fitness-related traits. Gryllus crickets exhibit a wing dimorphism mediating two distinct life-history strategies—long-winged crickets invest into flight capability while short-winged crickets do not. My results indicate that traits vary in their sensitivity to environmental stressors and stressor-stressor interactions (e.g., flight muscle). I only found support for non-additive costs or single-stressor costs of water and food limitation to fitness-related traits. Water availability had a larger effect on traits than food availability, affected more traits than food availability (wing dimorphism), and mediated the effects of food availability. Second, I investigated the role of life-history strategy in cost-mitigating strategies and further examined the costs (additive or non-additive) of multiple stressors to fitness-related traits, physiology, and behavior. I used the variable field cricket, Gryllus lineaticeps, to examine the costs of a simulated heat wave and water limitation. These stressors resulted primarily in single-stressor or non-additive costs to important traits (e.g., survival, final body mass, and total water content), extensive shifts in resource allocation priorities (e.g., reduced prioritization of body mass), and a limited capacity to conserve resources (heat wave reduced energy use only when water was available). Further, life-history strategy influenced the emergency life history stage (ELHS) because wing morphology and stressor(s) interacted to influence gonad and body mass, boldness behavior, and immunocompetence. Together, these two studies demonstrate that water availability and life-history strategy should be incorporated into future studies integrating important conceptual frameworks of stress (multiple-stressor framework and ELHS) across a suite of traits—from survival and life history to behavior and physiology.

Environmental change is a major threat to marine ecosystems worldwide. Understanding the key biological processes and environmental factors mediating spatial and temporal species' responses to habitat alterations underpins our ability to forecast impacts on marine ecosystems under any range of scenarios. This is especially important for calcifying species, many of which have both a high climate sensitivity and disproportionately strong ecological impacts in shaping marine communities. Although geographic patterns of calcifiers' sensitivity to environmental changes are defined by interacting multiple abiotic and biotic stressors, local adaptation, and acclimation, knowledge on species' responses to disturbance is derived largely from short- and medium-term laboratory and field experiments. Therefore, little is known about the biological mechanisms and key drivers in natural environments that shape regional differences and long-term variations in species vulnerability to global changes. In this thesis, I examined natural variations in shell characteristics, both morphology and biomineralisation, under heterogeneous environmental conditions i) across large geographical scales, spanning a 30° latitudinal range (3,334 km), and ii) over historical times, using museum collections (archival specimens from 1904 to 2016 at a single location), in mussels of the genus Mytilus. The aim was to observe whether plasticity in calcareous shell morphology, production, and composition mediates spatial and temporal patterns of resistance to climate change in these critical foundation species. For the morphological analyses, the combined use of new statistical methods and multiple study systems at various geographical scales allowed the uncoupling of the contribution of development, genetic status, and environmental factors to shell morphology. I found salinity had the strongest effect on the latitudinal patterns of Mytilus shape. Temperature and food supply, however, were the main predictor of mussel shape heterogeneity. My results suggest the potential of shell shape plasticity in Mytilus as a powerful indicator of rapid environmental changes. I found decreasing shell calcification towards high latitudes. Salinity was the best predictor of regional differences in shell deposition, and its mineral and organic composition. In polar, low-salinity environments, the production of calcite and organic shell layers was increased, while aragonite deposition was enhanced under temperate, higher-salinity regimes. Interacting strong effects of decreasing salinity and increasing food availability on compositional shell plasticity predict the deposition of a thicker external organic layer (periostracum) at high latitudes under forecasted future conditions. This response potential of Mytilus shell suggests an enhanced protection of temperate mussels from predators and a strong capacity for increased resistance of polar and subpolar individuals to dissolving water conditions. Analyses of museum specimens indicated increasing shell calcification during the last century. Deposition of individual shell layers was more closely related to temporal changes in the variability of key environmental drivers than to alterations of mean habitat conditions. Calcitic layer and periostracum showed marked responses to alterations of biotic conditions, suggesting the potential of mussels to trade-off between the deposition of calcareous and organic layers as a compensatory response to strategy-specific predation pressure. These changes in biomineralisation indicated a marked resistance to environmental change over the last century in a species predicted to be vulnerable, and how locally heterogeneous environments and predation levels can have a stronger effect on Mytilus responses than global environmental trends. My work illustrates that biological mechanisms and local conditions, driving plastic responses to the spatial and temporal structure of multiple abiotic and biotic stressors, can define geographic and temporal patterns of unforeseen species resistance to global environmental change.

Global biodiversity losses are being driven by human actions, and coral reef communities are not immune. Local anthropogenic stress and global climate change are rapidly changing coral reefs, through coral bleaching and mortality. How these stressors impact the biodiversity and community structure of corals on tropical reefs will not only affect the communities of fish and invertebrates that rely on coral reefs, but they could have lasting impacts on ecosystem functioning. The record-breaking marine heatwave caused by the 2015/2016 El Niño was superimposed on a strong local human disturbance gradient on Kiritimati, Kiribati, allowing for the investigation of how these combined disturbances affect coral communities. In Chapter 2, I investigated how soft coral cover varies with these disturbances and natural environmental factors, using benthic photoquadrats collected on Kiritimati’s forereefs from 2007 to 2019. Additionally, I conducted a literature review to establish what is already known about soft coral and disturbances, to compare Kiritimati data to that found in the literature. I show that soft corals are grossly understudied, with only a fifth (19/94) of coral studies presenting any results of heat stress effects on soft corals, and even fewer (5%) presenting taxonomic-specific results. On Kiritimati, prior to the 2015/2016 El Niño, soft corals were more common at sheltered sites with lower net primary productivity, but no effect of local disturbance was found. Soft corals were, however, highly vulnerable to heat stress, with a documented complete loss after the heatwave. I also show that soft coral skeletons persisted for years after the heatwave and provided substrate for hard coral recruitment. In Chapter 3, I examined how local and global stressors affected coral diversity, using community composition photoquadrat data collected from 2013 to 2017, and developed a conceptual framework for understanding effects of multiple stressors, when there are both discrete and continuous stressors. Coral alpha diversity (assessed as Hill diversity) exhibited a non-linear relationship with local anthropogenic stress, peaking at intermediate levels, and was negatively impacted by the marine heatwave, such that sites tended to decrease in both coral richness and evenness. Coral beta diversity (assessed as community composition turnover) was significantly impacted by both stressors, but sites exposed to higher levels of anthropogenic stress tended to experience less turnover during the heatwave. Explicitly considering the relationships between the two stressors, I found that it varied depending on the intensity of anthropogenic stress and the diversity metric (i.e., richness vs. composition) examined. For Hill-Richness, I found a tipping point at moderate levels of local anthropogenic stress, below which there was an additive response and above which the response tended towards synergy. In contrast, for Hill-Shannon and Hill-Simpson the responses were additive and there was an antagonistic effect between stressors for community composition. By using the frameworks outlined in this thesis for reporting changes to soft coral due to disturbances, and examining relationships between discrete and continuous stressors, we may better predict how reefs will look in the future and what actions will conserve and assist in the recovery of coral reef ecosystems. / Graduate / 2022-08-10

Climate change and introduced species are among the top five threats to freshwater systems face. Tropical regions are considered to be especially sensitive to the effects of climate change, while island systems are more susceptible to species introductions. Climate-driven changes in rainfall are predicted to decrease streamflow and increase flash flooding in many tropical streams. In addition, guppies (Poecilia reticulata), an invasive fish, have been introduced to many tropical freshwater ecosystems, either intentionally for mosquito population control, or accidentally because of the aquarium trade. This dissertation examines the effects of climate-driven change in rainfall and introduced guppies on stream structure (resource and invertebrate biomass and composition) and function (nutrient recycling) in Trinidad and Hawaii. In the first data chapter we used a time series to examine how nutrient recycling of guppies changes in the first 6 years after introduction to a new habitat and to examine drivers of these changes. We found that when guppy populations establish in a new environment, they show considerable variation in nutrient recycling through time. This resulted from changes in guppy density in the first two years of introductions, and changes in individual excretion in subsequent stages. In the following chapter we utilized a rainfall gradient that mimics forecasted, climate-driven changes in precipitation and resulting changes in streamflow to examine the effects of climate change on stream food resources and macroinvertebrates. We found that the drying of streams across the gradient was associated with a decrease in resource quality and a 35-fold decline in macroinvertebrate biomass. Invertebrate composition also switched to taxa with faster turnover rates. In the third data chapter we used this same space-for-time substitution approach to determine if climate-driven changes in stream structure also affected stream function. We showed that population nutrient recycling rates declined at the drier end of our rainfall gradient as a result of drops in population densities. We also found that under the current climate scenario, community excretion supplied up to 70% of the nutrient demand, which was ten-fold lower with projected climate changes in streamflow. Lastly, since freshwater ecosystems often face multiple human impacts, including climate change and invasive species, we wanted to understand how climate-driven changes in flow might alter the impact of introduced guppies on stream ecosystems. We selected several streams with guppies and several without guppies along the Hawaii rainfall gradient to examine if the effect of guppies changed with differences in streamflow. We found that the two stressors had synergistic effects on macroinvertebrate biomass and nutrient recycling rates. We concluded that climate change appeared to enhance effects of guppies, through direct and indirect effects. Overall, this dissertation shows that both climate change and species invasion can affect stream ecosystems at multiple levels of organization. This dissertation demonstrates that the effects of anthropogenic stressors are not static through time, and emphasizes the need and utility of using several methodological approaches when measuring the temporal effects of stressors. We also underline the significance of assessing multiple stressor interactions, as more than one stressor often impacts ecosystems. / Graduate / 2019-09-01