Cumulative Impacts of Multiple Stressors on Aquatic Communities: Novel Fish Predators in Warming Mountain Lakes

• Author / Creator

  MacLennan, Megan M

• Novel anthropogenic perturbations (i.e. stressors) often mediate the effects of each other, generating “ecological surprises”. While the cumulative impact of certain stressors on individual organisms and populations is becoming better understood, little is known about how their net effects transfer to higher levels of biological organization. To address this knowledge gap, I investigated the influence of species co-tolerance (i.e. the correlation between species tolerances to two stressors) and meta-community dynamics on the net effects of two common stressors of planktonic food webs in naturally fishless lakes, namely introduced trout (Oncorhynchus mykiss, Salvelinus fontinalis and Oncorhynchus clarkii) and higher temperatures. To help formulate my hypotheses, I analyzed trends in observational data using 1) a survey of naturally fishless and stocked lakes positioned along an elevational (and thus climatic) gradient and, 2) monitoring records of an alpine lake when stocked trout were absent and present across a 24-year period of climatic variation. My main hypotheses were that 1) non-native trout synergistically increases the effect of higher temperatures on planktonic communities of montane lakes, 2) positively correlated species tolerances to non-native trout and warming, driven by selection for the same trait by both stressors (i.e. smaller body-size), can help explain the influence of exposure sequence on their cumulative effects on species composition and functional structure, and 3) regional zooplankton imported from a diverse array of lakes can functionally compensate for local alpine species suppressed by non-native trout and warming. I experimentally tested these hypotheses using controlled manipulations of fish and temperature on planktonic food webs from naturally fishless mountain lakes. Empirical evidence from the spatial survey and outdoor mesocosm experiment following a two-factor, fully crossed design [(fish absent vs. present)  (ambient temperature vs. warmed)] with randomized blocking supported my first hypothesis by showing that the positive effect of higher temperatures on community biomass occurred only in the presence of non-native trout. The synergistic interaction between the stressors likely occurred because warming stimulated reproduction of smaller herbivores that were released from competition and predation in fish-stocked communities. Further, introduced trout stimulated primary production, likely by suppressing large efficient herbivores (Daphnia spp.), but also potentially by increasing nutrient recycling. As hypothesized, both non-native trout and warming selected for small body-size and planktonic communities of naturally fishless montane lakes showed positive co-tolerance to the stressors. Using a novel approach integrating species traits and co-tolerances and a one-factor experiment consisting of three stressor treatments (Unstressed, Warming then fish, and Fish then warming) with randomized blocking, I discovered that positive co-tolerance to trout and warming helped explain their net effect on the functional structure of communities. Together, the stressors extirpated larger species occupying higher trophic levels, which lacked tolerance to either stressor, while stimulating smaller co-tolerant species, regardless of exposure sequence. In the temporal survey and growth-chamber experiment following a three-factor, fully-crossed design [(size-selective predation absent vs. present)  (ambient temperature vs. warmed)  (local vs. local + regional species pools)] with randomized blocking, predation by non-native trout and higher temperatures each decreased community biomass in naturally fishless alpine lakes by suppressing large zooplankton. The experiment also revealed that declines of alpine species increased the establishment of regional zooplankton, including a diversity of montane species under warmed conditions. Yet, contrary to my hypothesis, the introduction of regional species did not alleviate the negative effect of fish predation and warming on alpine communities, suggesting that upward dispersal of montane species is unlikely to provide functional compensation to stressed alpine communities. My finding that non-native trout and warming had opposite effects on lower montane versus alpine lake communities highlights the challenging context dependency of their cumulative ecological impacts. Nevertheless, my integration of species co-tolerances with related key traits provides a promising tool to help predict the net effects of these and other stressors across ecological communities and habitats. A better understanding of the relationship between species tolerances to co-occurring stressors may also help prioritize management actions. For example, my discovery that non-native trout and higher temperatures similarly affect zooplankton communities suggests that fish removals may be best targeted in lakes that will experience less warming, such as those containing natural climate refugia (e.g. stable coldwater hypolimnia). Otherwise, climate warming may prevent planktonic food webs from recovering following the removal of introduced sportfish.

• Subjects / Keywords

  • Higher temperatures
  • Mountain lakes
  • Zooplankton
  • Trout
  • Multiple stressors

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R31J97G65

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.