The Influence of Sodium Chloride on the Performance of Gammarus Amphipods and the Community Composition of Microbes Associated with Leaf Detritus
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THE INFLUENCE OF SODIUM CHLORIDE ON THE PERFORMANCE OF GAMMARUS AMPHIPODS AND THE COMMUNITY COMPOSITION OF MICROBES ASSOCIATED WITH LEAF DETRITUS By Shelby McIlheran Leaf litter decomposition is a fundamental part of the carbon cycle and helps support aquatic food webs along with being an important assessment of the health of rivers and streams. Disruptions in this organic matter breakdown can signal problems in other parts of ecosystems. One disruption is rising chloride concentrations. Chloride concentrations are increasing in many rivers worldwide due to anthropogenic sources that can harm biota and affect ecosystem processes. Elevated chloride concentrations can lead to lethal or sublethal impacts. While many studies have shown that excessive chloride uptake impacts health (e.g. lowered respiration and growth rates) in a wide variety of aquatic organisms including microbes and benthic invertebrates). The impacts of high chloride concentrations on decomposers are less well understood. My research objective was to assess how increasing chloride concentrations affect the performance and diversity of decomposer organisms in freshwater systems. I experimentally manipulated chloride concentrations in microcosms containing leaves colonized by microbes or containing leaves, microbes and amphipods. Respiration rate, decomposition, and community composition of the microbes were measured along with the amphipod growth rate, egestion rate, and mortality. Elevated chloride concentration did not impact microbial respiration rates or leaf decomposition, but had large impacts on bacteria community composition. It did cause a decrease in instantaneous growth rate, and 100% mortality in the highest amphipod chloride treatment, but amphipod egestion rate was not significantly affected. The results of my research suggest that the widespread increases in chloride concentrations in rivers will have an impact on decomposer communities in these systems. ACKNOWLEDGEMENTS Support for this project came from a graduate student/faculty collaborative grant from the University of Wisconsin Office of Student Research and Creative Activity. I would like to thank the ERIC lab, Jenna Fossen, and Briana Harter for all their support in this project. Along with these individuals, I would like to thank Hannah Nauth and Nathan Nozzi for all their help without which this project would not have happened. I would also like to thank my graduate committee members, Dr. Bob Stelzer, Dr. Sabrina Mueller-Spitz, and Dr. Bob Pillsbury for all their help and for answering all the questions I had. ii TABLE OF CONTENTS Page LIST OF TABLES ....................................................................................................... iv LIST OF FIGURES .......................................................................................................v CHAPTER I – INTRODUCTION .................................................................................1 CHAPTER II – THE INFLUENCE OF SODIUM CHLORIDE ON THE PERFORMANCE OF GAMMARUS AMPHIPODS AND THE COMMUNITY COMPOSITION OF MICROBES ASSOCIATED WITH LEAF DETRITUS ............5 Introduction ........................................................................................................5 Results ..............................................................................................................15 Discussion ........................................................................................................24 Acknowledgements ..........................................................................................31 CHAPTER III – CONCLUSIONS ……………………………………… .................32 APPENDICES Appendix A. Genera that were present exclusively after exposure to a single chloride treatment ..................................................................................35 Appendix B. Sequence count data for all treatments (two replicates are indicated per treatment .....................................................................................38 Appendix C. Genera relative abundance (percentage) data for all Treatments (Two replicates are indicated per treatment) ................................53 Appendix D. Bacteria phyla relative abundance (percentage) data for all treatments (Two replicates are indicated per treatment) ..................................68 Appendix E: Two Additional Supplemental Figures ......................................70 REFERENCES ............................................................................................................73 iii LIST OF TABLES Page Table 1 Specific conductance data from chloride treatment solution ...............10 Table 2 Shannon-Weiner Diversity Index, Genera Evenness, and Richness ....19 iv LIST OF FIGURES Page Figure 1 Respiration rates (means+SD) from 6/6/18 after exposure to the chloride treatments ...............................................................................15 Figure 2 Percent leaf disk mass remaining on Day 6 (6/6/18) and Day 14 (6/14/18) (means+SD) .............................................................16 Figure 3 Dissolved organic carbon (DOC) (means+SD) ...................................16 Figure 4 Bacterial genera relative abundance after exposure to chloride treatments .............................................................................................20 Figure 5 Bacteria genera sequence counts after exposure to chloride treatments .............................................................................................21 Figure 6 Amphipod percent mortality after exposure to chloride treatments ....22 Figure 7 Amphipod instantaneous growth rates (means+SD)............................22 Figure 8 Amphipod egestion rates (means+SD) ................................................23 v 1 Chapter I Introduction Leaf litter decomposition in streams is necessary to break down allochthonous (i.e. inputs from outside the system) organic matter into smaller particle sizes that can be used as energy in ecosystems. Multiple organisms including microbes (bacteria and fungi) and macroinvertebrates (i.e. larger invertebrates such as worms and amphipods) break down this matter. Microbes will start decomposition by colonizing the leaf litter, which tends to make it more nutritious and available to macroinvertebrates who can consume organic matter (shredders). These shredders decrease the average particle size of the organic matter, which increases the available surface area for microbes to colonize and further facilitates decomposition. This breakdown helps carbon and energy move throughout the rest of the aquatic ecosystem (Tyree et al., 2016, Gulis et al., 2003). Water quality, including the chemical composition of streams, can affect how well this breakdown occurs. Changes in the ion concentrations of water can impact decomposition rates (Imberger et al., 2008). Chloride concentrations have been rising in many suburban and urban streams throughout the world in recent years (Gardner et al., 2010, Wallace et al., 2016, Corsi et al., 2015, Chapra et al., 2012). Chloride can enter streams, rivers, and other water bodies through anthropogenic sources such as surface water runoff from road salt, agriculture, mines, and wastewater treatment plants along with natural sources such as rock weathering, sea spray in coastal locations, and rainwater (Kaushal et al. 2018, Cañedo-Argüelles 2016, Cañedo-Argüelles et al., 2013). A dramatic increase in road salt 2 application during the last several decades is a major cause for rising chloride concentrations in streams and rivers, especially in urban areas (Kaushal et al., 2018). Once chloride enters streams or rivers, it can have many negative effects. These can include pH changes through variation in the number of base cations present, increases in heavy metal bioavailability and transport, and increases in lake stratification (Dugan et al. 2017, Kaushal et al. 2018). It can also cause mortality, changes in fertility and egg hatching, changes in sex ratios, decreased growth rates, and reduced larval recruitment in several animals including fish, amphibians, and invertebrates (Findlay 2011, Cañedo- Argüelles et al., 2016, Corsi et al., 2015, Corsi et al., 2010, Lambert et al., 2016). Chloride is able to cause these effects since it remains in solution in the water it enters instead of precipitating out like other ions. Chloride can upset the osmotic balance aquatic organisms have with the environment and cause them to expend more energy to keep this balance which can lead to negative effects such as reduced growth rates or mortality (Cañedo-Argüelles et al., 2014, Canhoto et al., 2017, Tyree et al., 2016, Findlay et al., 2011). Organisms in sub-lethal or lethal amounts (Corsi et al., 2010) can also take up chloride. How organisms are affected depends on how much chloride is in the water (i.e. its concentration) and if it is released in chronic or short-term periods. The length of time that chloride is at high concentrations can affect its impact on organisms and ecosystem recovery (Cañedo-Argüelles et al., 2014). Along with the time, how tolerant an organism is to elevated chloride can affect how well that organism can handle the chloride concentration in the water. The taxa that are more sensitive are more likely to have decreased growth or perish. This can affect the taxa composition (e.g. biodiversity) 3 of the ecosystem. Reduced biodiversity may have negative effects on organic matter decomposition rates, but there is not a consensus on the exact relationship