Variability in Invertebrate Trophic Networks along Stream Nutrient Gradients Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Travonya L. Kenly Graduate Program in The School of Environment and Natural Resources The Ohio State University 2018 Thesis Committee: Dr. S. Mažeika P. Sulliván, Advisor Dr. Lauren A. Pintor Dr. Carol Anelli 1 Copyrighted by Travonya L. Kenly 2018 2 Abstract Food webs, which describe biotic interactions and integrate energy pathways within ecosystems, represent one of the principal categories of ecological networks. However, the impacts of nutrient enrichment – a global stressor in aquatic ecosystems – on key characteristics of ecological trophic networks remain largely unresolved. We assessed associations between nutrient enrichment (nitrogen [N], phosphorus [P]) and aquatic invertebrate network characteristics at fourteen stream reaches in the Upper Big Walnut Creek watershed, Ohio (USA) across two seasons. Total N:P ratio was positively related to invertebrate density, richness, evenness, and connectance, but negatively related to the relative abundance of top predators; nitrate (NO3):phosphate (PO4) was positively associated with density as well as linkage density. Nutrient concentrations were also correlated with multiple descriptors of invertebrate networks, with total P and PO4 negatively associated with richness, linkage density, and connectance. NO3 was negatively related to connectance, while total N was negatively related to the proportions of basal and positively related to intermediate consumers. Species traits were linked to network properties. For instance, small body size was negatively related to linkage density, connectance, richness, and density; the proportion of bi- and multivoltine aquatic insects was positively associated with richness; and the relative abundances of collector- filterers were positively related to linkage density, connectance, and richness. These ii finding suggest that variability in nutrients can have strong impacts on ecological invertebrate networks, and that these effects are likely mediated by species traits. Understanding how ecological networks respond to nutrient enrichment will be an important contribution to interpreting community and ecosystem effects of nutrient enrichment in streams. iii Dedication I would love to dedicate this thesis to my mother. Without her, I wouldn’t be the woman and motivated student that I am today. She raised my twin and me on her own and shaped us into amazing women. She deserves nothing less than the world. Mom, you are my rock and foundation. And yes, I have friends. iv Acknowledgments First and foremost, I would like to thank my advisor, Dr. Mažeika Sullivan, for his expertise, encouragement, compassion, and absolute patience during my graduate career. I would have never made it this far without his guidance and I hope that one day I can be even a fraction as great of a scientist and mentor. I would also like to thank Kay Stefanik for her patience, kindness, and dedication to helping me defend on time. Next, I’d like to thank members of the STRIVE Lab and my fellow graduate students Kaitlin Kinney, Martha Zapata, Tiffany Atkinson, Kristen Diesburg, Jenna Odegard, Chelsey Nieman, Chelsea Crosby, Krystal Pocock, Levon Bajakian, Elizabeth Berg, Francisco Luque- Moreno, Lars Meyer, Ellen Comes, and Rebecca Czaja for their invaluable friendship, help in the lab and field, and overall, in life. I would also love to thank my non-scientist friends as they really helped me through my career as well. Calvin Ling, Aaliyah Watts, and Khara Bage, thank you for sitting with me many nights as I tried to fight through migraines and get work done and for helping me keep my sanity. Elisabeth Bonnell, thank you taking care of me through all my migraines and never complaining once about how they have inconvenienced you. Ashley Stewart and Evonne Turner-Byfield, thank you for the IHOP write nights and helping with data analysis. My friends are the light of my life. I would also like to thank my committee members, Dr. Lauren Pintor and Dr. Carol Anelli, for their encouragement and useful critiques of my research project. v Vita June 2011……………………………………William Allen High School May 2015……………………………………B.S. with Honors in Biology Cheyney University of Pennsylvania June 2015 to present………………………...Graduate Research and Teaching Associate, School of Environment and Natural Resources, The Ohio State University Fields of Study Major Field: Environment and Natural Resources Specialization: Ecosystem Science vi Table of Contents Abstract ............................................................................................................................... ii Dedication ......................................................................................................................... .iv Acknowledgments............................................................................................................... v Vita ..................................................................................................................................... vi List of Tables ................................................................................................................... viii List of Figures ................................................................................................................... iix Chapter 1. Background and Literature Review................................................................... 1 Chapter 2. Shifts in Invertebrate Network Structure Correspond to Nutrient Concentrations in Streams of central Ohio, USA ............................................................. 31 Appendix A: General Descriptions and Coordinates of Study Reaches .......................... 75 Appendix B: Macroinvertebrate Functional Feeding Groups, Feeding Guilds, and Diets ........................................................................................................................................... 76 Bibliography ..................................................................................................................... 90 vii List of Tables Table 1.1: Food-web properties of invertebrate trophic networks considered in this study. Modified from Woodward et al. (2005), Romanuk et al. (2006), and Hui et al. (2016). ................................................................................................................... 10 Table 2.1: Functional traits considered in this study and examples of documented responses to nutrient enrichment, along with references.. .................................... 59 Table 2.2: Summary statistics including mean and standard error (SE), along with minima (Min), medians (Med), and maxima of water-chemistry and stream-geomorphic characteristics by season across the fourteen study reaches of Big Walnut Creek watershed, Ohio. DO = dissolved oxygen, TP = total phosphorus, TN = total nitrogen. ................................................................................................................ 60 Table 2.3: Results from linear mixed-effects models with the random effects of Site (i.e., study reach) and Season, and with N:P ratio (model set A) and NO3:PO4 (model set B) as fixed effects (with Satterthwaite approximation for degrees of freedom). FCL = Food-chain length. R2m = marginal R2 and R2c = conditional R2. + indicates a positive relationship. -indicates a negative relationship. * = significance (p < 0.05); ^ = trend (p < 0.10). .................................................................................... 62 viii List of Figures Figure 1.1: Theoretical relationship among functional traits, network structure, direct and indirect effects of functional compostion, and ecosystem dynamics and function. From Gravel et al. (2016). ....................................................................................... 4 Figure 1.2: (a) A modeled food-web diagram with four trophic levels showing linkages among species. (b) The same food-web diagram partitioned into 15 modules of highly interacting species independent of their trophic position. From Anderson and Sukhdeo (2011). ............................................................................................. 11 Figure 1.3: Fourteen study reaches were located in the Upper Big Walnut Creek Watershed in central Ohio. The 14 stream reaches (represented as dots) drain into Hoover Reservoir. Study reaches were distributed across forested, agricultural, and developed land cover and use. Green = forested, red = urban/developed, yellow/brown = agricultural. ................................................................................. 15 Figure 1.4: Conceptual linkages between nutrient enrichment and ecological trophic networks... ............................................................................................................. 16 Figure 2.1: Land use in the Big Walnut Creek watershed. Study reaches repesented a mix of forested, agricultural, and developed/urban land-use and -cover types (Ohio EPA, 2005). Green = forested, red = urban/developed, yellow/brown = agricultural. ........................................................................................................... 63 ix Figure 2.2: Concentrations of total phosphorus (TP; mg L-1) for summer 2016 and spring 2017 for the fourteen study reaches of Big Walnut Creek watershed, Ohio... ..... 64 -1 Figure 2.3: Concentrations of phosphate