Detrital shadows: Evaluating landscape and species effects on detritus-based food web connectivity in Pacific Northwest estuaries Emily Russell Howe A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2012 Reading Committee: Charles Simenstad, Chair Daniel Schindler Andrea Ogston Program Authorized to Offer Degree: School of Aquatic and Fishery Sciences ©2012 Emily Russell Howe ii iii University of Washington Abstract Detrital shadows: evaluating landscape and species effects on detritus- based estuarine food web connectivity in Pacific Northwest estuaries Emily Russell Howe Chairperson of the Supervisory Committee: Charles A. Simenstad School of Aquatic and Fishery Sciences Estuaries are inherently open systems, linking together terrestrial, aquatic, and marine ecosystems. With fluid, permeable transitions (ecotones) marking the boundaries between these ecosystems, estuaries subsidize coastal food web productivity through the mediation of nutrient, material, and energy flux across ecosystem boundaries. Mechanisms governing the strength and scale of estuarine detritus-based food web connectivity, however, are poorly understood. For example, early estuarine descriptions suggest that extensive mixing and large-scale transport of organic matter occurs within estuarine systems, while recent evidence in estuarine detritus-based food webs has shown strong spatial gradients in the sources of organic matter assimilated by consumers across a diversity of scales. This suggests food webs are spatially compartmentalized in some estuaries, but strongly connected in others. Given that estuaries have experienced extensive structural and hydrological alterations over the past century, research describing the mechanisms of estuarine-supported subsidies is necessary if we are to provide informed guidelines for the conservation and restoration of estuaries and estuarine functions. In this dissertation, I describe the role of landscape context, consumer feeding mode, and consumer life histories in shaping the scale of food web connectivity in Pacific Northwest estuaries with implications for conservation and restoration strategies. Specifically, I examine detritus-based food webs, using multiple stable isotopes (δ13C, δ15N, δ34S) in combination with a Bayesian stable isotope mixing model to trace food web connections between organic matter (OM) sources and estuarine consumers. iv Chapter 1 identifies several factors influencing the degree of food web connectivity in Pacific Northwest estuaries. Both fluvial discharge and consumer feeding mode strongly influenced the strength and spatial scale of food web linkages observed in the estuarine systems we examined. To a lesser degree, seasonal shifts, and other estuary-specific landscape characteristics, such as marsh area or particle transport speed, can also influence the degree of food web linkages across space and time, often accounting for unexpected patterns in food web connectivity. Chapter 2 assesses whether passive (hydrologic) and active (behavioral) processes are the primary mechanism by which trophic energy flows across estuarine ecotones. We specifically compare passive OM transfer by estuarine circulation to the active trophic relay of OM via nekton movement by comparing isotopic and diet compositions of resident (bay pipefish, Syngnathus leptorhynchus) and transient (English sole, Parophrys vetulus) fishes in two estuaries with contrasting freshwater inflows. Our results indicate that both OM movement and organism movement enhance connectivity in Pacific Northwest estuaries. In the estuary exhibiting high fluvial discharge, water-advection plays a critical role in large-scale OM transport and delivery to adjoining ecosystems, while trophic relay by organisms may provide the more important vector of food web connectivity in the estuary exhibiting little to no fluvial discharge. The two mechanisms, however, certainly work in tandem to enhance food web connectivity across estuarine ecotones. In Chapter 3, I examine patterns in food web connectivity associated with restoration efforts within the Skokomish River estuary. We conclude that increasing ecosystem capacity for detritus production by restoring emergent marsh ecosystems can bolster support for detritus-based food webs, and that restoration actions that enhance connectivity across estuarine ecotones may achieve functional equivalency more rapidly than restoration projects exhibiting limited connectivity to the surrounding landscape. Together, the results presented in this dissertation demonstrate how physical and biological factors interact to affect food web connectivity in estuarine ecosystems. In light of the current, altered state of the world’s estuaries, restoration and conservation actions addressing ecosystem capacity and connectivity may effectively promote ecosystem function. v Contents Table of Figures ............................................................................................................................................ ix Table of Tables .............................................................................................................................................. x Preface .......................................................................................................................................................... 1 Chapter 1. Detrital shadows: stable isotopes reveal estuarine food web connectivity depends on fluvial influence and consumer feeding mode ...................................................................................................... 13 Abstract ................................................................................................................................................... 13 Introduction ............................................................................................................................................ 14 Conceptual Model and Objectives .......................................................................................................... 17 Approach ................................................................................................................................................. 18 Materials and Methods ........................................................................................................................... 19 Study sites and design ......................................................................................................................... 19 Primary producer collection ............................................................................................................... 19 Bioindicator organism translocation ................................................................................................... 20 Tissue preparation and isotope analysis ............................................................................................. 21 Mixing Model Diet Estimations ........................................................................................................... 22 Hydrodynamic characterization .......................................................................................................... 23 Statistical analyses .............................................................................................................................. 23 Results ..................................................................................................................................................... 25 Hydrodynamic characterization .......................................................................................................... 25 Isotope Signatures of OM Sources and Consumers ............................................................................ 26 Estimated diets of consumer indicators ............................................................................................. 27 Effects of landscape setting on food web connectivity ...................................................................... 29 Spatial scales of food web connectivity: cross-ecosystem, cross- ecotone, within-ecotone ............. 32 Food web compartmentalization among estuaries ............................................................................ 33 Effect of season on cross-ecosystem food web connectivity ............................................................. 35 Discussion................................................................................................................................................ 35 Cross-estuary differences in food web connectivity ........................................................................... 36 Cross-ecosystem connectivity ............................................................................................................. 39 Cross-ecotone connectivity ................................................................................................................. 44 Consumer feeding mode ..................................................................................................................... 45 Conclusion: Implications for delta restoration and management ...................................................... 48 Acknowledgements ................................................................................................................................. 52 Literature cited.......................................................................................................................................