Detrital shadows: Evaluating landscape and species effects on detritus-based food web connectivity in Pacific Northwest estuaries
Howe, Emily Russell
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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 trophic energy flux across ecosystem boundaries. Mechanisms governing the strength and scale of estuarine detritus-based food web connectivity, however, are poorly understood. Early estuarine descriptions suggest that extensive mixing and large-scale transport of organic matter occurs within estuaries, 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 spatial scales. This suggests food webs are spatially compartmentalized in some estuaries, but strongly connected in others possibly varying as a function of physical forces that transport organic matter across space, such as tidal action and fluvial discharge. Given the extensive structural and hydrological alterations of the coastal zone over the past century, research describing the mechanisms and scale of food web subsidies is necessary if we are to provide guidance for the conservation and restoration of estuaries and estuarine functions. In this dissertation, I describe the role of fluvial and tidal hydrodynamics, 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 in combination with a Bayesian stable isotope mixing model to trace food web connections among organic matter (OM) sources and estuarine consumers. 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 five estuaries examined. To a lesser degree, seasonal shifts in fluvial discharge, and other estuary-specific landscape characteristics, such as marsh area or particle transport speed, can also influence the lengths and strengths 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 (organism behavior) processes are the primary mechanism by which trophic energy flows across estuarine ecotones. I specifically compared 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, <italic>Syngnathus leptorhynchus<italic>) and transient (English sole, <italic>Parophrys vetulus<italic>) fishes in two estuaries with contrasting freshwater inflow regimes. Our results indicate that both OM movement and organism movement can enhance food web 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 nektonic 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. My findings illustrate 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 explicitly addressing ecosystem capacity and connectivity may more effectively promote ecosystem function.
- Fisheries