Ecosystem engineers are organisms that modify their physical and chemical surroundings in ways that shape the structure and function of ecological communities. Physically, they build biogenic structures that modify flow, light, and habitat complexity. Chemically, they change oxygen and pH levels through metabolic processes such as photosynthesis and respiration. These modifications can either facilitate the presence of associated species by creating favorable microhabitats or inhibit them by amplifying environmental stress. Understanding the circumstances under which and how these shifts occur has become increasingly important as climate change intensifies environmental variability in coastal ecosystems. Advancing our understanding of how ecosystem engineers shape their communities requires considering how external factors, particularly flow, mediate their influence on the surrounding environment. Driven by tides, waves, and currents, flow regulates water residence time and thus the accumulation or dispersion of biologically modified water. Yet despite its central importance, the role of flow in controlling the strength and direction of ecosystem engineering remains poorly understood.
This dissertation examines how local hydrodynamics influences the capacity of marine ecosystem engineers to modify their surrounding chemical environments. It focuses on two contrasting but complementary systems: an autotroph, bull kelp (Nereocystis luetkeana), and a heterotroph, mussels (Mytilus spp.). Looking across these systems provides a broader view of how different types of engineers—those that produce oxygen through photosynthesis and those that consume it through respiration—shape their local chemical environments. By studying both systems, this work links two aspects of ecosystem engineering: 1) oxygen production and depletion, and 2) explores how flow determines when these species have the potential to act as facilitators or inhibitors within their communities. I combined field observations with laboratory and field experiments to explore how flow dynamics interact with biological traits, such as canopy structure, density, and behavior, to determine when these engineers act as facilitators or inhibitors within their communities. Across chapters, the work progresses from identifying environmental controls on kelp-driven chemical modification (Chapter 1) to isolating mechanistic feedbacks between flow, mussel behavior, and chemistry (Chapter 2), and then investigating density effects on chemistry and behavior by out-planting manipulated mussel aggregations in natural conditions (Chapter 3).
Murie K., 2025. Flow as a mediator of ecosystem engineering: hydrodynamics shape chemical modification by kelp and mussel beds. PhD thesis, University of Washington. Thesis (restricted access).
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