151 pages ; Supplemental file(s) description: Graham Supplemental Material. ; In nature, interactions between a pathogen, susceptible host, and a suite of environmental conditions can influence disease transmission and spread. Here, I examine the impacts of the environment and microbial and herbivore communities on disease dynamics in Pacific Northwest eelgrass (Zostera marina). This is an especially tractable and important system for exploring these questions, given that the causative agent of seagrass wasting disease, Labyrinthula zosterae, is temperature-sensitive and culturable and given the value of eelgrass meadows and the ecosystem services they provide.In Chapter 1, I used a combination of field and lab approaches to explore the impact of wasting disease on eelgrass growth and belowground sugar reserves in natural eelgrass meadows. In Chapter 2, I examine the role of the eelgrass microbiome—bacteria and Archaea living on the surface of eelgrass leaves—in defense against disease. In Chapter 3, I conducted field surveys to determine how disease varies with depth and different environmental conditions. In Chapter 4, I used a range of experimental approaches to understand the role of eelgrass herbivores in disease transmission. Collectively, this work provides a deeper understanding of factors that influence disease dynamics in eelgrass meadows. As eelgrass are marine sentinels, indicative of the health of our oceans, understanding which factors influence disease spread and levels in nature can better inform the conservation and management of these valuable marine foundation species.