Phytoplankton form the base of aquatic foodwebs and play a fundamental role in the transformation of energy, carbon, and nutrients in aquatic ecosystems. Phytoplankton biomass and species composition can fluctuate rapidly, with ecological and even toxicological consequences, yet the factors that govern these dynamics in coastal and estuarine environments remains poorly constrained. This project establishes the PhytoChop Coastal Observatory, an autonomous instrument array designed to continuously monitor the composition and photosynthetic activity of the phytoplankton community. The observatory, located at Horn Point Laboratory?s research pier situated on a tidal sub-estuary of Chesapeake Bay, the Choptank River, will provide images and quantitative data describing phytoplankton composition and activity on a public-facing data portal. The project will provide data that will enable a deeper understanding of the factors that govern phytoplankton dynamics at time scales ranging from cellular division to seasonal succession and eventually to interannual scales. Engagement with end-users will take several forms, including the development of computer scripts for scientists, course modules for students, image libraries for everyone including the general public, and an internship to specifically engage with the oyster aquaculture community. <br/><br/>This proposal seeks to establish a state-of-the-art coastal observatory, PhytoChop, to support research and education on the community composition and ecophysiology of phytoplankton, including photosynthetic and mixotrophic protists and cyanobacteria. The PhytoChop Coastal Observatory will be housed at Horn Point Laboratory?s research pier drawing water from a mesohaline tidal sub-estuary of Chesapeake Bay, the Choptank River. The observatory will provide high frequency taxonomic and physiological data by combining an Imaging FlowCytobot (IFCB), a Fast-Repetition Rate Fluorometer (FRRF), and instrumentation to measure environmental parameters that constrain phytoplankton growth (temperature, light, nutrients). Data collection frequency will enable examination of the phytoplankton community from time scales spanning from cell division to seasonal succession, and eventually to interannual scales. The data stream resulting from the PhytoChop Coastal Observatory will be made publicly available in near-real time to be readily available to the scientific community at (https://hplmonitoring.umces.edu/). To help streamline data analysis in the classroom and beyond, we will deposit scripts (R, Python) that import and query PhytoChop Coastal Observatory data into a GitHub repository. Data resulting from this project is anticipated to benefit a broad scientific audience including phytoplankton ecologists and physiologists and coastal and estuarine ecosystem ecologists.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.