Chesapeake Bay, one the largest estuaries in the world, has been suffering from eutrophication and summer hypoxia for more than half a century. We studied the sources and transformation the key limiting nutrient, phosphorus (P), by characterizing its particulate and dissolved forms, using chemical extractions, nuclear magnetic resonance (NMR) spectroscopy, electron microprobe analyses, as well as phosphate oxygen isotope as a tracer.