CDOM plays an important role in the carbon cycling and biogeochemistry of coastal waters. It occurs naturally in aquatic environments primarily as a result of tannins released from decaying plant and animal matter, which can enter coastal areas in river run-off containing organic materials leached from soils. When present in high concentrations, it imparts a brown or yellowish color to water. Its presence can negatively impact fish populations by reducing dissolved oxygen concentrations to harmful levels and by releasing nutrients and metals that contaminate the water. Increased understanding of the role of CDOM will further our ability to manage and protect coastal ecosystems.