Southern California lagoons are complex, both in their environments and their management. In their natural states many estuaries will periodically open and close to the sea, but the strategy in recent decades has been to actively manage these lagoons to be largely open to improve water quality. Recently, however, scientists and managers have been reconsidering this one-size-fits-all approach. Managing a lagoon mouth to be continually open can be expensive, and a lagoon that is allowed to close may engender a unique set of species and environmental services. This fact sheet outlines how the study, supported by the National Estuarine Research Reserve System (NERRS), will focus on using existing knowledge and gather new data to provide managers with the information they need to improve the health of this resource.