Assessing confinement in coastal lagoons

► Confinement is a biological concept relevant in assessing and managing lagoons. ► Confinement cannot be measured, but it can be defined as a transport time scale. ► We present a first standard methodology for assessment of confinement. ► It can be assessed from the accumulation rate within a lagoon of a marine tracer. ► The adjoining sea and high frequency forcing should be explicitly included.