Plastic Debris Flowing from Rivers to Oceans: The Role of the Estuaries as a Complex and Poorly Understood Key Interface

After several investigations on plastic pollution in marine environments, the issue was raised in continental environments, mainly rivers. The latter was extensively studied as a potentially significant source of marine plastic debris. However, the estuaries represent a transition environment between continental and marine areas, making it a critical interface. Only limited studies were conducted on these environments that present the extra-complexity of being under the influence of variable and poorly understood dynamics. For instance, tidal cycles, salinity gradient, and maximum turbidity front are all estuarine characteristics that need to be considered when studying such an environment. The present review evaluates the current state of knowledge and assesses the way different studies deal with the estuarine specificities. It discusses the dynamics underwent by the plastic debris in estuaries. Ultimately, the question of the role of the estuaries, either as sources or sinks of plastic pollution, is raised.