The forgotten ones of ports: The filter feeders at the heart of siltation processes.

Siltation is a major concern in dynamic and complex ecosystems, such as ports. The mud must be regularly dredged to avoid disturbing the navigation channels. Sediments are carried by the waters entering the port and are partially trapped by harbour structures. Numerous studies have been conducted on the physical factors influencing siltation in port areas, whereas, few have focused on the role of biotic factors in mud formation. However, research in other contexts has shown that organisms that are abundant in pontoons, such as bivalves and tunicates, play an important role in this siltation process. All of these organisms belong to the filter feeder group. The sediments sucked in by the filter feeders are excreted in the form of faeces or mucus-bound pseudo-faeces. These waste materials, called bioproducts, settle efficiently and are involved in the composition of the mud. This study aimed to highlight the role of filter feeders in the siltation process in port areas and to determine the factors that influence the production of bioproducts by filter feeders. To investigate the role of filter feeders in the siltation processes, an experimental analysis was conducted in the largest marina in Europe (La Rochelle, France). It is divided into four basins with distinct filter feeder communities and environmental conditions, allowing for a detailed study of the environmental factors that influence the production of bioproducts. This analysis consisted of recovering and studying the bioproducts generated by the filter feeders using sediment traps fixed under pontoons. To explore the evolution of this biological production, 16 campaigns were conducted from January to March 2020 and May to July 2020. The total amount of dry matter produced was constant between seasons at approximately 130 g/m/d; marina-wide, this amount represents a total daily production of 3.2 tons. However, the production amount varies spatially and temporally in relation to marine hydrodynamics and the organisms involved. Bioproduction was taxon-dependent: areas abundant in oysters and mussels were the areas with the most pronounced bioproduction, whereas there was no significant relationship between bioproduction and the presence of tunicates or scallops. If we consider bioproduction on a seasonal scale, we can see that the campaigns with the greatest production correspond to the periods when the sediment supply was the highest, i.e. when the tidal range was the highest. The quality of the bioproducts (organic matter content) differed between seasons, which can be explained by both environmental and metabolic changes. Understanding the role of filter feeders in siltation processes appears to be essential in port environments that need to be regularly dredged to ensure safe navigation.