Estuaries are areas of high socioeconomic importance and, consequently, pollution sinks. For instance, 22 of the 32 largest cities in the world are located on them. One of the main sources of estuarine pollution is coming from industrial wastewaters. Among others, heavy metals discharged by industries are a group of toxicants very harmful on living organisms. Their exposure can cause disruption of the endocrine, reproductive and immune systems, neurological disorders and cancers.

Zinc (Zn) is one of the heavy metals with greater presence in estuaries due to the mining and industrial activity. There are many examples of estuaries presenting industrial pollution around the globe. Regarding Spain, Ría de Huelva, Pasajes Bay, Ría de Bilbao, Tarragona Harbour, Ría de Avilés or Cádiz Bay are examples of tidal waters under industrial pressure. Lastly, at the local scale of Cantabria, Suances Estuary and Santander Bay are receiving the majority of industrial discharges. Thus, estuarine sediments are a sink for metal contaminants derived from fluvial, marine and atmospheric sources. Whilst metal releases to estuaries have declined in recent years due to increased regulation and declining industrial activity; metals previously discharged, which are now locked up in intertidal sediments, remain a concern.
For example, there are indications that intertidal flats are already being eroded, due to climate change related sea level rise, in some locations. This erosion may result in the redistribution of historically contaminated sediment to locations, such as the mudflats, where it is more available to biota, such as the microphytobenthos (MPB). MPB are populations of microalgae and cyanobacteria associated with sedimentary bottoms, generally in the form of films or «biofilms». MPB and their extracellular polymeric substances (EPS) such as polymeric fibrils compose these biofilms. Diatoms dominate the MPB community in the mid-latitudes of the Northern Hemisphere. In addition to causing redistribution, climate change effects, such as increasing temperatures and storminess, may also alter the biomobilisation of metals to MPB. Increased concentrations of metals within the MPB could potentially increase their transfer to higher organisms through the food chain with potential impacts for biota and human health.

Therefore, it is necessary to deepen the estuarine characterization of the Zn exchange processes between the sediment and the water column in the presence of MPB because the studies carried out have tended to ignore the effects that MPB have on these processes. Often biofilm forms a relatively stable sediment surface and generates a binding force between sediment particles in the intertidal areas, known as sediment biostabilisation. This process attenuates the velocity current profile, increases the deposition and decreases the erosion. Moreover, the presence of MPB modifies the Zn diffusion and partitioning between the water column, the sediment layer and the interstitial water, known as Zn biomobilisation.