Discriminating between natural and human-induced shifts in a shallow coastal lagoon: A multidisciplinary approach

Coastal lagoons across the globe are increasingly experiencing undesired and sudden changes caused by human disturbances. Nevertheless, little is known about how long-term natural dynamics and anthropogenic impacts interact to drive the systems to their current state. This study discriminated natural- and human-induced changes in a shallow coastal lagoon near Punta del Este, Uruguay, using a multidisciplinary approach. The genesis of the water body resulted from a Middle Holocene marine transgression circa 6000 yr cal BP. From this time to about 4400 yr cal BP, a comprehensive set of proxies revealed a highly variable scenario, with alternating lagoon states linked to climatic variability. A Late Holocene hiatus was also identified in the sediment profile, linked to the barrier and inlet formation that caused a predominance of erosion over net deposition. Results showed that anthropogenic disturbances modified the landscape substantially and shifted the entire watershed to a previously unknown and undesired state. A freeway constructed in 1955 AD subsequently closed the lagoon's natural inlet and transformed the coastal water body into an artificial reservoir. After such significant human impact, the lagoon experienced a sharp decrease in water surface area along with an intensification of the siltation/eutrophication processes, as inferred from multiproxy data. Findings of this study are useful to identify early signals of human disturbances in pristine or less impacted coastal systems to assist mitigation measures and/or restoration actions.