With extreme sea-level events, including storms and tsunamis, projected to flood the coastal zone more often over the coming centuries, understanding their impacts is paramount. Whilst immediate impacts are well studied, we know relatively little about how coastal environments recover in the subsequent years and decades after such events. Here we investigate the response and recovery dynamics of the sedimentary system and biological environment to saltwater inundation into a coastal lake on Chiloé Island, northern Chilean Patagonia, from two tsunamis (in 1960 CE and 2670 cal yr BP) and as a consequence of earthquake-induced land-level change. We utilise an existing unique well-dated sediment record from Lake Huelde containing multiple tsunami inundation events, which provides an exceptional sedimentary archive and opportunity to improve understanding of the impacts of marine inundation on freshwater lake ecosystems by re-examining the sediments for new proxies. Analysis of the geochemistry of lacustrine sediments and the biota (algae, aquatic invertebrates and pollen) preserved within them, shows that the sedimentary and ecological responses to two tsunamis in 1960 CE and 2670 cal yr BP differed in nature and timing. We show that biota responded more immediately to the 2670 cal yr BP event, and impacts were longer lasting; saline anoxic bottom water can prevail for decades to centuries post-inundation. We suggest that key drivers of post-tsunami response and recovery are initially the rate of refreshment after saltwater inundation, and over the longer-term, connectivity to the ocean. In an area affected by earthquake-induced land-level changes, these can influence not only the establishment of sustained connection to the ocean, but can also bring the lake ecosystem within reach of storms and spring high tides, which can impact post-tsunami recovery. We therefore suggest that at Lake Huelde, relative sea (land)-level change, and hence tidal connection of the lacustrine system to the ocean, plays a critical role in ecosystem response.
