Hydrological change in the Coorong estuary, Australia, past and present: evidence from fossil invertebrate and algal assemblages

Abstract

Estuaries are defined by change; both on a diurnal basis, balancing tidal inflow with riverine outflow, and over hundreds or thousands of years through geomorphic evolution and sea level variability. However, contemporary management and protection underpinned by international agreements, such as the Ramsar Convention, presume that wetlands change within only limited ranges based on their contemporary conditions. Adaptive management strategies for cyclic or even directional change are more robust if underpinned by evidence from longer-term records, such as those obtained from palaeoecological records preserved in sediment cores. Such is the case for the Coorong lagoon in southern Australia; here analysis of assemblage changes of key invertebrates and algae preserved in the sediments reveals the variability of natural conditions well before the instrumental record. During the mid-Holocene to late Holocene and up until the mid-twentieth century, assemblage changes in microfauna and flora indicate variable salinity and water clarity associated with both fresh continental and marine water inputs. By contrast, in the south lagoon, the proxies indicate periodically enhanced salinity. The most significant changes in ecology and sedimentation are apparent after the 1950s; hydrological modifications have changed the influence of both continental and tidal water, producing conditions that are unusual in the long-term history of the site.