Biotic responses to extreme hypersalinity in an arid zone estuary, South Africa

The Groen Estuary does not function as a typical temporarily open/closed estuary for much of the time, due primarily to the high variability of its physico-chemical attributes. Extreme states are represented by an open mouth following a flood (low salinity) and extreme hypersaline conditions that develop after a prolonged period (years) of mouth closure and a high rate of evaporation from the water body. Available information suggests that biotic recruitment from the marine environment follows mouth breaching, but species disappear progressively as salinity increases and threshold salinity values are reached. A field survey in February 2015 indicated that at the time, salinity ranged from 223 in the lower reaches and < 10 upstream over a distance of < 1 km. The study provided new information on the estuary that includes the revised delineation of the lateral boundary. Habitat mapping from past aerial photographs showed that the open water surface area has decreased over time but the vegetated areas have remained stable. No benthic macroinvertebrates, mesozooplankton or fish were present. Together with low avian species counts and abundance it was concluded that the Groen Estuary represented a small, naturally stressed ecosystem in February 2015. Main stressors were extreme hypersalinity, relatively low water volume, anoxic sediments and a mouth that had remained closed for a relatively long period of time (years).