Tracing local hydrology and water source use of Eucalyptus largiflorens on the Calperum Floodplain using strontium, oxygen and deuterium isotopes

Abstract

Black Box trees (Eucalyptus largiflorens) across the Murray-Darling Basin are in critical condition due to high groundwater salinity and infrequent natural flooding. Geochemical tracers such as radiogenic strontium (87Sr/86Sr), oxygen-18 (𝛿𝛿18O) and deuterium (𝛿𝛿D) are considered useful in the understanding of catchment hydrology and plant water use, and in this study, 87Sr/86Sr, 𝛿𝛿18O and 𝛿𝛿D isotopes were used accordingly to better comprehend local hydrology and water use behaviour patterns of Black Box trees on the Calperum Floodplain, South Australia. Investigations were achieved by sampling and analysing local surface waters (Lake Merreti, Lake Clover, and River Murray), groundwater, soils (1.5 m depth) and plant material (stem water, and leaves) from two separate sites, north (Site 1) and south (Site 4). Considering the local hydrology, Lake Clover was composed of evaporated rainwater, while Lake Merreti was a relative mix of both evaporated rainwater and river water. Additionally, local rainfall sources appeared to vary overtime. Furthermore, groundwater showed no close relationship with rain water suggesting an alternative recharge source such as river water or remnant paleo-water. In terms of water use, linear mixing models using soil 87Sr/86Sr, leaf 87Sr/86Sr and stem water 𝛿𝛿18O inputs showed that Site 1 trees, on average, were predominately using rainwater (77%, 77% & 67%), while Site 4 trees used both rainwater (16%, 32% & 42%) and saline groundwater (70%, 62% & 58%), regardless of nearby lakes and streams. These findings have implications for future monitoring, and the management of outer floodplain Black Box populations that are unable to receive natural flooding inundation.