Understanding how the hydrology of the SOIs will change due to a changing climate is critical for predicting catchment and ecosystem response to future changes in rainfall, evaporation, and groundwater exchange. The role of groundwater in lake hydrology and hydrochemistry has not been identified, and its omission could have major implications for interpreting soil–water–air processes affecting lakes, and ultimately influencing how palaeoclimate lake records are interpreted. While isotopes have not been used widely across the SOIs, lake water chemistry surveys have applied traditional hydrochemical and biological approaches that highlighted the localised nature of geochemical drivers controlling the hydrochemical evolution of lake waters such as geology, sea spray contribution, vegetation, geographical location, and ice cover extent 7, 12, 13, 14, 15, 16, 17, 18, 19. However, until now this approach has been neglected in other SOIs and is a missed opportunity to monitor changes in catchment hydrology. While this study was limited to using only stable water isotopes, it demonstrated the enormous potential of using isotopic tracers for understanding island hydrology including identifying the sources of water to lakes. This study showed that lakes responded to seasonal change in rainfall and snowmelt, highlighting the importance of understanding the modern hydrology and hydrochemical characteristics of lakes for interpreting palaeoclimatic archives for SOIs 11. The only study to apply isotopic tracers to lake waters in the region used stable water isotopes (oxygen-18 and deuterium) in lakes across Signy Island (Fig. 1a,b) that are experiencing rapid, significant shifts in climate and terrestrial ecosystems 9, 10 has not been widely undertaken. Investigating the hydrochemistry and hydrology of lakes in the Southern Hemisphere high latitudes, including the Southern Ocean Islands (SOIs) (Fig. Lake water can be used to provide a deeper understanding of past environment and ecosystem information contained within lake sediments to understand long-term environmental change for a region 6, 7, 8. These techniques give us information on how catchments respond to rainfall, evaporation, groundwater influx and carbon and nutrient cycling 3, 4, 5. The application of isotopic tracers in water is essential for understanding lake hydrology and the fundamental processes driving lake water chemistry 1, 2. Future research will focus on long-term monitoring to understand seasonal, annual, and long-term variability to test fundamental hypotheses concerning ecosystem function and the consequences of environmental change on SOIs. Increasing temperatures and changing rainfall patterns predicted for the region will lead to shifts in nutrient cycles, and impact the island’s unique ecosystems. The hydrochemical and isotopic tracers suggest that lakes in lower elevations contain more terrestrial sourced ions that may be contributed from groundwater. In general, it was found that lakes at higher elevations are dilute and those located in lower elevation catchments have experienced more water–rock interactions. Lakes on the western side of the island are influenced by sea spray aerosols. These provide essential baseline data for hydrological, biological, and geochemical lake processes. Methods include stable carbon (δ 13C DOC: dissolved organic carbon and δ 13C DIC: dissolved inorganic carbon), oxygen (δ 18O), hydrogen (δ 2H) and strontium isotopic ratios ( 87Sr/ 86Sr) in water. Forty lakes were examined across Macquarie Island, using comparable methods to identify key environmental processes and their geochemical drivers. Here we present the first comprehensive, island-wide hydrochemical and isotopic survey of lakes on a SOI. The role of groundwater in lake hydrology and hydrochemistry has not been identified until now, and its omission will have major implications for interpreting soil–water–air processes affecting lakes. Historical work has highlighted the localised nature of geochemical drivers in controlling the hydrochemical evolution of lakes, such as geology, sea spray contribution, vegetation, geographical location, and ice cover extent. These techniques have not been widely applied to lakes in the Southern Hemisphere high latitudes, including Southern Ocean Islands (SOIs) experiencing rapid, significant shifts in climate. Isotopic and hydrochemical data from lakes provide direct information on catchment response to changing rainfall, evaporation, nutrient cycling, and the health of ecosystems.
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