Day 4

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Paper title On the potential of satellite Earth Observations for detecting regimes shifts in lakes
Authors
  1. Elisa Calamita Eawag, Swiss Federal Institute of Aquatic Science and Technology Speaker
  2. Clement Albergel European Space Agency - Climate Office
  3. Iestyn Woolway University of Reading
  4. Daniel Odermatt Eawag, Swiss Federal Institute of Aquatic Science and Technology
Form of presentation Poster
Topics
  • A7. Hydrology and Water Cycle
    • A7.06 EO for monitoring water quality and ecological status in inland waters
Abstract text Global warming affects ecosystems worldwide. Among others, climate change can trigger lake ecosystems shifts. Increasing trends for lake water temperature have been suggested from single case studies but also at the global scale. Increasing water temperature intensifies the thermal stratification of deep lakes, reducing the intensity of vertical mixing. Warming will thus likely alter the mixing regime of lakes substantially this century, as suggested by recent lake model simulations. This transition potentially leads to abrupt shifts in global lake ecosystems.
A reduction in mixing intensity and frequency can have severe implications for the entire lake ecosystem. For example, reduced deep water renewal hinders the vertical transport of oxygen from epilimnion to hypolimnion and can increase the extent and duration of seasonal hypoxia (low oxygen). Contrarywise, stratification suppresses nutrient resupply from the deep water to the surface layer. Both affect lake primary productivity and its entire food web. Records to characterize mixing regime anomalies and ecosystem shifts or their underlying mechanisms are scarce because they require long and dense time series, requirements often not met by traditional monitoring records.
We review and synthesize information on the detection of regime shifts in lakes worldwide. We suggest three main sources of data that can be used to detect lake ecosystem shifts: sediment coring, high-frequency in-situ measurements, and remote sensing. Remote sensing data started to be used for this purpose in a later stage but its potential to monitor several lakes at the same time allows studying a wider range of lakes. Our synthesis of the literature for more than 700 studies of lake regime shifts shows that to date remotely sensed time series of lake surface water temperatures (LSWT) have been mainly based on spatial averages of lake surface water temperature, neglecting the spatial dimensions of global LSWT products. However, the horizontal gradients could help a better understanding of the internal processes of lakes and the identification of lake mixing or ecosystem anomalies.
Seasonal overturning often occurs at different times across the lake. Thus, the spatial character of remotely sensed data can reveal important processes in freshwater systems and can help assess the long-term variability in the overturning behavior of large lakes in the context of climate change. However, limnologists have so far not extensively explored the spatially distributed character of remotely sensed data. We aim at developing a methodology to detect anomalies or shifts of lake ecosystems by using the spatial patterns of remotely sensed lake water properties (LSWT and ecological variables like turbidity and chlorophyll), and link such patterns to documented anomalies or shifts of lake ecosystems. Here, we exploit the CCI Lakes database from the standpoint of a limnologist, and with an advanced understanding of thermal forcing and ecosystem responses in lakes.