Detailed synthesis of in-situ and satellite derived LSWT over lakes globally, indicated rapid warming during summer. Lake Surface Water Temperature (LSWT) exhibits a rapid and direct response to climate forcing prominently induced by changes in air temperature, cloud cover, short wave radiation, latitude and the lakes morphometry 5. In Sweden, annual phytoplankton bloom in larger lakes was found to be anticipated by a month due to the early warming in spring 16. The study “Blooms like it hot” 12 explain climate change as a potent catalyst for the expansion of toxic blooms. The expansion of tropical bloom-forming cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) to the temperate lakes in the mid latitudes is triggered by global warming 15. Geographical expansion of toxic cyanobacteria is reported owing to the warming of lakes 11, 12, 13, 14. Global warming has a direct impact on thermal characteristics of lakes, influencing variations in the physical as well as biological characteristics 9, 10. Recent studies on a large number of lakes worldwide indicated a global trend of rapid warming related to climate change 5, 6, 7, 8.
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Thermal variations on the lake surface and epilimnion are crucial for key responses like vertical mixing and stratification, nutrient and oxygen dynamics, as well as spread and geographical expansion of biota 1, 2, 4. The surface temperature as being direct and sensitive to long-term changes in thermal structure of the lakes is a good indicator to understand the changes in the lake characteristics 1, 3. Any change in the surrounding catchment due to climate forcings will reflect on the physical, chemical and biological processes in lakes 2. Lakes are considered worldwide as sentinels of climate change 1, 2.
SUB ALPINE S SERIES
The new LSWT time series showed a significant annual rate of increase of 0.020 ☌ yr −1 (*P < 0.05) and of 0.036 ☌ yr −1 (***P < 0.001) during summer. We found the satellite derived homogenised LSWT being significantly correlated to in-situ data. The satellite derived LSWT maps were validated based on long-term monthly in-situ bulk temperature measurements in Lake Garda, the largest lake in Italy. Gaps in the temporal LSWT data due to the presence of clouds were filled by applying Harmonic ANalysis of Time Series (HANTS). We used a split-window technique to derive LSWT from brightness temperatures and a modified diurnal temperature cycle model to homogenise data which were acquired between 8:00 to 17:00 UTC. Precisely, we developed homogenised 1 km daily LSWT maps covering the last 30 years (1986 to 2015) combining data from 13 satellites. In this study, we introduce a new methodology to develop homogenised Lake Surface Water Temperature (LSWT) from multiple polar orbiting satellites.
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Availability of remotely sensed multi-spectral images since the 1980’s, which cover three decades of voluminous data could help researchers to study the changing dynamics of bio-physical characteristics of land and water.