Assessing water clarity status and long-term trends in North America’s largest lakes using ESA’s Ocean Colour Climate Change Initiative (OC-CCI) products

Abstract

Water clarity, as measured by Secchi disk depth (Z_SD) or diffuse attenuation (Kd), is an important indicator of a lake’s ecosystem state and can be reliably retrieved using satellite remote sensing. By combining data from multiple satellite missions, the European Space Agency’s Ocean Colour Climate Change Initiative (OC-CCI) aims to deliver stable, long-term, satellite data products suitable for trend assessments. Here we demonstrate the value of OC-CCI products for reporting on water clarity status and long-term trends in North America’s largest lakes. Extensive matchups between the OC-CCI Kd at 490 nm (Kd₄₉₀) and in situ Z_SD observations spanning 25 years enabled robust multi-lake validation of Z_SD retrievals over a wide range of water clarity conditions (R² = 0.9, MAPE = 29.6 %, BIAS = 6.6 %, N = 4297) providing a transferable model for large-scale mapping of inland water clarity. Significant differences in Z_SD retrieval uncertainty were observed between years, missions, and specific periods marking changes in the sensor datasets contributing to the OC-CCI products. Bias-correction of the OC-CCI Kd₄₉₀ provided confidence in the assumption of seamless continuity in this multi-mission dataset, thereby allowing long-term time-series analyses. Seasonal, inter-annual and inter-decadal variability and trends in lake-wide average Z_SD were subsequently evaluated for nine large lakes across Canada and the U.S. over the 1998–2023 period, capturing the timing and magnitude of significant shifts in water clarity conditions. Observations agree well with documented periods of ecosystem change in response to the cumulative impacts from harmful algal blooms, nutrient status, invasive species, and hydrological events.