Global inland waters trophic status from space observation: scientific advances and future challenges

Inland waters represent a significant share of the global freshwater resources and can have a profound impact on global climate and human well-being. By 2050, the world's population is expected to reach 9.7 billion, and one in four people is expected to suffer from water shortages. The United Nations has identified clean drinking water management as a Sustainable Development Goal (SDG 6) to address this major challenge. Eutrophication is a serious threat to inland water resources worldwide. The vigorous development of eutrophication research could greatly contribute to the achievement of SDG 6. To understand the trophic status in inland waters around the world, catchment and larger scale observations are necessary. In recent decades, advances in satellite remote sensing technology have improved our ability to assess the extent of the problem and understand underlying mechanisms of the eutrophication process. This review covers the basic concepts, key studies, and methodological frameworks for monitoring the trophic status of inland waters using satellite remote sensing across diverse spatial and temporal scales. Based on the pressure, status, response of eutrophication, we propose a conceptual framework that integrates index harmonization, a multi-model workflow, and guidance for index selection to enhance the effectiveness of remote sensing monitoring of trophic status in inland waters. The review (i) linked optical responses and biochemical processes of eutrophication, (ii) explained how to monitor trophic status using a multi-strategy approach with satellite retrieval algorithms, and (iii) discussed the significant importance of trophic status mapping for the healthy development of inland waters worldwide. Driven by remote sensing and various models, global multi-temporal and multi-spatial optical big data has played a significant role in the monitoring of inland water trophic status. Future research will focus on harnessing hyperspectral missions, developing high-quality atmospheric correction and addressing adjacency effects, advancing physics-guided machine learning, and integrating these approaches with process-based hydrological and biogeochemical models. These researches will continue to contribute to the assessment of risks in inland waters, the sustainable utilization and protection of water resources, and the active response to climate change and human pressures.