Anthropogenic and Climate-Driven Changes on Harmful Algal Blooms in Two Chinese Reservoirs

This study assessed the impacts of anthropogenic and climate change on harmful algal blooms (HABs) in the Dongzhang and Shanzai Reservoirs from 1990 to 2023, focusing on land use and land cover change (LULCC) analysis. Through supervised classification of Landsat imagery, the study examined changes in cropland, forest, shrub, grassland water, barren and built-up areas in the watersheds of both reservoirs and monitor HABs events. The Dongzhang catchment saw forest cover increase from 72.0% in 1990 to 75.3% in 2010 before decreasing to 71.7% in 2022, while cropland and built-up areas fluctuated. Similarly, Shanzai's forest cover declined from 94.4% in 1996 to 91.4% in 2022, with cropland increasing. Significant HAB events in Dongzhang Reservoir peaked in 1990, 2010 and 2022, affecting up to 40% of the reservoir's area in 2022. In Shanzai Reservoir, HAB events showed an increasing trend from 1996 to 2018, with a peak in 2018 when 40% of the reservoir's area was affected. Land use changes exacerbate HABs, as seen in declining forest cover and increasing cropland and built-up areas. Analysis using Landsat imagery showed HAB occurrences increasing from 1990 to 2023, with rising water temperatures indicating potential future increases. Addressing anthropogenic activities and climate change are vital to prevent and manage HABs, requiring improved monitoring and early warning systems. With remote sensing, water quality issues can be identified and managed. The findings emphasize the need for continuous monitoring, improved LULCC analysis, sediment pollution research and effective management strategies to protect water quality and ecosystem health under climate change. Recommendations include long-term monitoring, nutrient input investigations, modelling studies, early warning systems and adaptation strategies.