An improved coupled water quantity-quality-ecology model incorporating diurnal cycle as a key factor affecting algal...

Diurnal fluctuations in algal growth and decline are essential for understanding the dynamics and predicting algal blooms accurately in large river systems. This study analyzed environmental monitoring data during a spring algal bloom event from February to March 2022 in the Middle-lower Hanjiang River, China, and investigated the driving factors of diurnal variations. An improved coupled water quantity-quality-ecology model incorporating diurnal cycle was developed by proposing a novel light effect equation and integrating high-resolution hourly data, based on previous research and model calibration. This model can simulate river algal blooms and reveal the combined effects of the diurnal cycle on the algal growth and decline process. Model validation at the Zongguan cross-section showed satisfactory performance with the coefficient of determination (R²) greater than 0.70 and root mean square error (RMSE) less than 0.010 during the calibration and validation periods in simulating chlorophyll a (Chla) concentrations. The diurnal fluctuations in Chla concentration increased from a predawn minimum (usually between 4:00 and 8:00) to an afternoon peak (usually between 14:00 and 16:00), followed by a decrease. This pattern was driven by the combined effects of meteorological factors and interactions between algae and nutrients. During the bloom outbreak period, ammonia nitrogen (NH₃-N) was identified as the most important nutrient limiting factor. The above results enhance the understanding of mechanisms of algal growth and decline, and provide a scientific tool to support strategies for managing and mitigating algal bloom events in large river systems.