Spatial and Seasonal Change in Algal Community Structure and its Interaction With Nutrient Dynamics in A Gravel-bed Urban River

Abstract Harmful algal blooms frequently occur in urban rivers due to intense human activities. However, little is known about the change in algal community structure and its interactions with nutrient dynamics in gravel-bed urban rivers. In present study, water samples were collected from 25 sampling sites in a gravel-bed River Xin’an, China for five months over four seasons and a rainy month. The results showed that the harmful Cyanophyta accounted for 31.6 ± 24.1% of the total community in the hot season while Bacillariophyta contributed more than 60% to the community in the other three seasons. The nitrogen gas (N2) was supersaturated in the moderate and cold seasons but it was unsaturated in the hot season, along with high concentrations of nitrogen-fixing cyanobacteria (Anabaena), indicating that the nitrogen fixation capacity was strong and even stronger than denitrification and anammox in the hot season. However, nitrogen fixation was not the main source of nitrogen in the water column. The concentrations of nutrients and Chla in the downstream river were significantly higher than those in the upstream river (p