Effects of phytoplankton community and interaction between environmental variables on nitrogen uptake and transformations in an urban river

Abstract

Phytoplankton are not only the main bearer of the nitrogen cycle, but also a key link driving nitrogen cycle. However, most phytoplankton cannot directly use N 2 , and they must uptake nitrogenous nutrients (ammonium, nitrate, and urea) to meet their photosynthesis needs. We examined the uptake characteristics of several nitrogenous substrates using stable isotope technique and identifi ed the potential nitrogen transformations in the Fenhe River. Results revealed that spring phytoplankton community composed of mainly Fragilaria , Ulothrix , Microcystis , and Synedra . Urea can meet the spring partial nitrogen requirement of phytoplankton. The large uptake rate of urea depended on urease, chlorophyll a , and nitrate concentrations as shown in random forest models. Cyanobacteria explained more than 42.8% of the total abundance at all sites in summer. Upstream was dominated by Actinastrum , and Chlorella was relevant in the downstream section. The uptake rates of ammonium were higher than those of nitrate and urea. In addition, the random forest model demonstrated that ammonium, urease, and dissolved oxygen (DO) were the major contributors to the ammonium uptake rates. Ammonium was taken up preferentially in autumn and phytoplankton ( Cyclotella , Chlorella , and Pseudanabaena ) appeared to be able to respond to changes in nitrogen forms by adjusting their community composition. Structural equation models demonstrated that temperature-induced changes in DO directly aff ected the transformations of diff erent forms of nitrogen. At the same time, dissolved organic carbon can directly act on nutrients and then indirectly aff ect enzyme activity. There were great diff erences in the positive and negative eff ects of diff erent paths in the process of nitrate reduction to nitrite and then reduction to ammonium in time and space. These fi ndings provide a better understanding of the underlying mechanism of nitrogen uptake and the infl uences of interaction between environmental variables on nitrogen transformations in urban river ecosystems.