Trophic homogeneity due to seasonal variation in nitrogen in shallow subtropical lakes

Lakes play a crucial role in the nitrogen (N) cycle, and eutrophication disrupts the balance of the nitrogen cycle within lakes, including both the N removal process and the N supplement process. However, the mechanisms by which different nutrient levels affect seasonal nitrogen variations in the water columns are not clear, especially for long-term and large- scale studies. In this study, we used 206 independent spatial samples from a total of 108 subtropical shallow lakes from four surveys in the middle and lower reaches of the Yangtze River, as well as time-case study data from Lake Taihu and Lake Donghu of up to 23 and 14 years, respectively, to analyze the changes in summer TN compared to spring (delta TN). Delta TN was significantly negatively correlated with initial spring TN concentrations, with similar trends observed in both space and time. Furthermore, the slopes of spring TN vs. delta TN varied little across lakes in both time and space, suggesting a consistent relationship between initial spring TN and summer TN changes. When initial TN or TN: TP ratio was low, N fixation by algae played a significant role in compensating for summer N removal, thus mitigating summer N reductions; when TN was high or TN: TP ratio was high, ammonia stress reduced the compensatory effect of algae and denitrification played a significant role in summer N removal, thus increasing summer N reductions. Our study suggested that no matter what the initial conditions are, lakes tend to evolve towards a common nutrient status through biological regulation.