Pelagic cyanobacterial nitrogen fixation in lakes and ponds of different latitudinal zones.

Excess nitrogen (N) loading is one of the main factors causing eutrophication. Biological N fixation (BNF), as a main contributor to N loading, plays a critical role in the N cycle. The N2 fixation rate (N2fix) is regulated by many factors and is usually higher under conditions of N deficiency. Most studies have focused on the regulation of factors that influence the N2fix in specific aquatic ecosystems or artificial conditions, while fewer have focused on large scale such as the latitudinal distribution of N2fix. To understand the regulation of the N2fix in latitudinal zones, the key factors, and the underlying mechanism, we compared the N2fix in 27 lakes located in different latitudinal zones and analyzed the main regulators. The results showed that (1) heterocyst density (DHet) and the N2fix were highest in low-temperate lakes and were 2.5–2.7 and 11.6–22.1 times greater than in high-temperate lakes and tropical lakes, respectively, in the 99th quantile; (2) DHet increased and then decreased with increasing latitude and radiation, and peaked at 30.28° N and 2300 J/cm/d; (3) DHet was positively correlated with temperature and increased slightly with increasing temperature; (4) N2fix increased and then decreased with increasing latitude, temperature and radiation, and peaked at 38.8° N, 24.21 °C and 2120 J/cm/d. The results suggest that BNF could be regulated by larger scale factors, e.g., temperature and radiation in latitudinal scale. Compared with high-temperate lakes and tropical lakes, low-temperate lakes may face more difficulties in controlling eutrophication due to the potentially higher N loading from BNF under similar nutrient and morphometric conditions. [ABSTRACT FROM AUTHOR]