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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]

• Food directly promotes the tolerance of gastropods to ammonia. • Sediment and macrophyte decrease ammonia toxicity by ammonia absorption. • Sediment and macrophyte provide extra food for gastropods. • Sediment shelters gastropods from direct exposure to high ammonia concentrations. The toxic effect of unionized ammonia (NH 3) on aquatic organisms is receiving increasing attention due to the excessive nitrogen discharge to various surface waters. Researches have suggested the scale-dependence of NH 3 toxicity, being lower in field than under lab conditions. Such scale-dependence of toxicity is a big challenge to water quality criteria setting as the results solely from lab tests might not apply to natural ecosystems. Therefore, it is necessary to explore the underlying mechanism to understand the difference of toxicity across various spatial scales. In this study, we used the widely distributed gastropod Bellamya aeruginosa as the test animal and performed two 192-h microcosm experiments. Each experiment included a control and an ammonia addition treatment: N0(LC 50) & N+(LC 50), N0(LC 100) & N+(LC 100) (96-h LC 50 = 0.8 mg NH 3 N/L, 96-h LC 100 = 18.1 mg NH 3 N/L). Besides water-only, three potential key components (food, sediment, and submersed macrophytes) were included in the various treatments to mimic different complexity levels of aquatic ecosystems (Water, Water + Food, Water + Sediment, Water + Sediment + Macrophytes). The results showed that: 1) food directly improved the survival and growth of gastropods under expected lethal concentration of ammonia (96-h concentration of NH 3 N = LC 20 of the 96-h acute test); 2) sediment and macrophyte quickly decreased the ammonia concentration, mainly by sediment adsorption and macrophyte uptake, to alleviate the ammonia stress to gastropods and permitted them to survive and grow under expected lethal concentration of ammonia (96-h concentration of NH 3 N = LC 10 ∼LC 20 of the 96-h acute test); 3) sediment and macrophyte also provided additional food for gastropods; 4) under extremely high ammonia stress (i.e., 96-h LC 100 , food was left uneaten and macrophyte died, and gastropods could, therefore, not be released from ammonia stress. Our results demonstrate that under moderate ammonia stress, the introduction of extra ecosystem elements (food, sediment, and macrophytes) significantly improved the survival and growth of gastropods, mainly by enhancing their tolerance and by quickly decreasing the NH 3 concentration and thus toxicity. However, these introduced elements had little effect at very high concentration of ammonia (i.e., 96-h LC 100). Our findings add to the understanding of the reasons behind the previous observed scale-dependent toxicity of NH 3 on aquatic organisms and contribute to better decisions on the role of NH 3 in relation to water quality management. [Display omitted] [ABSTRACT FROM AUTHOR]