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Microcystis aeruginosa and Raphidiopsis raciborskii (previously Cylindrospermopsis raciborskii) are both common bloom-forming cyanobacteria which can coexist but alternatively dominate in freshwater ecosystems. To predict their blooming dynamics, we need to understand the potential environmental factors determining their succession. In the present study, we examined the pairwise competition of the three M. aeruginosa strains (FACHB905, 469 and 915) with one R. raciborskii strain (N8) at three temperature levels (16 °C, 24 °C, and 32 °C). We found that the competitive ability of three Microcystis strains were highly variable. M. aeruginosa FACHB905 was the strongest competitor among them which can finally exclude R. raciborskii N8 regardless of initial biovolume ratios and temperature levels. The competitive exclusion of N8 by 915 also was observed at 24 °C, but they coexisted at 16 °C and 32 °C. We observed that M. aeruginosa FACHB469 and R. raciborskii N8 were able to coexist under all the temperature levels, and M. aeruginosa FACHB469 was the weakest competitor among the three M. aeruginosa strains. Rates of competitive exclusion (RCE) showed that temperature affects the competition between three M. aeruginosa strains and R. raciborskii N8. M. aeruginosa strains always grew quickly at 24 °C and significantly enlarged its dominance in the co-culture system, while R. raciborskii N8 was able to maintain its initial advantages at both 16 and 32 °C. The competitive advantage of M. aeruginosa FACHB905 may be explained by allelopathic interactions through its allelochemicals and other secondary metabolites other than microcystin. We concluded that both strain difference and temperature can affect the competition between M. aeruginosa and R. raciborskii. Our results highlighted the complexity of cyanobacterial dynamics in waterbodies where there exist multiple strains.

Submerged macrophytes cannot be utilized directly by zooplankton. However, vegetation can serve as an organic carbon resource for heterotrophic bacteria, which are themselves accessible to zooplankton. We therefore hypothesize that submerged macrophytes supply a carbon source to zooplankton by increasing the availability of food such as heterotrophic bacteria. Here, we used stable carbon isotope (13C) labeling to trace the carbon flow from submerged macrophytes to zooplankton with a mesocosm experiment. The carbon stable isotope ratios of zooplankton and their potential food sources were analyzed in mesocosms planted with 13C-labeled Hydrilla verticillata (L. f.) Royle in comparison with the control in which plastic plants were planted. We found that all potential food resources of zooplankton, including phytoplankton, bacterioplankton, macrophyte-associated epiphyton and epibacteria, were significantly enriched with 13C in the presence of 13C-enriched H. verticillata. Zooplankton were significantly more enriched with 13C than phytoplankton, epiphyton and bacterioplankton but significantly less enriched with 13C than epibacteria. Based on a stable isotopic mixing model, we found a macrophyte carbon contribution of 30.5% to epibacteria and 14.7% to zooplankton. Our results indicated that macrophytes might be used by zooplankton as a carbon resource, mainly via a pathway involving epibacteria attached to macrophytes. [ABSTRACT FROM AUTHOR]

Phosphorus (P) loss from soil to water bodies is an important process associated with eutrophication of lakes and reservoirs. Phosphorus concentration in China's waterbodies changes geographically, increasing substantially with latitude. It is assumed that the geographical pattern results from change in soil retention and loss features of P. To test this assumption, we designed a soil column leaching experiment to measure the P retention capacity of three common types of soils in China: red, black and loess. Results showed that red soil possessed a much higher P retention capacity than black and loess soils. Red soil retained P primarily through binding by aluminium and iron, with half of the total retained P contributed by iron binding. Phosphorus loss in all three soils was influenced by pH of the used acid solution and the leaching rate. Specifically, acidification tends to promote soil P loss, whereas leaching rate decreases P loss. [ABSTRACT FROM AUTHOR]