Growth and grazing mortality of Synechococcus during its summer bloom in a brackish hypertrophic lake (Lake Sanaru, Japan)
摘要
In the picoplankton class, the cyanobacterial genus Synechococcus is ubiquitously distributed across a wide range of trophic states in aquatic ecosystems. Most Synechococcus species are grazed on by protists such as heterotrophic nanoflagellates (HNF) and ciliates. Although HNF is typically the main grazer of Synechococcus, few studies have investigated HNF grazing of Synechococcus in hypertrophic lakes. To evaluate the importance of Synechococcus biomass within the microbial food web, the current study was performed in the hypertrophic Lake Sanaru, where we determined the growth and grazing mortality of Synechococcus during a summer bloom using a dilution method. The abundance of Synechococcus was higher (1.2–8.5 × 106 cells mL−1) than in previous studies. Daily growth and grazing mortality rates suggested that a significant fraction (170–916 µg C L−1 d−1, 53–175%) of Synechococcus production was lost to HNF grazing in this lake. Synechococcus grew actively and grazed during the daytime and did not show marked growth or grazing mortality during the nighttime. Our findings reinforce the view that Synechococcus is an important food item for HNF in a wide trophic range of lakes, ranging from oligotrophic to hypertrophic. This is the first report of diel variations in the growth and grazing mortality of Synechococcus in a hypertrophic lake.