Horizontal variability and regulation of bacterial production in Lake Biwa, Japan

To clarify horizontal variability and regulation of bacterial production (BP), we investigated BP and environmental variables along three east–west transects (lines 12, 15, and 17) covering inshore/offshore stations in Lake Biwa, Japan, during four seasons using 15 N-labeled deoxyadenosine for measuring BP. In winter, surface BP along Line 12 (southern transect) was higher than Lines 15 and 17 (central and northern transects) and reflected the water-temperature distribution. Additionally, any nutrients and dissolved organic carbon did not correlate to BP, suggesting water temperature primarily regulated BP in winter. In spring, surface BP was higher at eastern inshore stations, near large agricultural fields, and was correlated with nutrient concentrations rather than water temperature, suggesting that the limitation shifted to nutrient availability. In summer, surface BP at offshore stations along Lines 15 and 17 was notably lower than the other stations, and the horizontal heterogeneity became largest (maximum to minimum BP ratio = 9.5, compared to 2.8 ~ 4.1 in the other seasons). The BP was also positively correlated to nutrient concentrations, especially phosphorus. Surface BP in autumn also showed higher values at eastern stations as well as spring and positively correlated to phosphorus concentration. Additionally, there was a negative relationship between BP and water temperature, suggesting that bacterial growth was enhanced by groundwater seepage at the eastern stations. The results suggest that the horizontal distribution is characterized by a north–south distribution with a temperature gradient in winter, and allochthonous nutrient loading determines horizontal BP variations in the other seasons in this lake.