Effects of nutrient supplies on the growth rates of planktonic bacteria in Uchiumi Bay, Japan

In the oligo- to mesotrophic waters of Uchiumi Bay, Japan, there is an intermittent physical event which supplies nutrients to the sea: bottom intrusion of deep, cool, nutrient-rich water that slides through just above the continental shelf. To determine the effects of nutrients supplied by this bottom intrusion on bacterial growth, we monitored seasonal changes in bacterial growth rates and conducted nutrient amendment experiments with single or mixed-nutrient additions of N and P. In addition, we examined the importance of organic matter supply for bacterial growth rates using glucose. Bacterioplankton bulk growth rates ranged between 0.37 and 1.86 d super(-1), with relatively high growth rates during the period of strong thermal stratification. Neither positive nor negative effects on bacterial growth rates were detected by amendments of nutrients or glucose. A significant positive relationship (p < 0.001) between cell density and production of bacteria was found, though the regression slope of bacterial cell density on bacterial production was relatively low (0.34). Our results suggested that bacterial growth rates in Uchiumi Bay are nearly saturated at ambient levels of nutrients and/or organic matter, and that bacterial abundance is subject to relatively weak bottom-up control.