Seasonal variation of phosphorus limitation of bacterial growth in a small lake

A series of bioassay experiments were performed from spring to autumn in a small dimictic lake (Deep Pond, Massachusetts) to examine the potential for bacterial growth limitation by organic carbon (glucose), inorganic nutrients (ammonium or phosphate), or both. The experiments demonstrated that phosphorus was the primary element limiting bacterial growth in Deep Pond during a large part of the summer. Significant increases (relative to controls) in bacterial cell volumes, protein production rates, and abundances were observed during 24-h incubations for samples amended with phosphate alone. Organic carbon was near colimitation for most of the samples, however, and dramatic increases in bacterial abundance and rates of protein production were obtained only when both substances (phosphate and glucose) were added together. There was no evidence for nitrogen limitation of bacterial growth during the study. Temperature was not an important determinant for bacterial production rates above 128C, but below 128C temperature acted to mute the effect of nutrient and organic carbon additions on production rates. Bacterial growth was not significantly increased by the addition of any combination of glucose, ammonium, or phosphate below 128C. A significant, albeit complex, effect of the microbial community on the bacterial response to nutrient/carbon enrichment was apparent in the samples. Substrate/nutrient supply and biomass removal by bacterivores both appeared to play a role in the outcome of the experiments. Pelagic heterotrophic bacteria play a fundamental role in aquatic ecosystems as regulators of the cycling of carbon and important nutrients such as nitrogen and phosphorus. Bacteria are the major consumers of dissolved organic matter (DOM) in the plankton. As such, they play vital roles in the recovery of organic matter, from detritus to living biomass, or in its remineralization back into inorganic compounds (Ducklow and Carlson 1992; Shiah and Ducklow 1994). Bacterial growth and biomass in aquatic ecosystems are regulated by a number of factors, such as temperature (Shiah and Ducklow 1994), predation (Caron 1991), substrate supply (organic and inorganic nutrients) (Cole et al. 1988; Rivkin and Anderson 1997), and viral infections (Proctor and Fuhrman 1992). The relatively low variability of bacterial abundance in aquatic ecosystems has given rise to the speculation that bacterial abundance is rather tightly regulated by the different gain and loss factors operating on bacteria (Sanders et al. 1992). Traditionally, organic carbon (for energy) has been considered the main factor limiting heterotrophic bacterial growth in pelagic environments. More recently, however, several investigators have demonstrated that bacterial growth in aquatic ecosystems can be limited by the availability of nitrogen and phosphorus rather than the supply of organic 1 Corresponding author ([email protected]).