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 12 degreesC, but below 12 degreesC 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 12 degreesC. 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.