Nutrient limitation of bacterioplankton growth in Lake Dillon, Colorado

Bacterioplankton biomass, production, and growth rate were measured over a 2-yr period in Lake Dillon, a mesotrophic Colorado reservoir. In addition, a multivariate statistical analysis and nutrient addition experiments were used to analyze the regulation of bacterioplankton growth in situ. Biomass ranged between 170 (winter) and 2,200 mg C m-* (summer); production ranged from 10 (winter) to 625 mg C m-* d-* (summer); annual bacterioplankton production was 47 g C m-2 yr-l in 1987 and 67 in 1988. Population growth rates ranged between 0.001 and 0.08 1 h I. Growth rates in the epilimnion were substantially below estimated potential rates, suggesting severe nutrient limitation during the period of stratification. Population growth rates were highly correlated with P concentrations but not with dissolved organic C concentrations. Bacterioplankton growth in the summer epilimnion responded strongly to the addition of P alone or in combination with N or labile organic C. The field data show that bacterioplankton arc frequently without suflicient nutrients to sustain maximum growth; the experimental and statistical analysts indicate that P, rather than organic C, is the critical nutrient for bacterioplankton growth in this lake. Organic carbon of algal origin is readily used by bacterioplankton (Brock and Clyne 1984). In addition, several workers have demonstrated a relationship between bacterioplankton abundance or production and phytoplankton production or biomass (e.g. Riemann and Sondergaard 1984; Bird and Kalff 1984; Cole et al. 1988). Although these observations suggest strong regulation of bacterioplankton by organic C, there is little experimental evidence to demonstrate that the flux of algal C directly constrains bacterioplankton production. Inorganic nutrients may also regulate bacterial activity, but have received less attention than organic C. The high affinities of bacteria for inorganic nutrients provides a significant competitive advantage (Currie and Kalff 1984a, b, c) and would seem to prcelude the possibility of inorganic nutrient limitation in planktonic bacteria under most circumstances. Also, bacteria sequester a ’ Present address: Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 180 15. Acknowledgments Bo Riemann, Russell Bell, and an anonymous reviewer provided suggestions for the improvement of this manuscript. This work was supported by NSF grant BSR 9020684. disproportionately large amount of P (Vadstein et al. 1988; Glide 1989; Jiirgens and Giide 1990) and N (Wheeler and Kirchman 1986; Kirchman et al. 1990) compared to other planktonic organisms. Although these studies support the presumption that bacteria compete favorably for inorganic nutrients, they do not show conclusively that planktonic bacteria grow under conditions of nutrient sufficiency. In fact, cellular P contents for natural populations suggest that bacterioplankton are frequently subsaturated with P and thus potentially limited by P deficiency (Vadstein et al. 1988; Vadstein and Olsen 1989). Activity or biomass of freshwater planktonic bacteria can sometimes be increased by adding inorganic P (Jones 1977; Cooncy et al. 1985; Peters et al. 1987; Toolan et al. 199 1). In addition, statistical evidence suggests that P may play an important role in determining differences in standing stocks of bacterioplankton across a wide variety of lakes (Bird and Kalff 1984; Shortreed and Stockner 1986; Currie 1990). However, because primary production is often stimulated by P (Schindler 1978), P may influence bacterioplankton indirectly by stimulating the rate at which C substrate is produced. Definitive identification of nutrient limitation requires an experimental approach