NUTRIENT LIMITATION OF BACTERIAL-GROWTH AND RATES OF BACTERIVORY IN LAKES AND OCEANS - A COMPARATIVE-STUDY

We conducted a series of dilution bioassay experiments to determine whether limitation of bacterial growth by nutrient elements (N and P) and organic substrates differs in marine and freshwater pelagic ecosystems. We also conducted Landry-Hassett dilution gradient experiments to assess whether rates of bacterivory differ substantially in the 2 ecosystem types. P addition stimulated significant (p < 0.05) responses at 5 of 6 marine and 4 of 5 freshwater sites, while N addition stimulated bacteria at 2 of 6 marine sites and 2 of 5 freshwater sites. Organic C (OC, added as glucose) was significant as a main effect in 4 of 6 marine and 2 of 5 freshwater experiments. Significant treatment interactions (N x P, OC x N, OC x P) occurred in several cases. Magnitudes of growth response to addition of the limiting nutrient were consistently greater in freshwater experiments. Rates of bacterivory did not differ (p > 0.05) for lake and marine systems sampled although data were extremely limited. Our results support the emerging view that mineral limitation of bacterial growth rates (as well as limitation by organic substrates) is widespread in both marine and freshwater ecosystems. However, differences in degree of response to enrichment and in growth rates in unenriched treatments in our marine and freshwater experiments suggested that severity of nutrient Limitation of growth in situ was considerably higher for freshwater bacteria than for marine bacteria. While our study was limited in scope and more data are clearly needed, based on these results we propose several hypotheses for the apparent difference in severity of nutrient limitation of bacteria in lakes and oceans.