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 wh ether limitation of bacterial growth by nutrient elements (N and P) an d organic substrates differs in marine and freshwater pelagic ecosyste ms. We also conducted Landry-Hassett dilution gradient experiments to assess whether rates of bacterivory differ substantially in the 2 ecos ystem types. P addition stimulated significant (p < 0.05) responses at 5 of 6 marine and 4 of 5 freshwater sites, while N addition stimulate d 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 m arine and 2 of 5 freshwater experiments. Significant treatment interac tions (N x P, OC x N, OC x P) occurred in several cases. Magnitudes of growth response to addition of the limiting nutrient were consistentl y greater in freshwater experiments. Rates of bacterivory did not diff er (p > 0.05) for lake and marine systems sampled although data were e xtremely limited. Our results support the emerging view that mineral l imitation of bacterial growth rates (as well as limitation by organic substrates) is widespread in both marine and freshwater ecosystems. Ho wever, differences in degree of response to enrichment and in growth r ates 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 an d oceans.