UNCOUPLING OF BACTERIOPLANKTON AND PHYTOPLANKTON PRODUCTION IN FRESH-WATERS IS AFFECTED BY INORGANIC NUTRIENT LIMITATION

Pelagic bacterial production is often positively correlated, or couple d, with primary production through utilization of autotrophically prod uced dissolved organic carbon. Recent studies indicate that inorganic N or P can directly limit both bacterial and phytoplanktonic growth. O ur mesocosm experiments, with whole communities from mesotrophic Calde r Lake, test whether this apparent bacterial-algal coupling may be the result of independent responses to limiting inorganic nutrients. In s ystems without N additions, numbers of bacteria but not phytoplankton increased 2- to 2.5-fold in response to P fertilization (0 to 2.0 mu m ol of P per liter); this resulted in uncoupled production patterns. In systems supplemented with 10 mu mol of NH4NO3 per liter, P addition r esulted in up to threefold increases in bacteria and two- to fivefold increases in total phytoplankton biomass (close coupling). P limitatio n of pelagic bacteria occurred independently of phytoplankton dynamics , and regressions between bacterial abundance and phytoplankton chloro phyll a were nonsignificant in all systems without added N. We describ e a useful and simple coupling index which predicts that shifts in phy toplankton and bacterioplankton growth will be unrelated (Delta bacter ia/Delta phytoplankton --> either +infinity or -infinity) in systems w ith inorganic N/P (molar) ratios of <similar to 40. In systems with hi gher N/P ratios (>40), the coupling index will approach 1.0 and close coupling between bacteria and phytoplankton is predicted to occur.