Uncoupling of bacterioplankton and phytoplankton production in fresh waters is affected by inorganic nutrient limitation

Pelagic bacterial production is often positively correlated, or coupled, with primary production through utilization of autotrophically produced dissolved organic carbon. Recent studies indicate that inorganic N or P can directly limit both bacterial and phytoplanktonic growth. Our mesocosm experiments, with whole communities from mesotrophic Calder Lake, test whether this apparent bacterial-algal coupling may be the result of independent responses to limiting inorganic nutrients. In systems without N additions, numbers of bacteria but not phytoplankton increased 2- to 2.5-fold in response to P fertilization (0 to 2.0 micromoles of P per liter); this resulted in uncoupled production patterns. In systems supplemented with 10 micromoles of NH4NO3 per liter, P addition resulted in up to threefold increases in bacteria and two- to fivefold increases in total phytoplankton biomass (close coupling). P limitation of pelagic bacteria occurred independently of phytoplankton dynamics, and regressions between bacterial abundance and phytoplankton chlorophyll a were nonsignificant in all systems without added N. We describe a useful and simple coupling index which predicts that shifts in phytoplankton and bacterioplankton growth will be unrelated (change in bacteria/change in phytoplankton leads to either +infinity or -infinity) in systems with inorganic N/P (molar) ratios of < approximately 40. In systems with higher N/P ratios (> 40), the coupling index will approach 1.0 and close coupling between bacteria and phytoplankton is predicted to occur.