Microzone formation: Its role in the enhancement of aquatic N₂ fixation1

Cyanobacterial‐bacterial aggregates form internal microenvironments (microzones) having metabolic and redox conditions distinct from those of ambient planktonic and benthic marine and freshwater habitats. Low turbulence accompanied by adequate supplies of dissolved organic matter (DOM) favored the formation of microzones. Increases in cyanobacterial and possibly bacterial N 2 fixation potentials paralleled the development of microzones, while increased turbulence and DOM deprivation greatly reduced both microzone formation and N 2 fixation potentials. Among North Carolina nearshore marine and freshwater systems examined, the formation and maintenance of microzones were shown to override both trace metal and phosphorus availability as factors regulating magnitudes of N 2 fixation. The degree to which development of microzone‐scale symbiotic cyanobacterial‐bacterial interactions occur in aquatic habitats in part determines the extent to which N 2 fixation can meet nitrogen demands in nitrogen‐depleted waters.