Microelectrode studies demonstrate the existence of persistent oxygen and pH gradients around flocculent, macroscopic marine particles known as marine snow. Oxygen is partially, but continuously, depleted within and around marine snow in the dark and can be completely depleted within large fecal pellets. Boundary layers hundreds of micrometers thick are maintained despite advection of fluid past the particles. The existence of chemical microhabitats on the scale of millimeters around macroscopic particles in the pelagic zone may significantly influence the distribution and activity of marine microorganisms and permit processes requiring low oxygen, including denitrification.