Annual production and consumption of oxygen were compared in large outdoor mesocosms differing only in the presence or absence of an intact benthic community and associated sediments. Both daily apparent production and nighttime respiration of oxygen were greater in tanks with a benthos. The fluxes of oxygen into, and dissolved inorganic nitrogen out of the bottom were also greater in tanks with an intact benthos. In tanks with a benthos, calculated gross system production increased 33% relative to tanks lacking a benthos. Depending on assumed O:N ratios only 45–60% of this increase was attributable to differences in the flux of inorganic nitrogen from the benthos to the water column. Nearly 40% was evidently fueled by higher rates of recycling in the water column. Between 3 and 17% of the difference in production could not be attributed to either source. The benthos apparently affects production in the water column not only by supplying nutrients directly, but also by enhancing rates of pelagic recycling.