Nitrogen biogeochemistry in an unpolluted estuary: the importance of benthic denitrification

Denitrification rates (N, production), and benthic ammonium, nitrate and oxygen fluxes were measured along a salinity gradient in Ochlockonee Bay (Florida, USA) over an annual cycle. Denitrification rates ranged from 0 to 210 pg-at N m-2 h-'. Lowest rates of denitrification were measured in late winter and highest rates in late spring. A major portion of the organic nitrogen cycled through the sedlments is denitrified; on an annual basis an average of 69 % of the benthic nitrogen flux is N2 and 31 % is ammonia, nitrite and nitrate. During summer the amount of nitrogen lost from the recycling pathway by denitrification is equivalent to about 40 % of the nitrogen requirements of the phytoplankton. The major source of NO: or NO: for denitrification is from nitrification in the sediments, not diffusion of NO? or NO; from the overlying water. Denitrification is a major removal mechanism for nitrogen in Ochlockonee Bay, and removes, on an annual basis, an amount of nitrogen equivalent to 54 % of the river input of dissolved inorganic nitrogen. N20 fluxes were small compared to N2 fluxes; N20/N2 ratios in March were less than 0.01.