Nitrification rates related to sedimentary structures in an Atlantic intertidal mudflat, Marennes-Oleron Bay, France

Rates of actual nitrification through acetylene blockage of NH4+ oxidation and potential nitrification were measured in 2 intertidal geomorphological structures, 'ridges and runnels', at Marennes-Oleron Bay, France. Nitrifica tion rates were 6 times higher in runnels as compared to ridges and their O -2 gradients with depth were different. Calculated N mineralization rates a veraged 1.66 mmol m(-2) d(-1) in runnels versus 0.29 mmol m(-2) d(-1) in ri dges: Over 70% of this produced NH4+ was nitrified and thus became potentia lly available for denitrification. In neither ridges nor runnels was there a significant correlation between potential nitrification and pH, E-h or po re water NH4+ (p > 0.05), but the correlation between potential nitrificati on and pore water NO3- was significant in runnels (p < 0.05). The presence of nitrifying bacteria below the depth of O-2 penetration in sediment sugge sts that mixing processes have a role in controlling nitrification in this mudflat.