Seasonal variation of nitrification along a salinity gradient in an urban estuary

Abstract

The rates of water column nitrification measured as nitrapyrin-sensitive rates of bicarbonate assimilation were monitoredalong the salinity gradient of the estuary of the RiverNervion (Bay of Biscay) from January to November 1993.Highest nitrapyrin-sensitive rates of bicarbonate assimilationwere found in the deep layer at stations located in the innerestuary during summer and autumn, coinciding with dissolvedoxygen concentrations generally lower than 1 mg O2 l−1. Possible explanation for this spatial distributionpattern are discussed. Axial and depth distribution of ammonia,nitrite and nitrate suggests that in the deep layer of theinner zone of the estuary nitrification and nitrate and nitritereduction are occurring concurrently. Nitrification showed asignificant positive correlation with temperature, but thiscorrelation was not very strong. It is argued that the wash outof nitrifying bacteria by rainfalls also plays a role in theseasonal variation of nitrification rates.