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Coupled nitrification‐denitrification in autotrophic and heterotrophic estuarine sediments: On the influence of benthic microalgae

Field data obtained from 18 European estuaries using the isotope pairing technique were analyzed for trends in relationship between activity of benthic microalgae and coupled nitrification‐denitrification. Kruskal‐Wallis tests and analyses of covariance performed on the field dataset showed strong statistical evidence for the hypothesis that sediments colonized by microalgae whose activity exceeds community respiration display lower rates of coupled nitrification‐denitrification than do heterotrophic sediments. In fully heterotrophic sediments, 90% of the measurements fell within the range 0–92 µmol N m−2 h−1 with a median of 20.3 µmol N m−2 h−1. In highly autotrophic sediments, 90% of the measurements fell within the range 0‐34 µmol N m−2 h−1, and the median was 4.2 µmol N m−2 h−1. The hypothesis was tested experimentally using 15N and microsensor (NO3−) techniques in prepared microcosms with and without algal activity. The results of the experimental studies were consistent with the hypothesis derived from the field data analysis. For the 15N study, coupled nitrification‐denitrification in alga‐colonized sediments was between 4 and 51% of the activity in sediments without algae activity, depending on the N load. For the microsensor study, there was no indication of net NO3− production in alga‐colonized sediments before addition of NH4−. In contrast, NO3− accumulated in the oxic zone of a similar alga‐free sediment. The experiments furthermore showed that compared to heterotrophic sediment, the presence of active microalgae might reduce the population of nitrifying bacteria capable of having an active metabolism. These bacterial populations could display diurnal variations in activity correlated with the diurnal variations in O2 penetration depth, however. The results showed that induction of nitrogen limitation of the nitrifying bacteria population is a major controlling mechanism of coupled nitrification‐denitrification in alga‐colonized sediments.