Assimilation and short-term processing of microphytobenthos nitrogen in intertidal sediments

Microphytobenthos (MPB) are an important nitrogen (N) sink in coastal systems, but little is known about the fate of N assimilated by MPB. We used an in situ N-15 pulse-chase experiment in intertidal, nonpermeable, sandy mud to trace the assimilation, transformation, and loss from the sediment of MPB-N over 31 d. Following assimilation, N-15 was tightly retained in surface sediments, with only up to 8.1% transported to 2-10 cm sediments over 31 d. MPB accounted for a considerable portion of the N-15 in surface sediments throughout the study (59% +/- 13%). Bacteria rapidly assimilated N-15 but accounted for only up to 17% of the N-15 within surface sediments. Of the assimilated N-15, 78.9% was lost from the sediment over 31 d. Resuspension was the dominant loss pathway (74.4%) and was primarily associated with minor flooding following a rainfall event at 8 d. Biological pathways for N export were far less important. Denitrification, during both sediment exposure and inundation, was the main biological pathway for N-15 loss (2.6% total), whereas there was little loss through ammonification and nitrification (NH4+ or NOx efflux; 1.4% combined) and dissolved organic nitrogen efflux (0.5%). It was estimated that the N incorporated by MPB within one emersion period would take 40 d to be entirely removed from the sediment. This study highlights the potential importance of MPB and intertidal sediments for the uptake and longer-term storage of coastal N, and the need to better quantify the impact of episodic flooding on coastal N budgets.