Advection Drives Nitrate Past the Microphytobenthos in Intertidal Sands, Fueling Deeper Denitrification.

Charles A Schutte,Paulina Huanca-Valenzuela,Hannah K Marchant,Dirk De Beer,Gaute Lavik

doi:10.3389/fmicb.2021.556268

Charles A Schutte, Paulina Huanca-Valenzuela + Show 3 more

Open Access

https://doi.org/10.3389/fmicb.2021.556268

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Abstract

Nitrification rates are low in permeable intertidal sand flats such that the water column is the primary source of nitrate to the sediment. During tidal inundation, nitrate is supplied to the pore space by advection rather than diffusion, relieving the microorganisms that reside in the sand from nitrate limitation and supporting higher denitrification rates than those observed under diffusive transport. Sand flats are also home to an abundant community of benthic photosynthetic microorganisms, the microphytobenthos (MPB). Diatoms are an important component of the MPB that can take up and store high concentrations of nitrate within their cells, giving them the potential to alter nitrate availability in the surrounding porewater. We tested whether nitrate uptake by the MPB near the sediment surface decreases its availability to denitrifiers along deeper porewater flow paths. In laboratory experiments, we used NOx (nitrate + nitrite) microbiosensors to confirm that, in the spring, net NOx consumption in the zone of MPB photosynthetic activity was stimulated by light. The maximum potential denitrification rate, measured at high spatial resolution using microsensors with acetylene and nitrate added, occurred below 1.4 cm, much deeper than light-induced NOx uptake (0.13 cm). Therefore, the shallower MPB had the potential to decrease NOx supply to the deeper sediments and limit denitrification. However, when applying a realistic downward advective flow to sediment from our study site, NOx always reached the depths of maximum denitrification potential, regardless of light availability or season. We conclude that during tidal inundation porewater advection overwhelms any influence of shallow NOx uptake by the MPB and drives water column NOx to the depths of maximum denitrification potential.

Highlights

Coastal ecosystems receive massive inputs of nitrogen from land that can fuel algal primary production and cause harmful algal blooms and hypoxia (Rabalais, 2002; Gruber and Galloway, 2008)
We studied how nitrate uptake by the MPB affects denitrification in an intertidal sand flat with permeable sediments and porewater advection during tidal inundation
The pattern of low potential denitrification rates near the sediment surface which increased to a maximum by 1.5–2 cm depth was identical in both seasons (Figure 2B)

Summary

Introduction

Coastal ecosystems receive massive inputs of nitrogen from land that can fuel algal primary production and cause harmful algal blooms and hypoxia (Rabalais, 2002; Gruber and Galloway, 2008). Advection can increase areal denitrification rates by an order of magnitude compared with diffusive transport in the same sediment (Gao et al, 2012). Diatoms are often an important component of the MPB (Longphuirt et al, 2009; Stief et al, 2013). The influence of the MPB on denitrification in coastal sediment has been investigated in the past under conditions where mass transport was driven by diffusion rather than advection (Nielsen et al, 1990; Risgaard-Petersen et al, 1994; Lorenzen et al, 1998; Sundbäck and Miles, 2000; Risgaard-Petersen, 2003). Algal ammonium uptake led to nitrogen limitation of nitrification, which in turn provided less nitrate substrate for denitrification, thereby slowing rates of coupled nitrification-denitrification (Risgaard-Petersen, 2003)

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