Kinetics of inorganic nitrogen turnover in a sandy seepage face on a subterranean estuary

Abstract

Subterranean estuary seepage faces are recognized as important reactive interfaces that regulate solute transport to coastal ecosystems via Submarine Groundwater Discharge (SGD). Here we describe benthic processes and rates driving the biogeochemical regulation of SGD-borne inorganic N loading into the Ria Formosa lagoon (Iberian peninsula) through a sandy seepage face. Maximum potential NO3− reduction rates, obtained by kinetic modeling of advection-controlled flow-through reactor experiments, ranged from 2.33 ± 1.06 to 14.17 ± 0.22 nmol cm−3 “bulk” sediment (bs) h−1. Maximum potential nitrification ranged from 0 to 7.5 ± 1.3 nmol cm−3 bs h−1 while potential ammonium assimilation was valued at 2.0 ± 0.3 nmol cm−3 bs h−1. These NO3− reduction rates are in good agreement with previous estimates obtained by diagenetic modeling of in-situ porewater NO3− vertical profiles at the same location. Potential NO3− reduction rates were very sensitive to temperature (Q10 = 3.5 ± 0.2). Porewater velocity seems to control net NO3− reduction rates, probably by determining solute distribution but also its supply to the microbial community by shaping the diffusive boundary layer around sediment particles. Nevertheless, NO3− reduction rates seem ultimately limited by organic matter availability at high velocities. Half-saturation constants of NO3− for NO3− reduction were low, suggesting that the NO3− reducing microbial community had high affinity for NO3−. In addition, our experiments provide evidence for the occurrence of alternative NO3− reduction pathways, including Dissimilatory Nitrate Reduction to Ammonium (DNRA) and apparent aerobic NO3− reduction within the shallow subsurface sediment layer (2–12 cm depth).