Zonations and oscillations via heterotrophic processes in tidal unvegetated aquifers

Abstract

Global-change drivers that alter tidal amplitude or sea level could directly impact coastal aquifers' seawater-microbe interactions. The semidiurnal or diurnal tide can have consequences for chemical redistribution, because it deepens oxygen and nutrients penetration depth, thereby increasing O 2 depletion via microbial respiration in sediments during groundwater circulation. However, chemical and microbial distributions in tidal aquifers remain unclear, in deference to their link with depth and lateral location. By combining the microbes-accumulated equation with the traditional groundwater flow model in coastal aquifers, we explicitly unravelled the heterotrophic zonation, the salinity zonation, and the oxic zonation of tidal aquifers. According to the model estimates, salt penetrates deeper than reactive chemicals, and the heterotrophic high biomass zone is presumably 2-3 m deep. Such depth of heterotrophic distribution is 2-3 orders of magnitude higher than phototrophic distribution.Additionally, heterotrophic biomass can exhibit faint oscillations, even under the tide forces, whereas dissolved chemicals fluctuate significantly with tidal pumps. Due to

Perturbations of heterotrophs and autotrophs in tidally dynamic coastal sediments: Development of a scalable model

“…In that case, the heavy BGSM‐A could occupy more than 32 GB of computer memory during calculation, and the calculation time of a single model could exceed 1 week. Thus, we simplified the reactions of AR, nitrification and denitrification of tidal aquifers based on our previous study (Yao et al (2022)) and ASM3 reactive model for fast calculation. The straightforward BGSM‐A here only includes one C source, two N sources and two immobile microbes, thus significantly improving the calculation speed.…”

“…(2) The decomposition rate of POC is limited by temperature, pH, the particle size of POC and microbial biomass. (3) The governing equations of the BGSM and BGSM‐A include the Darcy's equation, the convective‐diffusive equation of salt, the convective‐diffusive reaction equation of reactive nutrients, and the microbes‐accumulated equation (see Appendices and Yao et al (2022) for detail). As an independent study, immobile POC can be calculated by the microbes‐accumulated equation due to the nature of stillness.…”

“…Our simulations were performed in three steps: Step I : Variable‐density flow and salt transport were simulated together with a tidal oscillation boundary for 500 days to ensure a quasi‐steady state was reached, dynamically sustained near its unstable equilibrium position. This step was calculated using Equations and Equation () in Appendices (Yao et al (2022)). The simulation results of Step I represent the salinity distribution without microbial effects, which were further compared with the results of the following simulations. Step II : The reactive solutes and biomass variables were superimposed on the quasi‐steady state flow field from Step I, and then the reactive and biomass‐accumulative process was run for more than 150 days until a quasi‐steady state.…”

“…BGSM‐A introduces the reaction network from ASM3 into the basic model developed by Yao et al (2022). ASM3 provides a metabolic network for nitrification and denitrification.…”

“…The model formulation, the limitations of our assumptions, and the parameters used in our simulations are detailed in our previous study (Yao et al (2022)) and Appendices. As illustrated in Figure 1, the intertidal zone is located at the interface between the continent and the ocean.…”