| 28 Jul 2026
Oxygen Dynamics in Intertidal Sediments: Integrating High-Resolution Data and Reaction-Transport Modeling
Abstract. Predicting the response of O₂ dynamics in intertidal sediments to changing environmental conditions is essential for understanding and forecasting the impacts of climate change on coastal biogeochemical functioning. However, key environmental controls such as light, temperature and tidal regime (immersion vs. emersion) are only partially integrated into existing sediment biogeochemical models. Additionally, inversion-based approaches, which infer reaction rates from measured O₂ profiles without explicitly representing the underlying diagenetic mechanisms, cannot be used predictively. We developed a reaction-transport model for simulating O₂ dynamics in MPB-inhabited intertidal sediments at high spatial and temporal resolution under variable environmental forcing. The model was constrained and evaluated using laboratory microsensor measurements of O₂ concentrations and gross photosynthesis in muddy sediments from the Oosterschelde tidal bay (the Netherlands). Our analysis indicates that the studied MPB community is adapted to low irradiance and responds to changing light conditions through vertical migration. Reoxidation of reduced inorganic substances dominates sediment O₂ consumption at low irradiance, whereas aerobic mineralization becomes increasingly important at higher irradiance, with photorespiration representing an additional relevant O₂ sink under high-irradiance conditions. Additionally, both the O₂ producing and consuming processes show strong immersion-emersion and temperature response. Overall, the model is released as an open-source tool, providing a framework that can be adapted and refined for interpreting and predicting O₂ dynamics in intertidal systems under a broad range of environmental conditions.
This manuscript presents an interesting experimental and modelling study of O2 dynamics in microphytobenthos-inhabited muddy intertidal sediments, examining responses to light, temperature, and immersion-emersion conditions. The combination of high-resolution microsensor measurements with reaction–transport modelling is potentially valuable, and I particularly found the experimental dataset and the observed responses across the different environmental treatments interesting.
However, in its current form, I think the manuscript places too much emphasis on the predictive and mechanistic interpretation of the model. The model contains a relatively large number of parameters, many of which are fitted to the experimental observations rather than independently constrained, while several important quantities show substantial variability or cannot be directly verified. This makes some of the mechanistic conclusions derived from the model difficult to evaluate independently. I therefore recommend refocusing the manuscript more strongly on the experimental observations, with the model primarily serving as a framework for interpreting these observations and identifying processes and hypotheses for future investigation. In fact, I think one of the important conclusions emerging from the study is that we are still far from a robust mechanistic understanding of the processes controlling the MPB associated O2 dynamics in intertidal sediments. Such a refocusing would also provide an opportunity to substantially shorten and streamline the manuscript, which I believe would make the main findings clearer and the study more accessible and appealing to a broader readership.
Major comments
1. Model claims. The model is technically sound and nicely designed, but the difficulty is that the model goes substantially beyond what the measurements independently constrain. It has 34 parameters, with most estimated by fitting the same experimental dataset that the model is subsequently used to interpret. Some particularly influential parameters are weakly constrained. For example, light attenuation and the vertical MPB distribution cannot be independently determined from the measurements, so several of these parameters are fitted simultaneously to the O2-production profiles. Deep ODU concentrations vary by about an order of magnitude between cores but were not measured directly. Photorespiration was introduced because simulations without it systematically overestimated measured O2- concentrations. That does not mean photorespiration is unimportant, but the present data do not independently demonstrate that it is the process responsible for the discrepancy. Also the parameterization based on on industrial studies focusing on microalgae culturing. Similarly, the model requires a fivefold change in the ODU reoxidation parameter between dark and light conditions, from which the manuscript then infers depth-dependent differences in reoxidation. These are useful hypotheses, but I would be cautious about presenting them as mechanistically demonstrated results.
2. Sediment characterization. Given the considerable variability observed among replicate cores, I find the characterization of the experimental sediments rather limited, and I do not consider references to previous studies sufficient in this context. Important sediment properties that could directly affect O2 dynamics and model parameterization, including porosity, grain size, organic matter content, and the concentration and distribution of reduced compounds, are either assumed, inferred through model fitting, or referred to previous studies. This makes it difficult to determine whether the observed differences among cores primarily reflect physiological responses of the MPB community or variability in sediment properties. I therefore suggest that the authors provide all available sediment characterization for the experimental cores and discuss this limitation more explicitly.
3. Microphytobenthos Physiology. The physiology of the microphytobenthos is represented in an overly simplified manner. To give just a few examples, microphytobenthos can assimilate substantial amounts of nitrate and ammonium and, together with their vertical migration and DNRA, drive a dynamic redistribution of ODUs within the sediment. This can shift hotspots of metabolic activity, and thus oxygen consumption, vertically over time. Relatedly, it is unclear what happens when cells experience, for example, extreme temperatures or prolonged emersion, potentially leading to physiological stress or cell lysis. I would not necessarily expect such processes to be explicitly represented if the model were more clearly framed as a simplified model applicable to a specific range of conditions, rather than as a broadly applicable model of oxygen dynamics in intertidal sediments.
4. Microscale heterogeneity. Recent and older studies have shown that microscale heterogeneity can play an important role in intertidal sediments, including processes and structures occurring at spatial scales smaller than those resolved by the microsensors used here. Examples include localized organic matter accumulations and associated anoxic hotspots, porosity variability, but also small-scale heterogeneity during emersion, such as changes in the light field caused by scattering around water droplets. These processes and their potential implications for the interpretation of the measurements and model results should at least be discussed.
5. Advective terms. The authors mention in several places that the model does not apply to permeable, advection-dominated sediments. I consider this an important limitation that should be reflected more clearly in the title, abstract, and conclusions. Although this limitation is acknowledged in the manuscript, it can easily get lost, particularly in some of the broader concluding statements. The applicability of the model is therefore restricted to some intertidal sediments in which molecular diffusion dominates solute transport, rather than intertidal sediments in general.