Trade-Off Between Light Deprivation and Desiccation in Intertidal Seagrasses Due to Periodic Tidal Inundation and Exposure: Insights from a Data-Calibrated Model

Some seagrass species thrive in shallow intertidal zones globally, adapting to periodic tidal inundation and exposure with distinctive physiological traits and offering crucial ecosystem services. However, predicting the responses of intertidal seagrasses to external stressors is hampered by the complexity of the dynamic and harsh environments they occupy. Intertidal seagrass growth models, especially those incorporating dynamic physiological responses, are scarce in the literature. Our study comprehensively collated relevant data from the literature to parameterize the relationship between air exposure, seagrass leaf water content and photosynthetic efficiency to inform new growth rate functions for generalizable intertidal seagrass growth models. We tested the applicability of these model formulations for scenarios with varying physiological process assumptions, seagrass species, tidal conditions, meadow elevations and water turbidity. We found that neglecting air-exposed physiological responses (i.e., leaf water content loss and reduced photosynthetic efficiency) can substantially overestimate seagrass growth rates. We also observed a trade-off between light deprivation and desiccation on intertidal seagrass growth under specific tidal ranges and turbidity conditions. This can yield an "optimal" elevation where overall stress of desiccation (increasing with meadow elevation) and light deprivation (decreasing with meadow elevation) are minimized. The predicted optimal elevation, that is, the most suitable habitat for intertidal seagrass, moves upward as water turbidity increases. Our study provides conceptual and quantitative guidance for ecological modelers to include air exposure responses of intertidal seagrasses in coastal ecosystem models. The model also helps to evaluate the viability of intertidal seagrass habitats and inform decisions on coastal ecosystem management under changing environmental conditions. Some seagrasses grow in intertidal zones where they are periodically exposed or submerged due to the rise and fall of tides. However, predicting how these valuable ecosystems respond to such stresses in a highly dynamic and harsh environment is difficult. Our study collated relevant data from the literature to quantify the physiological responses of intertidal seagrasses to air exposure, and further developed intertidal seagrass growth models by incorporating these responses. We tested these models for different seagrass species and under various field conditions, and found a substantial decline in seagrass growth rates when considering air-exposure responses. We also observed that intertidal seagrass growth was affected by both light reduction (increasing with seagrass meadow elevation) and desiccation stress (decreasing with meadow elevation), and the most suitable habitat for seagrass growth is located at the elevation where the combined stress are minimal. The most suitable elevation for intertidal seagrass increases as water turbidity increases. Our model can help assess the suitability of intertidal seagrass habitats and inform decisions on coastal ecosystem management under changing environmental conditions. We developed an intertidal seagrass growth model by incorporating the physiological responses to periodic tidal inundation and exposure Neglecting air-exposure responses substantially overestimates intertidal seagrass growth rates A trade-off between light deprivation and desiccation on intertidal seagrass exists, which yields an "optimal" growth elevation