A novel 2D model of internal O2 dynamics and H2S intrusion in seagrasses

Seagrasses provide a physical connection between the water column and sediments by transporting photosynthetic- and seawater-derived oxygen to their roots and rhizomes. In this paper, we present a single-shoot reaction-transport model that incorporates the biological, chemical and physical processes in the water column, seagrass plant, and sediments and that simulates oxygen and hydrogen sulfide dynamics in the system. The model reproduces oxygen and sulfide patterns observed in laboratory manipulations and field measurements of Thalassia testudinum and Zostera marina. Model results reinforce experimental conclusions that (1) meristem oxygen is tightly coupled to water column oxygen and diel patterns of sunlight, (2) sediment sulfide enters the plant when plant tissues are hypoxic, and (3) internal sulfide is rapidly depleted once oxic seawaters are re-established or with the onset of photosynthesis. Sensitivity analysis further emphasizes that water column oxygen concentration has a strong influence on the minimum oxygen concentration and maximum sulfide concentration in the meristem at night. The model indicates that diffusion is the dominant transport process in the lacunae, though advective mass flow can account for nearly a quarter of oxygen transport during periods of increasing sunlight. In the model, biological sulfide oxidation and plant dissolved organic carbon exudation both play significant roles in determining patterns of sediment oxygen consumption and sulfide intrusion into the plant.