The effect of changing lagoonal volume on the amplitude of diel carbon chemistry variability within a seagrass lagoon at Dongsha Atoll, Taiwan

Ocean acidification (OA) caused by the oceanic uptake of anthropogenic CO₂ is likely to negatively impact a wide range of calcifying organisms, however, the magnitude of these impacts may vary markedly between different environments. The high productivity of seagrass meadows and their ability to alter local carbon chemistry has piqued interest in whether these settings can partially mitigate the negative effects of ocean acidification on nearby calcifying organisms. Additionally, these systems serve as an important sink of blue carbon. Here, we present measurements of seawater pH, total alkalinity (TA), dissolved inorganic carbon (DIC), CO₂ partial pressure (pCO₂), dissolved oxygen (DO), and flow velocities taken from a seagrass-dominated lagoon at Dongsha Island in the northern South China Sea over 5 days in June 2018. The relative importance of the physical and environmental drivers driving the observed variability in biogeochemical parameters were assessed by developing a biogeochemical box model of the system. The measured biogeochemical parameters exhibited distinct diel cycles, with pH and DO increasing during the day and decreasing at night, while other parameters presented an inverse trend. TA-DIC relationships revealed that organic carbon cycling in the lagoon was predominately controlled by photosynthesis and respiration. Importantly, variation in the diel amplitude of biogeochemical parameters between days was mainly controlled by tidally driven changes in lagoonal volume, and not by differences in biogeochemical fluxes during the study. These results are important to refine our ability to evaluate the capacity of seagrass systems to create local pH refugia for neighbouring marine calcifiers and underlines the importance of constraining a system’s fluid dynamics when investigating the biogeochemical processes within it.