The diurnal variation of dissolved oxygen in seawater is influenced by DOC excretion from seagrass and microbes
Tropical seagrass beds exhibit pronounced diel variations in dissolved oxygen (DO) and dissolved organic carbon (DOC), mediated by primary production, respiration, and microbial activities. However, near-future climate change scenarios predict elevated ocean temperatures with more frequent extremes. Tropical seagrass beds already inhabit ecosystems with wide environmental fluctuations and are therefore tolerant of current diel O2 and temperature extremes. Therefore, we examined two seagrass sites with contrasting hydrodynamics at Dongsha Island to elucidate the mechanisms influencing DO and DOC dynamics. Metabolic rates and DOC fluxes were quantified using in situ benthic chambers. Moreover, active microbial communities were characterized via the amplicon sequencing of 16S rRNA transcripts. During summer, seagrass beds predominantly exhibited net autotrophy, transitioning to a more heterotrophic state in winter. In contrast, unvegetated sediments remained net heterotrophic. Interestingly, DOC fluxes varied, with the sheltered lagoon acting as a net DOC source, whereas the open coast predominantly functioned as a sink. Diverse microbial taxa emerged, varying across locations and seasons and correlating with factors such as carbon, temperature, and DO. A surge in microbial activity over diel cycles likely drove the observed nighttime DO declines. Overall, contrasting trends were observed between the sheltered lagoon and open coast sites. As ocean temperatures continue to rise, the net heterotrophy and sulfidic conditions seen in the lagoon may represent the future for tropical seagrass ecosystems, underscoring the need to maintain coastal water quality and understand microbial community resilience.