Oceanic uptake of anthropogenic CO₂ emission leads to rapid ocean acidification (OA), making the assessment of its ecological impacts increasingly critical. Mesocosm-based experiments have proven to be an effective approach for understanding these impacts. We conducted a 38-day mesocosm experiment in a subtropical urban coastal area to investigate OA’s effects on local planktonic community. On Day 16, a higher trophic species (oyster) was introduced into the experimental system to examine its interactions with planktonic community. From Day 0 to Day 16, no significant differences were observed between ambient pCO₂ (AC) and elevated pCO₂ treatments (HC) in terms of phytoplankton biomass (Chlorophyll a, Chl-a) and community structure, primary production, bacterial abundance, or DOC concentration. After the addition of oysters, while phytoplankton biomass and primary production showed no significant differences between the two treatments, bacterial abundance and DOC concentrations were significantly higher in HC treatments. Given the increased oyster grazing rates under elevated pCO2, we speculate that this enhanced grazing rate may facilitate the transfer of phytoplankton carbon into DOC and non-Chl-a POC, thereby supporting higher bacterial abundance. This speculation is further supported by greater DIN utilization as well as higher POC and PON production in the HC treatment. Our results suggest that OA can affect organisms both directly and indirectly through interspecies interactions, emphasizing the importance of studying multiple trophic levels to fully understand OA's ecological consequences.  

ocean acidification