Comparative Responses of Coastal Arabian Sea Plankton to Different Ocean Acidification Simulation Methods

ABSTRACT

Ocean acidification (OA) is expected to alter marine phytoplankton communities by modifying seawater carbonate chemistry, with important implications for marine food webs and biogeochemical cycling. In this study, we tested the hypothesis that comparable pH reductions achieved through different acidification methods generate distinct carbonate chemistry conditions and consequently different biological responses. A 5‐day microcosm experiment was conducted using natural phytoplankton assemblages collected from the coastal Eastern Arabian Sea. OA was simulated by HCl addition (acid), CO ₂ enrichment (CO ₂ ), and acid–base manipulation. Changes in carbonate chemistry, nutrients, phytoplankton community composition, pigments, bacteria, microzooplankton, and organic carbon pools were monitored. As expected, the three manipulation methods produced significantly different carbonate chemistry conditions despite comparable pH reductions. Repeated‐measures ANOVA and Tukey's post hoc analyses confirmed significant treatment effects for pH, pCO ₂ , DIC, HCO ₃ ⁻ , CO ₃ ²⁻ , and total alkalinity. By Day 5, CO ₂ enrichment significantly increased total phytoplankton abundance and promoted microplankton dominance, driven by bloom‐like proliferation of Thalassiosira sp., whereas picoplankton declined significantly. Conversely, pennate diatoms, cyanobacteria, dinoflagellates, and coccolithophores exhibited weak or negative responses. Concurrent changes in phosphate, nitrite, autotrophic bacteria, and microzooplankton indicated that nutrient availability, microbial recycling, and grazing interacted with carbonate chemistry to regulate phytoplankton succession. Our findings demonstrate that phytoplankton responses in the oligotrophic Eastern Arabian Sea are governed by the combined effects of carbonate chemistry, nutrient dynamics, and ecological interactions rather than pH alone.