Drivers of biogenic acidification in oyster cultivation waters: macroalgal photosynthesis raises pH and carbonate saturation state, but not total alkalinity.

Biogenic acidification is a growing concern in large-scale bivalve aquaculture. To investigate its drivers and mitigation strategies, oysters were incubated in a continuous flow-through system with five chambers in series to simulate upstream-to-downstream water flow in a large-scale long-line oyster aquaculture system. Four stocking densities (1, 3, 6, and 9 ind. Chamber-1) and three water exchange rates (3.6, 5.4, and 7.2 L h-1) were tested. The mitigation potential of macroalgal photosynthesis was then evaluated. Results showed that biogenic acidification was observed in experimental waters and higher oyster stocking density intensified seawater acidification, with downstream chambers exhibiting more pronounced acidification than upstream chambers. Compared with the blank control, the terminal chamber at the highest density showed a pH decrease of 0.85 units, a pCO2 increase of 2698 μatm, and decreases in total alkalinity (TA) and TA - DIC of 333 and 349 μmol kg-1, respectively. Lower water exchange rate also exacerbated biogenic acidification. In the terminal chamber at the lowest water exchange rate, pH decreased by 0.52 units, pCO2 increased by 976 μatm, and TA and TA - DIC decreased by 140 and 188 μmol kg-1, respectively. Negative effects were observed: correlation analysis revealed a significant negative correlation between seawater pCO2 and the weight-specific CO2 excretion rate (P < 0.05), alongside a significant positive correlation between Ωcal and the weight-specific calcification rate (P < 0.05). Adding Ulva Lactuca L. to the acidified seawater progressively raised pH (7.75 to 8.06) and Ωcal (2.3 to 4.3) and lowered pCO2 (1306 to 585 μatm) over five days, approaching ambient levels, whereas TA remained at its post-acidification level despite a DIC drawdown of 133 μmol kg-1. CO2 removal therefore raises the pH and saturation -state caused by biogenic acidification but not the TA deficit, indicating that TA must be included when the mitigating effect of macroalgal photosynthesis is evaluated and that supplementary alkalinity sources may be required in large-scale bivalve aquaculture.