This dataset contains measurements of seawater pH and total alkalinity from a short-term factorial feeding experiment conducted independently of the main chronic alkalinity-enhancement exposure. The experiment tested whether the shellfish diet used to feed the oysters in the main experiment (Reed Mariculture SD1800, a concentrated microalgae feed) contributes to seawater total alkalinity, and whether this contribution depends on the presence of oysters. Four treatment combinations were run in parallel over a 9-day period (12 to 21 November 2024): unamended seawater without oysters or feed (control), seawater with food but without oysters (SD 1800), seawater with oysters but without feed (oysters) and seawater with both oysters and feed (oysters + SD 1800). Oysters in the unfed treatments were left entirely without feed for the full 9 days. Each treatment was sampled on six days (day 0, 1, 3, 5, 7 and 9) in triplicate. Seawater pH was measured with a pH electrode and total alkalinity by open-cell potentiometric titration, both on a Metrohm 855 Robotic Titrosampler.