Negative CO₂ emission technologies such as ocean alkalinity enhancement (OAE), in tandem with emissions reduction are necessary to keep the climate system below a critical tipping point. While the biogeochemical consequences of OAE remain poorly constrained, field deployment is accelerating in the commercial sector, leaving questions of ecosystem impact in the wake. In this study we conduct alkalinity perturbation experiments to capture the resultant impact to the organic carbon and calcium carbonate pools. We quantify shifts in dissolved/particulate inorganic (DIC, PIC) and organic (DOC, POC) carbon after the addition of three alkalinity sources – NaOH, CaO, and NaHCO₃ + CaCl₂ (to simulate dissolved limestone). These experiments are conducted with coastal sea water with enhancements of +500–1000 µmol kg^–1 alkalinity, and incubated in situ, to elucidate their impact over the short term, 0–4 days. Select experiments are also completely isolated from the light via bottle shading. With most treatments, there is no statistically significant CaCO₃ precipitation after OAE nor changes in organic matter production or consumption, relative to the untreated controls. One exception is when CaO addition enriches the water by 1000 µmol kg^–1, here we find a significant decline in POC production. ¹³C isotope spikes were added to trace C partitioning throughout the experiment. The seawater used in these experiments had a wide range of initial ambient PIC and POC, and within this wide range, a +500 µmol kg^–1 alkalinity in the form of NaOH, CaO or NaHCO₃ + CaCl₂ addition did not change the production or consumption of carbon in these waters.