The impact of NaOH, CaO, and [Ca 2+ ] + [HCO 3 − ] additions on PIC and POC formation in Los Angeles Harbor Waters

Abstract

Abstract. Negative CO2 emission technologies such as ocean alkalinity enhancement, in tandem with emissions reduction are necessary to keep the climate system below a critical tipping point. While the biogeochemical consequences of alkalinity enhancement 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 and organic carbon after the addition of three alkalinity sources – NaOH, CaO, and CaCl2 + NaHCO3 (to simulate accelerated weathering of limestone). These experiments are conducted with coastal sea water with enhancements of ∼ 500 and ∼ 1000 µmol kg−1 alkalinity, and incubated in situ, to elucidate the impact over 0–4 d. 13C isotope spikes were added to trace carbon partitioning throughout the experiment, and a few bottle experiments were completely isolated from the light via bottle shading to assess impacts on non-photosynthesizers. Despite a wide range in the initial ambient particulate carbon (both organic and inorganic) there was no statistically significant calcium carbonate precipitation or change in the organic carbon partitioning after a perturbation of ∼ 500 µmol kg−1 alkalinity in the form of NaOH, CaO or CaCl2 + NaHCO3. An increase of alkalinity by ∼ 1000 µmol kg−1 after CaO addition resulted in a statistically significant decrease of particulate organic carbon and dissolved oxygen relative to the control. To understand how a decrease in the particulate organic carbon may impact downstream trophic transfers, future studies should resolve the observed changes in particulate organic carbon by characterizing biomass within various particle class sizes with further filtration, microscopy, flow cytometry, and `omics analyses.