Calcium oxide changed carbon particles and oxygen in Los Angeles Harbor water

A field experiment found that a large alkalinity increase from calcium oxide lowered particulate organic carbon and dissolved oxygen.

Editorial illustration — not from the study.

Researchers tested three ways of increasing alkalinity in coastal seawater: sodium hydroxide, calcium oxide, and a calcium chloride–sodium bicarbonate mixture intended to simulate accelerated limestone weathering. The experiments used alkalinity increases of about 500 and 1,000 micromoles per kilogram and were incubated in the harbor for up to four days.

At the lower increase, none of the additions caused a statistically significant change in calcium carbonate precipitation or the distribution of organic carbon. At the higher increase, calcium oxide was associated with a statistically significant decrease in particulate organic carbon and dissolved oxygen compared with untreated water.

What changed in the water

The researchers found no statistically significant calcium carbonate precipitation or change in organic carbon partitioning after an alkalinity increase of about 500 micromoles per kilogram using sodium hydroxide, calcium oxide, or calcium chloride plus sodium bicarbonate. When calcium oxide raised alkalinity by about 1,000 micromoles per kilogram, particulate organic carbon and dissolved oxygen decreased significantly relative to the control. The experiments ran for 0–4 days, and carbon-13 tracers were used to follow carbon through the system.

Why the result matters

Ocean alkalinity enhancement is being considered as a way to remove carbon dioxide, but its effects on coastal ecosystems are not fully established. These results indicate that the material used and the size of the alkalinity increase may affect particulate organic carbon and oxygen in harbor water. Because particulate organic carbon can be transferred through food webs, the study points to the need to determine which organisms and particle sizes account for the observed change.

Evidence and open questions

This was a field-based alkalinity perturbation experiment using coastal seawater incubated in situ, with carbon-13 tracer additions and some light-shaded bottles to examine conditions without photosynthesis. The observations covered only 0–4 days and tested specific alkalinity sources and increases. The abstract does not establish the longer-term effects or the ecological cause of the particulate organic carbon decrease. The researchers say future work should use finer filtration, microscopy, flow cytometry, and molecular analyses to identify the biomass involved and assess possible effects on food-web transfers.

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