Award: OCE-1948914

The uptake of atmospheric CO2 by the oceans acidifies marine waters with detrimental effects on a variety of organisms, the trophic chain, and fisheries. The acidification of the oceans also affects the transformation of carbon and other important elements with a variety of implications on Earths climate. Although ocean acidification is relatively well characterized in the open ocean, the effect of atmospheric CO2 uptake on the coastal ocean is much more complex, as these environments are dynamic, process much more carbon that other systems, and are affected by annual variations in riverine inputs, photosynthesis, and organic matter degradation and carbonate mineral dissolution in the underlying sediment. In this project, the role of riverine sediment inputs on the ability to buffer or promote acidification in river-dominated margin sediments was investigated in the Louisiana shelf and slope near the Mississippi River delta, in the northern Gulf of Mexico (nGoM). This study demonstrated that the riverine discharge of organic and inorganic material influences the acidification of the continental margin seasonally. Although stations closes to the Mississippi delta generally mitigated acidification of the bottom waters year-round, episodic acidification events promoted by the physical mixing of sediment during peak discharge or storms were observed. The year-long release of acidity was also observed at stations located within the Mississippi plume, where the input of organic matter to the sediment was not significant enough to counteract acidification. In turn, continental slope sediments generally mitigated ocean acidification by promoting carbonate mineral dissolution. Finally, the continental shelf stations that were outside the Mississippi plume switched from acidifying the bottom waters during the low discharge seasons (fall and winter) to mitigating acidification of the waters after the spring peak discharge, as a result of the significant input of fresh marine organic matter synthesized during the spring bloom by the release of nutrients from the Mississippi watershed. These findings demonstrate that ocean acidification in coastal oceans exposed to significant riverine inputs is both enhanced and mitigated by the transformation of carbon in the underlying sediment. Last Modified: 03/25/2025 Submitted by: MartialTaillefert