Collaborative research: Sediment geochemical control on ocean acidification and carbon budget in a river dominated shelf system

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In many coastal regions fertilizer washed down rivers leads to high production of algae. Consumption of organic material produced by the algae in deeper waters or in sediments consumes oxygen, leading to low-oxygen "dead zones" in many places like the northern Gulf of Mexico. This process of nutrient input, algal growth, and loss of oxygen is called eutrophication. Consumption of the organic matter also releases dissolved carbon dioxide into the water, and contributes directly and indirectly to ocean acidification. In order to understand and predict this impact on acidification, it is important to understand how much of the organic matter degradation occurs in the water column versus in the sediments, how variable it is between seasons and years, and how it is affected by processes such as storms that disturb the bottom sediments. In this project, investigators from three institutions along with their graduate and undergraduate students will conduct a combination of field observations and computer modeling to address these questions in the northern Gulf of Mexico. They will share their results with the public through local outreach activities, and with secondary school students in Louisiana through partnerships and curriculum development with local teachers.The proposed research will use a combined observational and numerical modeling approach to better understand the role of shelf sediment in driving bottom water dissolved inorganic carbon (DIC) and pH dynamics and acidification at seasonal scales. Past and current studies have not addressed this mechanism. This current lack of knowledge makes it difficult to construct a comprehensive carbon budget for this region. The proposed research will (i) quantify the role of benthic fluxes in DIC production leading to acidification in the bottom water; (ii) determine the importance of the seasonally changing benthic DIC flux in acidifying the bottom water; and (iii) explore the importance of episodic resuspension events in modulating benthic fluxes of DIC. Seasonal sampling will be carried out in Louisiana shelf using state of the art benthic lander platform to record in situ sediment and bottom water oxygen consumption rates, organic matter remineralization rates, sediment oxygen penetration depths, benthic fluxes of DIC and accompanying pH drop. Bottom current velocities and turbidity will also be recorded in conjunction with sediment porewater and water column sampling. Complementing these detailed near-bed and seabed observations, we will utilize a recently developed coupled hydrodynamics, sediment transport and biogeochemistry model (HydroBioSed) to scale up observed estimates of benthic fluxes to an annual scale.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.