Quantification of sedimentary oxygen and carbon dioxide dynamics in a dry sandy beach affected by macroalgae deposition

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Sandy beaches provide habitat for endangered species, protect the coast, and are appreciated for their high social and economic values. They are facing escalating anthropogenic pressures through coastal development, nutrient increases and global warming, which in recent years led to the development of extensive seaweed blooms that deposit massive amounts of algae on sandy beaches worldwide. This project investigates the fate of the deposited algae and their influence on the biogeochemical environment of the beach.The decay of the algae locally depletes oxygen in the beach and produces toxic sulfides and high concentrations of ammonia that harm beach organisms, pollute nearshore waters and deter tourists. Research in this project quantifies biogeochemical and transport processes that control the algal decomposition. Generating these data is prerequisite for developing models that can predict the impact of the massive macroalgal deposition and thereby support decision making and coastal management. The project research is tightly linked to an educational component with instruction of graduate and undergraduate students in marine biogeochemical methods, and a dedicated outreach program informing the public about the seaweed issue and pathways to reduce this problem.The project produces data on the dynamics of oxygen and carbon dioxide distribution and interfacial gas flux in the dry, supralittoral zone (between high tide line and dunes) of sandy beaches and determines how distributions and fluxes are influenced by the degradation of the macroalgae. Organic carbon and nitrogen profiling characterizes the biogeochemical sedimentary environment at the two study sites, a carbonate and a silicate beach. A set of laboratory experiments relates gas fluxes to the degradation rate of buried macroalgal layers and the effects of gas exchange, temperature and moisture. The data will allow estimates of the time frame of the algal decomposition process and provide information on the degradation capacity of sandy beaches. Since beach sedimentary O2 and CO2 dynamics and fluxes are largely undescribed, the project data contribute to the mechanistic and quantitative understanding of carbon and nutrient cycling in sandy beaches.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.