| Summary: | Estuaries support many ecologically and economically important resources that are especially vulnerable to ocean acidification from rising anthropogenic CO 2 . However, complex local processes in estuaries complicate and may disguise long-term pH trends. For example, terrestrial nutrient runoff and marine coastal upwelling may exacerbate pH variability and declines. We investigated eutrophication impacts on acidification in a central California estuary, Elkhorn Slough, which receives high nutrient loads from intensive surrounding agriculture and upwelling of the California Current System. We examined drivers of acidification including nutrients, ecosystem metabolism, and upwelling by modelling pH trends over 20 years using a Generalized Additive Mixed Model at four sites from the National Estuarine Research Reserve Systemwide Monitoring Program and collected additional water samples to calculate aragonite saturation. Our models revealed acidification trends over two decades which were more pronounced near the marine inlet. Near the marine inlet, high nutrient levels and lower buffering are associated with the greatest rate of acidification in the estuary, which was four times greater than the trend from anthropogenic CO 2 alone. However, tidally restricted areas experienced a different acidification pattern. A tidally restricted site recorded higher mean pH and aragonite saturation and increased pH levels associated with stronger upwelling conditions supplying marine-sourced nutrients. Therefore, variable ecosystem metabolism and tidal cycles are threats to acidity in this location. The effects of enhanced seasonal cycles or long-term trends in different zones of the estuary have implications for monitoring with a temporal frequency and scale to capture coastal acidification risks in estuaries. Funding provided by: Drexel University Crossref Funder Registry ID: https://ror.org/04bdffz58 Award Number:
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