Biological and Physical Controls on Multidecadal Acidification in a Eutrophic Estuary

Abstract

Estuaries support ecologically and economically important resources that are vulnerable to ocean acidification from rising anthropogenic CO₂. However, complex local processes in estuaries complicate and may disguise long‐term pH trends. For example, terrestrial nutrient runoff and 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 modeling 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 were associated with the greatest rate of acidification in the estuary, which was four times greater than the trend from anthropogenic CO₂ alone. Compared to fully tidal sites, a tidally restricted site showed diminished pH declines over time because of higher mean pH and aragonite saturation levels, but greater diel and seasonal variability associated with cycles of ecosystem metabolism and tidal range. Therefore, short‐term drops of saturation 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 estuaries with a temporal frequency and scale to capture coastal acidification risks.