Biological and Physical Controls on Multidecadal Acidification in a Eutrophic Estuary

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. Increasing carbon dioxide in the atmosphere also impacts the chemistry of the ocean by lowering pH levels. The water quality indicator pH, a measure of the acidity, is important because pH changes are harmful to marine organisms that are critical for fishery industries. Many of these organisms live in estuary ecosystems, where pH levels are influenced by climate change as well as local conditions including nutrients coming from the ocean and human activities including nutrient pollution. These coastal conditions can worsen pH declines in estuaries. This study examined an estuary in central California, called Elkhorn Slough, that receives nutrient pollution from agriculture as well as nutrients from the ocean. We studied how pH and other signals of acidification have changed over 20 years in this estuary. We found that the change is happening faster here than in the ocean. Also, different locations in the estuary showed distinctive patterns of acidification. The site near the marine inlet changed faster over time, but others sites showed more variation between different seasons of the year. These results depended on the sampling location in the estuary, and therefore it is important that ongoing measurements of the pH capture multiple sites within estuaries. Metabolism of high nutrient loads in Elkhorn Slough, CA, enhances trends of coastal acidification compared to open ocean acidification High nutrient levels and lower buffering drive the greatest rate of acidification over two decades near the marine inlet of the estuary Temporal cycles of ecosystem metabolism and tides are threats to acidification in a tidally restricted site