Estuarine acidification mirrors the ocean trend and correlates with the El Niño Southern Oscillation

The pH of seawater has direct impacts on marine organisms and is one of several key variables for determining ecosystem-scale rates of carbon fixation, metabolism, and air-sea exchange. Based on previous estimates from four decades of monitoring data, the global ocean is acidifying at a rate of -0.0017 pH units yr^-1. However, in biologically active coastal environments where such longer-term pH data sets are limited to low resolution glass electrode measurements, analyses have focused on the shorter-term biogeochemical details of diurnal and seasonal variation. A better understanding of decade-scale trends in estuaries is needed and is achievable with available data. Here, Bayesian methods were used to combine previously reported pH trends with a multivariate time series analysis of a 25 year dataset (1996-2020) covering 16 fixed stations in the U.S. National Estuarine Research Reserve system. After temperature normalization, the estimated trend was -0.0013 pH units yr^-1. This estimate is less negative than previously reported but is also more precise, resulting in a 63% probability of a negative trend in unsampled estuaries. Over decadal time scales, biological response calculations predict demographically relevant impacts on early life stage mortality in bivalves and potentially other calcifying marine organisms. Additional multivariate analyses indicated strong association between the El Niño Southern Oscillation and estuarine pH both before and after temperature normalization. Although seasonal decoupling of biological production and respiration was expected to produce stronger pH seasonality at higher latitudes, no such pattern was detected either before or after temperature normalization.