Abstract
A four-decade dataset that spans seven estuaries along a latitudinal gradient in the northwestern Gulf of Mexico and includes measurements of pH and total alkalinity was used to calculate partial pressure of CO2 (pCO2), dissolved inorganic carbon (DIC), saturation state of aragonite (ΩAr), and a buffer factor (βDIC, which measures the response of proton concentration or pH to DIC concentration change) and examine long-term trends and spatial patterns in these parameters. With the notable exception of the northernmost and southernmost estuaries (and selected stations near freshwater input), these estuaries have generally experienced long-term increases in pCO2 and decreases in DIC, ΩAr, and βDIC, with the magnitude of change generally increasing from north to south. At all stations with increasing pCO2, the rate of increase exceeded the rate of increase in atmospheric pCO2, indicating that these estuaries have become a greater source of CO2 to the atmosphere over the last few decades. The decreases in ΩAr have yet to cause ΩAr to near undersaturation, but even the observed decreases may have the potential to decrease calcification rates in important estuarine calcifiers like oysters. The decreases in βDIC directly indicate that these estuaries have experienced continually greater change in pH in the context of ocean acidification.
Highlights
Unlike the acidification of the open oceans that is driven almost solely by the ocean’s uptake of atmospheric CO2, the changing carbonate chemistry of coastal and estuarine environments is modulated by several regional to local factors that are superimposed on global trends
We examined 54 total stations: one station in Sabine-Neches Estuary (SNE), 10 in Trinity-San Jacinto Estuary (TSJ), 13 in Lavaca-Colorado Estuary (LCE), two in Guadalupe Estuary (GE), five in, Mission-Aransas Estuary (MAE), 13 in Nueces Estuary (NE), and 10 in Laguna Madre Estuary (LM)
This study used the TCEQ Surface Water Quality Monitoring (SWQM)’s data record to examine longterm trends and spatial relationships in the carbonate system in the estuaries of the nwGOM. This dataset provides much more spatially and temporally extensive data than any current datasets collected by academic laboratories, and unlike many other regional monitoring programs, TCEQ has routinely monitored both pH and total alkalinity (TA), making this one of the longest running estuarine datasets in the world that allows the calculation of the complete carbonate system
Summary
Unlike the acidification of the open oceans that is driven almost solely by the ocean’s uptake of atmospheric CO2, the changing carbonate chemistry of coastal and estuarine environments is modulated by several regional to local factors that are superimposed on global trends. The high temporal and spatial heterogeneity in estuarine carbonate chemistry is driven by the complex interactions between watershed dynamics, riverine input of nutrients and organic matter, changing net ecosystem metabolism, changes to upwelling, mixing between and within reservoirs, and reservoir residence time (Frankignoulle et al, 1994; Cai et al, 2003; Feely et al, 2010; Hofmann et al, 2011; Lejart et al, 2011; Mongin and Baird, 2014; Wallace et al, 2014; Challener et al, 2016). It was previously demonstrated that the bays in the northwestern Gulf of Mexico (nwGOM - our study area) have experienced long-term declines in both pH and TA, which was hypothesized to be caused by long-term decreases in freshwater inflow of high-alkalinity rivers and accompanying decreases in TA delivery to the bays (Hu et al, 2015)
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