Long-Term Trends in Estuarine Carbonate Chemistry in the Northwestern Gulf of Mexico

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.

Aragonite saturation states in estuaries along a climate gradient in the northwestern Gulf of Mexico

“…These studies found decreases in pH, alkalinity, and Ω arag in most estuaries. Most of the long-term stations (with >20 years of record) in these estuaries have Ω arag trends of ~-0.01 --0.07 years −1 (Mccutcheon and Hu, 2022) amid the general warming trend of ~0.04 °C yr −1 (Montagna et al, 2011). The latter would have led to a slight increase in Ω arag (10 -3 yr −1 ) but far from countering the calculated decreasing trends.…”

“…The large changes in salinity, from hypersaline to brief, yet nearly complete freshening of the estuarine waters, along with flood-induced productivity change as well as ephemeral hypoxia (%DO saturation decreases to less than 20%) in limited areas within some of the estuaries, provided an opportunity to examine the "baseline" and temporal changes in Ω arag . Understanding Ω arag dynamics under these contrasting conditions in the context of multidecadal Ω arag trends found in these estuaries (Mccutcheon and Hu, 2022) is potentially important for managing and (Yao et al, 2022). The horizontal dashed line (blue) represents the open ocean TA value (2060 μmol kg −1 at salinity 36) (Hu et al, 2018).…”

“…In climate and ocean studies, the term "time of emergence" has been frequently used to describe the timescale for a secular trend to appear in time-series data (Hawkins and Sutton, 2012;Sutton et al, 2019;Turk et al, 2019). In a prior study, Mccutcheon and Hu (2022) calculated temporal trends of Ω arag in nwGOM estuaries using water chemistry data collected by the Texas Commission on Environmental Quality (TCEQ) that dated back to the 1970s. Based on Ω arag values obtained in this study, we explored the time scales for these trends to emerge for both secondary and primary bays of each estuary using Eq.…”

“…Using historical data collected by TCEQ, Hu et al (2015) and Mccutcheon and Hu (2022) explored multidecadal changes in estuarine carbonate chemistry in the nwGOM estuaries. These studies found decreases in pH, alkalinity, and Ω arag in most estuaries.…”

“…To better understand these ecosystems that host this important species and the habitats that they create, we examined a previously poorly studied water chemistry parameter, Ω arag , and other water quality data collected in four mid-coast estuaries in recent years. Then, using multidecadal change rates of Ω arag from a prior study (Mccutcheon and Hu, 2022), we also explored timescales beyond which Ω arag change will exceed its natural variability.…”