Surface Water CO2 variability in the Gulf of Mexico (1996–2017)

Abstract

Approximately 380,000 underway measurements of sea surface salinity, temperature, and carbon dioxide (co 2) in the Gulf of Mexico (GoM) were compiled from the Surface Ocean CO 2 Atlas (SOCAT) to provide a comprehensive observational analysis of spatiotemporal CO 2 dynamics from 1996 to 2017. An empirical orthogonal function (EOF) was used to derive the main drivers of spatial and temporal variability in the dataset. In open and coastal waters, drivers were identified as a biological component linked to riverine water, and temperature seasonality. Air-sea flux estimates indicate the GoM open (− 0.06 ± 0.45 mol C m −2 year −1) and coastal (− 0.03 ± 1.83 mol C m −2 year −1) ocean are approximately neutral in terms of an annual source or sink for atmospheric CO 2. Surface water pCO 2 in the northwest and southeast GoM open ocean is increasing (1.63 ± 0.63 µatm year −1 and 1.70 ± 0.14 µatm year −1 , respectively) at rates comparable to those measured at long-term ocean time-series stations. The average annual increase in coastal CO 2 was 3.20 ± 1.47 µatm year-1 for the northwestern GoM and 2.35 ± 0.82 µatm year −1 for the west Florida Shelf. However, surface CO 2 in the central (coastal and open) GoM, which is influenced by Mississippi and Atchafalaya River outflow, remained fairly stable over this time period. Since the onset of the Industrial Revolution, atmospheric carbon dioxide (CO 2) has increased by approximately 40% 1. The ocean is responsible for absorbing ~ 25% of anthropogenic CO 2 emissions, which modulates climate by reducing global warming 1. However, oceanic absorption of atmospheric CO 2 also leads to an increase in seawater partial pressure of CO 2 (pCO 2), and a decrease in seawater pH and calcium carbonate (CaCO 3) saturation state (Ω) 2,3. This process, termed ocean acidification, has numerous negative consequences for marine organisms, especially those that form CaCO 3 skeletons and shells (e.g. corals and shellfish), including a reduction in biogenic calcification and an increase in CaCO 3 dissolution 4-8. Gulf of Mexico (GoM) ecosystems support fisheries that are essential to the Gulf Coast and U.S. economies 9. The 2015 GoM seafood landings revenue was $858 million, which is almost 20% of the total U.S. landings revenue 9. Despite serving as a critical component for regional economic stability, the GoM is subject to numerous anthropogenic pressures that contribute to the decline of ecosystem health, including eutrophication, hypoxia, oil spills, warming, and acidification 10-12. Hypoxia in the northern GoM is driven by the spring flux of nutrients from the Mississippi and Atchafalaya River systems, combined with water column stratification that prevents mixing of bottom waters with oxygenated surface water 13-15. The production of CO 2 through respiratory processes also lowers bottom water pH (i.e., increases acidity) 11. Hypoxia and associated bottom water acidification are predicted to intensify due to increased nutrient loadings and organic matter production 16,17. Decadal declines in subsur...

A seamless ensemble-based reconstruction of surface ocean pCO<sub>2</sub> and air–sea CO<sub>2</sub> fluxes over the global coastal and open oceans

“…Along the coast, inputs of fresh water (decrease in salinity and increase in CO 2 solubility) and nutrients (biological activity and DIC drawdown) combine to strengthen CO 2 uptake (Arrigo et al, 2010;Yasunaka et al, 2016;Olafsson et al, 2021). uptake is also found over the western continental shelf where strong river discharges sustain high levels of biological productivity in particular during spring (Jamet et al, 2007;Kealoha et al, 2020). Weaker sinks or sources of CO 2 in the southwestern StA and the eastern subtropical gyre are primarily driven by high surface temperature and enhanced stratification .…”

Increasing coral calcification in Orbicella faveolata and Pseudodiploria strigosa at Flower Garden Banks, Gulf of Mexico

“…The rate and magnitude of OA at the FGB are unknown over the period covered by our cores. The threat of OA in the northwest Gulf of Mexico is poorly understood because seawater carbonate chemistry is influenced by the interplay of varying river discharge and the complex hydrodynamics of this region (Kealoha et al 2020b). Over the 10-yr period from 2007 to 2017, Hu et al (2018) reported a rate of OA at FGB that was greater than what has been measured in the Hudson and Robbin (1981) and Dodge and Lang (1983) D, Growth declines/anomalies associated with periods of high Atchafalaya discharge (1973-75, 1983-85, 1990-91, 1994, 1998, 2009-11).…”

“…Includes period from 1964 to 1970 that saw a rapid increase in Atchafalaya discharge ?, Growth anomalies that are not associated with a known event or environmental factor Coral Reefs open ocean. However, just to the east in the central regions of the open-ocean and coastal portion of the Gulf of Mexico, there has been no long-term trend in surface pCO 2 , SST, or salinity (Kealoha et al 2020b). The impact of runoff from the Mississippi and Atchafalaya Rivers can effectively erase any OA signal because runoff impacts surface productivity, as well as directly modifies seawater chemistry, both of which introduce variability that makes a long-term OA signal difficult to detect.…”

Temporal variability and driving factors of the carbonate system in the Aransas Ship Channel, TX, USA: a time series study

“…The GOM estuaries currently have less exposure to concerning levels of acidification than other estuaries because of their high temperatures (causing water to hold less CO 2 and support high productivity year-round) and often suitable river chemistries (i.e., relatively high buffer capacity) (McCutcheon et al, 2019;Yao et al, 2020). However, respiration-induced acidification is present in both the open GOM (e. g., subsurface water influenced by the Mississippi River Plume and outer shelf region near the Flower Garden Banks National Marine Sanctuary) and GOM estuaries, and most estuaries in the northwestern GOM have also experienced long-term acidification (Cai et al, 2011;Hu et al, 2015Hu et al, , 2018Kealoha et al, 2020;Mc-Cutcheon et al, 2019;Robbins and Lisle, 2018). This evidence of acidification as well as the relatively high CO 2 efflux from the estuaries of the northwest GOM illustrates the necessity to study the baseline variability and driving factors of carbonate chemistry in the region.…”