Tropical cyclone-induced coastal acidification in Galveston Bay, Texas

Coastal ecosystems are vital, providing numerous goods and services. However, they face numerous threats, including ocean acidification, which is particularly harmful to calcifying organisms like oysters and corals that build crucial reef structures. Ocean acidification results from the absorption of atmospheric CO2, lowering seawater pH and CaCO3 saturation state (Ω). Coastal acidification is further influenced by local processes such as river discharge and storms. While the physical damage of tropical cyclones on calcifying ecosystems is well-documented, their impact on coastal ocean chemistry is less understood. Rainfall and runoff associated with these cyclones can contribute to coastal acidification, potentially leading to short-term corrosive conditions for aragonite, a critical mineral for many marine organisms. With projections of increased tropical cyclone intensity and rainfall under climate change, understanding their effects on coastal acidification is crucial for environmental and resource management. Hurricane Harvey, the wettest tropical cyclone in US history, provided a unique opportunity to study this phenomenon in Galveston Bay, Texas, a region with valuable oyster reefs facing existing stressors and declining oyster populations. This study aimed to characterize the changes and recovery timescale of seawater carbonate chemistry in Galveston Bay following Hurricane Harvey, analyzing data collected before and after the storm to elucidate the mechanisms driving storm-induced acidification and its implications for calcifying ecosystems.

The literature extensively documents the negative impacts of ocean acidification on marine calcifying organisms, including slowed calcification and enhanced CaCO3 dissolution (Doney et al., 2020). Studies have shown the vulnerability of oyster larvae and spat (Miller et al., 2009) and adult oysters (Gazeau et al., 2007) to decreased aragonite and calcite saturation states. Coastal acidification, resulting from local processes interacting with global changes, is also a growing concern (Wallace et al., 2014). Previous research indicates that tropical cyclones can cause short-term coastal acidification events, potentially leading to aragonite undersaturation (Manzello et al., 2013; Gray et al., 2012). Projections suggest that these events will intensify under future climate scenarios (Knutson et al., 2010, 2015), highlighting the urgency of understanding their impacts on coastal ecosystems. Long-term acidification has been observed in several Texas estuaries, including Galveston Bay (Hu et al., 2015), further emphasizing the need to understand the role of tropical cyclones in exacerbating this issue.

This study analyzed carbonate chemistry data from Galveston Bay, Texas, collected from June 2017 through September 2018, encompassing pre- and post-Hurricane Harvey sampling periods. Samples were collected using Niskin bottles at various stations throughout the bay (Fig. 1). Parameters measured included salinity, temperature, pCO2, pH, Ωc, and Ωar. The VINDTA instrument was used for simultaneous analysis of total alkalinity (TA) and dissolved inorganic carbon (DIC), calibrated using certified reference materials. Other carbonate system parameters were calculated using CO2SYS software with appropriate constants for estuarine waters (Millero, 2010; Lueker et al., 2000). pCO2 was temperature-normalized to remove temperature effects on variations. Statistical analyses included ANOVA to identify significant differences between sampling periods and linear regression analysis to evaluate relationships between TA, DIC, and salinity. A fractional composition analysis was performed to determine the percentage of bay water composed of Gulf of Mexico seawater, Galveston-watershed river water, and rainwater. This analysis used TA as a conservative tracer, with fractions calculated using linear mixing equations and endmember values derived from various sources including the Texas Automated Buoy System (TABS) for Gulf of Mexico seawater and linear regressions of TA and DIC against salinity for river water. Rainwater DIC was estimated using two methods: one based on a pH and atmospheric pCO2 estimate and another using the WEB-PHREEQ application. A Deffeyes diagram was used to examine the influence of biological versus physical mixing processes on Galveston Bay carbonate chemistry.

Hurricane Harvey caused a significant coastal acidification event in Galveston Bay. Two weeks after landfall, average salinity dropped dramatically to 3.4 ± 3.8, indicating primarily freshwater conditions. Average pCO2 increased significantly to 985 ± 359 µatm, while pH decreased to 7.6 ± 0.2, representing a >200% increase in acidity. Aragonite undersaturation (Ωar < 1) was observed at all stations, and calcite undersaturation (Ωca < 1) was widespread. These conditions were not driven by temperature changes, as average temperatures remained relatively stable before and after the storm. The acidification persisted for at least three weeks, with elevated pCO2 and depressed pH measured in some areas. Although average pH and pCO2 values recovered to near pre-storm levels after three weeks, aragonite and calcite undersaturation remained significantly lower than pre-Harvey levels. By November 2017, carbonate chemistry parameters had returned to pre-Harvey levels. Analysis of freshwater sources revealed that the increase in freshwater fraction following Harvey was almost entirely due to increased rainwater input, with minimal change in river water fractions. This rapid influx of low-TA rainwater overwhelmed other factors such as enhanced respiration and CaCO3 dissolution. A Deffeyes diagram showed that all data points lay within the uncertainty range of the mixing lines, confirming that changes in carbonate chemistry were overwhelmingly caused by physical mixing and dilution of seawater by rainwater rather than biological effects.

The study demonstrates that Hurricane Harvey caused a severe, prolonged coastal acidification event in Galveston Bay, driven primarily by the influx of low-TA rainwater. The extended duration of the acidification, exceeding three weeks, is largely attributed to the prolonged release of stormwater from reservoirs designed to mitigate inland flooding. The findings support the hypothesis that increased tropical cyclone rainfall, as projected under climate change, will pose a significant threat to coastal ecosystems, especially calcifying organisms like oysters that form aragonite. While enhanced respiration and CaCO3 dissolution may have played secondary roles, the massive influx of rainwater was the dominant driver of the observed acidification. The significant oyster mortality observed after Harvey highlights the vulnerability of these ecosystems to such events. The relatively rapid recovery of Galveston Bay's carbonate chemistry suggests a degree of resilience, yet the potential for increasingly frequent and intense storms underlines the need for careful management and conservation strategies.

This study conclusively demonstrates the potential for severe and prolonged coastal acidification events triggered by intense tropical cyclones. The Galveston Bay case study reveals the significant impacts of Hurricane Harvey on carbonate chemistry, resulting in widespread undersaturation and high oyster mortality. The dominant factor was the large volume of low-alkalinity rainwater, compounded by extended reservoir releases. Future research should investigate the long-term ecological consequences of such events and explore adaptation strategies to mitigate the impacts of climate change on coastal ecosystems. Improved understanding of the interplay between storm intensity, rainfall, and freshwater runoff is crucial for accurately predicting and managing the risks of coastal acidification.

The study's primary limitation is the lack of continuous, high-frequency carbonate chemistry data immediately following Hurricane Harvey's landfall. The sampling schedule prevented a precise characterization of the very early stages of acidification, but still captures the majority of the impact. While the study focuses on Galveston Bay, the generalizability of its findings to other coastal systems may depend on specific factors such as watershed characteristics, reservoir management practices, and the intensity and duration of storm events.