Water column carbonate chemistry data (typically total alkalinity or TA, total dissolved inorganic carbon or DIC) and hydrographic data collected during various research cruises from 2006 to 2019 were retrieved as detailed in Table 1. This study focused on the continental shelf region of the nwGoM, specifically between 27-30°N latitude and 90-98°W longitude (Figures 1, 2). Most sampling stations were located in areas with water depths less than 150 m (Figure 3), with the most significant recorded depth being 1250 m. The spatial distribution of stations across the study area was not uniform, with a higher density of sampling near the Louisiana coast (between 28-29°N, 90-93°W) and around FGBNMS (approximately 28.3°N, 94.5°W; Figure 1). The Texas coast featured more sparsely spaced stations, and the sampling usually did not cover the outer shelf. In addition, sampling dates were unequally distributed across seasons, with most of the data collected in the summer and fall months and much fewer spring and no winter data from the Texas coast by the time when this manuscript was written (Figures 1, 2).

The second part of the study aims to provide a comprehensive overview of nwGoM species potentially sensitive to CO₂ changes with vulnerabilities supported by the literature. The selection and inclusion of these potentially vulnerable species were based on the following merits:

All species were selected from the collection Felder and Camp (2009). Each section was divided by phyla, and phylum was assessed using a preliminary literature search. We scanned through 77 phyla and 15,419 species to apply the abovementioned constraints. Three thousand four hundred eighty species (3480) were recorded as locationally relevant. Each species within a given phylum with locational relevancy was searched in Google Scholar or Research Rabbit with the keywords “(genus) CO₂” and “(genus) ocean acidification.” All species with peer-reviewed articles that experimentally addressed or reviewed direct exposure to elevated pCO₂ on any biological function were pulled and used in this report. Of the 3480 locationally relevant species, this review found relevant data on a subset of 35 species. Those species with one or more reports of exposure to increased pCO₂ were listed in Table 2. Sections within this survey were refined by class, as some phyla may have had locationally relevant classes with no relevant literature, and species identified can be found in the data provided. The literature search was conducted in January 2024.

Carbonate chemistry parameters (pH, Ω_arag, pCO₂) were calculated using the CO2SYS program, with TA and DIC as input parameters, along with silicate and phosphate concentrations when available. In this study, carbonic acid dissociation constants, bisulfate dissociation constant, hydrofluoric acid dissociation constant, and borate-to-salinity ratio from Lueker et al. (2000); Dickson (1990); Perez and Fraga (1987), and Lee et al. (2010), respectively, were used for carbonate speciation calculations. All reported data are in situ temperature and pressure. The pH values are presented on the total scale.

Fourteen classes and 35 species were identified in experimental conditions of CO₂ enrichment (Table 2). Among these were vertebrates belonging to the classes Actinopterygii and Chondrichthyes, along with invertebrates such as Malacostraca, Copepoda, Cirrepedia, Bivalvia, Cephalopoda, Gastropoda, Polyplacophora, Scyphozoa, and Scleractinia. Rhodophycea, Florideophyceae, and Bacillariophyceae were also included. Biological responses ranged from calcification, survival (larval & adult), growth, settlement, and gut microbial communities (Table 2). Exposures to elevated CO₂ level were also highly variable. Most attempted to provide an effect curve from future predictions of oceanic CO₂ conditions resulting from climate change scenarios detailed in the Intergovernmental Panel on Climate Change (IPCC, 2013) report. This effect curve also extended much further than these predictions and simulated extreme values (+2000 µatm)—with one study reaching levels shown at hydrothermal vents (231,280 µatm). Exposure times also ranged from the lowest 0.2 days to the highest at 187 days (Table 2).

Of the taxa reported, 68.5% showed a negative directionality response to increased pCO₂, whereas 31.4% showed a neutral or mixed neutral response (positive or negative). Only 11.4% of reported taxa showed a positive response to elevated pCO₂. The taxa here only represent 1.0% of the total we recorded in the nwGoM region, and inconsistencies in experimental methods, pCO₂ levels, and exposure times may be challenging for one-to-one comparisons. Also, these taxa are present in the nwGoM, but most studies in this report were done elsewhere, which could potentially introduce bias with regionally derived tolerances. We recommend a survey driven toward manipulation experiments under relevant controlled pCO₂ and exposure times to these taxa regionally sourced from the nwGoM.

Rhodophyta or red algae are distinctly red due to phycoerythrin pigments–allowing for greater photosynthetic capacities in deeper waters (Fredericq et al., 2009). Red algae play a crucial role in reef cementation, accretion, and sedimentation, providing a settlement substrate for sessile invertebrates, including coral larvae (McCoy and Kamenos, 2015; McNicholl et al., 2020; Nelson, 2009). Elevated levels of pCO₂ in seawater have been observed to directly affect certain rhodophytes or calcifying reef algae species, increasing the energetic costs associated with calcification. However, McNicholl et al. (2020) examined Jania adhaerens and found that daytime calcification was less impacted compared to nighttime dissolution, where dissolution was more pronounced in a high pCO₂ (1881-1524 µatm) environment (Table 2). Indirectly, the impacts of elevated pCO₂ have also been observed to increase environmental competition, with macroalgae outcompeting crustose coralline algae species for space (McCoy and Kamenos, 2015; Nelson, 2009).

Florideophyceae is a class of red algae containing most species found within the phylum Rhodophyta and is essential in both photic and benthic marine zones. Like the class described above, calcifying Florideophyceae provides a settlement substrate for benthic marine organisms (De Carvalho et al., 2022). Shown in Neosiphonia harveyi species, lower temperatures and elevated pCO₂ (800-1500 µatm) positively impacted the coralline algae by increasing and supporting photosynthetic processes and growth rates (Olischläger and Wiencke, 2013). Porzio et al. (2011) showed that many species within Florideophyceae continue to photosynthesize and grow in conditions of increased pCO₂ (pH 7.8). Approximately 95% of the algal species studied in the examination by Porzio et al. (2011) could tolerate and survive in conditions of 7.8 pH (Table 2). However, other work show elevated pCO₂ acting in reducing tissue growth (McCoy and Kamenos, 2015). The Hydrolithon spp. will respond negatively to increased pCO₂ (900 ppm) which has been shown to exhibit reduced calcification (Semesi et al., 2009).

The FGB sanctuary in the nwGoM is home to 24 specific species of Scleractinia corals, contributing to the rich coral diversity in the area. The coral cover within the West and East FGB has been noted as some of the most pristine in the region, with approximately 50% to 80% total coverage (Garavelli et al., 2018; Hickerson et al., 2012; Johnston et al., 2016). Coral calcification facilitates benthic complexity, supporting marine biodiversity and building large reef structures, but these communities could be threatened by ocean acidification with reduced calcification (Jokiel et al., 2014; Page et al., 2017; DeCarlo et al., 2017). Also, coral’s biphasic life history strategies present challenges in assessing tolerances due to fundamentally different physiologies dependent on life stage (Gleason and Hofmann, 2011).

Studies have examined the effects of pCO₂ increases on carbonate-based organisms, including corals. Albright and Langdon (2011) found that increased pCO₂ (560 & 800 µatm) significantly impacted larval metabolism, settlement, and post-larval growth in Porites astreoides (Table 2). Madracis mirabilis, forms dense clusters and provides shelter for marine microfauna and flora (Jury et al., 2010). In a study conducted by Jury et al. (2010), researchers demonstrated that Madracis auretenra (mirabilis) exhibited varying responses to ocean chemistries associated with increased pCO₂ (1480 µatm), showing a nonlinear relationship between chemistry and resistance levels that is not yet fully understood. Additionally, Albright et al. (2010) observed a significant reduction in fertilization, larval settlement, and growth of Acropora palmata caused by increased pCO₂ (560-800 µatm; Table 2), further emphasizing the potential negative impacts of increased pCO₂ on coral reproduction and development.

Polyplacophorans or chitons are marine grazers characterized by a dorsal shell composed of eight articulating plates formed with aragonite (Sigwart and Carey, 2014). Increased environmental pCO₂ could impact marine grazers by increased metabolism and subsequent feeding (Burnell et al., 2013; Sigwart and Carey, 2014). The exclusive formation of chiton shells from aragonite renders them susceptible to the effects of hypercapnia (i.e., increases in environmental pCO₂; Sigwart and Carey, 2014). A study conducted by Sigwart et al. (2015) found no change in the fracture strength required to break chiton shells under hypercapnia (2000 µatm; Table 2). Suggesting that chitons may possess specific attributes that reduce or mitigate the effects of increased pCO₂, even with predominantly aragonitic shell composition.

The class Gastropoda, is comprised of a diverse group of marine mollusks, and exhibit varying responses to hypercapnic scenarios. The genus Aplysia is a sea hare that is found in benthic habitats in the nwGoM (Moroz, 2011). Most sea hares possess an internal shell that protect internal organs composed primarily of aragonite (Carey et al., 2016). In a study by Carey et al. (2016), it was found that the cosmopolitan sea hare species Aplysia puncata could maintain normal calcification rates under increased pCO₂ (2800 µatm), although their metabolic rates were significantly impacted. Studies on species within the genus Haliotis (abalone) by Auzoux-Bordenave et al. (2019) demonstrated that raised pCO₂ environments (740-1400 µatm) significantly impacted shell growth and mineralization (Table 2). Furthermore, larval shell responses of the slipper limpet Crepidula fornicate were examined by Noisette et al. (2014), revealing an overall reduction in shell length under increased pCO₂ conditions (1400 µatm; Table 2; Bogan et al., 2019; Kriefall et al., 2018; Reyes-Giler et al., 2021).

Bivalves are essential animals in marine food webs and contribute to benthic biodiversity. Typically, bivalves synthesize their shells out of calcite, a polymorph of calcium carbonate. These calcification processes, which are of significant concern in several other taxa (as discussed i.e., corals, snails, limpets, and chitons), could be affected by increased pCO₂. Marine clams belonging to the genus Macoma were examined for their susceptibility to increased pCO₂. The life history strategies of this species were impacted by elevated pCO₂ at 1454 μatm (Van Colen et al., 2012), and it was found that pCO₂ at > 500 μatm (600–1650 μatm) impacted larval settlement as well (Jansson et al., 2015). Crassostrea virginica, or the eastern oyster, shows variable responses to increased pCO₂ (800-3500 μatm), some showing regulatory compensation and others showing decreased calcification (Beniash et al., 2010; Miller et al., 2020; Waldbusser et al., 2011). At levels ~3500 μatm, Beniash et al. (2010) reported decreased calcification, juvenile mortality, and soft body growth. Larval growth was slowed substantially in pCO₂ manipulations at ~800 μatm, as Miller et al. (2020) showed. Also, pH manipulation by CO2 bubbling significantly decreased calcification with a reduction of ~0.5 units from Waldbusser et al. (2011).

The bivalve Pinctada spp. was examined, and significant upregulation of acid-based control in lower pH environments (7.5 & 7.8) resulted in higher metabolic demand (Li et al., 2016). Another study found weaker shells and deformations in outer growth patterns due to lower pH (7.8 & 7.6; Welladsen, 2010). A separate analysis found significant differences in calcium content and organization of nacre caused by decreased pH (7.4; Liu et al., 2017). Two studies examining Argopecten irradians found significantly reduced larval survival at increased [CO₂] (650 ppm and 1987 μatm respectively; Talmage and Gobler, 2009; White et al., 2013). The reduction in larval survival was significant after one day of exposure (1987 μatm), showing pulse vulnerability to increased pCO₂ (White et al., 2013).

The genus Spisula was investigated, and researchers found increased energy costs due to increased pCO₂ exposure (2163 μatm; Pousse et al., 2020). The surf clam larvae showed decreased growth rates in high pCO₂ of 1243 µatm but resistance to moderate levels of 821 µatm (Meseck et al., 2021). In Mercenaria spp., larvae exhibited extreme mortality and delayed growth due to increased concentration of CO₂ ([CO₂]) conditions (650 ppm; Talmage and Gobler, 2009). However, some studies suggest these species are relatively tolerant to moderate pCO₂ levels of ~800 µatm (Dickinson et al., 2013; Matoo et al., 2013). The genus Panopea spp. larvae were investigated, and proteomic shifts were found due to exposure to decreased pH (7.1). Metabolism, cell cycle, and protein turnover rates were all impacted by this decreased pH level (Timmins-Schiffman et al., 2020).

Barnacles are ecologically important contributing significantly to marine food webs and intertidal coastal communities. Larvae are generally planktonic and are exposed to pelagic open water before being distributed intertidally via ocean currents. Amphibalanus spp., a common barnacle species, has been examined in several studies in the past decade (Eriander et al., 2016; McDonald et al., 2009; Nardone et al., 2018; Pansch et al., 2013). Increased pCO₂ ranging from 970 to1600 µatm were found to have no effects on this barnacle genus’s larval or adult stages (Table 2). However, some explored the variability of these responses and showed variations between individuals (Pansch et al., 2013). There is also an increasing need for research in this field as there are very few experiments on barnacles exposed to elevated pCO₂ (Eriander et al., 2016).

The phylum Echinodermata has 512 species in the nwGoM; Asteroidea comprises around 7% of the global echinoderm diversity (Felder and Camp, 2009; Table 3). Echinoderms play a significant role in marine food webs due to their grazing habits and interactions with other species. Their skeletons are composed of magnesium-rich calcite, particularly vulnerable to increased pCO₂ (Schram et al., 2011). In the sea star Luidia clathrata, a common species in the nwGoM, decreased pH and increased pCO₂ (780 µatm) had little to no effect on the species’ regenerative capacity or chemical constitution (Schram et al., 2011). However, various studies observed that increased temperature significantly impacted Asteroidea species more than reduced pH or increased pCO₂ did on physiological responses (Carey et al., 2016; Curtis et al., 2021; Parajuli, 2023).

The blue crab Callinectes sapidus has been subjected to many experimental trials regarding the increase in pCO₂ due to its abundance and ecological/economic importance (Giltz and Taylor, 2017; Longmire et al., 2022). Researchers have found decreased claw pinch force (pH 7.0) and decreased larval survival (pH 7.8) due to exposure to increased pCO₂ (Giltz and Taylor, 2017; Longmire et al., 2022). One study on the Portunus spp. looked at biomarkers following acute exposure (24 hrs) to extreme levels of pCO₂ like those found at hydrothermal vents, and found all pCO₂ ranges above normal levels to be stressful environments (2203-231287 µatm), with complete mortality in 72 hours when pH was less than 5.5 (Jeeva Priya et al., 2017), although this condition is not expected for the nwGoM coastal waters.

Smaller crustaceans like the hermit crab Clibanarius spp. have been shown to decrease larval survival due to exposure to acidified seawater through in situ measurements of pH swings (Tomatsuri and Kon, 2019). The commercially viable stone crab Menippe spp. has also been examined when exposed to increased pCO₂ (596 µatm), and researchers found geotactic differences in larval swimming patterns due to decreased pH, which could be detrimental to overall distribution and larval supply (Gravinese et al., 2019). Lower seawater pH (7.5) also showed delayed larval embryonic development in Menippe spp. by over 24% of the original time (Gravinese, 2018).

Diatoms are ecologically significant primary producers contributing nearly 25% to the world’s carbon fixation rate (Felder and Camp, 2009). Marine diatoms support higher trophic levels and represent the base of marine food webs (Qu et al., 2018). Sobrino et al. (2014) found that elevated pCO₂ (1050 ppm) decreased multiple physiological mechanisms such as esterase (metabolic) activity, radical oxidative stress (ROS), cell death, and DNA damage (Table 2). This study also noted that high pCO₂ conditions increase phytoplankton sensitivity to solar irradiance, carbon fixation, and photosynthesis rate. These changes in physiological responses indicate that elevated pCO₂ affects the photosynthetic structure’s downregulation and carbon concentrating mechanisms (CCMs) in marine diatoms (Sobrino et al., 2014).

Contrary to the findings of Sobrino et al. (2014), other studies have found that elevated pCO₂ (1050 ppm) does not play a significant role in diatom composition, cell size, or physiological function (Table 2). Qu et al. (2018) found that instead of CO₂ concentration, temperature and nitrate availability played more significant roles in the growth, carbon export, and physiology of larger diatoms such as Coscinodiscus spp. In experimental groups that combined elevated pCO₂ (400 & 800 µatm) and warmer temperatures, carbon fixation rates were enhanced, suggesting that OA may provide negative feedback to increasing atmospheric CO₂ concentrations (Qu et al., 2018). For experimental treatments that combined elevated pCO₂ with optimal temperatures of 30°C and higher, it was found that the increased pCO₂ (800-1400 µatm) depressed growth rates in the diatom species Thalassiosira pseudonana (Laws et al., 2020). In addition, Goldman et al. (2017) mention that enriched pCO₂ effects on diatom photosynthesis, growth, and respiration will likely be minor for the end-of-the-century predictions of pCO₂ concentrations ranging from 800-1400 µatm.

Marine copepods are primary consumers and play an integral role in marine food webs (Habibi et al., 2023; Wang et al., 2018). Almost 70% of the ocean’s metazoan biomass is allocated to marine copepods and zooplankton (Habibi et al., 2023). Copepods are challenging to assess because of stage-dependent and species-dependent responses to increased pCO₂ (Wang et al., 2018). A species of zooplankton Acartia located in the nwGoM was tested in mesocosm experiments to identify plasticity across generations and found that female size decreased with a possible threshold of pCO₂ (884 – 1413 µatm) for these species (Vehmaa et al., 2016). Calanus spp. were tested with low to high pCO₂ (320-1700 µatm) levels, and there were no effects on developmental rate, dry weight, C and N mass, and oxygen consumption rate, indicating possible tolerance (Bailey et al., 2017). Some conflicting research has shown population differences in physiological tolerances for Calanus spp (Thor et al., 2018). Transcriptomic analysis of differentially expressed genes revealed in Parvocalanus crassirostris that decreased pH (7.9-7.7) affected several biological pathways related to cellular processes (Habibi et al., 2023). In this study, an increase in pCO₂ resulted in 17 upregulated and 31 downregulated physiological pathways, suggesting physiological stress responses to increased pCO₂ (Habibi et al., 2023).

Aurelia aurita, a well-studied species of Scyphozoan, is found worldwide in various environments (Algueró-Muñiz et al., 2016; Baumann and Schernewski, 2012; Falkenhaug, 2014; Luparello et al., 2020). This species response to pCO₂ increases (800-4000 µatm) has been well studied, with findings suggesting both larval and adult stages of Aurelia spp. exhibit some level of resistance to acidification (Algueró-Muñiz et al., 2016; Hall-Spencer and Allen, 2015; Winans and Purcell, 2010). However, Winans and Purcell (2010) observed a decrease in statolith sizes in adult medusae of Aurelia spp. under low pH (7.9-7.2) conditions. This reduction in statolith size could potentially impact the ability of adult medusae to orient themselves in the water column. It is debated whether Scyphozoans, including Aurelia spp., will benefit from increased pCO₂ by exploiting open niches and potentially becoming top trophic predators (Richardson and Gibbons, 2008).

Cephalopods are essential for marine ecosystems as both predator and prey of several taxa. Cephalopods act as prey sources for many commercially important fish species like red drum while supporting fisheries directly being fished by local fishermen for food or bait (Kaplan et al., 2013). Recent investigations concerning the cephalopod species Doryteuthis pealeii, commonly found along the nwGoM reef shelf, have revealed the detrimental impacts of elevated pCO₂ (2200 µatm) on larvae rearing (Kaplan et al., 2013). Kaplan et al. (2013) evaluated various biological responses, such as the time duration to hatching, aragonite statolith sizes, and overall body size in this species, and found negative developmental/physiological responses to CO₂ enrichment (2200 µatm). Additional studies analyzed the disparities observed in the egg-laying behavior of D. pealeii, which primarily occurred around a depth of 50 m (Zakroff et al., 2019), emphasizing the importance of understanding taxa-specific reproductive ecology and vulnerability linked to [CO₂] concentrations (1300 ppm; Table 2). The effects on larvae were contingent upon the clutch and were typically only observed when [CO₂] surpassed 2200 ppm (Zakroff and Mooney, 2020).

Chondrichthyes are significant in almost all marine ecosystems and maintain ecosystem balance through predation. Elasmobranchs within Chondrichthyes have effective acid-base buffering capacities, which may allow them to have higher tolerances to elevated pCO₂ levels (Rummer et al., 2020). In near-future elevated pCO₂ conditions, shark embryo development was largely unaffected, although there were clear adverse effects on behavioral lateralization (turning left or right), hunting ability, growth, aerobic potential, and prey detection (Rosa et al., 2017). Odor tracking, swimming ability, hunting behavior, and other similar abilities were also negatively impacted by increased pCO₂ (~1000 µatm) and a decrease or total loss of lateralization (Zemah-Shamir et al., 2022). Contrary to these findings, Green and Jutfelt (2014) found that the small-spotted catshark, Scyliorhinus canicula, did not have adverse effects in growth, aerobic scope, or metabolic rate when exposed to 990 µatm pCO₂ for four weeks (Table 2). This species displayed increased lateralization when exposed to pCO₂ rather than decreased or complete loss, as shown in other studies (Green and Jutfelt, 2014). An increase in lateralization is not necessarily a positive effect, though, as most changes from the “natural” state of fish physiology likely mean it is compensation for other adverse effects it is experiencing.

Similar to the findings of Zemah-Shamir et al. (2022) and Rosa et al. (2017); Pistevos et al. (2015) found that elevated pCO₂ and temperature had adverse, synergistic effects on metabolic efficiency, growth rates, and olfactory mechanisms in sharks. A reduction in the olfaction mechanisms in sharks can result in various adverse consequences, as smell is the primary sense that sharks utilize to find prey. Di Santo (2019) conducted a study that exposed Leucoraja erinacea embryos to an increased temperature of 20°C and pCO₂ conditions of 1100 µatm (Table 2). This research showed that in the elevated pCO₂ conditions, mineralization increased in the jaws and cartilage of the crura in L. erinacea (Di Santo, 2019). Proper mineralization in elasmobranchs is essential because it can impact feeding and swimming abilities, directly affecting several skeletal elements’ strength and stiffness (Di Santo, 2019). Additionally, the stress that resulted from high-temperature conditions on the development and formation of embryos was intensified by elevated pCO₂ (Di Santo, 2019; Rosa et al., 2017).

The class Actinopterygii within the phylum Vertebrata consists of ray-finned bony fishes that comprise a large part of marine food webs and are crucial to the health and maintenance of marine ecosystems. As predator and prey, bony fishes contribute to ecosystem balance with bottom-up or top-down control of the abundance of organisms in the marine environment. It was documented that increased pCO₂ will likely have impacts on behavioral and sensory mechanisms in Actinopterygii as well as potentially interfere with the metabolic rate in marine fishes (Wang et al., 2022). It has been shown that elevated [CO₂] (~1000 ppm) can increase reproduction in fish through indirect effects on fish reproductive tissues (Nagelkerken et al., 2021). Other indirect effects from increased [CO₂] (~1000 ppm) were observed, such as the alteration of territory defenses shown by mature males, investment into gonads in both males and females and the altering of the abundance of parental males in areas with naturally increased [CO₂] levels (Nagelkerken et al., 2021).

A study published by Cattano et al. (2020) found that elevated pCO₂ (402 to 952 µatm) can cause shifts in benthic habitats, significantly impacting fish communities (Table 2). A reduction in benthic complexity (structural rugosity) in coral reefs was observed in reefs subjected to these increased pCO₂, resulting in a diminution of corresponding food webs and habitats (Cattano et al., 2020). The simplification of marine habitats due to increased pCO₂ may adversely affect fish communities by forcing them to relocate or risk their survival in a changing ecosystem. As discussed above, the increased impact of pCO₂ on marine benthic organisms could significantly affect indirectly reliant species.

Several studies have noted that increased pCO₂ (ranging from 600 to 1900 µatm) directly affects various fish species stress response, behavior, metabolic rate, and neurosensory mechanisms (Esbaugh, 2018; Heuer and Grosell, 2014; Servili et al., 2023). The alteration of any of these physiological processes can decrease fitness, reducing species population abundance in the long term. Studies have shown that otolith growth has increased in many species of marine fish exposed to high pCO₂ conditions (reviewed by: Heuer and Grosell, 2014; ~1600 µatm; Kwan and Tresguerres, 2022). Otoliths are calcium carbonate structures found in the inner ear of teleost fishes that aid in sensory detection and balance, and the overgrowth of these concretions can lead to hearing impairments, balance issues, and reduced survival success (Kwan and Tresguerres, 2022). The increase in otolith growth is believed to stem from hypercapnia in fish, which can result in increased CO₃^2- levels in the endolymph that may lead to enlarged otoliths (Kwan and Tresguerres, 2022).

Enriched seawater CO₂ effects can also lead to disturbances in the acid-base regulation in marine fish. Maintaining a stable pH in the blood, intracellular, and extracellular fluids is critical for maintaining homeostasis and proper physiological functions in marine vertebrates. Acid-base regulation is done mainly through the respiratory and circulatory systems in fish, and an internal increase in pCO₂ due to externally elevated pCO₂ levels (from 600 to 1900 µatm) can lead to the acidification of internal fluids (Heuer and Grosell, 2014). To combat the fluctuation in pH and pCO₂ levels internally, an increase in HCO₃^- in the blood plasma can help buffer additional acidity that is present and help maintain homeostasis (Brauner and Baker, 2009; Heuer and Grosell, 2014). Even with this acid-base regulation, fish exposed to elevated pCO₂ (from 600 to 1900 µatm) still show increased HCO₃^- and pCO₂ in their extracellular fluids, which can lead to various complications in physiological processes (Heuer and Grosell, 2014).

Sciaenops ocellatus is an extremely valuable species in the nwGoM ecologically, recreationally, and commercially. At pCO₂ levels of 1000 µatm in S. ocellatus was shown to result in consistent hyperventilation that can alter the acid-base regulation functionality and lead to a reduction in plasma HCO₃^- retention requirement by nearly 40% (Esbaugh, 2018). This reduction in necessary HCO₃^- retention means that S. ocellatus could effectively remove some excess pCO₂ from the body via the respiratory system. However, this energy allocation to hyperventilation likely means a tradeoff is occurring downstream for another physiological process (Esbaugh, 2018). Furthermore, Menidia beryllina, a significant food source for many larger fish in the nwGoM, showed a 73% reduction in 10-day survival after exposure to 780 µatm of pCO₂ (Esbaugh, 2018). In larvae reared at 1800-4200 µatm pCO₂, wide-spread tissue damage and other developmental delays were seen (Esbaugh, 2018). The species-specific studies showed differences in survival rates and maintenance of physiological mechanisms between species, life stages, and individuals within the same species.

According to a report from 2007, the GoM is home to nearly 1000 species of parasitic organisms belonging to Trematoda, Cestoda, and Dicyemida (Felder and Camp, 2009). Trematodes have approximately 3575 potential host species, including Fishes, Aves, Reptiles, Mammals, and Marine Mammals. Conversely, Cestodes have roughly 9774 likely host species, including Chondrichthyes, Actinopterygii, Marine Mammals, Bivalves, Gastropods, and Scyphozoans.

It is worth noting that parasites and relationships with pCO₂ increases have been highly understudied, with no identified articles linking the two before 2011 (Poulin et al., 2016). This knowledge gap presents an intriguing area of interest for marine ecologists, as the effects of increased pCO₂ on host-parasite interactions remain largely unknown (MacLeod and Poulin, 2012; Poulin et al., 2016). The infectivity rates of parasites and their responses to decreases in oceanic pH levels are also widely unknown.

From our survey of 3,480 locationally relevant species, only 35 (1.0%) had published data on responses to elevated pCO₂. Of these, 68.5% showed negative physiological or developmental responses, 31.4% exhibited neutral or mixed responses, and just 11.4% responded positively (Figure 8). Interspecific interactions may be disrupted substantially by the decline of ecologically important taxonomic groups shown here (Figure 8). Moreover, 83% of species shown here occupy the top 20 m of the water column, placing them within the depth range of the highest seasonal variability we observed. These findings suggest that many nwGoM taxa may already experience several events of geochemical extremes annually—patterns that are largely absent from global-scale projections.