Removal of dissolved inorganic carbon from seawater for climate mitigation: potential marine ecosystem impacts

Abstract

Electrochemical technology can be used to remove inorganic carbon from seawater and facilitate the removal of carbon dioxide (CO₂) from the atmosphere. Electrochemical ‘Direct Ocean Carbon Capture and Storage’ (DOCCS) is a marine carbon dioxide removal (mCDR) method that removes atmospheric CO₂ by releasing low-carbon seawater into the surface ocean, where it re-equilibrates with the atmosphere and stores atmospheric CO₂. At the point of release, DOCCS discharge has low concentrations of dissolved inorganic carbon (DIC) and high pH, potentially causing unintended marine environmental impacts; however, its chemistry moves progressively towards that of ambient seawater as it dilutes and re-equilibrates with the atmosphere. To date, there are no published studies that investigate the impact of DOCCS discharge on marine ecosystems. Research from relevant analogues, where biological responses to low-DIC and/or high-pH seawater are investigated, provides some insight into potential DOCCS impacts. Despite this, significant evidence gaps remain. These evidence gaps are discussed alongside DOCCS-specific recommendations for future environmental impact research. Understanding the potential risks/benefits to marine ecosystems from discharge of low-DIC and high-pH seawater is critical to: (i) support licensing applications; (ii) develop any necessary mitigating actions; (iii) determine the net benefit of mCDR approaches; and (iv) stimulate informed public discourse about the acceptability of such approaches.

1 Introduction

1.1 Marine carbon dioxide removal

Analysis of emissions scenarios has shown that meeting the Paris Agreement goal to avoid global average temperature rise of more than 1.5°C above preindustrial levels will require not only the reduction of emissions but also the use of negative emissions technologies that can remove carbon dioxide (CO₂) from the atmosphere (Bach et al., 2019; Masson-Delmotte et al., 2018). The ocean is the largest inorganic carbon reservoir in the Earth’s ocean–atmosphere-biosphere system (Ciais et al., 2014), estimated to hold approximately 38,000 Gt C, which is over 60 times the estimated carbon content of the atmosphere (Friedlingstein et al., 2023; Intergovernmental Panel on Climate Change (IPCC), 2023; Majumdar and Deutch, 2018). The ocean’s capacity to hold carbon draws attention to its potential use as a sink for atmospheric CO₂ within climate mitigation strategies. Marine Carbon Dioxide Removal (mCDR) aims to accelerate the natural CO₂ sequestration processes of the ocean through a range of different techniques (Bach et al., 2024). mCDR methods are typically less well understood (Smith et al., 2024) than terrestrial CDR methods at present, but mCDR has inherent advantages over land-based methods, including less competition for space (Johnson et al., 2024). All mCDR techniques manipulate the marine environment in some way and therefore have the potential to cause unintended environmental impacts.

Here, we discuss the potential marine environmental impacts of a mCDR approach that removes atmospheric CO₂ by electrochemically removing CO₂ from seawater. At the time of writing, the electrochemical removal of CO₂ from seawater does not have an established name, and in the literature has been referred to as direct ocean capture (DOC) (Karunarathne et al., 2025; Al Yafiee et al., 2024), direct ocean removal (DOR) (Cross et al., 2023), electrochemical direct ocean capture (eDOC) (Aleta et al., 2023), and electrochemical ocean alkalinity cycling (OAC) (Eisaman, 2024). In this report, it will be referred to thereafter as Direct Ocean Carbon Capture and Storage (DOCCS) as this is used in the ‘State of CDR’ report (Smith et al., 2024). In this report, we will introduce and discuss some of the potential unintended environmental impacts that could be linked to DOCCS technology and highlight key knowledge gaps.

1.2 Direct Ocean Carbon Capture and Storage (DOCCS)

The vast majority of ocean carbon is stored in the form of dissolved inorganic carbon (DIC) (Equation 1) (Carroll et al., 2022). DOCCS achieves atmospheric CO₂ removal by electrochemically, or otherwise, adjusting seawater to extract its DIC (Bach et al., 2024). DOCCS generates seawater depleted in DIC, which is released into the marine environment where it re-equilibrates with the atmosphere by ‘refilling’ with atmospheric CO₂ (Halloran et al., 2025). DOCCS essentially uses the ocean’s natural carbon sequestration capability as a ‘pump’ or ‘pass through’ to draw CO₂ out of the atmosphere (Aleta et al., 2023; Eisaman, 2024). Methods to remove DIC from seawater using DOCCS technology are broadly split into two: one which removes DIC in the form of a gas; and the other in the form of a solid (Aleta et al., 2023; Eisaman et al., 2018; National Academies of Sciences, Engineering, 2022). This report will focus its discussion on the former method, which due to significant energetic advantages and simplified output stream handling, is the approach being piloted today (Aleta et al., 2023).

Equation 1: Total seawater DIC (μmol kg^-1) is the sum of three different inorganic carbon species, carbon dioxide (CO_{2 (aq)}), bicarbonate (HCO₃^-_(aq)), and carbonate (CO₃^2-_(aq)).

In electrochemical DOCCS, electrolysis of incoming seawater is used to physically divide it into two separate seawater streams enriched in hydrogen (H⁺) and hydroxide (OH^-) ions, respectively, which can be used to manipulate the inorganic carbon chemistry of large volumes of seawater (Aleta et al., 2023; de Lannoy et al., 2018; Digdaya et al., 2020; Eisaman et al., 2012, 2018; Karunarathne et al., 2025; National Academies of Sciences, Engineering, 2022; Sharifian et al., 2021). This is because the balance of inorganic carbon species (CO₂, HCO₃^-, and CO₃^2-) influences the pH of seawater and vice versa (Cornwall et al., 2012; Millero, 2000) (Figure 1). The addition of acid (H⁺) to lower seawater pH to around 4, converts 100% of seawater DIC into CO₂ (Aleta et al., 2023) (Figure 1), which can be liberated and vacuum stripped from seawater as a gas (de Lannoy et al., 2018). Removed CO₂ is purified for long-term geological storage or utilised in other ways such as in synthetic fuel production (Eisaman et al., 2018), however it should be noted that any CO₂ released from the use of DOCCS-derived products or services must ultimately be re-captured and stored to be considered CDR (Aleta et al., 2023). Acidic seawater that is depleted in DIC is then recombined with the electrochemically dissociated base (OH^-) to restore seawater alkalinity to natural levels (Eisaman, 2024). The addition of base to low-DIC seawater increases the pH beyond the seawater’s original pH (Eisaman, 2024), due to depleted levels of inorganic carbon, and specifically carbonic acid, in that seawater. Treated water is then released into the marine environment where the atmosphere and ocean naturally equilibrate, drawing down atmospheric CO₂ to replace the removed carbon (Aleta et al., 2023), typically over a period of months to years (Halloran et al., 2025).

Figure 1

Several factors make DOCCS an attractive approach for CDR. By volume, seawater contains approximately 100-150 times more inorganic carbon than air (Karunarathne et al., 2025), meaning that removal efficiency could be higher than in Direct Air Carbon Capture and Storage (DACCS). Further to this, rather than needing to pump air across a sorption surface, DOCCS carbon removal takes place over a large surface area downstream of the point of decarbonised water release (Al Yafiee et al., 2024). Carbon removed by DOCCS has the potential to be extremely durable, as the CO₂ gas that is removed from seawater and then purified can be deposited in long-term geological storage (Smith et al., 2024). However, the efficacy and sustainability of this approach is still being researched (Al Yafiee et al., 2024). After re-equilibration with the atmosphere, seawater released by DOCCS is chemically indistinct from surrounding ambient seawater (Eisaman, 2024). This offers advantages over other mCDR techniques like Ocean Alkalinity Enhancement (OAE), which results in increased DIC, alkalinity and pH after equilibrium has been reached (Eisaman, 2024). Unlike DOCCS, OAE could also result in the addition of chemical impurities to seawater associated with mineral feedstocks (Bach et al., 2019; Ferderer et al., 2024; Flipkens et al., 2024; Geerts et al., 2025; Guo et al., 2025; Hutchins et al., 2023; Xin et al., 2024). That said, versions of OAE that use base electrochemically generated from seawater avoid this issue (Marín-Samper et al., 2024; Oschlies et al., 2025). Before re-equilibration with the atmosphere, seawater released into the surface ocean following DOCCS will have a changed inorganic carbon chemistry: characterised by low DIC concentrations and high pH (Eisaman, 2024), which could impact marine ecosystems.

1.3 Aims and objectives

The potential marine ecosystem impacts associated with releasing low-DIC, high-pH seawater into the marine environment currently represents one of the biggest unknowns and barriers towards large-scale deployment of DOCCS technology (Digdaya et al., 2020; Karunarathne et al., 2025; National Academies of Sciences, Engineering, 2022). First this report describes how DOCCS impacts the inorganic carbon chemistry of seawater, including how it changes downstream of the plant. Key perturbations of the inorganic carbon chemistry are then linked to potential marine ecosystem impact by referring to relevant research that may provide insight. Finally, the report highlights key knowledge gaps and provides recommendations for future research on DOCCS marine impacts.

2 Influence of DOCCS on seawater inorganic carbon chemistry

At the point of release into the marine environment, DOCCS discharge has low DIC concentrations and a high pH (Eisaman, 2024). Table 1 describes the inorganic carbon chemistry of seawater from the Western English Channel under ambient conditions (Kitidis et al., 2012) (inflow) and following 90% DIC removal from DOCCS (outflow). pH is high because of the addition of the basic (OH^--enriched) seawater to the low-DIC seawater stream. Total alkalinity (TA) remains at ambient levels because the alkalinity change associated with the addition of base (OH^-) during pH manipulation balances that from the acid (H⁺) added to remove the carbon. The net effect is that inorganic carbon speciation is perturbed in DOCCS discharge (relative to ambient seawater, see Table 1), with a higher relative proportion of DIC in the form of CO₃^2- and a smaller relative proportion as HCO₃^- and CO₂ (Figure 1). While the impacts of DOCCS on seawater inorganic carbon chemistry are generally consistent (low DIC and high pH) across different ocean regions, natural differences in salinity, temperature, DIC, and TA causes some variation of pH and DIC speciation for DOCCS discharge. In the supplementary materials, we reproduce DOCCS inorganic carbon chemistry calculations for Tropical and Polar waters.

2.1 The impact of dilution and CO₂ uptake on the inorganic carbon chemistry of DOCCS discharge

DOCCS discharge (low-DIC, high-pH seawater) released into the marine environment will become diluted by mixing with ambient seawater, such that the chemistry downstream of the discharge point eventually becomes indistinct from normal seawater (Eisaman, 2024) (Figure 2). In parallel to dilution, uptake of atmospheric CO₂ by DOCCS discharge will return DIC (concentration and speciation) and pH to that of ambient seawater (Figure 3). The extent to which low-DIC, high-pH chemical perturbations persist in the marine environment will depend on the scale and frequency of DOCCS discharge, as well as the level of mixing and magnitude of air-sea CO₂ flux.

Figure 2

Figure 3

Figure 4 presents the inorganic carbon chemistry of DOCCS discharge as it dilutes with ambient seawater. As DOCCS discharge mixes with ambient seawater, DIC and TA both change at a predictable rate, because both behave linearly with respect to mixing (Dickson et al., 2007; Wolf-Gladrow et al., 2007). As DIC and TA concentrations for both DOCCS discharge and ambient seawater are known (Table 1), their resulting concentrations in any mixed sample can be calculated based on the dilution ratio (Table 2). TA is the same in both DOCCS discharge and ambient seawater (Table 1), so the concentration remains constant as the two mix (Figure 4 and Table 2). DIC is lower in DOCCS discharge compared to ambient seawater (Table 1) and so will increase as the two are mixed (Figure 4 and Table 2). While DIC and TA are considered conservative with respect to mixing (Wolf-Gladrow et al., 2007), pCO₂, bicarbonate, carbonate, pH, and the calcium carbonate saturation state (Omega calcite and Omega aragonite) are not because of the non-linear nature of inorganic carbon system equilibria (Emerson and Hedges, 2008; Wolf-Gladrow et al., 2007). CO₂ system solvers, such as ‘seacarb’ (used in this report) (Gattuso et al., 2024), can be used to predict the full inorganic carbon system when seawater DIC, TA, temperature, salinity, and pressure are known (Dickson et al., 2007). It is therefore possible to determine the full seawater inorganic carbon system at different dilution ratios of DOCCS discharge to ambient seawater (Figure 4). The dilution trends of the inorganic carbon system are comparable across different ocean regions, though there are subtle differences in pH and DIC speciation (see Supplementary materials for DOCCS inorganic carbon system calculations in polar and tropical seawater).

Figure 4

One phenomenon that calculations of seawater inorganic carbonate chemistry (Figure 4) do not account for is the formation of mineral precipitates that can occur in high-pH seawater. Aleta et al. (2023) has shown that a variety of solid carbonate and hydroxide precipitates are possible with dolomite (CaMg(CO₃)₂) precipitation dominating below pH 9.7, and artinite (Mg₂CO₃(OH)₂) and brucite (Mg(OH)₂) dominating above pH 9.7. The precipitation of carbonate and hydroxide minerals could result in the long-term removal of alkalinity (Bach, 2024; Gately et al., 2023; Halloran et al., 2025; Schulz et al., 2023), consequently reducing the pH and altering the speciation of inorganic carbon. It is important that DOCCS minimises precipitation in discharge because the removal of alkalinity would limit the capacity of DOCCS discharge to take up atmospheric CO₂ (Halloran et al., 2025). Understanding how mineral precipitation could impact seawater chemistry in DOCCS discharge after it is released into the marine environment has not yet been thoroughly investigated.

To reduce the level of chemical perturbation in DOCCS discharge, DOCCS technology could hypothetically make use of specialist discharge systems that dilute DOCCS discharge and or encourage CO₂ uptake before it is released into the marine environment. For instance, active sparging and passive cascades (National Academies of Sciences, Engineering, 2022), could be used to promote gas exchange in treated seawater, before DOCCS discharge is re-introduced into the marine environment. It should however be noted that pre-equilibration essentially turns DOCCS into DACCS and therefore no longer sidesteps DACCS’ challenges around movement of large volumes of air and the pressure drop associated with CO₂ sorption (Erans et al., 2022).

2.2 Are DOCCS discharge conditions novel?

Surface seawater has a naturally stable pH of about 8.1 due to its strong buffering capacity facilitated by high alkalinity (Hansen, 2002; Jiang et al., 2019; Middelburg et al., 2020). Despite seawater’s ability to withstand changes in pH, changes in pH and therefore inorganic carbon chemistry do occur. Ocean acidification, the term used to describe the gradually decreasing average ocean pH since the industrial revolution (Doney et al., 2009), and the consequent shifts in the inorganic carbon system (Raven et al., 2005), is occurring globally in response to rising atmospheric CO₂ (Gattuso and Hansson, 2011). In addition, more localised but even greater magnitude acidification regularly occurs in coastal regions as a consequence of accelerated microbial respiration (and associated oxygen removal and CO₂ production) driven by the aftermath of anthropogenic eutrophication when algal blooms crash (Baumann, 2019). DOCCS discharge, which is low-DIC and high-pH seawater, represents the opposite change in inorganic carbon chemistry to that observed during ocean acidification (see Table 3).

As demonstrated by the DOCCS process, seawater pH can also be increased. OAE, another emerging mCDR technique, which achieves CDR by increasing seawater alkalinity, also generates high pH in seawater (Bach et al., 2019). Seawater manipulated by DOCCS and OAE experience the same inorganic carbon system shift due to high pH (Figure 1) (Halloran et al., 2025). However, seawater DIC remains high or increased in OAE, whereas seawater DIC is reduced in DOCCS discharge (Table 3) (Halloran et al., 2025).

High-pH seawater conditions can also occur naturally without any human intervention. In eutrophic marine environments where rates of primary production are high, DIC is consumed through photosynthesis and if the CO₂ is not restored via air-sea exchange, for example, due to restrictions on mixing or convection of that water, lower pCO₂ levels result, and pH is elevated (Hendriks et al., 2014; Hinga, 2002; Krause-Jensen et al., 2016; Legrand et al., 2018; Søgaard et al., 2011). Eutrophic marine environments may be a useful natural analogue for DOCCS, as high pH is achieved by DIC removal rather than by alkalinity adjustments. However, high rates of primary production also drive other changes in seawater chemistry such as nutrient removal (Kemp et al., 1997; Tilman et al., 1982) and oxygen addition (Cloern et al., 2014; Herfort et al., 2012). In natural analogues, low-DIC, high-pH seawater conditions will persist until DIC levels are replenished via a range of processes (e.g., CO₂ production through respiration, air-sea CO₂ exchange, or by mixing with water enriched in DIC). Previous work has discussed natural analogues with respect to various mCDR techniques, but not for DOCCS (​Bach and Boyd, 2021).

3 How could DOCCS discharge impact marine organisms?

To date there have been no studies that have investigated the specific environmental impacts of low-DIC, high-pH seawater on marine organisms in the context of DOCCS. In the absence of technology-specific environmental impact studies, existing research, primarily from ocean acidification, OAE and eutrophic marine environments studies may provide insight into potential DOCCS marine impacts. Here we outline the key chemical perturbations associated with DOCCS discharge (based on the inorganic carbon chemical perturbations shown in Table 1 and Figure 4) and provide an initial perspective of the potential impacts to marine organisms based on studies that experimentally manipulate the inorganic carbon chemistry of seawater. The scope of the discussion is constrained to studies investigating biological impacts of chemical perturbations most relevant to the inorganic carbon chemistry regime of DOCCS discharge.

3.1 Low DIC

The reduction of DIC in DOCCS discharge is a significant perturbation to seawater chemistry that is required in order for DOCCS technology to achieve mCDR (Halloran et al., 2025). Low DIC in seawater could impact marine organisms that rely on DIC for important cellular processes such as photosynthesis, calcification, and acid–base regulation. Studies have shown that phytoplankton can become limited at DIC concentrations < 0.5 mM (Berge et al., 2010; Clark and Flynn, 2000; Hansen et al., 2007; Søderberg and Hansen, 2007). These experiments were able to manipulate DIC levels without changing the pH, thus removing pH as a confounding factor in their experiments.

This was an improvement on earlier studies such as those by Riebesell et al. (1993) and Chen and Durbin (1994), which investigated the impact of carbon limitation on phytoplankton by increasing pH. DIC limitation of growth and photosynthesis has also been observed in a range of macroalgal species (Borowitzka, 1981; Flores-Moya and Fernández, 1998; Frost-Christensen and Sand-Jensen, 1990; García-Sánchez et al., 1994, 2016; Lignell and Pedersen, 1989) with two studies showing significant reductions in macroalgal growth when seawater DIC was reduced to 150 μM (Andría et al., 2001; Jiang et al., 2016). Reduced calcification in macroalgae and marine bivalves has also been demonstrated under DIC limiting conditions (Borowitzka, 1981; Thomsen et al., 2015).

3.2 High pH

Internal pH plays a key role in many aspects of cell biology (Putnam, 2001). If an increase in seawater pH associated with DOCCS discharge influences internal pH in organisms, it could impact a wide range of biochemical processes (Putnam, 2001).

3.2.1 Impacts to internal physiology

Ocean acidification research has shown that decreasing pH, driven by an increasing concentration of carbonic acid in seawater, has the potential to impact marine organisms through a variety of physiological pathways, such as acid–base regulation, metabolism, and calcification (Fabry et al., 2008; Guinotte and Fabry, 2008; Pörtner, 2008; Pörtner et al., 2004).

It is worth stating that for many animals, in particular fish, the elevated environmental pCO₂ (rather than the seawater pH itself) is thought to be the cause of most of the impacts observed (Brauner et al., 2012; Wilson, 2020). This is because all aquatic animals, by the necessity for respiratory gas exchange, are permeable to gases, hence as environmental CO₂ rises, internal (blood and tissue) CO₂ rises in parallel (Brauner et al., 2019; Wilson, 2020). In turn, this elevation of internal CO₂ results in at least temporary internal acidification (reduced pH) termed “acidosis” (Esbaugh, 2017; Fabry et al., 2008). However, many marine animals including fish and crustaceans recover blood and intracellular pH rapidly (hours) through acid–base regulation processes that primarily involves excretion of excess H⁺ or uptake of additional HCO₃^- via the gills (Brauner et al., 2019; Fehsenfeld and Weihrauch, 2017; Wilson, 2020). Internal pH regulation is assumed to have an energetic cost that can therefore potentially divert energy away from other important physiological processes like growth, swimming, and reproduction, functions that are known to be impaired by elevated CO₂ (Lefevre, 2019; Munday et al., 2019; Skov, 2019).

CO₂-driven ocean acidification is also known to interfere with ecologically relevant sensory processes such as olfaction, which may be via a direct effect of seawater pH change on external-facing receptors in aquatic animals (Porteus et al., 2018, 2021).

Given the well-established impacts of relatively small reductions in seawater pH and elevations in CO₂ on marine organisms, it is highly likely that the opposite scenario, high seawater pH and reduced CO₂, will result in similarly important physiological impacts to those identified in ocean acidification research. While there is far less research on the impacts of high pH on marine organisms, research investigating the environmental impacts associated with OAE and eutrophic marine environments may provide insights. For example, published research suggests that marine primary producers such as phytoplankton (Berge et al., 2012; Chen and Durbin, 1994; Hansen, 2002; Oberlander et al., 2025; Pedersen and Hansen, 2003; Rai and Rajashekhar, 2014; Søderberg and Hansen, 2007; Søgaard et al., 2011; Taraldsvik et al., 2000), macroalgae (Borowitzka, 1981; Frost-Christensen and Sand-Jensen, 1990; Jiang et al., 2018; Lignell and Pedersen, 1989; Menéndez et al., 2001; Tee et al., 2015), and seagrass (Invers et al., 1997) have a growth/performance upper limit of ca. pH 9. Experiments investigating the impacts of high pH on non-phototrophic marine organisms are less common but also demonstrate impacts above pH 8 (Camatti et al., 2024; Comeau et al., 2017; Cripps et al., 2013; Hansen et al., 2017). Several of these studies highlighted acid–base regulation, membrane transport and metabolic processes as potential impact pathways at high pH (Cripps et al., 2013; Hansen, 2002; Pedersen and Hansen, 2003).

The only research to investigate the impact of elevated pH on higher trophic levels comes from studies focused on brackish and freshwater ecosystems, where high pH can develop due to underlying geochemistry (Brauner et al., 2012) or episodically as a result of eutrophication and high daytime photosynthesis rates (Scott et al., 2005). There is evidence to suggest that high pH could impact the internal physiology of marine fish. For example, excretion of nitrogenous waste, a critical function in aquatic organisms, becomes inhibited in freshwater fish at high pH, resulting in the toxic accumulation of ammonia in the blood (Thompson et al., 2015; Wilkie and Wood, 1991, 1996; Wood, 1993, 2022). However, marine fish may be more tolerant to these impacts, as research shows that toxicity at high pH decreases with higher freshwater calcium concentrations (Iwama et al., 1997; Wilkie and Wood, 1996; Yesaki and Iwama, 1992). Calcium concentration averages 10 mM in seawater compared to a global median of 0.65 mM for inland waters (Pinheiro et al., 2021).

Ion transport at the gills is also inhibited at high pH, in particular the uptake of sodium and chloride ions, which can result in the gradual reduction of these two essential ions in freshwater fish (Thompson et al., 2015; Wright and Wood, 1985). In marine fish, removal of sodium and chloride ions by active transport is required to mitigate their continuous inward diffusion from seawater (Wood, 2022). High seawater pH exposure in marine fish could therefore have the opposite effect, whereby sodium and chloride gradually accumulate, reaching potentially toxic levels. In addition, elevated environmental pH is associated with an internal alkalosis in freshwater fish (Wilkie and Wood, 1996) and reduced blood pCO_2. This is the opposite outcome to that described above for fish exposed to ocean acidification but necessitates similar acid–base regulatory processes (but opposite direction) to restore internal pH, that will require energy with potential knock-on effects for other energy requiring processes.

3.2.2 Exacerbation of DIC limitation due to elevated pH

At high pH the relative concentrations of CO₂ and HCO₃^- decrease while the relative concentration of CO₃^2- increases (Figure 1). This means at high pH, CO₂ and HCO₃^- comprise a smaller fraction of DIC, which is already reduced (due to DIC removal) in DOCCS discharge (Table 1 and Figure 4). The combined impact of a reduced proportion of CO₂ and HCO₃^- in an already reduced pool of DIC, could exacerbate DIC limitation for some marine organisms, particularly for processes like photosynthesis, calcification, and acid–base regulation that use CO₂ and HCO₃^- (Findlay et al., 2011; Raven, 1994; Wilson, 2020). As highlighted in Bach et al. (2019) and Faucher et al. (2025), some studies have shown that under high-pH conditions phytoplankton growth is impacted when pCO₂ falls below ca. 100 μatm (Bach et al., 2011, 2015; Faucher et al., 2025; Sett et al., 2014). However DIC was greater than 1.3 mM in all these studies. Research investigating the performance of marine dinoflagellates in eutrophic marine environments, where pH is increased by the removal of DIC, have shown that the impacts of low DIC can be exacerbated when seawater is at high pH (Hansen et al., 2007; Søderberg and Hansen, 2007). Both studies showed that the ‘growth compensation point’ (the concentration of DIC required for maintenance of cells), increased with pH, indicating that DIC became limiting at a higher concentration as pH increased.

CO₂ is generally considered the main inorganic carbon substrate for terrestrial plant photosynthesis (Cooper and Filmer, 1969). However, CO₂ is less readily available to marine plants than terrestrial plants because the diffusion of CO₂ in seawater takes longer than in air (Reinfelder, 2011; Sun et al., 2023). To adapt to the challenge of CO₂ limitation, marine primary producers have developed Carbon Concentrating Mechanisms (CCMs) that enhance inorganic carbon utilisation (Raven et al., 2011). CCMs commonly make use of the HCO₃^- pool, through carbonic anhydrase, an enzyme which converts HCO₃^- to CO₂ (Beer et al., 2002; Reinfelder, 2011; Sun et al., 2023). In marine environments where CO₂ becomes naturally depleted over time (e.g., rock pools), marine organisms with CCMs can continue functioning even when the concentration of CO₂ is significantly reduced (Maberly, 1990; Murru and Sandgren, 2004). CCMs may help some marine producers tolerate DOCCS discharge conditions, as CO₂ concentrations are lower than in ambient seawater (Table 1 and Figure 4). However, at very low dilution factors (ca. < 1), CCMs are unlikely to provide much benefit as the total pool of both CO₂ and HCO₃^- is significantly reduced (Figure 4).

3.2.3 Increased calcium carbonate (CaCO₃) saturation states

CaCO₃ saturation state is a term used to describe how saturated seawater is with respect to CaCO₃ minerals. The CaCO₃ saturation state is ‘supersaturated’ when >1 and ‘undersaturated’ when <1 (Feely et al., 2004, 2008; Gangstø et al., 2008). When seawater is saturated, Ca²⁺ and CO₃^2- ions are plentiful and assuming no complicating factors, mineral precipitation can occur inorganically. When seawater is undersaturated, CaCO₃ dissolution is thermodynamically favoured. Calcifying marine organisms require additional carbon to build their CaCO₃ structures, and saturated conditions provide a favourable environment for this to occur (Langdon et al., 2000). However, as saturation states decreases and become undersaturated, any exposed CaCO₃ mineral structure can be subject to dissolution and the organism must use more energy to continue to calcify in these less favourable conditions (Findlay et al., 2011; Wood et al., 2008). In high-pH seawater a greater fraction of DIC is in the form of CO₃^2- (Figure 1). At a dilution factor of ~ 0 - 2, the concentration of CO₃^2- ions in DOCCS discharge temporarily increases above ambient levels (Figure 4), which increases the CaCO₃ saturation state (Figure 4). Several studies have shown biological calcification increases in response to increasing TA and pH (Albright et al., 2016; Borowitzka, 1981; Gore et al., 2019). However, research by Bednaršek et al. (2025), which uses calcification response data from ocean acidification studies to try and predict how marine calcifiers may respond to high pH during OAE, suggests that the response of marine calcifiers to increasing pH is likely to be varied. The high CaCO₃ saturation state observed at low dilutions (dilution factor ca. > 0 to 2) of DOCCS discharge could reduce dissolution of existing CaCO₃ structures, potentially countering some of the projected impacts of ocean acidification on very local scales.

4 Key knowledge gaps and research suggestions

Based on what is currently known about the response of marine organisms to low DIC and/or high pH, its plausible that DOCCS discharge could impact marine organisms when the degree of seawater CO₂ removal is high and any pre-dilution before discharge is low. However, major gaps in understanding regarding DOCCS environmental impacts remain. Addressing the knowledge gaps to support the responsible development and regulation of DOCCS requires a collaborative and coordinated response.

4.1 Lack of empirical environmental impact data for DOCCS

There is currently no published research that directly studies the environmental impacts of DOCCS on marine organisms/ecosystems. The combined impact of low DIC and high pH is central to understanding how marine organisms may be impacted by DOCCS discharge waters. Ocean acidification and OAE research highlights that marine organisms could be sensitive to inorganic carbon chemistry changes generated by DOCCS, however none of these experiments replicate the conditions experienced under DOCCS so should be used only to generate hypotheses.

The most relevant data on potential DOCCS impacts comes from studies investigating the performance of photosynthesising marine organisms in naturally occurring eutrophic environments. Various techniques were used to manipulate inorganic carbon chemistry in these studies, often holding either DIC or pH constant while adjusting the other (Andría et al., 2001; Borowitzka, 1981; Hansen et al., 2007; Søgaard et al., 2011). Holding one variable constant while changing another allows researchers to examine the independent effects of low DIC and high pH on marine organisms but does not address their combined impact. Many experiments that raise pH and hold DIC constant do so by adding alkalinity, which is similar to an OAE experiment (Rai and Rajashekhar, 2014). Other studies combined the effects of low DIC and high pH and assessed the impacts using “drift” experiments. Drift experiments allow DIC and pH to covary while photosynthetic organisms grow in a closed system (Berge et al., 2012). An alternative approach generates low-DIC and high-pH conditions by acidifying seawater, removing DIC as CO₂, and adding alkalinity (Hansen et al., 2007; Søderberg and Hansen, 2007). This technique, which allows for precise control of DIC and pH, and does not alter any other seawater chemistry, resembles DOCCS discharge conditions most closely. The combined approach of seawater DIC removal and alkalinity addition is likely to be the most appropriate technique for simulating DOCCS discharge conditions for environmental impact experiments as it requires minimal equipment and can be achieved in most laboratories.

While analogues of DOCCS discharge conditions are useful for directing research, they do not provide enough evidence to make strong conclusions about potential DOCCS environmental impacts. As DOCCS environmental impact experiments are currently in their infancy (CNN, 2025; BBC News, 2025) early research should investigate the short-term physiological response of key marine organism groups, both photosynthetic (phytoplankton, macroalgae) and non-photosynthetic (bivalves, fish), to short term DOCCS discharge exposure, at a range of dilution levels. Investigating direct and immediate impacts, could help early identification of important ecological sensitivities to DOCCS discharge, and shape future environmental impact research priorities.

The long-term marine ecosystem impacts of sustained exposure to low-DIC, high-pH conditions generated by DOCCS have not been studied. Short-term studies on individual marine organisms are in early development (CNN, 2025; BBC News, 2025) but there is no data on the ecosystem-wide, chronic changes that may unfold. Ocean acidification research has shown that stressors can cause long-term shifts in biodiversity, organism behaviour, and ecosystem services that are not picked up by acute or single-generation laboratory experiments (Dupont et al., 2008; Howald et al., 2022; Pedersen et al., 2014). Ultimately, long-term laboratory studies, similar to those conducted for ocean acidification research, will be needed to determine if chronic impacts arise due to sustained exposure to low-DIC, high-pH water.

Maintaining ‘non-equilibrated’ DOCCS conditions may be challenging in long-term laboratory studies as DOCCS discharge will have a natural tendency to draw in atmospheric CO₂, changing the experimental conditions. Potential CO₂ uptake by non-equilibrated seawater is also highlighted in ‘Guide to best practices in Ocean Alkalinity Enhancement Research’ (Iglesias-Rodríguez et al., 2023; Schulz et al., 2023). Simulating long-term DOCCS discharge conditions in a laboratory, may require either; a closed-system design, where the experiment is closed off from the atmosphere; or the continuous supply of freshly generated low-DIC, high-pH seawater into the experiment. Further, inorganic mineral precipitation will occur at high pH (discussed more in Section 4.2), which could cause the seawater chemistry to evolve through the experiment, again pointing to the need for mesocosms with a continuous supply of fresh decarbonised water (Halloran et al., 2025).

The positive or negative response of individual species to low-DIC, high-pH seawater must be considered within the context of complex and highly connected marine ecosystems as the net ecosystem response to DOCCS discharge may differ from the sum of individual species responses. Predicting secondary impacts (i.e., the impacts within an ecosystem that occur indirectly as a result of primary impact) due to low-DIC, high-pH seawater will be as challenging as it was for ocean acidification and OAE research. Net ecosystem impact studies will require a multifaceted approach that includes the collection of empirical data from laboratory- and field-based experiments, long-term and continuous environmental monitoring (including baseline monitoring), and ecological modelling. Prior to field experiments, single and multi-species mesocosm studies, as has been done for OAE (Bhaumik et al., 2025; Faucher et al., 2025; Sánchez et al., 2024), should be conducted to capture and explore the short to medium term response of natural ecosystems to DOCCS discharge and ensure that the risk of environmental impact in field experiments is low.

4.2 Chemical uncertainty for DOCCS discharge

Figure 4 demonstrates the expected theoretical inorganic carbon chemistry in DOCCS discharge at different mixing points downstream. However, as briefly mentioned in Section 2.1, the modelling used to determine the inorganic carbon chemistry does not consider inorganic precipitation that occurs in high-pH seawater. Inorganic precipitation could impact the marine environment through the removal of essential nutrients and/or by physical disturbance (Gately et al., 2023). The removal of dissolved ions via inorganic precipitation will have a marked effect on the inorganic carbon chemistry and ion concentration of DOCCS discharge (Gately et al., 2023; Halloran et al., 2025), changing its chemical profile.

For the former, inorganic precipitation can result in the removal of DIC and TA from the solution (Gately et al., 2023; Halloran et al., 2025; Ramírez et al., 2025), which will have subsequent impacts to the pH, and the relative proportions of the inorganic carbon species. Modelling of the inorganic carbon chemistry that does not consider inorganic precipitation may risk mis-characterising the inorganic carbon chemistry of DOCCS discharge when pH is high (ca. >9). To obtain accurate measurements of DIC and TA during DOCCS environmental impact experiments, seawater samples will need to be filtered to remove any precipitate, as has been suggested for OAE inorganic carbon chemistry measurements (Schulz et al., 2023).

pH can also influence the bioavailability of nutrients in seawater. Ocean acidification research has shown how low pH can change the bioavailability of micronutrients and lead to potential biological impacts in marine organisms (Cheriyan et al., 2024; Hoffmann et al., 2012; Millero et al., 2009; Stockdale et al., 2016). There is far less evidence for how high pH could impact the bioavailability of nutrients in seawater. At high pH, mineral precipitation can result in the reduction of dissolved nutrients such as Ca, Mg, Fe, Si, and P in seawater (Aleta et al., 2023; Gately et al., 2023; Moras et al., 2022), which could limit important physiological processes such as calcification (Dixon-Anderson, 2021; Ries, 2010). Experiments investigating the performance of phytoplankton in high-pH environments have briefly discussed the possibility of high pH directly impacting nutrient availability, though tend to attribute biological impacts to different mechanisms of the pH effect like intracellular impacts and CO₂ limitation (Hansen, 2002; Hansen et al., 2007; Lundholm et al., 2004). Alternatively, phytoplankton research by Olsen et al. (2006), demonstrated that a photosynthesis-induced increase in seawater pH to 9, resulted in up to 20% of the phosphate in the system precipitating out of solution.

To develop a better understanding of how high pH influences inorganic carbon system dynamics and nutrient availability in DOCCS discharge, laboratory experiments and modelling should be combined to characterise the inorganic carbon chemistry and nutrient profiles of DOCCS discharge at a range of dilution levels. This is particularly important in scenarios where the degree of DOCCS carbon removal is high, and the level of dilution is low because high pH levels and abiotic mineral precipitation are more likely.

4.3 The impact of spatial scale

The extent and duration to which low-DIC, high-pH conditions persist in the marine environment following the release of DOCCS discharge will be determined by the rate of mixing with ambient seawater and by the rate of atmospheric CO₂ uptake by DOCCS discharge (Bednaršek et al., 2025; Schulz et al., 2023). The uptake of CO₂ by DOCCS discharge is expected to occur over the course of months to years (Halloran et al., 2025) and is influenced by physical, chemical, and biological factors (Halloran et al., 2025). Dilution of DOCCS discharge is likely to happen over much shorter timescales, though this is scale dependent and influenced by the outflow rate and the local hydrodynamic conditions.

At scales comparable to other point source-pollution outflows, such as sewage outfalls or waste discharge from ships, DOCCS discharge is expected to dilute quickly, posing little risk to the marine environment (Hunt et al., 2010; Lewis and Riddle, 1989; Loehr et al., 2006; Tate et al., 2016; Water Research Centre Limited, 2024). At these scales, low-DIC, high-pH perturbations may only exist within a short distance (e.g., a few meters) of the point source. If DOCCS technology was scaled by several orders of magnitude, dilution would occur at a much slower rate (assuming DOCCS discharge was subject to the same mixing), and low-DIC, high-pH conditions could persist in the marine environment for longer and over greater spatial scales.

For prospective DOCCS sites, plume dilution modelling (Fennel et al., 2023; Hunt et al., 2010; Premathilake and Khangaonkar, 2019) can provide insight into how DOCCS discharge is expected to dilute and disperse within the marine environment. Dilution model outputs can then be coupled with inorganic carbon chemistry data (Figure 4) to predict the extent of chemical perturbation at different locations within the plume affected area. Biological response data from laboratory impact experiments, can also be integrated at this stage, enabling the assessment of environmental risk across the plume-affected area. Modelling dilution under different outflow scenarios enables early evaluation of the chemical perturbation and potential environmental impact as outflow rates are increased towards climate-target relevant scales.

4.4 Context dependent variability

The environmental impacts of DOCCS discharge will vary considerably based on the context in which they are deployed. In Section 4.3, the impact of scaling DOCCS technology on environmental risk was briefly discussed, however, there are many other context-dependent factors that will determine the extent to which DOCCS technology could impact the marine environment. For example, an offshore location may avoid water discharged by DOCCS interacting with particularly sensitive (e.g., benthic or coastal) environments. Some offshore marine organisms such as microbial, plankton, and higher-trophic mobile species could still be impacted, though some mobile species may be able to avoid DOCCS discharge conditions by moving into unaffected areas (Tierney, 2016). Conversely, proximity of DOCCS to productive coastal areas potentially risks exposing sensitive benthic marine ecosystems (e.g., seagrass and coral reef ecosystems), that are fixed in their position, as well as disrupting complex biogeochemical cycling. That said, there is an argument that coastal DOCCS deployment poses less environmental risk than offshore, as our scientific understanding of coastal zones far exceeds that of the open ocean (Bridges et al., 2023; Mishonov et al., 2024).

Also important to consider is the temporal context of DOCCS discharge release. The extent of environmental impact associated with DOCCS discharge may change over different timescales. For example, DOCCS discharge may have a larger impact on marine primary producers during the day, when photosynthesis is the dominant physiological process, compared to the night when rates of photosynthesis naturally decline (Harding et al., 1981). Seasonality could also change the course of environmental impact, by driving variability in physical, chemical, and biological conditions over the course of the year. On longer time scales, understanding how the potential environmental impacts of DOCCS discharge interact with aspects of climate change like ocean acidification and rising sea surface temperatures will also be important.

5 Conclusion

There has been very little study of the response of marine ecosystems to low-DIC and/or high-pH seawater. Understanding the potential environmental impacts is crucial for the overall development of DOCCS technology, but particularly for environmental safety and the social license to operate, especially at large scales and/or at high intensities. Existing research, discussed in Section 3, shows that marine organisms may be sensitive to low-DIC and/or high-pH seawater conditions. DOCCS discharge could therefore cause unintended environmental impacts at or near the point of discharge. Potential environmental impacts are likely to be more severe when the degree of CO₂ removal is high (ca. >90%) and when DOCCS discharge is released at large scales. It would be irresponsible to deploy DOCCS technology at large scales, relative to the rates of dilution, until further work is conducted. Studies that assess the response of individual marine species to naturally generated low-DIC, high-pH conditions in eutrophic environments offer an interesting perspective on the potential impacts of DOCCS, but do not fulfil the necessary evidence requirements to build a comprehensive environmental risk assessment. An environmental impact evidence base from a range of marine organisms and ecosystems is urgently required to assess the impacts of DOCCS discharge. A key challenge is to generate an environmental impacts evidence base for DOCCS quickly enough to prevent harm to marine ecosystems from inappropriate operation, while enabling this technology to scale, if and where appropriate, to contribute to the mitigation of harm caused by climate change.

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