September 2025 | Oceanography
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most recent scientific research has focused on the impact of OA
and low pH on marine species, high pH can also significantly
impact them (Pedersen and Hansen, 2003; Mos et al, 2020;
Bednaršek et al., 2025). Further research is needed to confirm the
safe levels of alkalinity and pH that can be released without caus-
ing harm; such investigations would produce critical data for ide-
alized dosing operations and identification of alkalinity addition
limits in different oceanic regions.
DOCCS uses electrochemical processes to remove DIC from
seawater by adding acid to convert all DIC into CO2, and then
using a stripping process to remove and capture the CO2, before
finally adding alkalinity back in to elevate total alkalinity (TA) to
original levels (Karunarathne et al., 2025). This approach results
in the discharge of treated seawater back into the ocean that now
contains lower DIC, ambient alkalinity (noting the dominant
alkalinity at the end of the process is OH– rather than HCO3
– and
CO3
2–), but higher pH. This low DIC, high pH discharge water has
potential to mitigate OA in the direct vicinity of the discharge
plume (“DIC removal, no TA addition” line in Figure 2; see also
Figure 3). As atmospheric CO2 re-equilibrates into the low CO2
plume, the carbonate chemistry returns to the original values and
therefore any mitigation effect is confined to small spatial and
temporal scales. As with OAE, further research is needed to fully
elucidate the potential of DOCCS to either mitigate OA or indeed
have its own environmental impacts from aspects such as elevated
pH or low CO2 and bicarbonate water.
The biological mCDR approaches of ocean nutrient fertiliza-
tion and macroalgal cultivation are considered to be CDR tech-
niques because they directly remove DIC through biological pri-
mary production. The manipulation of seawater chemistry in
these cases, as a result of photosynthetic activity, results in local
drawdown of CO2, elevation of pH, and consumption of nutri-
ents (especially phosphate and nitrates, but also silica in diatoms
and other minor nutrients), which has a small impact on alka-
linity (Wolf-Gladrow and Klaas, 2024; “Photosynthesis” line in
Figure 2; see also Figure 3). Thus, in the immediate surround-
ing surface waters, both macroalgal cultivation and nutrient fer-
tilization have the potential to ameliorate OA. However, due to
the smaller changes in alkalinity and its higher variability, the
potential for co-benefit at a large scale seems limited (Berger
et al., 2023). Some research has shown that macroalgal cultiva-
tion can mitigate the impact of OA on calcifying bivalves that are
grown in close proximity to the macroalgae (Wahl et al., 2018;
Young and Gobler, 2018). However, contrasting results suggest
coral calcification decreases in the presence of macroalgae (Isaak
et al., 2024). There is also evidence that, while macroalgae sys-
tems photosynthesize during the day (increasing pH), nighttime
respiration produces CO2, and decreases pH (Hirsh et al., 2020;
Ricart et al., 2021), and the resulting diel variability in pH can
be problematic for organisms not already adapted to this regime.
More research is required to understand the potential impact at
much larger scales.
Especially important to consider for these biological
approaches, but indeed for all approaches that make use of stor-
ing carbon at depth in the ocean, is what happens to that carbon
once in that location. The deep ocean already has higher concen-
trations of carbon than upper layers due to organic carbon rem-
ineralization and global circulation patterns. There is widespread
evidence that OA is not limited to the surface. The ocean inte-
rior is also acidifying, with some areas acidifying at much faster
rates than others (e.g., Fassbender et al., 2023; Müller and Gruber,
2024). This ocean interior acidification is resulting in an expan-
sion of undersaturated conditions with respect to calcium car-
bonate (aragonite and calcite) minerals, which will impact cal-
cifying organisms in the deep sea and throughout the water
column (e.g., Feely et al., 2024). By intentionally adding more car-
bon, either directly as liquid CO2 (a method not discussed in this
paper) or indirectly as organic matter from phytoplankton or
macroalgae (or land-grown biomass) that can be remineralized
back to CO2 (“Respiration” line in Figure 2), there is potential to
increase this deep ocean interior acidification impact. Subsurface
remineralization processes that are enhanced by eutrophication
in coastal regions also contribute to interior acidification. Excess
nutrients lead to rapid phytoplankton or macroalgal blooms in
surface waters that then sink and are remineralized in subsurface
waters, consuming oxygen and releasing CO2. Remineralization
can be amplified in estuaries and river plumes where mixing-
induced minimum buffering zones can locally amplify OA sig-
nals (e.g., Van Dam and Wang, 2019; Cai et al., 2021). In the
open ocean, the eutrophication phenomenon can result in large
hypoxic and anoxic zones (e.g., Feely et al., 2024; Rose et al., 2024).
Artificial upwelling has the potential to both counteract the
efficacy of its own mCDR mechanism and cause more surface
OA, because the deep waters that are being brought to the surface,
while rich in nutrients, are also rich in carbon. Natural upwell-
ing events that occur off the west coasts of the continents (also
known as eastern boundary upwelling systems) have demon-
strated that upwelling can amplify OA (Feely et al., 2008, 2016)
and impact organisms (Barton et al., 2015). Bringing carbon-rich
water to the surface shortens the otherwise longer-term storage
time of the carbon by exposing it to the atmosphere and poten-
tially negates any uptake of carbon that occurs through enhanced
biological productivity.
Any impact of mCDR on marine organisms or ecosystems
could also have knock-on consequences for the carbon cycle,
including direct implications for CO2 sequestration, but also for
indirect feedbacks to OA. This is a knowledge gap that needs to
be addressed within the mCDR research effort. These potential
impacts include the broader alterations in seawater chemistry
whose impacts on marine life are not fully understood (Meyer
and Spalding, 2021; Bednaršek et al., 2025). For example, shifts
in nutrient biogeochemical cycling, production of climatically
important gases, and particle flux dynamics can play significant
roles in changing the speciation of the carbon system in seawater