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Ocean acidification does not make all phytoplankton decline. As seawater absorbs human-produced carbon dioxide, its chemistry changes, and different plankton species respond differently: some grow faster, some slower, and others show no measured growth-rate response under the conditions studied. Those changes can affect the food available to grazers and other marine life, but the scale and direction of the effects depend on the species, life stage, and environment.
How ocean acidification changes seawater
When the ocean absorbs carbon dioxide (CO2) produced by human activities, the balance of dissolved carbon compounds in seawater shifts. Seawater pH falls, and carbonate ions become less available. That matters especially to organisms that use carbonate to build and maintain calcium carbonate shells or other structures.
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Phytoplankton are a diverse group of photosynthetic organisms, and not all of them build calcium carbonate structures. For many species, the relevant effects are therefore not simply a direct problem with shell-building. Acidification may instead affect growth, survival, physiology, or other traits, with responses varying among species and conditions.
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What happens to phytoplankton
Growth responses vary by species
NOAA’s summary of experimental research reports that growth rates increased for some studied phytoplankton species, decreased for others, and showed no measured sensitivity in still others. These findings do not support a single prediction that phytoplankton as a whole will increase or decrease as the ocean acidifies.
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Composition and food value may also change
NOAA also reports that elemental composition changed in some species in the experiments it summarizes, while other species showed no such change. Researchers are investigating whether acidification can alter which phytoplankton species are present and the nutritional content of the organisms consumers eat. Those are possible pathways to food-web effects, not established outcomes that apply everywhere.
Marine phytoplankton produce over half of the planet’s oxygen, according to NOAA’s Ocean Acidification Program; the source does not specify a year for that statistic. It describes phytoplankton’s broad role in oxygen production, not a measured forecast of how acidification will change oxygen levels.
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How effects can move through the marine food web
Phytoplankton sit near the base of marine food webs. Zooplankton graze on them, and plankton in turn provide food for animals ranging from fish to whales. If acidification changes the growth, survival, species mix, or nutritional characteristics of plankton, consumers may encounter a change in food supply or quality.
This is an indirect pathway: changing seawater chemistry can affect particular plankton species or traits, which may then influence the organisms that eat them and the larger food web. The result is not inevitable or uniform. It depends on which organisms are present, how they respond, and the conditions in a given ecosystem.
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Regional prey bottlenecks
In its vulnerability assessment for the Gulf of Alaska and Bering Sea, NOAA describes some lower-trophic-level prey as food-web “bottlenecks”: they funnel energy from phytoplankton to larger organisms. The assessment names krill, pteropods, and copepods among examples in those regions and identifies food-web disruption as an expected primary route for acidification effects on some fish and marine mammals. That regional assessment should not be treated as a prediction for every ocean ecosystem.
Examples show why species and life stage matter
Pteropods: shell effects in a calcifying plankton animal
Pteropods are small swimming snails that live as zooplankton, not phytoplankton. They build shells and are prey for fish and marine mammals in high-latitude ecosystems. NOAA reports partially dissolved pteropod shells at high-acidification locations along the U.S. West Coast, as well as laboratory confirmation that North Pacific pteropods are sensitive to acidification. This is a specific example involving a calcifying animal; it does not show that all phytoplankton respond in the same way.
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Pacific krill: a Puget Sound result differs by life stage
NOAA summarizes a study of Pacific krill (Euphausia pacifica) in Puget Sound in which lower pH did not affect egg hatch, but slowed larval development and decreased survival. NOAA suggests this species may live near the limits of its pH tolerance in Puget Sound. The finding concerns this species, place, and life-stage responses; it cannot be generalized to all krill, locations, or stages of development.
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What a developing carbon-cycle finding does—and does not—show
A 2025 NOAA summary of Barrett et al. reports an increase in ocean surface total alkalinity of 0.072 ± 0.023 μmol per kilogram per year. The authors proposed a biological feedback associated with this trend and estimated that it corresponded to about 0.20 PgC of additional human-emitted carbon absorbed by the ocean since the 1990s. The summary says more data are needed to quantify the feedback and its impacts. These figures describe a developing carbon-cycle finding, not a quantified forecast of phytoplankton or food-web outcomes.
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How to interpret claims about plankton impacts
A useful study or claim should make clear which organism and response it concerns. Evidence about shell condition in pteropods, larval survival in Pacific krill, and phytoplankton growth are not interchangeable. When assessing a reported effect, look for:
- Organism: Is it a phytoplankton species, a zooplankton animal, or another consumer?
- Life stage: Are eggs, larvae, or adults being measured?
- Exposure and place: What carbonate-chemistry conditions and region are involved?
- Measured response: Does the evidence concern growth, survival, shell condition, composition, or nutrition?
- Evidence type: Is it a laboratory experiment, a field observation, or a regional assessment?
Keeping these distinctions in view avoids two misleading conclusions: that acidification affects every plankton species in the same way, or that one observed biological response proves a particular food-web outcome is inevitable.
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