New Paper: Acidification and CO2-Driven Conditions in the Estuary and Gulf of St. Lawrence During 2024
- OA CoP
- 13 minutes ago
- 8 min read
The Estuary and Gulf of St. Lawrence (EGSL) is one of the largest and most productive estuarine/marine systems in the world. This region supports unique ecosystems and productive fisheries that have been critical sources of food and culture for millennia, while also being a major historical transportation channel for Eastern North America. However, the EGSL is experiencing rapid changes to water conditions, including acidification, associated with climate change much faster than global averages. We interviewed Dr. Martine Lizotte, Research Scientist in Ocean Biogeochemistry at Fisheries and Oceans Canada (DFO), Associate Professor at the Université du Québec à Rimouski – Institut des Sciences de la Mer de Rimouski (UQAR-ISMER), and Chair of the Canadian National Committee for the Scientific Committee on Oceanic Research (CNC-SCOR), on her important work to monitor the changes in the EGSL and its real-world impacts on the vital ecosystems and fisheries in the region.
What is your background?
"My research centers on carbonate system dynamics, ocean acidification and hypercapnia in the Estuary and Gulf of St. Lawrence (EGSL). Much of this work is grounded in the Atlantic Zone Monitoring Program (AZMP, see Galbraith et al. 2026) and contributes directly to ecosystem-based fisheries management. A key output of that contribution is the “Ecosystem Summaries” for the EGSL, a synthesis that integrate physical, chemical, and biological indicators into a coherent picture of ecosystem state and trends, giving fisheries managers the climate-informed context they need to assess stocks and support decision-making under Canada's Ecosystem Approach to Fisheries Management. Another recent product of this monitoring work is the first technical report on acidification and CO2-driven conditions across the EGSL, documenting increased carbonate system vulnerability at depth, persistent hypercapnia in deep waters, and emerging regional hotspots of reduced buffering capacity (Lizotte et al. 2026). Beyond monitoring, I'm involved in several interdisciplinary projects spanning phytoplankton responses to ocean acidification, microparticle characterization, coccolithophore carbon sequestration, and a transdisciplinary filmmaking project that uses sensory cinema to reveal the hidden microbial world of the St. Lawrence hypercapnic-hypoxic zone. I'm also an active contributor to DFO working groups on mCDR, ocean chemistry, and climate change."
Could you give us a little bit of background on your research project/area of study?
“The St. Lawrence Estuary and Gulf are facing a kind of perfect storm of climate stressors: the water is warming, becoming more acidic, and losing oxygen, often all at once…”
"The St. Lawrence Estuary and Gulf are facing a kind of perfect storm of climate stressors: the water is warming, becoming more acidic, and losing oxygen, often all at once, particularly in the deep layers. My work tries to understand how these changes affect the tiny organisms at the base of the food web, mainly phytoplankton, and what that means for the broader ecosystem and the fisheries that depend on it. Because if the foundation shifts, everything built on top of it may shift too. Part of that work feeds into long-term monitoring programs that help managers track ecosystem health over time. I'm also part of a transdisciplinary filmmaking project that takes that idea further, because getting knowledge out of scientific reports and into the hands of broader audiences, whether that's policymakers, coastal communities, or the general public, is just as important as generating it in the first place."
What was the motivation or inspiration for this research?
"A core motivation behind this work is the need for monitoring that goes beyond detecting chemical change in isolation. Tracking carbonate system indicators over time is essential, but the real value for marine resource managers comes from translating those signals into biological terms. A shift in pH tells you something is changing. Knowing that CO2 levels have crossed a threshold known to potentially impair key marine organisms, or that the habitat window for shell-forming species is compressing, tells you something is at risk. The technical report was built around that principle, using biologically-relevant indicators alongside traditional carbonate chemistry to give resource managers a more direct line of sight between what the water is doing and what that means for the species they're responsible for. Long-term monitoring makes this translation possible by revealing not just where conditions stand today, but how fast they're moving and where the trajectories could be headed."
What was the main question of this research?
"The central question driving this research is straightforward: how are CO2-driven conditions currently manifesting across the Estuary and Gulf of St. Lawrence, where are they changing fastest, and where are marine organisms most exposed? The EGSL is a chemically complex system where acidification doesn't follow a simple surface-to-bottom pattern. Physical circulation, biological activity, and the accumulation of respiratory CO2 at depth all interact to create conditions that can be far more severe than what surface measurements alone would suggest. Systematic monitoring of acidification parameters in the EGSL has been underway since 2014, but building a clear picture of spatiotemporal variability across the full carbonate system, and identifying where thresholds relevant to marine life are being crossed, requires sustained, multi-indicator tracking. That's what this report sets out to document, and it's intended as the first in a series of annual reports that will allow us to follow these conditions through time, track how they evolve, and catch emerging vulnerabilities."
How did you conduct this research or how did you go about answering your question?
"The data behind this report come from ship-based surveys conducted each year in spring, summer and fall across several regions of the EGSL, from the Lower Estuary to the Magdalen Shallows, covering both shallower coastal areas and deep channels that exceed 200 metres. At each station, water samples are collected at multiple depths and analyzed in the lab for a suite of carbonate chemistry parameters, including pH, total alkalinity, and dissolved inorganic carbon. From those core measurements, we derive a broader set of indicators that speak more directly to biological risk, including CO2 partial pressure, calcium carbonate saturation states relevant to shell-forming organisms, and a buffering capacity index that tracks how well the water can resist further acidification. A high-frequency monitoring station in Rimouski, sampled roughly weekly during summer and monthly through the rest of the year, adds temporal resolution that seasonal surveys alone can't provide. Taken together, this multi-region, multi-depth, multi-indicator approach is what allows us to move beyond a single snapshot of conditions and start identifying trends, hotspots, and the depths and areas where biological thresholds are most at risk of being crossed."
What were the main findings of your work?
"The clearest signal in the data is that deep waters, particularly around 200 metres depth and deeper, are under sustained and intensifying stress. Across all regions of the EGSL intersected by deep channels, multiple indicators of carbonate system health converged on the same message: buffering capacity is declining, CO2 levels are rising, and the chemical conditions that shell-forming and CO2-sensitive organisms depend on are deteriorating, a pattern consistent since ca. 2020. The Estuary is the most severely affected region, where all five biological stress thresholds tracked in the report are simultaneously exceeded at depth, with some indicators reaching record lows in 2024 (Figure 3). A clear regional gradient emerges, from the Estuary and Northwest Gulf at the more vulnerable end, to the Centre Gulf and Magdalen Shallows at the less affected end, though no region is entirely spared. Surface waters tell a more complex story, shaped by short-term biological and physical variability, making trends harder to pin down without longer records."
Did you find anything unexpected?
“Perhaps the most striking aspect of these findings is not that conditions are deteriorating, but how fast.”
"Perhaps the most striking aspect of these findings is not that conditions are deteriorating, but how fast. While historical carbonate chemistry data for the EGSL are sparse, some early pH measurements from the deep waters of the Estuary, dating back nearly a century, give us a rare glimpse into a long-term perspective. Those records suggest that deep-water pH was around 7.85 at the time. Today the annual average sits around 7.55. That seemingly small numerical shift translates to more than a 100% increase in acidity in less than 100 years, a rate that far outpaces the global surface ocean, which has seen roughly a 40% increase in acidity since pre-industrial times and is already considered alarming. The EGSL's deep waters are changing much faster, driven by a combination of incoming waters already preconditioned with anthropogenic CO2 and the steady accumulation of respiration-derived CO2 in poorly ventilated deep layers."
What is the one take-home of this work that you want everyone to know or remember?
“In less than a century, acidity in the deep [St. Lawrence] Estuary has more than doubled, a rate that far exceeds global trends.”
"The chemistry of deep waters in the St. Lawrence is changing, and it's changing fast. In less than a century, acidity in the deep Estuary has more than doubled, a rate that far exceeds global trends. The Estuary is the most affected region, but no deep area is untouched. And yet, biology is rarely as straightforward as chemistry. Organisms don't simply respond in lockstep to declining pH, and a growing body of work, including a recent national synthesis of ocean acidification responses across Canada (Barclay et al. 2026), is revealing a wide and sometimes surprising range of tolerances across species. Some organisms may be better equipped than we expected, particularly in naturally dynamic systems like the EGSL where life has always had to contend with chemical variability. Through my own Phytozoa project, I'm actively investigating how phytoplankton and their trophic connections respond to these conditions, while Dr. Daniel Small and other colleagues at DFO in the Quebec region are tackling the same questions for commercially important crustaceans and other key species. Together, we're building the knowledge base that managers will need to make informed, targeted decisions as conditions continue to evolve."
“Together, we're building the knowledge base that managers will need to make informed, targeted decisions as conditions continue to evolve.”
Anything else you’d like to say?
"Initiatives like this one (the Canadian OA CoP - Research Recap series) matter. Getting scientists to step outside their technical comfort zone and explain their work in plain language is a genuinely valuable exercise, both for the audiences we reach and for us as researchers. It forces clarity of thought. So thank you for the invitation and for creating a space where science can travel a little further than the journal page."
Read the report:
Lizotte, M., Blais, M., Chassé, J., Galbraith, P. S., Hébert, A.-J., Starr, M. 2026. Acidification and CO2-Driven Conditions in the Estuary and Gulf of St. Lawrence During 2024. Can. Tech. Rep. Hydrogr. Ocean. Sci. 410 : vi + 71 p.
Learn more about Dr. Lizotte and her work:
References
Barclay KM, Gurney-Smith HJ, Ahmed M, Christian JR, Cyr F, Duke PJ, Else BGT, Gimenez I, Lizotte M, Reader MC, Roth M, Rutherford K, Starr M, Steiner NS, Turner J, VanderZwaag DL and Evans W (2026) Ocean acidification in Canada: the current state of knowledge and pathways for action. Front. Mar. Sci. 13:1761703. doi: 10.3389/fmars.2026.1761703
Galbraith, P.S., Blais, M., Lizotte, M., Bélanger, D., Casault, B., Clay, S., Layton, C., Penney, J., Gabriel, C.-E., Ringuette, M., Azetsu-Scott, K., Chassé, J., Coyne, J., Devred, E., Johnson, C.L., Maillet, G., Shaw, J.-L., Starr, M. 2026. Oceanographic conditions in the Atlantic Zone in 2025. Can. Tech. Rep. Hydrogr. Ocean. Sci. 420 : vii +49 p.
Lizotte, M., Blais, M., Chassé, J., Galbraith, P. S., Hébert, A.-J., Starr, M. 2026. Acidification and CO2-Driven Conditions in the Estuary and Gulf of St. Lawrence During 2024. Can. Tech. Rep. Hydrogr. Ocean. Sci. 410 : vi + 71 p.