Johanna Vega-Sequeda1, Gwénaëlle Chaillou1, Fanny Noisette1
1Université du Québec, Canada
Coastal marine ecosystems exhibit large pH and carbonate chemistry variations, sometimes promoting corrosive conditions (Ω<1) superimposed to global ocean acidification (OA). These conditions challenge the growth and calcification of marine calcifiers like crustose coralline algae (CCA), key ecosystem engineers in rocky reefs. Natural pH and carbonate chemistry regimes in subarctic coastal zones of the Saint-Lawrence system are barely characterized, while likely affected by different sources of influence such as riverine acidic waters or intense metabolic activity. This study assessed environmental dynamics in two shallow rocky areas covered by CCA and compared CCA growth rates in these locations.
From summer to fall 2024, continuous (pH, temperature, practical salinity) and discrete measurements (alkalinity, DIC) of environmental variables were collected in Rimouski, and only continuous data were taken in Port-Cartier, Canada, to characterize natural fluctuations. In addition, CCA fragments (cf. Lithophyllum sp.) from Rimouski were placed in both places to compare in situ growth rates (thickness).
Preliminary results showed similar high pH dynamics in both sites, with daily amplitude extreme ranging from nearly 1 to 0.05 units. Higher daily values happened in Rimouski in August and in Port-Cartier in September. Seasonally, Rimouski reached low pH values around 7.3 (September) and 7.6 in Port-Cariter (July-August), exceeding projected open ocean pH for 2100. In Rimouski, Ω varied between extreme values around 1.0 unit, during the study, with undersaturation observed for several days in a row in September. The metabolism of productive shallow ecosystems likely drives these fluctuations. CCA growth rates were around 2.0 µm d-1 during warmer-productive months, surpassing those of similar cold-water species, focusing solely on summer growth.
These results suggest that this CCA can support a wide range of pH fluctuations and even corrosive conditions. This work underscores the need to understand dynamic coastal pH environments, and the local acclimatization of calcifiers, enhancing their resistance to global change. Especially which process allows them to maintain net growth under these conditions.
Biography
A marine biologist from Colombia, she is currently pursuing her PhD in Oceanography at the University of Quebec at Rimouski. With extensive experience in researching and monitoring diverse marine ecosystems—including seagrass beds, rocky shores, and coral reefs—she is committed to advancing knowledge in her field. Her current research focuses on the impacts of ocean acidification, aiming to understand how declining pH levels affect coastal ecosystems and their vulnerable organisms.