Flow-driven micro-scale pH variability affects the physiology of corals and coralline algae under ocean acidification

Area of Science:

• Marine Biology
• Oceanography
• Climate Change Science

Background:

• The diffusive boundary layer (DBL) surrounding marine calcifiers can influence their response to ocean acidification (OA).
• Natural pH variability within the DBL may be a critical factor in coral and coralline algae resilience to OA.

Purpose of the Study:

• To investigate how flow velocity, pH, and light intensity interact to affect DBL conditions and the physiological responses of corals and coralline algae to OA.
• To determine if low flow conditions can create a refuge from OA for these organisms.

Main Methods:

• A 27-week experiment using 12 flumes with controlled flow velocities, pH, and light.
• Two coral species with different morphologies and one coralline alga were studied.
• Geochemical proxies, physiological measurements, and micro-probe analyses were employed to assess DBL conditions and organismal responses.

Main Results:

• Low flow velocity did not mitigate the negative impacts of low pH on calcification for all species.
• Flow velocity exhibited species-specific effects, enhancing calcification in two species.
• Low pH in the calcifying fluid (pH_cf) was observed under low flow and low light conditions for corals, and pH_cf was significantly influenced by DBL pH in species capable of modifying it.

Conclusions:

• Sub-tropical coralline algae face severe OA impacts in low light and low flow habitats.
• Certain coral species are also more vulnerable to OA in low flow environments.
• The study underscores the context-dependent nature of OA effects, varying significantly with habitat conditions and species composition.