Anthropogenic CO2 emissions are expected to increase ocean acidity and temperature over the coming century. Under the Shared Socioeconomic Pathway (SSP) 5–8.5 scenario, mean surface ocean pH is projected to decrease by approximately 0.39 units, while sea surface temperature is expected to rise by 2–4 °C between 2010 and 2100. Sea pens (Octocorallia) are important bioindicators of soft-sediment habitat quality and associated benthic communities; however, their responses to these stressors remain considerably less studied than those of their hexacoral counterparts.
This thesis investigates the individual and combined effects of ocean acidification and warming, based on SSP5–8.5 projections, on the sea pen Pennatula phosphorea (Linnaeus, 1758), collected from the Swedish Gullmarsfjord. During winter and summer 2025, four pH treatments (8.0, control; 7.8, present-day low value; 7.4, projected low value for 2100; and 7.0, extreme scenario) and four temperature treatments (7.7 °C, winter control; 14 °C, summer control; 17 °C, present-day high value; and 20 °C, projected high value for 2100) were assessed. Response variables included behavioural traits (colony burrowing, colony inflation, polyp opening and bioluminescence emission), survival, and bioluminescence substrate levels (coelenterazine concentration and maximum light intensity, Lmax). Three hypotheses were tested: (1) ocean acidification and warming would negatively affect all response variables, with stronger effects under combined exposure; (2) prolonged exposure would result in either progressive deterioration or acclimation of behavioural traits; and (3) larger individuals would be less affected by stressors than smaller individuals.
The effects of acidification and warming varied among response variables, seasons, and whether stressors were applied individually or in combination. Overall, pH 7.8 produced few significant effects, except on burrowing behaviour, whereas pH 7.4 and 7.0 generated effects ranging from negligible (p > 0.1) to highly significant (p < 0.01), generally with greater impacts at pH 7.0. Similarly, temperatures of 17 and 20 °C elicited responses ranging from negligible to highly significant, with the strongest effects observed at 20 °C for quantitative measures of bioluminescence substrates. Combined exposure generally amplified the observed effects, suggesting that tolerance to one stressor may be compromised by the presence of another. No consistent evidence of behavioural acclimation or progressive deterioration was detected over time. Contrary to the initial hypothesis, larger individuals appeared more susceptible to environmental stressors than smaller individuals.
Craciun C., 2026. Combined effect of ocean warming and acidification on the sea pen “Pennatula phosphorea”. MSc thesis, University of Louvain. Thesis (restricted access).
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