The ecological effect of CO2 on the brown algae Fucus serratus and its epibionts: From the habitat to the organismic scale.
Carbon dioxide plays a central role in the functioning of organisms and ecosystems. For autotrophs, it is the substrate for photosynthesis while for heterotrophs it is a waste product of respiration. For two centuries Human activities, are responsible for an increase from 280 to 380 μatm of the atmospheric pCO2. A further increase up to 1000 μatm is predicted for the 21th century. The ocean surface and the atmosphere are at the equilibrium for CO2. The CO2 dissolving in seawater reduces the pH and increase of corrosiveness of water for shells and skeletons made of calcium carbonates. Thus, this process of ocean acidification is expected to have detrimental effects on calcifying organism. On the contrary, marine autotrophs are supposed to (slightly) benefit from this extra supply of CO2. In this thesis, we aimed at assessing the influence of CO2 on members of the nearshore macrophytes meadows of the Baltic Sea, an ecosystem naturally exposed to elevated water acidity. In a first part, we investigated the natural variations of the carbonate system in a meadow during three weeks of July, August, and September 2011 in a sheltered bay of the Western Baltic. We observed important day night dynamics together with wider scale variations (days to weeks) of magnitude exceeding future climate change predictions. We were able to explain the variations by the action of light and wind speed and direction. Light drives the uptake and release of carbon by photosynthesis and respiration of the meadow and wind influences the upwelling of offshore hypercapnic seawater. In a second part, we investigated the growth response to elevated pCO2 of one of the main primary producer of the meadows, the brown algae Fucus serratus, in laboratory experiments. The algae were incubated under ambient pCO2, actual upwelling pCO2 and future upwelling pCO2. We observed an increase of growth of 20 % at the pCO2 expected for the year 2100 and up to 50 % at pCO2 possibly occurring during future upwelling events (4000 μatm). However, the effect was transient and a limitation of growth by nutrients occurred after about 20 days. In the third part, we tested the effect of at the same three pCO2 on the growth and recruitment of the main members of the sessile associated communities of Fucus serratus: the calcifying and non-calcifying bryozoan Electra pilosa and Alcyonidium hirsutum and the calcifying tubeworm Spirorbis spirorbis. We tested the hypothesis of greater sensitivity of calcifyers to acidification and found a resistance of all the tested organisms to the future ambient pCO2. In contrast, at the highest pCO2 tested (future upwelling), we observed in Spirorbis severe shell corrosion, reduction of growth and collapse of recruitment. The growth rates of the worm settlings were assessed at light and dark under the three experimental pCO2. A 40 % enhancement of growth was observed at light at any pCO2, possibly due to the algal photosynthetic reduction of pCO2 / increase of pH in the boundary layer surrounding the algal thallus. Our study illustrates the possibility of facilitation between species to resist ocean acidification.