ALBERT

Description: Ocean acidification (OA) is generally assumed to negatively impact calcification rates of marine organisms. At a local scale however, biological activity of macrophytes may generate pH fluctuations with rates of change that are orders of magnitude larger than the long-term trend predicted for the open ocean. These fluctuations may in turn impact benthic calcifiers in the vicinity. Combining laboratory, mesocosm and field studies, such interactions between OA, the brown alga Fucus vesiculosus, the sea grass Zostera marina and the blue mussel Mytilus edulis were investigated at spatial scales from decimetres to 100s of meters in the western Baltic. Macrophytes increased the overall mean pH of the habitat by up to 0.3 units relative to macrophyte-free, but otherwise similar, habitats and imposed diurnal pH fluctuations with amplitudes ranging from 0.3 to more than 1 pH unit. These amplitudes and their impact on mussel calcification tended to increase with increasing macrophyte biomass to bulk water ratio. At the laboratory and mesocosm scales, biogenic pH fluctuations allowed mussels to maintain calcification even under acidified conditions by shifting most of their calcification activity into the daytime when biogenic fluctuations caused by macrophyte activity offered temporal refuge from OA stress. In natural habitats with a low biomass to water body ratio, the impact of biogenic pH fluctuations on mean calcification rates of M. edulis was less pronounced. Thus, in dense algae or seagrass habitats, macrophytes may mitigate OA impact on mussel calcification by raising mean pH and providing temporal refuge from acidification stress.

Description: Numerous studies have been conducted on the effect of ocean acidification on calcifiers inhabiting nearshore benthic habitats, such as the blue mussel Mytilus edulis. The majority of these experiments was performed under stable CO2 partial pressure (pCO2), carbonate chemistry and oxygen (O2) levels, reflecting present or expected future open ocean conditions. Consequently, levels and variations occurring in coastal habitats, due to biotic and abiotic processes, were mostly neglected, even though these variations largely override global long-term trends. To highlight this hiatus and guide future research, state-of-the-art technologies were deployed to obtain high-resolution time series of pCO2 and [O2] on a mussel patch within a Zostera marina seagrass bed, in Kiel Bay (western Baltic Sea) in August and September 2013. Combining the in situ data with results of discrete sample measurements, a full seawater carbonate chemistry was derived using statistical models. An average pCO2 more than 50 % (~ 640 µatm) higher than current atmospheric levels was found right above the mussel patch. Diel amplitudes of pCO2 were large: 765 ± 310 (mean ± SD). Corrosive conditions for calcium carbonates (Ωarag and Ωcalc 〈 1) centered on sunrise were found, but the investigated habitat never experienced hypoxia throughout the study period. It is estimated that mussels experience conditions limiting calcification for 12–15 h per day, based on a regional calcium carbonate concentration physiological threshold. Our findings call for more extensive experiments on the impact of fluctuating corrosive conditions on mussels. We also stress the complexity of the interpretation of carbonate chemistry time series data in such dynamic coastal environments.

Description: It has been speculated that macrophytes beds might act as a refuge for calcifiers from ocean acidification. In the shallow nearshores of the western Kiel Bay (Baltic Sea), mussel and seagrass beds are interlacing, forming a mosaic habitat. Naturally, the diverse physiological activities of seagrasses and mussels are affected by seawater carbonate chemistry and they locally modify it in return. Calcification by shellfishes is sensitive to seawater acidity; therefore the photosynthetic activity of seagrasses in confined shallow waters creates favorable chemical conditions to calcification at daytime but turn the habitat less favorable or even corrosive to shells at night. In contrast, mussel respiration releases CO2, turning the environment more favorable for photosynthesis by adjacent seagrasses. At the end of summer, these dynamics are altered by the invasion of high pCO2/low O2 coming from the deep water of the Bay. However, it is in summer that mussel spats settle on the leaves of seagrasses until migrating to the permanent habitat where they will grow adult. These early life phases (larvae/spats) are considered as most sensitive with regard to seawater acidity. So far, the dynamics of CO2 have never been continuously measured during this key period of the year, mostly due to the technological limitations. In this project we used a combination of state-of-the-art technologies and discrete sampling to obtain high-resolution time-series of pCO2 and O2 at the interface between a seagrass and a mussel patch in Kiel Bay in August and September 2013. From these, we derive the entire carbonate chemistry using statistical models. We found the monthly average pCO2 more than 50 % (approx. 640 μatm for August and September) above atmospheric equilibrium right above the mussel patch together with large diel variations of pCO2 within 24 h: 887 ± 331 μatm in August and 742 ± 281 μatm in September (mean ± SD). We observed important daily corrosiveness for calcium carbonates (Ωarag and Ωcalc 〈 1) centered on sunrise. On the positive side, the investigated habitat never suffered from hypoxia during the study period. We emphasize the need for more experiments on the impact of these acidic conditions on (juvenile) mussels with a focus on the distinct day-night variations observed.

Description: 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.

Description: The impact of ocean acidification on benthic habitats is a major preoccupation of the scientific community. However, the natural variability of pCO2 and pH in those habitats remains understudied, especially in temperate areas. In this study we investigated temporal variations of the carbonate system in nearshore macrophyte meadows of the western Baltic Sea. These are key benthic ecosystems, providing spawning and nursery areas as well as food to numerous commercially important species. In situ pCO2, pH (total scale), salinity and PAR irradiance were measured with a continuous recording sensor package dropped in a shallow macrophyte meadow (Eckernförde bay, western Baltic Sea) during three different weeks in July (pCO2 and PAR only), August and September 2011.The mean (± SD) pCO2 in July was 383±117 µatm. The mean (± SD) pCO2 and pHtot in August were 239±20 µatm and 8.22±0.1, respectively. The mean (± SD) pCO2 and pHtot in September were 1082±711 µatm and 7.83±0.40, respectively. Daily variations of pCO2 due to photosynthesis and respiration (difference between daily maximum and minimum) were of the same order of magnitude: 281±88 µatm, 219±89 μatm and 1488±574 µatm in July, August and September respectively. The observed variations of pCO2 were explained through a statistical model considering wind direction and speed together with PAR irradiance. At a time scale of days to weeks, local upwelling of elevated pCO2 water masses with offshore winds drives the variation. Within days, primary production is responsible. The results demonstrate the high variability of the carbonate system in nearshore macrophyte meadows depending on meteorology and biological activities. We highlight the need to incorporate these variations in future pCO2 scenarios and experimental designs for nearshore habitats.

Description: Mussel and seagrass beds are characteristic of the shallow nearshores of the western Baltic Sea, forming a mosaic of habitat. The diverse physiological activities of seagrasses and mussels are affected by seawater carbonate chemistry and in return locally modify it. Seagrass photosynthesis decreases seawater CO2 concentration at daytime and increases it at night through respiration. This dynamic creates very favorable chemical condition to calcification at daytime but turn the habitat corrosive to calcium carbonates at night. In contrast, mussel respiration releases CO2, with the potential of locally changing the carbonate chemistry, turning the environment more favorable for photosynthesis by adjacent seagrass. Mussel calcification has the potential for reducing alkalinity (uptake of CO32-) thereby increasing CO2 concentration in seawater and reciprocally once dead, due to the dissolution of the shells. To capture these interactions between habitats, we 1) used a combination of state-of-the-art technologies to create an in-situ high precision carbonate sensor suite and 2) deployed it in August and September 2013 at the interface between a seagrass and a mussel patch submitted to series of down- and upwelling events.

Description: Seaweeds are key species of the Baltic Sea benthic ecosystems. They are the substratum of numerous fouling epibionts like bryozoans and tubeworms. Several of these epibionts bear calcified structures and could be impacted by the high pCO2 events of the late summer upwellings in the Baltic nearshores. Those events are expected to increase in strength and duration with global change and ocean acidification. If calcifying epibionts are impacted by transient acidification as driven by upwelling events, their increasing prevalence could cause a shift of the fouling communities toward fleshy species. The aim of the present study was to test the sensitivity of selected seaweed macrofoulers to transient elevation of pCO2 in their natural microenvironment, i.e. the boundary layer covering the thallus surface of brown seaweeds. Fragments of the macroalga Fucus serratus bearing an epibiotic community composed of the calcifiers Spirorbis spirorbis (Annelida) and Electra pilosa (Bryozoa) and the non-calcifier Alcyonidium hirsutum (Bryozoa) were maintained for 30 days under three pCO2 conditions: natural 460±59 µatm, present-day upwelling1193±166 µatm and future upwelling 3150±446 µatm. Only the highest pCO2 caused a significant reduction of growth rates and settlement of S. spirorbis individuals. Additionally, S. spirorbis settled juveniles exhibited enhanced calcification of 40% during daylight hours compared to dark hours, possibly reflecting a day-night alternation of an acidification-modulating effect by algal photosynthesis as opposed to an acidification-enhancing effect of algal respiration. E. pilosa colonies showed significantly increased growth rates at intermediate pCO2 (1193 µatm) but no response to higher pCO2. No effect of acidification on A. hirsutum colonies growth rates was observed. The results suggest a remarkable resistance of the algal macro-epibionts to levels of acidification occurring at present day upwellings in the Baltic. Only extreme future upwelling conditions impacted the tubeworm S. spirorbis, but not the bryozoans.