Seawater carbonate chemistry, nutrients, chlorophyll and diatom Proboscia alata during experiments, 2012

The combined effects of different light and aqueous CO2 conditions were assessed for the Southern Ocean diatom Proboscia alata(Brightwell) Sundström in laboratory experiments. Selected culture conditions (light and CO2(aq)) were representative for the natural ranges in the modern Southern Ocean. Light conditions were 40 (low) and 240 (high) µmol photons/m**2/s. The three CO2(aq) conditions ranged from 8 to 34 µmol/kg CO2(aq) (equivalent to a pCO2 from 137 to 598 µatm, respectively). Clear morphological changes were induced by these different CO2(aq) conditions. Cells in low [CO2(aq)] formed spirals, while many cells in high [CO2(aq)] disintegrated. Cell size and volume were significantly affected by the different CO2(aq) concentrations. Increasing CO2(aq) concentrations led to an increase in particulate organic carbon concentrations per cell in the high light cultures, with exactly the opposite happening in the low light cultures. However, other parameters measured were not influenced by the range of CO2(aq) treatments. This included growth rates, chlorophyll aconcentration and photosynthetic yield (FV/FM). Different light treatments had a large effect on nutrient uptake. High light conditions caused an increased nutrient uptake rate compared to cells grown in low light conditions. Light and CO2 conditions co-determined in various ways the response of P. alata to changing environmental conditions. Overall P. alata appeared to be well adapted to the natural variability in light availability and CO2(aq) concentration of the modern Southern Ocean. Nevertheless, our results showed that P. alata is susceptible to future changes in inorganic carbon concentrations in the Southern Ocean.

Seawater carbonate chemistry and pteropod Limacina helicina antarctica shell dissolution during experiments, 2012

Anthropogenic ocean acidification is likely to have negative effects on marine calcifying organisms, such as shelled pteropods, by promoting dissolution of aragonite shells. Study of shell dissolution requires an accurate and sensitive method for assessing shell damage. Shell dissolution was induced through incubations in CO2 enriched seawater for between 4 and 14 days. We describe a procedure that allows the level of dissolution to be assessed and classified into three main types: Type I with partial dissolution of the prismatic layer; Type II with exposure of underlying crossed-lamellar layer, and Type III, where crossed-lamellar layer shows signs of dissolution. Levels of dissolution showed a good correspondence to the incubation conditions, with the most severe damage found in specimens held for 14 d in undersaturated condition (Ohm ~ 0.8). This methodology enables the response of small pelagic calcifiers to acidified conditions to be detected at an early stage, thus making pteropods a valuable bioindicator of future ocean acidification.

Seawater carbonate chemistry and physiological response of the Mediterranean crustose coralline alga Lithophyllum cabiochae to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO2 and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near-ambient conditions of irradiance, at ambient or elevated temperature (+3 °C), and at ambient (ca. 400 µatm) or elevated pCO2 (ca. 700 µatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO2 but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO2. Conversely, photosynthesis was strongly affected by pCO2 with a decline under elevated pCO2 in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO2. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO2 but decreased with rising temperature under elevated pCO2. Surprisingly, the highest rates in summer were found under elevated pCO2 and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO2. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO2. However, there is likely a trade-off with other physiological processes. For example, photosynthesis declines in response to increased pCO2 under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.

The influence of seawater pH on U/ Ca ratios in the scleractinian cold-water coral Lophelia pertusa

The increasing pCO2 in seawater is a serious threat for marine calcifiers and alters the biogeochemistry of the ocean. Therefore, the reconstruction of past-seawater properties and their impact on marine ecosystems is an important way to investigate the underlying mechanisms and to better constrain the effects of possible changes in the future ocean. Cold-water coral (CWC) ecosystems are biodiversity hotspots. Living close to aragonite undersaturation, these corals serve as living laboratories as well as archives to reconstruct the boundary conditions of their calcification under the carbonate system of the ocean. We investigated the reef-building CWC Lophelia pertusa as a recorder of intermediate ocean seawater pH. This species-specific field calibration is based on a unique sample set of live in situ collected L. pertusa and corresponding seawater samples. These data demonstrate that uranium speciation and skeletal incorporation for azooxanthellate scleractinian CWCs is pH dependent and can be reconstructed with an uncertainty of ±0.15. Our Lophelia U / Ca-pH calibration appears to be controlled by the high pH values and thus highlighting the need for future coral and seawater sampling to refine this relationship. However, this study recommends L. pertusa as a new archive for the reconstruction of intermediate water mass pH and hence may help to constrain tipping points for ecosystem dynamics and evolutionary characteristics in a changing ocean.

Seawater acidification by CO2 in a coastal lagoon environment: Effects on life history traits of juvenile mussels Mytilus galloprovincialis

The carbonate chemistry of seawater from the Ria Formosa lagoon was experimentally manipulated, by diffusing pure CO2, to attain two reduced pH levels, by -0.3 and -0.6 pH units, relative to unmanipulated seawater. After 84 days of exposure, no differences were detected in terms of growth (somatic or shell) or mortality of juvenile mussels Mytilus galloprovincialis. The naturally elevated total alkalinity of the seawater (= 3550 µmol/kg) prevented under-saturation of CaCO3, even under pCO2 values exceeding 4000 µatm, attenuating the detrimental effects on the carbonate supply-side. Even so, variations in shell weight showed that net calcification was reduced under elevated CO2 and reduced pH, although the magnitude and significance of this effect varied among size-classes. Most of the loss of shell material probably occurred as post-deposition dissolution in the internal aragonitic nacre layer. Our results show that, even when reared under extreme levels of CO2-induced acidification, juvenile M. galloprovincialis can continue to calcify and grow in this coastal lagoon environment. The complex responses of bivalves to ocean acidification suggest a large degree of interspecific and intraspecific variability in their sensitivity to this type of perturbation. Further research is needed to assess the generality of these patterns and to disentangle the relative contributions of acclimation to local variations in seawater chemistry and genetic adaptation.

Seawater carbonate chemistry and calcification of two mediterranean cold-water coral species in a laboratory experiment

Deep-water ecosystems are characterized by relatively low carbonate concentration values and, due to ocean acidification (OA), these habitats might be among the first to be exposed to undersaturated conditions in the forthcoming years. However, until now, very few studies have been conducted to test how cold-water coral (CWC) species react to such changes in the seawater chemistry. The present work aims to investigate the mid-term effect of decreased pH on calcification of the two branching CWC species most widely distributed in the Mediterranean, Lophelia pertusa and Madrepora oculata. No significant effects were observed in the skeletal growth rate, microdensity and porosity of both species after 6 months of exposure. However, while the calcification rate of M. oculata was similar for all colony fragments, a heterogeneous skeletal growth pattern was observed in L. pertusa, the younger nubbins showing higher growth rates than the older ones. A higher energy demand is expected in these young, fast-growing fragments and, therefore, a reduction in calcification might be noticed earlier during long-term exposure to acidified conditions.