Grazing rates and body mass of Calanus finmarchicus and Calanus glacialis incubated under elevated pCO2

It is currently under debate whether organisms that regulate their acid-base status under environmental hypercapnia demand additional energy. This could impair animal fitness, but might be compensated for via increased ingestion rates when food is available. No data are yet available for dominant Calanus spp. from boreal and Arctic waters. To fill this gap, we incubated C. glacialis at 390, 1120 and 3000 µatm for 16 days with Thalassiosira weissflogii (diatom) as food source on-board RV Polarstern in Fram Strait in 2012. Every four days copepods were sub-sampled from all CO2 treatments and clearance and ingestion rates were determined. During the SOPRAN mesocosm experiment in Bergen, Norway, 2011, we weekly collected C. finmarchicus from mesocosms initially adjusted to 390 and 3000 µatm CO2 and measured grazing at low and high pCO2. In addition, copepods were deep frozen for body mass analyses. Elevated pCO2 did not directly affect grazing activities and body mass, suggesting that the copepods did not have additional energy demands for coping with acidification, neither during long-term exposure nor after immediate changes in pCO2. Shifts in seawater pH thus do not seem to challenge these copepod species.

Physiological responses and scope for growth upon medium-term exposure to the combined effects of ocean acidification and temperature in a subtidal scavenger Nassarius conoidalis

Physiological responses (ingestion rate, absorption rate and efficiency, respiration, rate, excretion rate) and scope for growth of a subtidal scavenging gastropod Nassarius conoidalis under the combined effects of ocean acidification (pCO2 levels: 380, 950, 1250 µatm) and temperature (15, 30 °C) were investigated for 31 days. There was a significant reduction in all the physiological rates and scope for growth following short-term exposure (1-3 days) to elevated pCO2 except absorption efficiency at 15 °C and 30 °C, and respiration rate and excretion rate at 15 °C. The percentage change in the physiological rates ranged from 0% to 90% at 15 °C and from 0% to 73% at 30 °C when pCO2 was increased from 380 µatm to 1250 µatm. The effect of pCO2 on the physiological rates was enhanced at high temperature for ingestion, absorption, respiration and excretion. When the exposure period was extended to 31 days, the effect of pCO2 was significant on the ingestion rate only. All the physiological rates remained unchanged when temperature increased from 24 °C to 30 °C but the rates at 15 °C were significantly lower, irrespective of the duration of exposure. Our data suggested that a medium-term exposure to ocean acidification has no effect on the energetics of N. conoidalis. Nevertheless, the situation may be complicated by a longer term of exposure and/or a reduction in salinity in a warming world.

Biomineralization changes with food supply confer juvenile scallops (Argopecten purpuratus) resistance to ocean acidification

Future ocean acidification (OA) will affect physiological traits of marine species, with calcifying species being particularly vulnerable. As OA entails high energy demands, particularly during the rapid juvenile growth phase, food supply may play a key role in the response of marine organisms to OA. We experimentally evaluated the role of food supply in modulating physiological responses and biomineralization processes in juveniles of the Chilean scallop, Argopecten purpuratus, that were exposed to control (pH 8.0) and low pH (pH 7.6) conditions using three food supply treatments (high, intermediate, and low). We found that pH and food levels had additive effects on the physiological response of the juvenile scallops. Metabolic rates, shell growth, net calcification, and ingestion rates increased significantly at low pH conditions, independent of food. These physiological responses increased significantly in organisms exposed to intermediate and high levels of food supply. Hence, food supply seems to play a major role modulating organismal response by providing the energetic means to bolster the physiological response of OA stress. On the contrary, the relative expression of chitin synthase, a functional molecule for biomineralization, increased significantly in scallops exposed to low food supply and low pH, which resulted in a thicker periostracum enriched with chitin polysaccharides. Under reduced food and low pH conditions, the adaptive organismal response was to trade-off growth for the expression of biomineralization molecules and altering of the organic composition of shell periostracum, suggesting that the future performance of these calcifiers will depend on the trajectories of both OA and food supply. Thus, incorporating a suite of traits and multiple stressors in future studies of the adaptive organismal response may provide key insights on OA impacts on marine calcifiers.

Impact of medium-term exposure to elevated pCO2 levels on the physiological energetics of the mussel Mytilus chilensis

This study evaluated the impact of medium-term exposure to elevated pCO2 levels (750-1200 ppm) on the physiological processes of juvenile Mytilus chilensis mussels over a period of 70 d in a mesocosm system. Three equilibration tanks filled with filtered seawater were adjusted to three pCO2 levels: 380 (control), 750 and 1200 ppm by bubbling air or an air-CO2 mixture through the water. For the control, atmospheric air (with aprox. 380 ppm CO2) was bubbled into the tank; for the 750 and 1200 ppm treatments, dry air and pure CO2 were blended to each target concentration using mass flow controllers for air and CO2. No impact on feeding activity was observed at the beginning of the experiment, but a significant reduction in clearance rate was observed after 35 d of exposure to highly acidified seawater. Absorption rate and absorption efficiency were reduced at high pCO2 levels. In addition, oxygen uptake fell significantly under these conditions, indicating a metabolic depression. These physiological responses of the mussels resulted in a significant reduction of energy available for growth (scope for growth) with important consequences for the aquaculture of this species during medium-term exposure to acid conditions. The results of this study clearly indicate that high pCO2 levels in the seawater have a negative effect on the health of M. chilensis. Therefore, the predicted acidification of seawater associated with global climate change could be harmful to this ecologically and commercially important mussel.

Ocean acidification affects the phyto-zoo plankton trophic transfer efficiency

The critical role played by copepods in ocean ecology and biogeochemistry warrants an understanding of how these animals may respond to ocean acidification (OA). Whilst an appreciation of the potential direct effects of OA, due to elevated pCO2, on copepods is improving, little is known about the indirect impacts acting via bottom-up(food quality) effects. We assessed, for the first time, the chronic effects of direct and/or indirect exposures to elevated pCO2 on the behaviour, vital rates, chemical and biochemical stoichiometry of the calanoid copepod Acartia tonsa. Bottom-up effects of elevated pCO2 caused species-specific biochemical changes to the phytoplanktonic feed, which adversely affected copepod population structure and decreased recruitment by 30 %. The direct impact of elevated pCO2 caused gender-specific respiratory responses in A.tonsa adults, stimulating an enhanced respiration rate in males (> 2-fold), and a suppressed respiratory response in females when coupled with indirect elevated pCO2 exposures. Under the combined indirect+direct exposure, carbon trophic transfer efficiency from phytoplankton-to-zooplankton declined to < 50 % of control populations, with a commensurate decrease in recruitment. For the first time an explicit role was demonstrated for biochemical stoichiometry in shaping copepod trophic dynamics. The altered biochemical composition of the CO2-exposed prey affected the biochemical stoichiometry of the copepods, which could have ramifications for production of higher tropic levels, notably fisheries. Our work indicates that the control of phytoplankton and the support of higher trophic levels involving copepods have clear potential to be adversely affected under future OA scenarios.

Tolerance of juvenile Mytilus galloprovincialis to experimental seawater acidification

Coastal ocean acidification is expected to interfere with the physiology of marine bivalves. In this work, the effects of acidification on the physiology of juvenile mussels Mytilus galloprovincialis were tested by means of controlled CO2 perturbation experiments. The carbonate chemistry of natural (control) seawater was manipulated by injecting CO2 to attain 2 reduced pH levels: -0.3 and -0.6 pH units as compared with the control seawater. After 78 d of exposure, we found that the absorption efficiency and ammonium excretion rate of juveniles were inversely related to pH. Significant differences among treatments were not observed in clearance, ingestion and respiration rates. Coherently, the maximal scope for growth and tissue dry weight were observed in mussels exposed to the pH reduction delta pH=-0.6, suggesting that M. galloprovincialis could be tolerant to CO2 acidification, at least in the highly alkaline coastal waters of Ria Formosa (SW Portugal).