Ocean acidification causes bleaching and productivity loss in coral reef builders.

Ocean acidification represents a key threat to coral reefs by reducing the calcification rate of framework builders. In addition, acidification is likely to affect the relationship between corals and their symbiotic dinoflagellates and the productivity of this association. However, little is known about how acidification impacts on the physiology of reef builders and how acidification interacts with warming. Here, we report on an 8-week study that compared bleaching, productivity, and calcification responses of crustose coralline algae (CCA) and branching (Acropora) and massive (Porites) coral species in response to acidification and warming. Using a 30-tank experimental system, we manipulated CO(2) levels to simulate doubling and three- to fourfold increases [Intergovernmental Panel on Climate Change (IPCC) projection categories IV and VI] relative to present-day levels under cool and warm scenarios. Results indicated that high CO(2) is a bleaching agent for corals and CCA under high irradiance, acting synergistically with warming to lower thermal bleaching thresholds. We propose that CO(2) induces bleaching via its impact on photoprotective mechanisms of the photosystems. Overall, acidification impacted more strongly on bleaching and productivity than on calcification. Interestingly, the intermediate, warm CO(2) scenario led to a 30% increase in productivity in Acropora, whereas high CO(2) lead to zero productivity in both corals. CCA were most sensitive to acidification, with high CO(2) leading to negative productivity and high rates of net dissolution. Our findings suggest that sensitive reef-building species such as CCA may be pushed beyond their thresholds for growth and survival within the next few decades whereas corals will show delayed and mixed responses.

Prey Capture by the Sea Anemone Metridium senile (L.): Effects of Body Size, Flow Regime, and Upstream Neighbors.

The sea anemone Metridium senile is a quantitatively important passive suspension feeder in hard-bottom communities on the west coast of Sweden and occurs in aggregations with different size distributions. This study tests the hypothesis that different polyp sizes have different optimal flow regimes maximizing prey capture. Results showed that prey capture by M. senile is a function of both flow regime and polyp size, and different optimal flow regimes exist for different size classes. Large anemones had a maximum feeding efficiency at the slowest flow, medium-sized anemones at moderate flow, and small anemones at moderate- to high-flow regimes. Small anemones showed consistently higher feeding rates (per unit of biomass and area of tentacle crown) at all velocities above 10 cm s-1 and exhibited less flow-induced deformation of the tentacle crown, suggesting that small anemones are better at feeding in moderate- to high-flow habitats. Different vertical projections of large and small anemones in the boundary layer could only partly account for differences in feeding success among size classes. Feeding rate was also a function of upstream conspecifics, declining asymptotically to 30% of the maximum rate. The negative effects of neighbors on feeding in aggregations with clonal rather than polyp growth appear to be compensated for by the generally higher feeding efficiency of small polyps.