Award: OCE-1519401

Ocean acidification (OA) is a major threat to the persistence of many marine organisms, particularly those using CaCO3 to make body parts, such as shells, carapaces, and internal skeletons. These include oysters, clams, mussels, corals, crabs, sea urchins, sea stars, and calcifying marine plants. To date much evidence indicates that many of these taxa face reductions in abundance under levels of OA forecast for future decades. Both the ecological and commercial importance of most of these taxa demand that marine scientists carry out research to understand the ways in which calcifying taxa will respond to future levels of OA (because of time lags and the sheer volume of the ocean, increases are inevitable for many decades). In the funded research, we focused on the corallinaceous algae. The ecology of this algal group is relatively poorly understood, yet it is global in distribution and common on hard bottom substrata within the photic zone of the ocean. We were particularly interested in the community role of turfy coralline algae, which are ubiquitous in benthic hard-bottom habitats of the northern California Current System. Algal turfs form an important "understory" layer on the shore that lies beneath taller "canopy" algae, suggesting that potentially strong competitive or facilitative (negative or positive) effects may underlie patterns of abundance and diversity of algal-dominated communities. The upright and basal crust portions of turfy coralline algae have a high proportion of calcium carbonate in their thalli, and prior research has suggested that this morphological aspect can deter herbivory. Our research project asked three general questions. First, how important are coralline turfs to the structure and dynamics of rocky intertidal communities? Second, what is the influence of ocean acidification on turfy coralline algae? And third, what is the likely community and ecosystem effect of alteration of coralline abundance and diversity? We used four approaches to address these questions: (1) field observations at multiple rocky intertidal sites to quantify the distribution and abundance of corallines and their potential interactors, including other algae, surfgrass, and herbivores. (2) Field experiments to test the influence of competition, facilitation, and herbivory on coralline algae. (3) Testing the effect of current and future levels of OA on the performance of turfy coralline algae using an OA mesocosm built in part with NSF funds. We examined the effects of both constant and variable levels of CO2 inputs. And (4) determining how different species of erect coralline algae responded to elevated acidification. Salient results included: (1) coralline turfs are the primary settlement substrata for the dominant intertidal kelp Saccharina sessilis. Thus, coralline turfs facilitate the settlement of the kelp, even though this means that the kelp displaces the corallines settled upon and those thalli around them (kelp holdfasts occupy about 100 cm2 of rock/coralline-occupied space), (2) Field experiments testing the interaction between coralline turfs and Saccharina indicate that the turfs also facilitate survival of the adult kelp plants. This effect did not vary among sites, indicating independence from oceanographic or local physical conditions. (3) Mesocosm (lab) experiments indicated that the performance of Corallina vancouveriensis, one of the species common in coralline turfs is strongly influence by increasing CO2: calcification decreases while photosynthesis increases. The former response occurs because the calcite and aragonite saturation levels decrease with increasing CO2, making it increasingly difficult to form the internal skeleton. The latter is a direct response to CO2, which is the primary chemical used by all plants to produce new tissue. This effect occurs with both constant levels of elevated CO2 and varying levels ranging down to the extreme lows forecast for future decades. (4) Other mesocosm exp...