Background/Question/Methods
Increasing global concentrations of atmospheric CO₂ are predicted to decrease ocean pH by increasing production of carbonic acid, thereby potentially impacting marine food webs severely, but empirical data documenting ocean pH dynamics over time are limited and in situ studies of ecosystem response are lacking. Starting in 2000, we measured ocean pH and other key physical parameters at 30 minute intervals with a Hydrolab multiprobe during spring and summer at Tatoosh Island, Washington. We constructed a mechanistic regression model relating changes in ocean pH to relevant physical and biological variables (surface atmospheric CO₂, water temperature, upwelling, time of day, regional phytoplankton chlorophyll, salinity, Pacific Decadal Oscillation index). Using our long-term dataset on intertidal community dynamics, we examined annual variation in transitions among species to probe impacts of pH change, and constructed Markov Chain models with these transitions to predict ecosystem response to reduced pH. 
Results/Conclusions

Ocean pH varied substantially over the study period.  Our model explained 70% of the variation in pH, which declined significantly with increasing atmospheric CO₂ and upwelling, increased with temperature and regional chlorophyll, and varied with the diurnal photosynthesis/respiration cycle.  The rate of decline in pH documented in our data, 0.045 units per year, is an order of magnitude faster than predicted by current models of pH change based on physical processes.  Patterns of transitions among sessile species varied significantly among years and were associated with changes in pH.  In general, transitions involving calcifying species indicated reduced performance, whereas those involving non-calcifying taxa indicated enhanced performance.  Markov Chain models predict that reduced pH will cause a shift in ecosystem structure, reducing dominance of calcareous mussels and goose barnacles, and increasing abundance of calcareous acorn barnacles and non-calcareous fleshy algae.  Hence, reduced calcification plays a role ecosystem response, but the complex web of species interactions complicates the impacts.  Our results indicate that ocean pH decline is ongoing and may be occurring at a faster rate than previously suspected, that such changes have ecological consequences, that models of ocean pH should integrate biological processes, and that more comprehensive data collection and analysis is needed to adequately understand ocean pH dynamics and their consequences.