Award: OCE-1756208

Our project included an unprecedented manipulation of both temperature and CO2 levels in natural tide pools for up to 6 months to determine the impacts of two main perturbations of climate change (warming and ocean acidification) on coastal ecosystems. Using a factorial design -- in which pools were either unmanipulated, warmed, acidified, or both warmed + acidified -- we evaluated effects on the population sizes of marine species, community diversity, and ecosystem functions, including productivity. In total, we intensively studied 36 tide pool ecosystems, containing a total of >65 species of plants and animals, across 3 years (1 year pre-manipulations & 2 years of manipulations), 4 seasons, and during daytime and at night. Prior to the climate manipulations, we made several key discoveries related to our observations of changes in environmental conditions and species in these ecosystems over time. Multiple discoveries indicated the degree to which marine producers can mitigate climate change by modifying the marine environment (for example, absorbing excess CO2 and reducing acidification through photosynthesis). First, while we expected that variation in pH over time would be driven by the most dominant species (a marine seaweed), this hypothesis was only supported when the seaweed was studied in isolation but not when it was living amidst the natural tide pool community. Thus, dominance does not necessarily indicate impact. Second, we found major seasonal changes in pH dynamics, with greater potential for primary producers to reduce ocean acidification in the summer than in the winter. Third, our work highlighted that community respiration rates in coastal ecosystems differed dramatically between night and day due to variation in temperature and oxygen concentrations, with implications for how we measure ecosystem responses (such as productivity) to global change. Simulated climate change led to clear alterations in the physiology of species in the tide pools, while impacts on community diversity were less extreme. Specifically, we found that the most common shellfish in the pools (the blue mussel) developed thinner and weaker shells when exposed to acidified conditions, yet this effect appeared to be diminished when they were simultaneously exposed to warming. We also found that physiology of the most common seaweed in the pools (the Oregon pine seaweed) was altered by our climate manipulations, and these effects persisted after the manipulations were ended. Seaweeds exposed to higher temperatures and elevated CO2 concentrations had lower photosynthetic efficiency at low light levels and higher photosynthetic performance at high light levels than those in unmanipulated tide pools. Across the tide pools and two years of manipulations, we observed the greatest mortality in pools exposed to simulated climate change that were located in the hottest part of the habitat (on the higher shore), particularly when the experimental manipulation coincided with a marine heat wave along the U.S. West Coast. This suggests that some of these ecosystems are existing at their environmental stress limits, and that climatic disturbance can push conditions past these limits to drive mass mortality. This project was very much a team effort, which provided opportunities for participants to be involved in collaboration and training across all levels, including undergraduate and graduate students who participated in fieldwork and lab analyses as well as K-12 students who attended our education and outreach programs. Last Modified: 09/10/2022 Submitted by: Kristy J Kroeker