Award: OCE-1904185

The combined effects of warming temperatures and intensifying ocean acidification have the potential to dramatically alter marine communities of algae and animals. Studying the effects of these two stressors (warmer temperature and declining pH) can be done in a variety of controlled laboratory settings with single species or groups of species, but learning about the impacts on whole communities of many species in a natural setting will ultimately be more informative. For this project, we worked in a natural marine system, shoreline tide pools, that could be experimentally manipulated during low tide. We began by monitoring water quality parameters (temperature, pH, alkalinity, dissolved oxygen) and the numbers and identity of more than 65 species of algae and animals across all four seasons in a set of 36 tide pools on a shoreline in southeastern Alaska, adjacent to the town of Sitka. During the first year we gathered baseline data before performing any experimental manipulations. In Years 2 and 3 we used a subset of the tidepools to warm the water and lower pH by bubbling CO2 into the tidepool in a factorial design, so that we had tide pools that were only warmed, pools that only had CO2 bubbled into them, pools that had both warming and CO2 bubbling, and a set of control tide pools with no temperature or CO2 manipulation. During two summer seasons, we maintained warming and CO2 bubbling treatments and carried out period surveys of the algae and animals in the tide pools, along with measuring their primary productivity and community respiration. When we made measurements in the tide pools before our experiments began, we found that algae could affect pH in these pools, but that when those algae were part of a diverse community their impact on pH was reduced, particularly in the spring and winter. During the long days of summer, algae could mitigate some of the effects of tide pool acidification, but shorter day lengths outside of summer resulted in less of an impact of those photosynthesizing algae. When we raised tide pool temperatures and lowered pH by bubbling CO2 into the pools, we found changes in some of the key species, but overall community diversity was not adversely affected. Blue mussels, which rely on a calcium carbonate shell for protection, tended to grow thinner, weaker shells when the water was acidified, but warming of the water could offset some of this response. For the photosynthesizing algae, warming and elevated CO2 (lower pH) tended to raise photosynthetic efficiency in pools with abundant light, but resulted in lower efficiency in pools with lower light levels. For both the animals and the algae, mortality was highest in tide pools that were higher on the shore, which will typically result in longer isolation from the ocean and warmer temperatures on hot days. The organisms living in those highest pools may already be experiencing stressful conditions under current-day weather conditions, and when we further increased temperature and CO2 concentrations, those communities experience greater mortality, or emigration of mobile animals. As part of this project, we created a number of open source electronic hardware tools that allowed us to carry out the manipulations, gather environmental data, and collect physiological data for animals in the tide pools. The designs and software for these products have been made freely available, along with publications detailing the process for developing or modifying these experimental tools. This project was a collaborative effort among faculty and students from three universities (UC Irvine, UC Santa Cruz, and San Diego State University), and allowed for cross-pollination among the different lab groups, which ultimately led to broader training experiences for our trainees. Our project team also participated in outreach efforts, particularly with the local Sitka AK community through the Sitka Sound Science Center. These experiences included putting together informational articles and laboratory teaching modules designed for high school and middle school students, both in person and online during the pandemic quarantines. Last Modified: 11/08/2022 Submitted by: Luke P Miller