Award: OCE-1736891

Our project examined how a variety of factors - including herbivores, nutrients, and temperature - shape plant communities in tide pools and how these effects change across locations. Understanding these interactions and effects is critical because the focal communities (which we colloquially call "plants" but are really composed of a complex group of bacteria, algae, and other associated microorganisms) are poorly understood but are probably responsible for a substantial fraction of the productivity and nutrient transfer in coastal marine systems. Large seaweeds get a lot of credit, but they start their lives as microscopic members of the assemblages we studied, so the processes we studied - many of which were invisible to the naked eye - have ramifications for the large-scale structure of rocky shore communities. And microbes uniquely mediate many steps in nutrient cycling. We found that nutrient additions affect grazing by herbivores in northern California but not in southern California. We hypothesized that these differences were associated with temperature, predicting that experimental warming in northern California would disrupt the effects of nutrients on herbivory. This was the case: in contrast to unwarmed tide pools, where nutrient additions enhanced consumption of primary producers, there were no effects of herbivores on producers in warmed tide pools. We have also documented - in an unprecedented way - effects of herbivores, nutrients, and temperature on the complex "biofilm" assemblages that coat the rocks on the shore. These assemblages contain bacteria, microscopic algae (including the young forms of larger seaweeds) and young invertebrates and serve as starting point for overall community development. Quantifying these effects was a substantial undertaking, requiring collection and analyses of hundreds of samples, and provided a rarely-seen glimpse into the "black box" of these critical but seldom-assessed microbial biofilms on rocky shores. These assemblages are key conduits of nutrients and energy and likely participate in the majority of food chains involving grazing mollusks in these systems. We highlight key roles of herbivores in mediating bacterial diversity, nutrients in mediating algal diveristy, and effects of herbivores, nutrients, and warming on biofilm community composition. Our work supported trainees at all levels, including undergraduates, graduate students, postdoctoral researchers, and technicians, and trainees were involved in all aspects of the work. Results were disseminated in scientific publications (3 published papers thus far), at conferences, and in invited lectures, and they have been incorporated into undergraduate teaching in several courses. Our work also provided opportunities for public outreach, including numerous interactions with people at our publicly accessible field sites, QR codes attached to bolts near our experiments that directed people to a website, and examples of our experimental set-up incorporated into docent displays at the public entrance to Corona del Mar State Beach. Last Modified: 12/17/2021 Submitted by: Matthew E Bracken