Award: OCE-1904184

Rocky shorelines can be highly productive habitats, with algae, microbes, and animals making their homes on the wave-swept rocks. The growth of photosynthesizing algae and microbes is affected by numerous factors, such as the supply of nutrients from the ocean, the temperature of the water, and the effects of herbivorous animals that eat the algae and microbes. For our project, ?Collaborative Research: Context-dependency of top-down vs. bottom-up effects of herbivores on marine primary producers?, we carried out a series of experiments to measure how these three factors (nutrients, temperature, herbivores) affect algae and microbial growth under current-day conditions, and under potential future conditions produced by climate change. With the help of undergraduate and graduate student trainees from San Diego State University, San Jose State University, and UC Irvine, PI Luke Miller and his collaborators at UC Irvine, PIs Matt Bracken and Adam Martiny, carried out experiments in tide pools on the shore where we manipulated the amount of nutrients by adding nitrogen and phosphorus fertilizers, raised temperatures using battery-powered heaters, and raised or lowered herbivore grazer abundances. The addition of slow-release fertilizers allowed us to simulate what tide pools might be like if the coastal ocean is impacted by increased winds blowing along the coast that promote upwelling, a process that draws nutrient-rich waters up to the surface where they can wash onto the shoreline. By heating our experimental tide pools, we attempted to simulate what conditions might be like a few decades in the future when the coastal ocean warms due to climate change. This approach required the development of a new electronic heating system that would turn on tide pool heaters during the daily low tide period and turn them off during high tide when the pools were washed by waves. Through the efforts of our trainees, the batteries for these heaters were changed daily for four weeks during the experiment. Finally, by changing the numbers of grazing herbivorous snails and limpets in the pools, we could simulate the impact that the loss or addition of extra grazing animals might have on algae and microbial communities in the future. To measure the impact of these experimental changes, we measured the growth and identity of algae and microbial species in response to these changes, as well as the levels of nutrients retained in the pools, the temperatures in the pools, and the oxygen production and consumption in the pools. We have found that the types of microbial species present in the tide pools changed in response to the addition of nutrients, but that this effect is enhanced when herbivore grazers are reduced in number. The effect of a small increase in temperature (~1.5°C) does not appear to change the abundance or identity of the microbial community to a great degree, but further work remains to identify potential effects. The current results and future results stemming from the ongoing analysis of this work will be an important contribution to shoreline ecology and climate change studies, as this work will help identify which types of species are most impacted, for better or worse, by changes in the coastal ocean and the kinds of organisms that live there. During the course of this project, we trained four undergraduate and two graduate students in field experimental techniques, and provided opportunities for those students to gather their own data alongside the experiment. We incorporated data and results from this project into classroom teaching at San Jose State University and San Diego State University, as well as in seminars delivered to scientific audiences. Last Modified: 01/27/2021 Submitted by: Luke P Miller