Council on Ocean Affairs, Science and Technology

Ocean Warming, Acidification, Purple Urchins and Bull Kelp

Marine Science


Increasing levels of carbon dioxide (CO2) in the atmosphere from anthropogenic greenhouse gas emissions are raising global temperatures and changing ocean chemistry. Ocean waters are becoming more acidic (increased acidity of seawater is represented by lower pH values), which presents challenges for marine organisms, especially those that build their skeletons and shells out of calcium carbonate (CaCO3). These organisms may have to expend significant amounts of energy to cope with increased water temperatures and ocean acidification, leaving them with less energy for growth, feeding and reproduction. Over time, this can lead to population decreases and subsequent shifts in ecosystem dynamics.

In Northern California, bull kelp is the foundation of highly productive kelp forest ecosystems, providing food and habitat for many economically important marine species and creating hot spots of marine biodiversity. Kelps can affect the chemistry of the seawater around them through photosynthesis and may even be able to reduce the acidity of the water surrounding them. Urchins, one of the major grazers of kelp, have calcareous shells that are sensitive to changes in seawater pH. Recently, Northern California bull kelp forests have been rapidly reduced to unproductive urchin barrens by a multitude of climate-driven events and widespread disease that wiped out many sea stars, the urchins’ main predator. Humboldt State University faculty members Dr. Paul Bourdeau, assistant professor of biological sciences, Dr. Brian Tissot, professor of biological sciences and director of the HSU marine laboratory, and Dr. Eric Bjorkstedt, adjunct professor of fisheries biology, are working together to better understand the dynamic interactions among ocean acidification, bull kelp and purple urchins.

HSU graduate and undergraduate students play a large role in this research. In summer 2018, graduate student Kindall Murie and three HSU undergraduate dive assistants completed 134 dives to characterize spatial variation in kelp and urchin abundance and collected more than 300 water samples in and out of kelp forests at two locations in Northern California. Data collected from this project will form the basis for Murie’s master’s thesis.

The team also has developed a multistressor laboratory mesocosm system that can simultaneously manipulate pH, dissolved oxygen (DO) and temperature to mimic current and predicted future ocean conditions to determine how they may affect urchin grazing on bull kelp. This design and implementation of this mesocosm system has been logistically difficult but has provided HSU graduate and undergraduate students the opportunity to gain valuable experience in circuit schematics and building, computer program coding and experimental design.

Preliminary results indicate that bull kelp can ameliorate local conditions. Even in small, remnant stands of bull kelp, daytime pH and DO were higher in kelp canopies, where photosynthesis primarily occurs, than near the seafloor or in nearby urchin barrens where no kelp was present. These differences were as great as they are in and around large persistent kelp forests off the coast of Washington, demonstrating that even small fragments of bull kelp forest may provide potential refuge from ocean acidification for marine species living in the kelp canopy. This information can help natural resource managers prioritize areas for conservation and restoration efforts to mitigate the local impacts of climate change.