The choreography connecting kelp forests to the beach

Shorebirds forage in kelp wrack on a beach in California. CREDIT: JENNY DUGAN, UC SANTA BARBARA
Shorebirds forage in kelp wrack on a beach in California. Credit: Jenny Duggan, UC Santa Barbara

The Santa Barbara Channel’s kelp forests and its sandy beaches are intimately connected. Giant kelp, the foundation species of rocky reefs, serves as a major part of the beach food web as fronds of the giant seaweed break away from the forest and are transported to the beach. But the relationship goes deeper.

CPR Survey Research Fellow Dr Lawrence Sheppard from the Marine Biological Association (MBA) was part of an international team of scientists who published a paper in the Proceedings of the National Academy of Sciences. The team demonstrated that kelp forests can do more than supply food to tiny, hungry crustaceans living in the sand. They can also influence the dynamics of the sandy beach food web.

The study uncovers the role of synchrony (simultaneous action) in the beach food web, with broader implications as the climate shifts in ways that might change how linked ecosystems perform their functions.

“The amount of kelp on the reef changes through time in a way where the peaks and low points in abundance across several kelp forests are matched together,” said lead author Jonathan Walter, a senior researcher at the University of California, Davis, and its Center for Watershed Sciences. “That’s what we refer to as synchrony. It is related to the ability of systems to persist in the face of changing environmental conditions. A little asynchrony allows systems to be resistant to fluctuations and therefore more stable.”

Image credit: Monica Pessino, Ocean o' Graphics, UC Santa Barbara.
(1) Local and (2) regional kelp production provide wrack; (3) waves fragment and dislodge kelp, (4) beach width mediates the wrack retention; (5) wrack on beaches mediates predator (shorebird) abundance by subsidizing the intertidal prey community. Image credit: Monica Pessino, Ocean o’ Graphics, UC Santa Barbara.

Revealing synchrony’s role in these ecosystems fills a key knowledge gap in our understanding of the connection of reef and beach.

Though a natural and ubiquitous phenomenon, synchrony and its implications are not yet fully understood.

The research team sought to understand whether and how kelp wrack (detritus) could affect the beach ecosystem’s dynamics. For instance, how might species respond to the changing environment, and how resilient is the beach ecosystem to disturbances?

To address these questions, the study used long-term data from UCSB’s Santa Barbara Coastal Long Term Ecological Research site, which is supported by the National Science Foundation. The team’s model was built on a time series of wind, wave, wrack, and beach-width data at five sandy beaches over 11 years.

It revealed patterns of synchrony — where the abundance of kelp wrack on beaches could be explained by kelp abundance in the forest, wave action, and beach width fluctuating together. At the longest timescales, kelp forest biomass and beach width were the biggest drivers of kelp wrack on the beaches.

Importantly, the researchers found this synchrony crossed from ocean to shore. The abundance of predatory shorebirds, like sandpipers and plovers, lagged behind the deposition of wrack on beaches.

The study was funded by the Santa Barbara Coastal Long Term Ecological Research, National Science Foundation, McDonnell Foundation and Humboldt Foundation.

Read the full paper: 10.1073/pnas.2310052120