Saturday June 25th: Hopkins Seaweed Survey by Mark Denny

ffeddersenBathymetry Survey

One hypothesis of ROXSI is that the drag on a lush carpet of benthic seaweeds could add substantially to the dissipation of wave energy on rocky shores. To approach that question, we needed to quantify the abundance of seaweeds at our experimental sites. Fortunately, last Saturday (June 25) was a beautiful day for surveying seaweeds: minimal swell and negligible wind. Our divers managed to complete video transects at both the Asilomar and China Rock sites. And the results were definitive! The seafloor at both sites is overrun with purple sea urchins, likely a result of the recent die-off of predatory sea stars, and the urchin stampede has browsed the seaweeds down to bare rock. It’s as barren as anyone has ever seen. So we can, with assurance, answer our question, at least for this year at these sites. No seaweeds, no seaweed-induced dissipation. Any energy dissipation we measure won’t be due to benthic algae!

A lot of urchins!