Dive Dry with Dr. Bill

#219: An Asian Beauty... Not!

A few months ago I was diving off the Empire Landing Quarry when I observed a slender, graceful beauty from Japan in the water along the boulder breakwater. As luck would have it, this was not my dream dive buddy. In fact, it was a menace newly arrived on our shores. I'm referring to the potentially "deadly" (although not to humans) brown alga known scientifically as Sargassum filicinum. Because it comes from Asia, it has no English common name that I could uncover through my research, and unfortunately I'm not fluent in any of the Asian languages.

This alga is native to Japan and Korea, but was discovered in Long Beach in 2003. Scientists assume it arrived on the West Coast due to transport on or in container or other ships bringing goods in from the exotic Far East. In April of this year it was discovered in the region of the Isthmus by Dr. Kathy Ann Miller of UC Berkeley. She and Dr. Jack Engle based at UCSB with The Tatman Foundation discovered several other Catalina locations for this invasive, weedy alga.

To date I have seen it at Twin Rocks, Blue Car Wreck and Garibaldi Reef in addition to Empire Landing. On dives this past weekend, I discovered it has also infested the Casino Point dive park. In several of these locations it forms very dense stands. Obviously this weedy exotic seaweed has spread quickly around our island. "Weedy" species are those which can quickly invade sites, especially ones that are ecologically disturbed, and rapidly reproduce and disperse. When I was vice president of the Catalina Conservancy, one of our biggest ecological problems on the island were the approximately 200 (out of about 600) plant species that were not native to the island. The effort to fight these required recognition of which plants could be controlled, and a prioritized approach to containing or eradicating them.

Recent arrivals like this seaweed often receive high priority for control since it may be possible to wipe them out before they gain a foothold. However, the success and rapid spread of this admittedly beautiful alga has resulted in a distribution from the West to the East End on the leeward coast of Catalina. In many of the sites I have observed, the dense thickets of this alga will be very difficult to remove, if eradication is possible at all. Such efforts will require the cooperation of many divers, and I'm sure the Catalina Conservancy Divers will be there to lend a hand.

Why do scientists like me worry about such invasions? First, this species has no natural controls since the snails and other critters which eat it in Japan and Korea most likely didn't book a boat ticket to our coast on the same vessel that brought it. Perhaps local invertebrates and fish will find it a palatable addition to their diet. So far the only interaction I've seen was a giant kelpfish which built its nest in this Sargassum. Second, when these invaders quickly spread in disturbed areas they may out compete our native seaweeds, including the giant kelp, and significantly alter our native species and ecosystems.

Scientists, by nature, create hypotheses which ask questions that can be tested using the scientific method to formulate theories. Now I am about as "hypothetical" and speculative as any scientist, so I'll offer a few of mine in hopes someone with more patience in conducting experiments will test them. I mentioned that weedy species often are successful in disturbed sites, locations which have experienced some alteration due to factors like storms, high temperatures or human influence. Let's see how disturbance may have affected this invasion.

One of my hypotheses is that the absence of giant kelp, caused by the extremely warm waters of last summer, is partially responsible for the quick spread of this invasive alga. The giant kelp canopy usually shades its understory, keeping light levels very low. When the giant kelp canopy disappears as it did this year, light at the bottom is more abundant and many different species can gather enough energy to photosynthesize, grow and reproduce. So perhaps the ecological disturbance caused by the very warm water has led to the conditions that gave the Sargassum filicinum its opportunity to spread.

My second hypothesis is that the extremely thick patches of S. filicinum will seriously affect which native algal species can grow in the future. These dense understory thickets almost completely cut off light reaching the substrate (aka the rocky bottom). The spores and reproductive stages of most algal species (not to mention the invertebrates) attach to the rocks, and require enough light so they can photosynthesize and grow. If the Sargassum cuts off light, then these native species may be out competed by it. Perhaps even the spores and young gametophytes (male and female "plants") of our giant kelp will be affected as well.

You can bet I'll be keeping a close eye on what develops over the next six months. If our giant kelp is itself shaded out, it may only grow at depths beyond those that Sargassum favors (about 40'). Perhaps once it establishes its surface canopy, the giant kelp may extend over, and shade the Sargassum in the shallower waters. Ecosystems are very dynamic and this will be an interesting issue to watch unfold. Perhaps scientists, the CCD and other divers will find effective ways to control this alien. Stay tuned for future reports by this newspaper's underwater correspondent... blub, blub, blub. In the meantime, I'll also keep my eyes peeled for that other slender, graceful beauty I hope to discover in our waters... or by traveling back to the fascinating (and warm) waters of SE Asia!

© 2006 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles" or "Gentle Giants: Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Close-up of Sargassum, dense thicket choking out other marine life;
plants in the 5-7' range swaying in the surge.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia