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Dive Dry with Dr. Bill

#613: A Real Example of Biodiversity

I'm generally supportive of the California Dept. of Fish & Wildlife, although with respect to a number of regulations I don't think they go far enough to ensure healthy populations of fish and invertebrates in our waters. Although most abide by the rules, there are those who won't unless the consequences are high. I certainly hope none of my readers possess more than seven lobster at a time... including the ones in your freezer! Fortunately the ones in your stomach don't count against the possession limit!

Over my 45 years of diving Catalina waters, I have seen the decline of many species... but also a few positive signs such as the recovery of the giant sea bass and the white sea bass. I was very supportive of the Marine Life Protection Act (MLPA) and the marine protected areas (MPAs) it finally created throughout our State. I do take issue with the boundary of the Casino Point MPA since its outer limit is too close to shore to really create adequate protection for the lobster which are hoop netted legally at its boundary.

However, I have one major criticism of CDF&W, and that is with respect to its inaction on what I consider one of the worst ecological impacts on our coast in my many decades of diving here. Yes, I'm talking about the invasion of our waters by the Asian seaweed Sargassum horneri. In today's column I'm going to give a little explanation about why this exotic species is so devastating to our native ecosystems, especially during warm water episodes when our giant kelp (Macrocystis) forests are decimated.

As I've written previously, the warm water, low nutrients and hurricane surge have left the island's kelp forests from Long Point to the East end largely decimated. There is no longer any kelp canopy left in the dive park, or much kelp period. Near the harbor mouth a few kelp "plants" may be seen lying on the bottom and that's about it. With the kelp canopy gone, there no longer is anything blocking sunlight from reaching the bottom. Thanks to this, the Sargassum has exploded and is totally covering most of the rocky areas in the park.

When a single species comes to dominate an ecosystem like this, ecological biodiversity plummets. I'm proud to say that one of my favorite Harvard professors (E. O. Wilson) coined the word biodiversity back in 1988 although the concept itself has been around for a long time. Scientifically, biodiversity can be measured a number of ways. Perhaps the simplest way is to simply count the number of species present in a given ecosystem or region. We refer to this as "species richness."

Obviously when a single species like Sargassum comes to dominate an area, the number of algal species decreases. Of course other species of seaweed are present, so the species richness doesn't actually drop to one, but their numbers are very small. Biologists have also created more revealing measures of biodiversity that take into account the number of each species present in a system. Using these metrics, biodiversity in a system dominated by a single species (but containing small numbers of other species) becomes more critical.

Pete Vanags, a technical diver from the mainland came over to the island recently and asked if I'd dive with him. Of course I prefer to dive solo so no one sees me flailing around in the water, and most divers wouldn't be interested in the kind of experimentation I planned to do on my dives. Pete was willing to tackle the project and had a background in scientific diving.

Together we carefully removed all the non-native Sargassum from several rocky surfaces, leaving (hopefully) only the native species that were able to grow under its dense cover. I filmed before and after shots of these areas and the results were not unexpected... but I hope shocking enough to startle others into taking action against this invader. Once the Sargassum was removed, there were very few native algae growing on the rocks. In fact they appeared nearly bare. The biodiversity of our kelp forest ecosystem had greatly declined due to the appearance of this one non-native invader!

There were a few patches where more of the natives were present, but generally the Sargassum almost totally dominated the substrate. The sad thing about this is that the "seed" for the recovery of our giant kelp forests is the tiny microscopic stages of giant kelp on the rocks. These begin to grow when sunlight reaches them, such as when a storm clears out a patch of the mature kelp canopy. The same thing happens in terrestrial forests. However, when sunlight is blocked from reaching the microscopic kelp due to the dense Sargassum, they cannot grow.even when water temperatures cool and nutrients return.

Even when the microscopic stages of kelp begin to grow, or new fronds start up from a denuded holdfast, the Sargassum may put a stop to this and thereby to the recovery of the kelp forests. As the young kelp grows up toward the surface and penetrates through the Sargassum, the kelp often gets entangled in the exotic seaweed and its blades are ripped off. With few blades to conduct photosynthesis, growth of the individual kelp (and therefore recovery of the kelp forest) is slowed if not stopped altogether. CDF&W should be focusing effort on finding ways to control this invader so that the giant kelp can recover and our native seaweeds can retake the rocky bottom and return our ecosystems to healthy levels of biodiversity. This is important not just to ecologists like myself, but to divers who enjoy diving here and anglers who seek a kelp bass for dinner... but end up getting their line tangled in the Sargassum instead.

© 2014 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Test area dominated by Sargassum and after clearing the non-native; Sargassum-dominated landscape
and giant kelp (Macrocystis) entangled in Sargassum.

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia