Years ago when I was Vice President of the Santa Catalina Island Conservancy, I was saddened to discover how little respect the island's plant life received from most people. When we tried to explain the importance of removing feral animals like pigs and goat from the island to ensure the survival of many of our unique plant species, many questioned why we would value plants over animals. When the Conservancy was formed by the Wrigley and Offield families, its primary stated goal was to preserve native species and ecosystems. Pigs and goats are not even native to the North American continent. They were brought here from Europe by early explorers including that genocidal "discoverer" of the New World, Christopher Columbus.
Plants are a critical element in the stability of any ecosystem. Being photosynthetic organisms, they capture sunlight and convert it into carbohydrates used not only for their own growth, but to nourish the herbivores which in turn feed the predators. For this reason they are called "primary producers." Without plants and their abundant energy source from our local star the Sun, our ecosystems would probably not be able to develop the diversity of life we see today. Of course not only do these primary producers create food for critters, they also create oxygen as a by-product of photosynthesis.
In marine ecosystems there are relatively few true plants. These include eelgrass (Zostera marina) and species of surfgrass (Phyllospadix). Both of these are true plants that have vascular systems to transport food and they produce flowers and seeds. Back when I first learned marine biology, algae (seaweeds) were also considered plants. However in the millennia between my first classes on the subject and today, seaweeds have been demoted. There is still much confusion about where they belong in our human classification scheme of living things, but many biologists place them in the Kingdom Protista. It is hard to consider a microscopic single celled amoeba and our giant kelp as relatives, but...
Last week I wrote about the various types of algae and how their pigments affect the depths they are usually found at. Algae or seaweeds are the main primary producers in marine environments, serving the same role as true plants in terrestrial ecosystems. They utilize photosynthesis to create food and oxygen. Without them our undersea habitats would be far less diverse and interesting. Here in our waters, the giant kelp forests demonstrate this very effectively. Although not as obvious on tropical coral reefs, algae are still very critical. The difference is that many of them are embedded within the coral polyps and so are less obvious to our eyes.
Since my readers are all highly intelligent (why else would they gaze upon my columns?), most of you know that our giant kelp (Macrocystis pyrifera) is limited in its depth range. In clear waters it may be found as deep as 130 ft, but in turbid murky waters where light doesn't penetrate as far it may not descend below 60 ft. Beyond these depths where shading from the giant kelp canopy is not present, we find a host of deeper water kelps. They include elkhorn or elk kelp (Pelagophycus porra), oarweed (Laminaria farlowii) and sea collander kelp (Agarum fimbriatum).
These three species (and others) are characterized by extremely large blades ("leaves") that may stretch tens of feet in length. At the depths they are found, light is much less intense than that captured by the giant kelp canopy near the surface. Therefore, the "solar collectors" used by these photosynthesizers need to be much larger to capture as much ambient sunlight as possible.
Laminaria and Agarum possess a single blade that generally rests along the ocean floor. There they are subject to getting covered by soft sediments such as sand and silt, which obviously affects their ability to photosynthesize. Elk kelp largely solves this problem by utilizing a float bulb or pneumatocyst to lift its blades off the bottom so they don't get inundated. In fact, elk kelp actually forms deep water kelp forests at times.
I have written frequently about the loss of giant kelp during the height of the extremely warm water episode last summer and fall. Many of my dives during that period took me down to 100 ft where these kelps usually dominate. It was sad to see that even at that depth they were affected by the warm water and had diminished noticeably.
In the past I've noticed large circular bite marks taken out of giant kelp and elkhorn kelp blades. Recently I sent pictures of these to Dr. Paul Dayton, noted kelp forest ecologist at Scripps in San Diego. He was surprised to see the images and passed them around to several of his colleagues. No one could figure out what critter was responsible. During our exchanges, we talked about what white abalone feed on and I mentioned the sea collander kelp (Agarum). Paul was surprised at that and informed me that this species possesses chemical defenses in the form of polyphenols just like the invasive Asian Sargassum does. I guess that explains why I don't remember seeing the strange bite marks on that kelp species. I'm still wondering why there are bite marks on the dinners I prepare!
© 2015 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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Sea collander kelp (Agarum) and oarweed (Laminaria); elkhorn kelp (Pelagophycus) and blade showing strange bite marks
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