Dive Dry with Dr. Bill

075: Where's the Lettuce?

I'm rapidly approaching my 800th logged dive since "leaving" the Conservancy 3 1/2 years ago. Two of my favorite dive buddies, Jenefer and Danielle, are due out to dive next weekend and will join me on that "landmark" dive. Some time this year I'll log the 1,000th dive since June 30, 2000, and by then I think I'll feel comfortable enough with all the underwater footage I've shot to start editing it into a commercial series on kelp forest ecology. After all, it's about time I start making some real money out of all this fun! Every month like clockwork the bank requests a large piece of me to pay off the mortgage, and prices in Vons seem to rise like the tide (but rarely fall).

The past few days we've been blessed with some really nice weather... great for warming up during my surface intervals after diving in the 57 degree water. Forecasts suggest it will continue through next weekend, making the diving and topside fun unseasonably nice. It will also be a good transition between our cold water kelp forests and the tropical waters and coral reefs of Belize and Honduras where I'll spend most of the next two months! Half of that time I'll be serving on board one of the Lindblad cruise ships, the other half I'll be diving on my own in places like Caye Caulker, Lighthouse Reef, and Roatan.

While diving our kelp forests, the interactions between species are reasonably well known to me. After 35 years here they should be! In addition to the nearly invisible plankton, the bases of the food chains here are quite evident. Huge "salads" of giant and elkhorn kelp abound as well as the thick turf of the smaller algae on the rocks. Any self-respecting vegetarian, be it invertebrate or fish, has an easy time of it here. And in turn, they become food for those preferring more "meaty" fare.

For several decades my fear of flying kept me from diving other countries including the tropics. Of course being a teacher at Toyon with a meager salary didn't help much either. While at the Conservancy, I never took vacations due to my commitment to the job. Once I cured that fear and "earned" my freedom from the office, I quickly began exploring tropical waters. I've been fortunate to dive SE Asia, Australia and the South Pacific while backpacking through those regions.

Several things immediately struck me about the tropics. The diversity of species and the beautiful colors that distinguish them was expected. The tremendous clarity of tropical waters, in part a result of low nutrient levels and plankton productivity, was also anticipated. What I didn't expect, was my reaction to the near absence of algae, large or small, in those waters. With relatively low plankton abundance and very little obvious vegetarian fare on the menu, what were so many different species feeding on?

Cold water kelp survives in areas where maximum temperatures generally don't exceed 68-70 degrees. Corals require minimum temperatures above that to survive. Therefore kelps are generally found on either side of the equator at temperate mid-latitudes while coral straddles the equatorial band. Of course if global warming is the reality I believe it is, some day Catalina waters may be more tropical and our giant kelp replaced by coral reefs. As our President says, we will adapt to climate changes (rather than controlling our greenhouse gas emissions as we should). Hmmm... his notion sounds more like evolutionary theory than what I'd expect from him.

So what are all those colorful tropical species munching on? One another of course, but there must be a base for any food chain. A group of organisms that can produce their own food... from the sun like photosynthetic land plants, algae and phytoplankton or by chemical means as bacteria and vent organisms do. Yet when one looks around the tropics, one sees mainly animals which feed on one another, and extensive coral reefs. We know where the beef is... where's the lettuce?

The key is something many of us learned in high school biology. Corals have symbiotic algae living within their tissues called zooxanthellae. Warm waters contain far fewer nutrients than cold waters. Since nutrients are limited in the tropics, these tiny algae utilize nutrients from the wastes produced by the coral polyps. In turn, they produce oxygen and food in the form of carbohydrates which the polyp can use. Now that's what I call working as a team.

Many tropical fish like parrotfish, wrasses and triggerfish have strong beak-like jaws and are known to "feed" on coral. They take large bites out of the reefs. However, recent information indicates that many of these fish have long digestive tracts which are typical of vegetarians rather than meat eaters. Many scientists believe they are algae feeders rather than digesting the coral polyps in their hard protective calcium carbonate skeletons. I believe they are feeding on the zooxanthellae within the coral polyps, perhaps in addition to the polyps themselves.

So one base of the food chains in the tropics may well be the tiny little algae embedded in the bodies of the coral animals. Quite a departure from the abundant 200 foot giant kelp that our opaleyes, halfmoon, snails and others fed on. I'll keep my eyes out while I'm in Belize and the Honduran Bay Islands to see if I can gather further evidence of this... or perhaps new information that will add depth to my embryonic knowledge of tropical coral reefs. After all, I'd better get a grip on these systems before the Dive Park fills with beautiful corals and we are warmed by the "winter" sun in a few decades!

© 2004 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.

Dr. Bill bridges the gap between the cold water kelps and the
warm water tropical corals (elegant and giant pictured here)!

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia