Dive Dry with Dr. Bill

#160: Written in Stone

I am a marine biologist and ecologist by training and by preference. I enjoy observing the many species of marine life, especially their interactions (you know, the old M & M thing... munching and mating). However, one cannot fully understand marine life just by observing the species themselves, or even their interactions. There are a number of other factors involved in a real study of their ecology. This includes the physical aspects of their environment whether it be water temperature, salinity, dissolved oxygen or the very nature of the substrate they attach to or walk on.

While diving Yellowtail Point just past the buildings at the Empire Landing Quarry, I became intrigued by the physical environment of the reef more than the life forms I observed. The geology here was different from that of many other dive sites I've visited on Scuba Luv's King Neptune dive boat the past two months. The reef itself was a scattered boulder field, and in that sense it was similar to other boulder reefs around the island. However, the boulders themselves were different.

First, there did not appear to be the wealth of marine life living on the boulders that is often seen elsewhere. There was relatively little algae or seaweed growing on these rocks which was somewhat unusual. When I did see it growing, it appeared to be limited to a single type of brown algae and only grew on the flat upper surfaces of these boulders. I didn't see the giant keyhole limpets and other species from sites like the two quarries or Ship Rock. What I did find was an abundance of barnacles growing on top of the rock. These filter-feeding crustaceans; relatives of the lobster, crab and shrimp; were very prominent. Perhaps they kept the rock surface clear of other life. But I had to wonder why they dominated these rocks.

Second, the boulders were very unusual in that they came from metamorphic rock that apparently originated as sedimentary rock. You could easily see the many layers in which the sand or silt or mud was laid down by sediment falling out of the water column to the bottom. The layered sedimentary rock was then subjected to metamorphosis due to increased pressure and possibly temperature. However the layers here were not contorted as might be expected from violent geologic activity such as volcanic events or plate tectonics. The layers were too well ordered for that.

In my Harvard days, the basic geology course was laughingly referred to as "Rocks for Jocks." In other words, it was a "gut" (easy) course that football players and other athletes took for an easy passing grade. I skipped it and took more advanced courses in geology like invertebrate paleontology (fossils). Later, whether at the Toyon school or the Conservancy, I've always had someone on staff who was far more conversant in geology than I am. I always relied on them to supply the answers I needed regarding rocks and geologic history of the island. Heck, even my first real girlfriend here on Catalina was a geologist (now a petroleum geologist making the big bucks from what we pay the oil companies for our gasoline). Now I am all by myself (sniff) and have no knowing geologist to consult. That is all to say I can't tell you, my readers, just what kind of rocks these were!

The message in this column is that rock types can affect the biological species in an underwater habitat just as they affect the plant species on land. Catalina is a complex geologic formation. There is the 20 million year old quartz diorite porphyry that dominates the Avalon end of the island, the metamorphics from plate tectonic activity that dominate the region around the Airport and extend to the Empire Landing Quarry area, the 100+ million year old metamorphosed sedimentary rocks that dominate the West End, and volcanic rocks at various locations around the island. Each of these rock types, and the subcategories within them, have different characteristics relative to hardness, porosity, mineral content, etc. These differences can be enough to alter the type of marine ecosystem that emerges in their presence. So, in some senses the marine life of a dive site may be "written in stone."

© 2005 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. Buy my "Munching and Mating in the Macrocystis" DVD so you can take Dr. Bill home with you... we'll both be glad you did!

Unusual boulders forming the reef at Yellowtail Point.

This document maintained by Dr. Bill Bushing.
Material and images © 2005 Star Thrower Educational Multimedia