Dive Dry with Dr. Bill

059: My Hypothesis on Poor Vis and Great Whites

This is one summer I would have preferred to be in Hawaii, Thailand or the Caribbean where the water is warm and clear. Not only did our local economy dip a bit, the diving conditions were the worst I can remember in 35 years of on-again-off-again diving on Catalina. Other long-term divers I've spoken with agree. If I didn't have to get my gills wet every few days, I might not have dived most of the summer. Of course I also have to keep my dive buddies happy, don't I ladies?

Local and visiting divers have asked me, as a marine biologist, why we're experiencing such poor visibility and winter-like temperatures at depth. On a recent dive I was lucky to see 8-12 feet ahead of me and the bottom temperature dipped to 55 degrees- and I wasn't that deep. This time of year we should be experiencing our warmest waters and good visibility. After months of watching this nonsense, I have formed a hypothesis that not only explains these conditions but also the increased sightings of great white sharks. Read on.

A scientific hypothesis is actually just a theory based on a set of observations in the "real" world. I'm referring to underwater, not the world of jobs and commerce... of which I obviously know little. A scientist formulates a theory to explain the causes of the observed events or conditions. My Funk & Wagnalls (yes, I actually own one) defines an hypothesis as "an unproved scientific conclusion drawn from known facts." "Let's look at the facts, ma'am," as Sergeant Joe Friday used to say.

The poor visibility ("vis" in diver terminology) appears to be caused by plant material due to the strong green color. Phytoplankton blooms are not uncommon along our coast in spring and early summer as daylength increases and nutrients from colder, deeper water mix near the surface. Plants including planktonic ones need sunlight and nutrients (marine plants already have plenty of water of course). Normally these blooms last a week or two, but this year they seem to have lasted forever. Why? Water temperatures were abnormally low earlier this summer. Colder water contains more nutrients than warmer water. Therefore abundant nutrients in the water column and increasing daylength allowed longer plankton blooms.

As the summer progressed, surface temperatures suddenly jumped up 5-10 degrees in a short time (two weeks). This warming should have depleted nutrients and stopped the bloom. However we had a thermocline (the abrupt boundary between warm surface and cold deeper water) that was fairly shallow- as little as 17 feet based on my dive computer readings. This thermocline rose and fell throughout the entire summer. When the cold water rose higher, it replenished nutrients in the surface waters and maintained the plankton bloom for weeks on end. Vis has generally been worse in the upper 40-60 feet where sunlight is strongest.

We've also had a lot of surge throughout the summer, not just during the normal hurricane season off Baja. At times I've felt I was in a winter storm underwater with strong surge tearing seaweeds (even large kelps) from the rocks. The rapid increase in water temperature in August also triggered a quick die-off of giant kelp near the surface, a natural but usually slower process during the gradual warming in a normal summer. Lots of seaweed and kelp detritus covered the ocean floor where it was ground into small particles by the surge and abrasion. This explains the presence of a lot of particles in the water in addition to the much smaller phytoplankton, further decreasing "vis."

The pea soup conditions have not been limited to Catalina. Most of southern California out to San Clemente Island experienced it. Red tides and strong bioluminescence caused by the dinoflagellates dominated mainland coastal waters. The regional scale of these events coupled with unusual weather globally suggest larger phenomena are at work. The jet stream which affects large scale weather patterns has been erratic this year. The Pacific Ocean has also entered a normal period of cooler water with more cold water upwelling enhancing nutrients, plankton and many fish populations.

So how do the great white sharks fit in? As mentioned, poor visibility dominated the southern California coast. Even when I'm diving in the Dive Park under low vis, I often have to surface to get my bearings since I can't see my usual landmarks if they are more than 10 feet away. Although sharks use many senses including chemical and electrical to locate food, based on the occasional attacks on surfers vision plays a role. Perhaps the great whites are experiencing a similar problem and need to surface to get their bearings... or their sea lions. This may also help explain the fatal attack on the swimmer at Avila Beach.

Of course hypotheses should be tested using the scientific method before being accepted as scientific "fact." Should some foundation wish to fund the research, say to the tune of a quarter million dollars (okay, how about $25,000? a measly $25???), I'd be happy to test this hypothesis. Makes more sense than funding research to determine whether monkeys can compose the Great American Novel on a computer. Until my hypothesis is properly tested, it will remain my educated opinion. Take it or leave it.

© 2003 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.

Image caption: Bat ray, garibaldi, gorgonian or soft coral, and sheephead as seen during
this summer's poor visibility in the Casino Dive Park.

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