This summer and fall our waters have been rather toasty with surface temperatures still in the low 70s in late October. Being the warm water wussie that I am, I enjoy being able to dive comfortably for hours in a wetsuit. In fact on a recent dive to the incredible depth of 5 feet to film our new whitetail damsel, the minimum water temperature was a balmy 74 F. I lasted more than two hours in those conditions and only surfaced when my camcorder battery died.
I remember back to some of the great warm water episodes during my years here on Catalina. The 1982-84 El Niño was incredible as I free dove for hours without turning too deep a shade of blue. A milder El Niño in the late 1990s was also pleasant although I was often tied to my desk at the Catalina Conservancy even on weekends and couldn't take a lot of time to dive. Then in 2006 surface temperatures reached 79 F for about three weeks and it was downright toasty down to depths of 150 ft. Of course these warm temperatures are not necessarily good for the full-time residents of our temperate kelp forests.
Any time water temperatures rise above 68 F for a period of 2-3 weeks, our giant kelp forests become nutrient starved and disappear. That is happening today in portions of the dive park, although the areas closer to the harbor mouth where deeper, colder, nutrient-rich water is present are still healthy. Over the past seven years such periods are often are accompanied by a surge in the density of the highly invasive Asian seaweed Sargassum horneri. Strangely enough the language in the Marine Life Protection Act that established our new marine protected areas (MPAs) around the state apparently does not distinguish between native and non-native species so I'm still struggling with the Department of Fish and Game to get a permit to weed out this noxious invader.
It is not just the seaweeds that are affected by temperature changes like these. During the previous El Niños and warm water episodes, I've observed significant die-offs of echinoderms including sea urchins and sea stars (or starfish for the non-PC crowd). Similar die-offs have been observed in other regions including the tropics. A number of different mechanisms have been suggested as the cause including toxins from red tide plankton blooms, greatly diminished food resources for the urchins and bacterial diseases.
The plankton in a red tide that could be responsible for urchin die-offs produce toxins that may be harmful or even lethal to both marine life and critters (including us) that feed on marine life that ingests it and biomagnifies it up the food chain. However, our recent warm waters are unlikely to produce red tides, which tend to be far more rare on Catalina's coastline than along mainland shores. Warm water contains very small concentrations of the nutrients responsible for plant plankton growth and blooms, as well as for larger algae like our giant kelp. Thus red tides are even more unlikely during warm water events along our steep slopes and can largely be ruled out as a possible cause.
Die-offs of seaweeds such as giant kelp due to elevated temperature reduce the food supply available for sea urchins and other critters like abalone. They are largely dependent on drifting kelp fragments for their munchies. The region between Long Point and the East End has some of the warmest water on our island's 54-mile coastline. However, I first noticed the die-off of sea urchins before the kelp really succumbed to the higher temperature and diminished nutrients. Although their food supply is diminishing in these warmer areas, and is undoubtedly affecting sea urchins now, the die off of the echinoderms began before that of the kelp.
Warmer water favors the production of bacteria. This is why we keep food cold in refrigerators. Bacteria growth rates increase with temperature and die-offs of kelp and other marine life provide increased food for these decomposers. While diving the tropical waters of Fiji as a guest of Jean-Michel Cousteau at his resort in Savusavu back in 2001, I used a pair of fins that cut into my ankles leaving open wounds. These wounds quickly became infected and the result was rather ugly as bacteria multiplied, feeding on my tissues and fluids! Fortunately when I travel, I carry the antibiotic Cipro which quickly wiped out the ugly infection.
Sea urchins and other marine life may also become infected by naturally occurring bacteria in the ocean. Back in 1986 researchers Gilles and Pearse at UC Santa Cruz looked at naturally occurring bacteria in the infections detected on purple sea urchins (Strongylocentrotus purpuratus). They identified 14 different types of bacteria in these lesions, although only two species were deemed capable of initiating such infections.
When urchins die in large numbers, one often detects the characteristic lesions from these infections on their hard exoskeletons (called tests). There is a darkened region where necrosis (tissue death) has occurred, surrounded by swollen tissue where the infection is active. These infections are easily seen because the urchin loses its spines in the region of the disease. The bacterial infection was often greatest in the tissues at the base of the spines. Many of the dead urchins exhibiting extensive lesions have lost all the protective spines on their tests (exoskeletons), while some have just lost the spines in the vicinity of the infection. That led other researchers to call this the "bald sea urchin disease." Fortunately I am not even close to being prematurely bald. At my latest Harvard class reunion, the undergraduates applauded as I passed by carrying our class banner. When I asked my roommate why, he said they were commenting that I still had a full head of dark hair unlike the majority of my classmates.
Gilles and Pearse did their study using urchins from cooler central California waters, and collected their specimens in 1981-82 largely prior to the intense El Niño of 1982-84. The die-offs I remember seeing back then occurred during that El Niño event. Mild to moderate bacterial infections occur naturally even in otherwise healthy urchin populations. Given the influence of temperature on bacterial growth and reproduction, it is quite likely that bacterial infections intensified during such warm water episodes.
One of the conclusions by Gilles and Pearse was that some form of stress was probably necessary for the infection to take hold initially. This could be due to an injury caused by a sheephead or lobster attack. It could also happen through careless action by a SCUBA diver who tries to pry an urchin out of its rocky hole where they hide from the predators abundantly present in the protected dive park. Unlike areas on the mainland and in the northern Channel Islands were overfishing or cooler water keep urchin predator populations unnaturally low, we have abundant urchin munchers in the protected dive park. Therefore urchins are not a problem there and divers usually only see them hiding in the rocks during the day. Please let them be... they are part of a healthy ecosystem here.
© 2012 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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Healthy purple urchin, diseased urchin showed dark, necrotized infection; dead urchin tests
filmed off Ship Rock during the 2006 warm water episode.
This document maintained by
Dr. Bill Bushing.
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