Dive Dry with Dr. Bill

#218: In Hot Water

This summer there was no need for me to travel to the tropics to enjoy warm water diving. With surface temperatures up to at least 79 degrees, I felt like I was reliving the 1982-83 El Nino. While this unusual elevation in water temperatures pleased many WWW's (Warm Water Wussies) who dove our usually cool temperate kelp forests this summer, it was a different story for many of the critters that live here and prefer our usually cool waters.

Many of you are aware that giant kelp prefers water temperatures below 68 degrees. Warmer waters contain fewer of the nutrients that kelp needs to survive. A "starving" kelp forest begins to deteriorate after a few weeks of such elevated temperatures. Instead of watching kelp fronds grow at their peak rate of 24" per day, much of the kelp's biomass (the stipes and blades, equivalent to the stems and leaves of a terrestrial tree) aged and fell prey to various diseases in their weakened state. Although senescence or aging of kelp fronds is a natural event once they've lived out their roughly half year of life, new fronds constantly grow up from the submerged holdfasts to replace the dying ones. This year the high temperatures caused most of the stipes and blades to die off, often leaving nothing but the holdfast and a very scraggly kelp "forest." Warm temperatures even at the depth of the holdfast apparently prevented any regrowth of new fronds to replace the lost ones.

Now there are the tiny young stages of giant kelp waiting to sprout once pulses of nutrient-rich cold water return. Eventually much of the luxuriant forest will return. In fact, we are already seeing recovery at the cooler water sites around Catalina. However, the loss of the kelp forest has ecological repercussions for many other species. Their three-dimensional kelp forest habitat has largely disappeared and with it a vital food source for many species. In addition, the elevated water temperatures can directly affect the invertebrates, fishes and even marine mammals of our region. The plant or phytoplankton is also affected by the lack of nutrients, and its decline adds to the upheaval by affecting those invertebrates and fish that prefer their "greens" in small "pieces."

Normally kelp goes through a continual process of growth and decline. Once individual fronds die and are cast off, they become food for invertebrates like abalone and sea urchins. In healthy systems, these species have a continual supply of food in the form of drift kelp and detritus. When most of the kelp dies all at once, there is initially a virtual smorgasbord for them... followed by very little available food. Our recovering abalone population needs to be well-nourished to reproduce. If sea urchins cannot find enough drift algae to eat, they may come out of their hiding places and try to munch what is left of the once healthy kelp forest.

Less obvious are the effects on many filtering species that remove plankton from the water as their primary menu item. Sponges, clams, mussels, sea anemones, sea fans, barnacles, worms, tiny bryozoa and others are deprived of their food source when phytoplankton declines. As they starve, the predators that munch on them lose their food source as well. This includes fish, crabs, shrimp, nudibranchs and other species. Our once highly productive waters support fewer and fewer individuals until the base of the food chains; the kelp, phytoplankton and other algae; are replenished.

Unusually warm waters have direct effects on many species. During the 1982-83 and 1997 El Ninos, we lost many of the sea stars (starfish), sea urchins and sea cucumbers in our waters to a bacterial "plague" known as wasting disease which was accelerated by the warm waters. Starfish almost literally disintegrated before our eyes due to the impact of the Vibrio bacterium. I have observed signs of this during the summer, with many sea urchins lying dead and disintegrating on the bottom without any signs of predation by sheephead or other "munchers." I saw what appeared to be the beginning of the same thing in starfish and a few unlikely candidates like sheep crabs.

Even if they are not affected by disease, many cold water species become stressed when temperatures exceed their preferred range. This includes fish like rockfish and surfperch as well as invertebrates including certain crabs, abalone, cup corals and sponges. Those capable of moving can seek relief in the colder, deeper waters... while attached forms have to "tough it out" if they can. I've found that temperatures on my deepest dives last summer were slightly colder than what I experienced at the same sites the previous winter. Of course warm water species like garibaldi, sheephead, lobster, and black urchins may enjoy their new "climate" while it lasts. Some like the orangethroat pikeblenny, Guadalupe cardinalfish, rainbow scorpionfish and mantis shrimp may actually extend their ranges further north during such episodes.

So our regional kelp forest ecosystems are subject to continual changes due to variations in temperature and other physical factors. All systems are dynamic and must respond to such changes... short-term or long-term. Although I believe in global warming, it does not appear to be the driving mechanism behind our balmy water temperatures this summer. Waters in the Channel Islands have been on a gradual warming trend since the late 1970's, although it is believed this is part of a natural cycle and will eventually reverse. Perhaps by the time it does, and I age a bit more, I'll have to consider a search for warmer waters with frequent trips or extended stays in the tropics at least during our winter months. After all, to paraphrase Jimmy Buffet... "It's 80 degrees somewhere..."

© 2006 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. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles" or "Gentle Giants: Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

A senescing kelp blade and a stipe with all its blades missing; broken holdfasts on the bottom...
all that was left of some of our kelp forests this past summer.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia