Dive Dry with Dr. Bill

#126: Plumes and Blooms

As I write this column, the temperatures in Los Angeles are forecasted to reach the lower 80's in January. If this keeps up, we'll soon wish that the rains of the previous few weeks return again... NOT! Fortunately the island survived the record series of storms with just damaged roads and leaking ceilings rather than loss of life. After two weeks topside, I was anxious to get my gills wet once again. While I expected less than ideal visibility, I wasn't really prepared for what I encountered.

I planned to dive the wreck of the Valiant with friends from the mainland, but they didn't make it. I suited up, swam out to one of the orange drop down buoys, adjusted my gear and started my descent. Once underwater, all I could see was the rope that anchored the buoy and a disturbing void. The bottom, only 25 feet below, was not visible... nor were any fish. Finally a a few blacksmith swam into view... just a few feet away. Some divers had reported visibility of 15-20 feet earlier in the day, but I estimated only 5-8 feet on entry. It was a bit disorienting not to see the bottom as I descended. Now I know how those quarry divers in the Midwest feel since they experience similar conditions regularly. By the end of my second dive, visibility had dropped to about 2-5 feet and it was like being in a green "white out," reminding me of winter blizzards in my home town of Chicago.

Once I got out of the shallows and down to depths past 50 feet, things cleared up a little. There might have been 20-25 feet visibility by some standards, but not by mine. As a videographer, I measure water clarity in terms of the farthest distance I can clearly film. The only subjects I could capture were ones within a few feet of my lens. Anything beyond that would be a murky shadow. Always looking for the silver lining in any cloud, I started filming the horrible conditions instead. I can always use such footage to show how variable water clarity and dive conditions can be. Of course the images associated with this week's column will not exhibit much in the way of aesthetics, but they "clearly" document what I experienced!

Both my dives that day were to a maximum depth of 70 feet. The "panorama" at that depth had a very strong green cast to it... almost like diving in slightly diluted Andersen's split pea soup. Green? Yes, the reduced visibility was not due just to suspended sediments as is often the case in winter. The water would be brown if that were the case. There were billions and billions of tiny microscope phytoplankton suspended in the water column. But this is January... such blooms of plant plankton more frequently occur in the spring and summer, not the depth of winter. This is the time of year when visibility off our island is best. Why now?

The answer can be found by returning to the thrilling storms of yester year (2004) and early January. Only the visually (or mentally) impaired missed the sight of tons of our island soil washed into the sea by the runoff from these record storms. You know, the brown in the raging waters. This soil contains not just pulverized rock, but nutrients that are much needed for "plant" growth. The ironwood's loss became the algae's gain. Runoff from the island flowed into the sea, forming plumes of sediment (and nutrient) rich water. Looking at satellite images one could see that even plumes from the mainland were extending as far as Catalina. Our waters were becoming nutrient enriched after months of decline during the warm summer and fall when plankton consumed the supply.

Had our skies been cloudy, this might not have created the conditions I witnessed. Instead, they were a beautiful blue (like mother ocean) and dominated by this large yellow ball of gas radiating sunshine over the island. The optimum requirements for any photosynthetic organism were being met in our coastal waters- rich nutrients and plenty of sunshine. The plumes soon triggered blooms and the visibility was doomed... at least until the nutrients are exhausted and the algae "starve."

The spring and summer plankton blooms are normally triggered by increasing day length, and nutrients reaching shallow coastal areas from deeper waters due to upwelling or a rising thermocline. More sunlight means more energy for photosynthesis. Earth's day length in the northern hemisphere has been increasing slowly since the winter solstice (~ Dec 21st), but not enough to trigger a bloom by itself. The combination of the stormy days followed by extended sunshine created these somewhat unique conditions. First the ocean was enriched with nutrients and then provided with light as an energy source to drive the bloom. Of course this productivity will increase food supplies for fish and invertebrates as the animal plankton feed on the phytoplankton, then on one another. Even if you are not a diver, you may have observed the plankton bloom when you flushed your toilet in the dark late at night. Those bursts of light were not because the bowl was "sparkling clean," but due to the increased plankton!

There is a downside to these winter rains. No, I'm not referring to the fact I couldn't see "Ray" at the Avalon Theatre. The sediments that closed the road to the Casino also had an effect on marine life. The poor visibility and low penetration of light to the bottom also reduced photosynthesis in deeper water algae and kelp. As I dove, I could see (if I were close enough) that these seaweeds were covered with sediments that had settled on their surfaces. Shaking a small kelp would release a cloud of these fine sediments. Such a coating also reduced the light reaching the tissues of the algae, and therefore its ability to photosynthesize.

Of course these adult life forms can survive on their stored food for the time it will take the water to clear up. In greater peril are the microscopic "young" stages of the giant kelp and other algae. Although mature giant kelp may be 200 feet long with most of the tissue in the upper canopy, the tiny gametophyte and sporophyte stages must live at the bottom. Sediments there can scour and abrade these sensitive microscopic stages, or bury them, causing them to die off. So although the plant plankton are enjoying abundance, other photosynthetic algae may be suffering... along with divers like myself! At least the Thompson Reservoir at Middle Ranch is overflowing its spillway and the freshwater drought has ended. Hopefully we as a city will plan more effective conservation programs despite this temporary abundance so we don't face another period of rationing in a few years.

© 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.

Garibaldi (top left) and blacksmith (top right) appearing through the murk;
visibility at its best (bottom left) and worst (bottom right).

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