Dive Dry with Dr. Bill

#6109: The Rise and Fall of the Kelp Forest

Two weeks ago I wrote about spending time with the group of Japanese divers and scientists from Shibuya Diving Industry. The primary reason Masanobu Shibuya had come to southern California back in July when we met was to film our giant kelp (Macrocystis pyrifera) forests and other seaweed. We kept n touch through his interpreter Hazuki Yasuhara via e-mail so they were aware there were major changes in the dive park due to the elevated temperatures, low nutrients and storm surge from multiple hurricanes since they were here last. Still I think Masanobu was shocked to see how little kelp remained in the dive park compared to his previous visit.

For those who haven't been in the water there, our once tall and proud kelp forests that draw divers from all over the world are nothing but a few stragglers barely hanging on. The only evidence of these once lush kelp beds are the decaying holdfasts attached to the rocks and devoid of any fronds. The few that did survive the ecological onslaught are senescing (aging) even faster than this old geezer.

The blades ("leaves") on the few remaining kelp fronds are torn and tattered. In fact many of them are missing from the stipes ("stems") completely. Even the solitary "plants" that bear a decent load of blades are lying on the bottom because their pneumatocysts ("float bulbs") are not able to keep them upright in the water column. True kelp canopy is a thing of the past and the barren landscape is dominated by the "understory" seaweeds, mostly the non-native Asian invasive Sargassum horneri (which I refer to as "devil weed").

On his visit here last July Masanobu had been surprised to see the Sargassum so prevalent in our dive park. Back in his homeland and other parts of Asia, it is a commercially important alga. Initially he had no idea it had spread to southern California and proliferated as much as it has in the 11 years since its introduction in Long Beach. In Japan it is declining in abundance so scientists are interested in finding out why.

When we weren't diving, we talked about what was happening ecologically in the dive park and elsewhere due to this unusual combination of events. I explained my thoughts about how the warm water created a situation with very low nutrients causing the kelp to weaken early in the summer season. There is a well-known relationship between water temperature and nutrient levels that I don't completely understand. Once temperatures exceed 68° F (20° C) for a period of a week or two, nutrient levels plummet. We call this an inflection point. Our waters have been higher than 68° F for months now creating low nutrient conditions under which giant kelp can not survive.

Then in late summer the strong storm surge from Hurricane Marie easily ripped out much of the weakened kelp (not to mention the damage it did to our local infrastructure and businesses). Certainly the frequency of the hurricanes south of the border is linked to the warm water episode we are in. If this is a prelude to what will happen in the future due to global climate change, our native marine life could be in big trouble.

Many of the abalone that were seen in the park lately were "buried" by the rocks moved about in the surge. Even if this didn't crush them, it made it more difficult for them to obtain food. These abalone usually feed on drift kelp such as the blades that slough off as they age. With little kelp left in the park, they are probably hungry enough to eat a horse (as my Mom used to say about me). I've been feeding them chunks of the invasive Sargassum which they aren't supposed to like due to chemical defenses (polyphenols) against herbivores in that seaweed. Heck, when I'm starved I'll even eat brussel sprouts!

I've mentioned before that our starfish (sea stars for the P.C. crowd) and sea urchins have been dying due to an apparent disease in the first case and the warm water and possible disease in the second. I've recently seen sea cucumbers that appear to be diseased as well. Warm waters favor the growth of bacteria and other pathogens and may weaken their hosts making disease a more serious problem during episodes like this.

I've also observed strangely colored sea hares (big shell-less snails) in the park. The red algae they normally eat seems to have died out. Since the pigments in that seaweed are what the sea hare uses for its body and ink color, the pink ones I'm seeing may be feeding on other algae that aren't as vividly red. Divers also report seeing far fewer of the sea hares this year than would be expected in cooler times.

Returning to the effect of the low nutrient levels on giant kelp, this makes it difficult for the microscopic young kelp "plants" to grow and reestablish our forests. It is hard to grow when you don't get much "food" (like nitrates and phosphates). Combine that with a third factor, the thick growth of the invasive Sargassum horneri which overshades and chokes out the small kelp before it gets a chance to grow tall enough to get adequate sunlight. The relatively few kelp youngsters that do rise above the Sargassum often get entangled in the invasive seaweed causing the kelp blades to tear off, further limiting photosynthesis and growth.

Masanobu is a seaweed specialist in Japan and very interested in our ecological dilemma here. He plans to return in late winter to study and film the Sargassum when it reaches its peak. We hope it will give him further insight into how we might control the Sargassum in our waters and halt the decline of it in Japan where it is native and a commercially important seaweed.

© 2014 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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Dead kelp holdfast and scraggly survivor; giant kelp entangled in Sargassum
and Sargassum-dominated landscape of the dive park today..

This document maintained by Dr. Bill Bushing.
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