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Dive Dry with Dr. Bill

#624: Sargassum Cascade, Part I

As a marine ecologist, I'm well aware of what my academic peers refer to as a trophic cascade. Pull your chairs up to your desks and take out your notebooks because I'm a'gonna learn ya something today. No skipping class! There are two types of trophic cascades. One occurs when you change the abundance of a top predator. It is referred to as a "top down trophic cascade." Let's go back to the mid-1970s during the hysteria after the movie "Jaws" was released. By the way... some scenes were filmed at Torqua Springs about a mile from my old marine biology lab. I'm sure you wondered why our giant kelp was seen in the underwater scenes off New England! People went crazy and started killing sharks... not just great whites but all kinds of sharks. And most of them didn't end up in shark fin soup.

With fewer sharks present in the ocean, the critters they preyed upon such as seals, sea lions and fish had the pressure of predation released a bit. Since fewer prey were getting munched, their populations increased. Of course once the number of sea lions increased, the number of fish that they consumed increased so the populations of those prey species decreased. A change in one level of the food chain affected the populations of the others. Of course nothing is that simple... other factors in the rebounding of seals and sea lions included their recovery from many decades of hunting by man for their oil back in the days before petroleum was discovered.

There are also "bottom up trophic cascades" where a change in primary producers such as seaweed affects the levels above them in the food chains. In general these can be very serious events. When an important species of seaweed is removed from an ecosystem by natural disaster, overharvesting or other means; the food supply for the entire ecosystem generally drops. Therefore there should be fewer plant eaters (herbivores) and, up the chain, fewer predators as well.

Now don't worry... I'm not going to throw a surprise quiz at you like I used to do to my Toyon and UCSB students. However let's look at the ecological dilemma created in our waters over the past 10 years and especially during the last 6-8 months. I've written many times over the years about the serious threat posed by the invasive Asian seaweed Sargassum horneri that entered North American waters in 2003 from Japan and arrived on Catalina in the winter of 2005-06.

This year our native giant kelp (Macrocystis) forests were largely gone by September. Warm water temperatures and very low nutrient levels caused the giant kelp to weaken and die. In August the storm surge from Hurricane Marie ripped out much of the weakened kelp along our coast. With no shading by the usually thick kelp canopy, the understory Sargassum began growing thanks to the existence of plenty of light. It grew incredibly thick, sucking what nutrients were left from the water and preventing light from reaching the microscopic giant kelp plants on the rocks and triggering their growth once temperatures cooled a bit.

Now I'm sure you are wondering where Dr. Bill is going with all this. So am I! How does the disappearance of our giant kelp and the explosion of the non-native Sargassum have anything to do with a trophic cascade? Well, let's add another piece to the puzzle. Sargassum contains some nasty chemicals known as polyphenols. Well, at least many of our local marine critters think they are nasty since they have not co-evolved with these plants. Instead they've had access to the tender, juicy blades of our giant kelp which they find far more palatable based on my observations.

Now I've set the stage for a slightly unusual example of a bottom up trophic cascade. Our native giant kelp, which our local critters love to chow down on, has largely disappeared due to the "perfect storm" of warm water, low nutrients and storm surge last year. In its place a somewhat foul tasting non-native seaweed has come to thoroughly dominate much of the submarine rocky substrate above about 70 ft where the Sargassum generally diminishes. In essence we have replaced one tasty seaweed food source almost completely with a distasteful one, essentially removing a major source of nourishment for the kelp forest ecosystem.

When this fascinating story continues next week. I will detail some of the impacts of this ecological replacement as it progresses through the levels of our fairly complex kelp forest food webs. Until then, try to find something to occupy your mind so it doesn't fall apart due to the anticipation of... the rest of the story (with apologies to Paul Harvey).

I also want to welcome all the divers who have come over for the special dive park clean-up on Friday, February 6th, to remove the debris from the sailboat that went down in the December storm. Just wish you could each remove a few thousand pounds of Sargassum as well... but CA Fish & Wildlife still won't let us. Grrrrr.

© 2015 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 kelp forest and canopy during previous years in the dive park and
divers (for scale) in Sargassum dominated dive park today.

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia