Dive Dry with Dr. Bill

#664: Eco-Catastrophe Part Two: The Herbivores

Last week I outlined how the persistent warm water period of the last 18 months has affected our local marine algae, the base of most marine food chains. The huge body of warm water resting off SoCal and northern Baja, called The Blob (actually one of three), has helped create a very significant change in the seaweeds here due to the lack of nutrients associated with warm water. Two years ago we had a diverse assemblage of different algae and a fairly healthy kelp forest. Today we have an ecosystem in which most of the native algae are gone or greatly diminished. Instead, the non-native and highly invasive Asian seaweed (Sargassum horneri) now dominates much of our leeward coast. Although I haven't taken any plankton tows, I assume the lack of nutrients has impacted the plant or phytoplankton as well. How has this affected the herbivores that feed on algae?

When I go out to a restaurant (on a divebum's budget?), I like to se a menu full of different choices. Some algae eaters are like that, too; but many are more specialized and have a narrow range of food preferences. Our abalone, sea urchins, top snails, kelp crabs and even some omnivorous fish like opaleye seem to greatly prefer giant kelp (Macrocystis pyrifera). It is mild, soft and chewy unlike some seaweeds that are bitter and tough. I know because I used to cook with kelp... my kelp muffins were considered pretty good back in the 1970s. Sea urchins favor giant kelp too, which reaches them as drift when the blades drop off as they senesce or age. Kelp crabs seem to feed mostly on Macrocystis but occasionally take other seaweeds as a snack. Kelp snails climb the tall kelp and munch on it, but they will also chow down on southern sea palm kelp (Eisenia arborea). Wavy top snails love it and capture the drift blades, but will also eat a few other seaweeds.

You all know that giant kelp is highly prolific and grows at rates of a foot a day or more when sunlight and nutrients are plentiful. It can supply a lot of food to the algae eaters and remain quite healthy under the proper conditions. However, when it dies out along with many other of our native seaweeds, it eliminates a major source of food to power the marine ecosystem. I have observed and filmed abalone that apparently died of starvation (and a few that were poached when they came out of hiding to seek food).

The warm water has directly killed sea urchins and indirectly done so by making them vulnerable to diseases that are more prevalent when waters warm up. This affected the northern species (the reds and purples) that don't tolerate warm water, but not the black urchins that are affiliated more closely with warm southern waters. Our local urchin species have not co-evolved with the Asian seaweed that dominates our waters now. By that I mean they didn't "grow up" together. Species that evolve with one another often develop the ability to feed on, or defend against, other species over time. Urchins are a major herbivore on Sargassum in Asia. I have not observed sea urchins like the purple and red feeding on it... but then most of those are dead and gone. The reason is the Sargassum or "devil weed" produces bitter chemicals known as polyphenols. that our local species find distasteful.

Now I have been able to force feed abalone some of the invasive Sargassum, but probably only because they are starving due to lack of giant kelp. Green abalone seem to tolerate it better than the pinks. But the Sargassum is a very tough alga. It does not break off easily (unless I rip it out) and therefore does not form drift that might feed urchins, abalone and other herbivores. In my dives over the last 6-8 months I do not remember seeing a single southern kelp crab. Usually they are present in numbers on my night dives. It seems their diet is too specialized to survive the changes in algal composition. Kelp snails have been rare as well. Opaleye, which graze on kelp but also add invertebrates to their diet in our colder waters, are present in lower numbers and attack what little giant kelp is left.

So many of our invertebrate and fish species have been seriously impacted by the near disappearance of giant kelp and many other native seaweeds. "Plants" are important... very important! Warm, nutrient-poor waters coupled with the dominance of the non-native Sargassum here have created a substantial shift in the first trophic ("feeding") level in our ecosystem. This first level is the primary producers, the organisms which trap sunlight and convert it into forms that other species can feed on. But as I've mentioned this one-two combo has also affected another group of primary producers, the "plant" plankton. When phytoplankton diminish, there is less food for the animal plankton (zooplankton) that feed on them. That affects any critter that feeds on plankton!

Such critters include the soft corals or gorgonians here, many species of tube worms and bivalves like scallops. With less plankton present in our waters, they have less to feed on as suspension or filter feeders. These critters are also hit by the one-two punch in another way. The Sargassum which dominates our rocky reefs and even grows on soft bottoms down to at least 100 ft impacts both types of munchers. It grows so dense that water cannot easily pass through the understory above the rocky reef where the worms and bivalves are attached. Less food for them is present and their filtering efficiency is reduced by the dense algal growth. In the case of the soft corals, many of them have begun to die and others have been affected by entanglement with the Sargassum and also overgrown with other algae that do better in low nutrient conditions.

So we have not just lost much of our giant kelp and other native seaweeds, we have lost the food source that powers much of our marine ecosystem. Those species of invertebrates and fish which are directly dependent on these native algae are impacted as well. What happens on one level within an ecosystem usually affects all other levels, in this case through a bottom-up ripple. Next week I will outline some of the impacts on the next level of munchers, the predators. YOU could be next!

© 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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Starving green abalone feeding on "devil weed" and sea urchin killed by warm water and disease;
southern kelp crab (left) and wavy top snail (right) feeding on giant kelp.

This document maintained by Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia