DIVE DRY WITH DR. BILL #830: I NEED AN UNDERSEA WEED WHACKER!
Since I returned to King Neptune's realm weeks ago, I've been focused on two things while I dive. One is getting up close and personal with our California wreckfish (aka giant "sea bass"), Stereolepis gigas. I've spent several dives just kneeling on the bottom watching a group of six of these gentle giants from a few feet away. Nothing like looking into the eye of one of them... unless I finally find my mermaid beneath the waves.
The second thing I've been focused on is weeding out the highly invasive Asian seaweed Sargassum horneri, or as I called it a decade ago, devil weed. This non-native species arrived on our West Coast from the Seto Inland Sea in Japan around 2003. It was first spotted in Long Beach Harbor by Drs. Jack Engle and Kathy Ann Miller. I first saw it at the Empire Landing Quarry during the winter of 2005-06. Sad to think that it might have been transported here by a dive boat.
This cool water brown alga thrives here from late fall to late spring and although an annual species (dying off each year), it may produce a second generation in a single year. During warm water periods when our giant kelp (Macrocystis pyrifera) is stressed and dies, the devil weed may totally dominate the substrate. During cooler water periods, the kelp often flourishes and its thick canopy limits Sargassum growth by trapping sunlight before it reaches the understory.
When I first observed it in our waters, I thought we had a chance to control its spread. I wrote articles about the threat it posed and sent an educational video to the Fish & Game Commission in hopes they would take quick action. Apparently all they did was form a committee that never met. I was a bit livid at their inaction and started weeding it out as much as I could.
As an unofficial "experiment," diver Pete Vanags and I completely weeded several large boulders of the Sargassum and I observed what happened over the following months. I was shocked to find that very few of our native seaweeds colonized the nearly bare rock. It led me to hypothesize that the devil weed may possess some allelopathic compound that inhibits the growth of other seaweeds. I hope someone will eventually test this hypothesis.
Many of my recent dives have involved hours of weeding this nasty seaweed from our dive park. Many have asked how I do this, some concerned that just pulling and releasing it may cause it to spread further. About a decade ago I consulted with several Japanese marine biologist about this. They said that studies in Japan (where it is native and used for food, fertilizer and biofuel) indicate that allowing it to drift away does not result in significant dispersal of the species. Of course by now it has spread everywhere around the island anyway. I wish we could organize weeding events in the dive park and other marine protected areas to at least control it in these sensitive sites.
To help insure that the drifting Sargassum does not reproduce elsewhere, we focus on weeding out the alga when it is still immature. In its earliest stages it is a low growing roseate of fern-like blades. As it grows, it develops air bladders (similar to the float bulbs or pneumatocysts of giant kelp) and rises upright into the water column. These are the best stages to remove. Finally when it matures reproductively, it develops receptacles that contain the eggs and can reproduce.
Below you can see images of healthy algal turf in the absence of devil weed. It is composed of many different species of brown, red and green algae. This biodiversity is important in maintaining ecological stability. The second set of images show a landscape completely dominated by a monoculture of Sargassum. Very little biodiversity under those conditions. Herbivores such as snails and crabs that feed on our native seaweeds have few options. I have successfully hand fed devil weed to some abalone, but they greatly prefer tender, juicy giant kelp blades.
With the dense covering of devil weed, many predators that would chow down on the snails and crabs, such as sheephead and octopus, have trouble finding their prey. The dense undergrowth also limits water circulation over the rocky reef. Filter feeders such as worms may have trouble getting enough plankton and organic matter to sustain them. In short, the monoculture of devil weed poses a devilish ecological problem.
© 2019 Dr. Bill Bushing. For the entire archived set of over 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: A natural, biodiverse algal turf in the dive park; two rows of devil weed monocultures during the cooler months; immature Sargassum; air bladders and reproductive receptacles.
Since I returned to King Neptune's realm weeks ago, I've been focused on two things while I dive. One is getting up close and personal with our California wreckfish (aka giant "sea bass"), Stereolepis gigas. I've spent several dives just kneeling on the bottom watching a group of six of these gentle giants from a few feet away. Nothing like looking into the eye of one of them... unless I finally find my mermaid beneath the waves.
The second thing I've been focused on is weeding out the highly invasive Asian seaweed Sargassum horneri, or as I called it a decade ago, devil weed. This non-native species arrived on our West Coast from the Seto Inland Sea in Japan around 2003. It was first spotted in Long Beach Harbor by Drs. Jack Engle and Kathy Ann Miller. I first saw it at the Empire Landing Quarry during the winter of 2005-06. Sad to think that it might have been transported here by a dive boat.
This cool water brown alga thrives here from late fall to late spring and although an annual species (dying off each year), it may produce a second generation in a single year. During warm water periods when our giant kelp (Macrocystis pyrifera) is stressed and dies, the devil weed may totally dominate the substrate. During cooler water periods, the kelp often flourishes and its thick canopy limits Sargassum growth by trapping sunlight before it reaches the understory.
When I first observed it in our waters, I thought we had a chance to control its spread. I wrote articles about the threat it posed and sent an educational video to the Fish & Game Commission in hopes they would take quick action. Apparently all they did was form a committee that never met. I was a bit livid at their inaction and started weeding it out as much as I could.
As an unofficial "experiment," diver Pete Vanags and I completely weeded several large boulders of the Sargassum and I observed what happened over the following months. I was shocked to find that very few of our native seaweeds colonized the nearly bare rock. It led me to hypothesize that the devil weed may possess some allelopathic compound that inhibits the growth of other seaweeds. I hope someone will eventually test this hypothesis.
Many of my recent dives have involved hours of weeding this nasty seaweed from our dive park. Many have asked how I do this, some concerned that just pulling and releasing it may cause it to spread further. About a decade ago I consulted with several Japanese marine biologist about this. They said that studies in Japan (where it is native and used for food, fertilizer and biofuel) indicate that allowing it to drift away does not result in significant dispersal of the species. Of course by now it has spread everywhere around the island anyway. I wish we could organize weeding events in the dive park and other marine protected areas to at least control it in these sensitive sites.
To help insure that the drifting Sargassum does not reproduce elsewhere, we focus on weeding out the alga when it is still immature. In its earliest stages it is a low growing roseate of fern-like blades. As it grows, it develops air bladders (similar to the float bulbs or pneumatocysts of giant kelp) and rises upright into the water column. These are the best stages to remove. Finally when it matures reproductively, it develops receptacles that contain the eggs and can reproduce.
Below you can see images of healthy algal turf in the absence of devil weed. It is composed of many different species of brown, red and green algae. This biodiversity is important in maintaining ecological stability. The second set of images show a landscape completely dominated by a monoculture of Sargassum. Very little biodiversity under those conditions. Herbivores such as snails and crabs that feed on our native seaweeds have few options. I have successfully hand fed devil weed to some abalone, but they greatly prefer tender, juicy giant kelp blades.
With the dense covering of devil weed, many predators that would chow down on the snails and crabs, such as sheephead and octopus, have trouble finding their prey. The dense undergrowth also limits water circulation over the rocky reef. Filter feeders such as worms may have trouble getting enough plankton and organic matter to sustain them. In short, the monoculture of devil weed poses a devilish ecological problem.
© 2019 Dr. Bill Bushing. For the entire archived set of over 800 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: A natural, biodiverse algal turf in the dive park; two rows of devil weed monocultures during the cooler months; immature Sargassum; air bladders and reproductive receptacles.
