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Monday, February 6, 2017


An amazing amount of Giant Kelp (Macrocystis pyrifera) washed up on local beaches during the last two days.  Although abundant in the Monterey area, Giant Kelp is less common in Bodega Bay.  I don't think I've ever seen this much Macrocystis on beaches in Bodega Bay (during the last 12 years).

The photo above shows the rippled or corrugated blades of Giant Kelp.

Below, note its relatively loose and sprawling holdfast (the portion of the kelp that attaches to the substrate):

(Why is the holdfast purple?  Maybe someone out there can provide some ideas?)

There was some large Bull Kelp (Nereocystis luetkeana) mixed in with the Giant Kelp.  Bull Kelp is the more common large kelp in our area.  In the photo below, the Bull Kelp is tangled up with the Giant Kelp, but you'll recognize Bull Kelp's very large float (pneumatocyst) and the smooth and shiny blades (the Bull Kelp blades also lighter in color than the Giant Kelp blades):

Here's a photo comparing the holdfasts of the two species:

The Giant Kelp holdfast is on the left and the Bull Kelp holdfast is on the right. 

I had wondered if this kelp might have been ripped out by the powerful 20-21 January storm, but upon closer inspection, I noticed quite a few pelagic barnacles (Lepas sp.) attached to the kelp, indicating it had been at sea for quite some time.  [There's a lot going on in this photo, so I've highlighted a good example of a pelagic barnacle with a circle.]

Based on the amount of kelp, its age, and strong recent currents from the south, I'm still guessing that this Giant Kelp originated from sites to our south.  But I'm not sure why so much has arrived on our beaches!  (Let me know if you have some ideas!)


Al in Penngrove said...

Could this abundance of loose kelp be related to the explosion of sea urchins?

Jackie Sones said...

Hi, Al! Thanks for your question! It's an interesting topic. As a generality, when the abundance of urchins increases, the amount of kelp often *decreases.* But I'm guessing you're wondering whether the urchins could loosen the kelp from its attachment point resulting in more drifting kelp?

This gets into some of the details of how sea urchins eat. Although I don't have a lot of subtidal experience, my understanding is that sea urchins will eat the kelp blades first. And my guess would be that it's not likely that the entire kelp individual would break free while the sea urchin is grazing the blades. (But someone can correct me if I'm wrong!) Also, because these are very large kelp specimens and the holdfasts are also present in most of the individuals that are washed ashore, it's an indication that the kelps were ripped off the bottom, rather then being grazed and broken off above the holdfast.

I'm leaning towards this being a sign of (a) decent kelp growth at sites to our south, (b) some large storms/wave impacts in those areas at some point during the last year, and (c) strong northward-flowing currents or oceanographic patterns bringing this drift kelp to shore.

I'm interested in other ideas, though, so feel free to make other suggestions.

P.S. Just a note -- Although increased urchin abundances have been reported to our north, I haven't heard about their status to our south (e.g., whether increased urchin abundances have also been noted in sites around Monterey Bay).