Join me in the cold, dark, life-sustaining NE Pacific Ocean to discover the great beauty, mystery and fragility hidden there.

What on Earth is “Alternation of Generations”?

Bull Kelp forest, July 4, 2020 near Telegraph Cove.

Have you ever wondered how it can be that Bull Kelp forests largely die off in the winter but then reappear in the spring?

Have you ever wondered about the light patches in the fronds of Bull Kelp as seen in the three images below?

I hope that’s enough of a hook for you to want to know more about the remarkable reproduction of most algae / seaweeds (and ferns and mosses).

Their reproduction involves two versions of the same species. The parent generation looks nothing like their offspring but DOES look like their great-offspring’s generation i.e. there is alternation of generations.

Those light patches are spore packets in the fronds of Bull Kelp!  They drop to the bottom of the ocean, release spores which create a completely different, very tiny version of Bull Kelp (asexually) which then makes the big, long version of Bull Kelp (sexually).

Below you have my attempt at further explaining this aided by a really good graphic.

And for the super science nerds (I see you), more detail from Wernberg et al regarding reproduction in kelp forests: “Recruitment involves multiple microscopic stages (i.e. gametophytes and juvenile sporophytes). Because the sperm has to find an egg, male and female gametophytes must settle in close proximity at densities of ca. 1 square millimetre in order to secure fertilization (Reed, 1990). Gametophytes and microscopic sporophytes can persist in the kelp forest understory for weeks to months, where they serve as a ‘seedbank’ (Hoffman & Santelices, 1991). Microscopic sporophytes start growing once stimulated by high light (Reed & Foster, 1984) .  . . Recruitment into the adult population takes anywhere from a few months to 2-3 years depending on the species and local conditions (Pedersen et al, 2012, Teed, 1990). Most juvenile plants succumb to predation, stress, or self-thinning within the first year, but some individuals can remain viable for years without growing (Sjotun, Christie, & Helge Fossa, 2006) until space and light become available.” 

And you thought  your sex life was complicated! 🙂

Note that of the giant kelp species found off our coast from California to Alaska, Bull Kelp (Nereocystis luetkeana) is an annual whereby most sporophytes die off every year. Giant Kelp (Macrocystis pyrifera) is perennial whereby the sporophyte does not die off at the end of the growing season.

If you have read to this point, congratulations! You are amongst the few humans who may have a good comprehension of alternation and generations. You know what is in the photo below.

Spore packet in the frond of Bull Kelp that washed up on the beach. It always makes me smile to see them while diving, knowing that they will make the forest grow anew. 

You also know how it can be that Bull Kelp forests reappear in the spring. There were never really gone. The species was there all along, but just in a different version  / generation.

With regard to growth rate, the stipe (stem-like structure) of Bull Kelp can grow up to a maximum height of 36 m. The stipe would have to grow an average of 17 cm a day to reach this length in the 210-day growing period (Druel).  If you include the growth of the fronds (the leaf-like structures), the maximum growth rate has been documented to be at least 25 cm per day (Duncan). Giant Kelp grows even faster and bigger = up to 30.5 cm at day to heights of 53.4 m.

Do know that there is concern about diminishing kelp forests due to impacts of changing ocean conditions  / climate change  / Sea Star Wasting Syndrome. Some sea star species are recovering but the Sunflower Star is not whereby there are more of their prey, including urchins which graze on kelp forests.

Sporophyte of Bull Kelp photographed yesterday. It’s the time of year when the Ocean is all the more soupy with life. It means that the more typical photos of beautiful kelp forests are difficult to obtain because the water is thick with gametes, phytoplankton, larvae and other zooplankton. Bu this is the time of year that the kelp forests are at maximum productivity – as habitat, food, oxygen producers and carbon dioxide absorbers. When it is colder, there is better visibility because there is less sunlight for phytoplankton growth and reduced cues for reproduction of marine invertebrates.


Sources: 

More good sources to understand alternation of generations:

16 Responses to “What on Earth is “Alternation of Generations”?”

  1. Jan Kocian

    I am glad I read the whole article. I learned something I didn’t know. The spore packets ! Thank you. Also I see from your photos you suffer the same visibility as I do here on Whidbey. 🙂

    Reply
    • The Marine Detective

      Well that made my day Jan! We only have the poor vis on shore dives generally. There’s some much water rushing in out and of the passes that even in summer the vis on boat dives is quite good.

      Reply
  2. Meg OMahony

    Great post! May I use some of your photos & info (with proper referencing, of course) in my slides for the Bio class I’m teaching this summer. Real examples kick butt over textbook diagrams!!

    Thanks! -definitely for the interesting post -hopefully for your permission

    Meg O’Mahony

    Sent from Meg’s iPhone

    >

    Reply
  3. Michael Jackson

    Nicely done, Jackie! Very clear explanation of a complex phenomenon!

    Reply
  4. Victoria

    You never cease to create a sense of wonder in these posts. Thank you for all you do!

    Reply
  5. Celia Lewis

    Ahhh, another mystery explained!! I always find it fascinating how plants manage to reproduce in different environments. This was so clear. Merci buckets!

    Reply
  6. Maggie Conners

    Great information. You take wonderful photos of Bull Kelp!!

    Reply
  7. Morbeau

    I remember studying this in botany school, but we never talked about bull kelp. That’s what happens when you live in Alberta – no kelp, and few ferns to talk about. 🙂

    Thanks for the refresher!

    Reply

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