Aging old growth kelp

You think old growth trees are cool…..well so is ‘old growth kelp.’

Me and my wonderful-awesome research assistant Tanya Prinzing have been counting the rings on many samples of the kelp Pterygophora californica over the past couple of weeks. Why? Because ecologist Dr. Jane Watson and others have shown that the annual growth rings can be used to estimate the age of individuals. Jane also showed in her PhD thesis that Pterygophora can reveal information about sea otter recovery.

When sea otters return to an area, they often consume most of the sea urchins in that area, which releases subtidal algae species from grazing pressure (ie. being munched!). Following what is often a first year of colonizing annual species, longer lived species tend to dominate the substrate. One such species is Pterygophor californica – a common undersstory species that can live up to 15+ years (which is really old for a kelp!).

Because urchins are removed all at once, the algae species recruit onto the new substrate in one rather similar-aged cohort. Jane showed in her PhD thesis that if you sample P. californica populations from these deeper areas (7-10m) where sea urchins once were, the “modal age” (ie. most common age encountered in the population) of the kelp stand correlates rather well with the estimated time of sea otter appearance. Hence, determining the age of the ‘old growth’ kelp can help us to estimate when sea otters may have arrived in an area (because in remote coastal areas….we’re not always around to witness when that happens).

We would like to know this information for our sites on the Central Coast of B.C……and so….we are counting kelp rings to hopefully give us more information. Stay tuned for results….

Pterygophora samples

Our collection of Pterygophora californica samples, freshly harvested from several sites on the Central Coast



Tanya and I count rings on our super cool briefcase light table. Coolest piece of lab gear we own!


Pterygophora stipes

Here are some examples of the cross-sections of the P. californica stipes. The rows show stipes from older (top) to younger (bottom) individuals.