Mid-May had a fantastic tide series and I was able to explore the rocky intertidal twice. My first early morning stop was on a Sunday at Luffenholtz Beach, near Trinidad California in Humboldt County. I’ve intermittently tidepooled in the Trinidad area over the years, usually going to the rocks just below the old lighthouse. Back in the Bay Area later that day I woke up early again on Monday to visit Mile Rock Beach at Lands End in San Francisco.
Besides the time to photograph and identify seaweeds, both visits were excellent reminders of the role of disturbance in shaping intertidal communities, although each site provided different examples. At Luffenholtz, the extensive intertidal was lush with red algae and kelps, that was until I came upon a small nearly barren area tucked between large rocks which was full of urchins. A bit later, perhaps only 30 meters away, I came upon a second barren, this one much larger than the first. It too was full of urchins. The thick algal canopies that were common elsewhere in the intertidal, only persisted on a few of the highest rocks because the dense urchins, hunkered down in the low intertidal zone, had mowed virtually every rock clean.
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Lush intertidal flora at Luffenholtz Beach. |
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Lush algae above, urchin barren below at Luffenholtz. |
The spread of urchin barrens in the Northeast Pacific, and their decimation of some local kelp forests, is not a new story, but it is still a little shocking when I see examples of this with my own eyes. Multiple visits to the rocky intertidal at Carmel Point near Monterey over about two decades also brought the urchin barren story home to me when I found that a similarly lush area of low intertidal had been dramatically altered by a local urchin explosion.
Luffenholtz was windy in the morning though I did not think too much of it. Wind is common on the PNW coast. However, the wind was still blowing in the Bay Area a few hundred miles to the south by Sunday afternoon and it continued into Monday morning for my next intertidal visit in San Francisco. In the famously foggy city, Monday morning’s wind brought in relatively warm air out of the northeast, keeping fog away and providing temperatures that felt a little atypically warm. This warm, and probably drier, air in fact imposed a terrestrial stressor on organisms that are accustomed to being covered with cool saline water. While many species that live in the higher zones of the intertidal are adapted to fairly long periods of being exposed to air, heat, and wind, low intertidal species are generally less tolerant because they spend most of the time submerged.
Wandering Mile Rock Beach for a couple hours I saw shriveled up low intertidal kelps (Laminaria sinclairii) and stressed out delicate red seaweeds. Some of the red algae may have been literally bleaching before my eyes as they turned from a rich purple red to a sickly orange color. Desiccation stress of this nature may not be terribly frequent; after all it requires that a daytime low tide coincide with strong dry winds.
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Wilting kelps, Laminaria sinclairii, at Mile Rock Beach, San Francisco. |
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Bleaching red algae at Mile Rock Beach. |
On top of these local or regional stressors, organisms are facing another pervasive disturbance – global climate change. Recently, the Northeast Pacific Ocean has been anomalously warm. Whether directly tied to climate change or not, anthropogenic warming certainly excerbates natural oceanic climate cycles, and adds another layer of environmental stress to the other local factors organisms have to deal with from time to time.
Disturbances are a natural part of ecosystems, shaping short-term ecological dynamics and influencing the evolution of populations and species. They can be short-term or long-term, ocean-basin-wide or happen at very small scales. Yet each can affect the composition of coastal communities in some way. A ripped out patch of large mussels allows smaller species to recruit, or annual sea palm kelps to get a foothold in the middle intertidal zone. An unusually warm wind event during a tide or two will lead to die off of less tolerant algae that may open up space for new propagules to settle.
