Boiler Bay

Boiler Bay rocky intertidal (north of the
small sandy cove). Is this the boiler from
which the site gained its name?

Although I lived in Oregon for several years (including a year and a half on the coast), I did less exploration of the rocky intertidal during those years than I would typically do during the same time span living in California. Oregon’s exceptionally scenic coastline features dunes, sandy beaches, rocky shores, and numerous estuaries. It was the latter ecosystem that I explored the most, working up and down the coast to research Oregon’s tidal wetlands.

For my last rocky shore visit of this month’s spring tide series, I wandered about the shores of Boiler Bay, a small cove north of Depot Bay on the central Oregon coast. A short steep trail leads to a small sandy beach at the head of the cove, with rocky intertidal benches to the north and south. Boiler Bay is apparently a well studied area, as evidenced by bolts and other obvious scientific interventions along the shoreline. I believe it may be a study site for Bruce Menge’s long-term study of Oregon’s rocky intertidal communities. South of the cove there was even a collection of dozens of small pools cut into the bedrock, too uniform in size and location to not be the hard work of a former research project, perhaps the sweat and tears of a graduate student dissertation from years past.

Various evidences of scientific interventions to study the rocky shore at Boiler Bay.

After observing for a few hours, an obvious ecological story of the Boiler Bay rocky intertidal is that of plant versus herbivore. Along the rocky benches south of the cove, large areas – dominated by purple urchins (Stronglyocentrotus purpuratus) in the thousands – are clearly claimed by herbivore. I don’t ever recall seeing intertidal urchins in such high densities anywhere else I’ve wandered on the west coast. The power of the urchins to shape this segment of coastline has even formed a geologic imprint: many of the urchins rest in small cavities carved into the rocks, the gradual work of spines that have eroded the mudstone over however many generations.

Small bull kelp in a tide pool at Boiler Bay.

Large fleshy seaweeds are prolific in other areas, away from the urchins. This means they are more abundant at higher shoreline elevations where urchins probably can’t tolerate the prolonged exposure to air, or in lower tidal areas where urchins perhaps aren’t able to gain high densities for other reasons. The large kelp Saccharina sessile grows in abundance higher on the shore out of reach of the grazing hordes, while Alaria marginata and Nereocystis luetkeana (bull kelp) occur in low intertidal pools or other fortuitous havens of safety. The elegant bull kelps typically occurred in aggregations as small to medium plants that will never reach the much larger stature that plants forming thick forests offshore will attain.

There is one notable kelp species that seems more impervious to the grazing menace of the purple urchins: Costaria costata. Having various common names like seersucker or five ribbed kelp, this striking species consists of a single, highly ruffled brown blade growing out of the base of the plant. Within the extensive urchin enclaves of the low intertidal at Boiler Bay, Costaria was present in remarkable abundance. The plants were typically small, and often tattered, but it was basically the only large seaweed common in the “barrens” where the purple urchins ruled.

Costaria costata sporophytes with abundant purple urchins in the low intertidal zone.

Costaria’s secret may lie in part from being an annual species. With quick growth in the spring, some plants may escape herbivory long enough to produce spores to continue the life cycle. Another possibility is that Costaria is a less preferred food source for the hungry urchins. Alternatively, competition might limit Costaria to urchin-dominated areas. In areas where perennial kelps like Laminaria setchellii or Pterygophora californica are present, perhaps the annual Costaria seldom gains the upper hand in competition with the established plants for space and light.

One important ecological force structuring coastal rocky shore ecosystems in Oregon however, lays obscured in history. Before being hunted severely to near extinction, the sea otter of the northeastern Pacific was a key predator of urchins, keeping grazers in check and promoting kelp abundance. This example of a marine “trophic cascade” was described decades ago in work done by ecologist Jim Estes. In a trophic cascade, healthy predator populations keep herbivore abundance low which favors primary producer populations. Today however, sea otters are essentially absent from Oregon’s coastline, so a key link in the historic web of connected coastal species is missing. Are Oregon urchin populations much higher today than they were several hundred years ago when otters were present? Whatever combination of mechanisms underlies the spatial patterns of distribution of urchins and Costaria, the observations suggest an interesting story.

Sponge in a low intertidal pool.

Large red urchins, Stronglyocentrotus franciscanus, were also relatively common at Boiler Bay, though their numbers are dwarfed by their smaller purple cousins. These grazers tended to be underwater (e.g., in pools), perhaps less tolerant of being exposed to air than their congener. Other intertidal invertebrates included beds of mussels, chitons, a few species of sea stars, and aggregating and solitary anemones. I observed no nudibranchs, but did see a few bright yellow sponges in the low intertidal and a lovely crab which I may try to identify when I am back in the company of a good reference book. 

Low intertidal crab at Boiler Bay.