Weeds in the Intertidal Garden

Out of long, long, consideration of the parts he [Darwin] emerged with a sense of the whole. Where we wished for a month on station, and took two days, Darwin stayed three months. Of course, he could see and tabulate. It was the pace that made the difference, and in the writing of Darwin, as in his thinking, there is this slow heave of a sailing ship, and the patience of waiting for a tide.
– Steinbeck speaking of Darwin, in the Sea of Cortez, 1941.

In the California marine intertidal we have prominent rockweeds and turfweeds that arrogate broad swaths of rock surface, preventing other species from settling in the space they occupy, and often becoming the predominant species in an intertidal community. We have wondered how long the intertidal has been overrun by these marine algae, and how settlement could be accomplished by animals when so much space is taken by “weeds.”

At the same time our surveys presented this abundance of plants, we found an apparent lack of biodiversity at sites where one or more of several species ̶ barnacles, mussels, sea stars, shore crabs, and sea urchins ̶ appeared absent from the intertidal community, and whether biodiversity is impacted by this rank population of algae.Missing species by survey site

 Intertidal communities. Numerous communities on the California coast have been identified as having a higher coverage of marine algae than invertebrates (Ocean Science Trust, 2013), and this finding is consistent with our experience surveying intertidal ecology of the central California coast this Spring (2014). At four of our survey sites we found the middle intertidal zones dominated by turfweed (Endocladia muricata: aka Brillo Pad, Nail Brush, Pot Scrubber, tufted red algae), stunted Turkish towel (Mastocarpus papillatus), and much of the rock surface was encrusted with tar spot algae (Petrocelis). Turfweed in places covers as much as 90 percent of rock surfaces. E. muricata briefly yields dominance to black pine rockweed (Neorhodomela larix) that blankets about half of the mid-tidal rock shelf surface at Fitzgerald Marine Reserve, but is overwhelmed in Gerstle Cove by two different unidentified strains of rockweed that together occupied 40 percent of Zone 2 in our survey area and 20 percent of Zone 3.

Average algae density by survey site
Are marine algae the same kind of noxious invaders as terrestrial weeds which, we are told, are harmful and have no redeeming value, spread quickly through the garden, guzzle water and nutrients, take up space better plants might use, propagate profusely and are difficult to remove? In the course of our survey, we have reviewed the publications of several researchers on marine algae, and E. muricata in particular, as well as on biodiversity in intertidal communities, and the experience they report has brought us to a more considered understanding of the dynamics of life between the tides.

“Longtime Californ’.” We mentioned in our Pacific Grove survey that “the predominance of Endocladia muricata in the intertidal of the sites surveyed to date is unexpected for someone returning to the shoreline after thirty years away,” the implication being that we did not recall there being so much algae in the intertidal and thought it might be a recent phenomenon. However, examination of older photos of the California intertidal shows marine algae present in abundance similar to what we have now. This can be seen by comparing photos in the 3rd edition of Between Pacific Tides, first published in 1939, with photos taken for our surveys.DSCN0616 #3

DSCN0205DSCN0612 #2

Our photo from Coral Street in Pacific Grove shows the dense cover of Endocladia there today, and an equally dense cover can be seen in Ricketts’ photos taken at Tomales Point and Point Lobos back in 1939. Hunt’s 2006 paper on Endocladia has a similar comparison, photographs of the same location at Hopkin’s Marine Station in 1947 and in 2006.

Competition for space. Turfweed (E. muricata ) is known to compete for space with barnacles (Forde and Raimondi, 2003) and with mussels and rockweed (N. larix; Friday Harbor, 2014). Expanding mussel patches may displace Endocladia (UCSC, 2012, Endocladia), but when mussels are removed from a site by predation or environmental disturbances, turfweed will move into the empty space and start new growth; turfweed will even grow on top of mussel shells (Friday Harbor, 2014). Marine herbivores such as limpets compete with algae by grazing on spores and other recruits of algae and invertebrates that attach to open rock surfaces, maintaining open space as meadow for grazing (Walder, 1999); owl limpets in particular are reported to be very aggressive in clearing open space (Ricketts, 1985). On one hand, the activity of these herbivores prevents algae from colonizing, and checks the expansion of turfweed, but at the same time prevents other algae from competing for space with turfweed. When grazers are not present in an Endocladia assemblage, algae spreads and open space declines (Walder, 1999). Substantial growth of E. muricata may imply the relative absence of herbivorous gastropods, or numbers insufficient to significantly retard the growth of the algae. At the same time, abundance of E. muricata may prevent recruitment and settlement of herbivorous gastropods.

Turfweed mitigates settlement. Far from impeding settlement in the littoral, turfweed and rockweed have been found to provide habitat for many neighbors that shelter under the algae’s canopy to avoid the stress of wave action, desiccation, and being eaten by limpets, including even mussel larvae. Amphipods, Petrocelis, M. papillatus, and Sylvetia have been found together with turfweed, and one researcher (Glynn, 1965) has been cited as reporting between 60 and 90 species sheltering in E. muricata in Monterey Bay (Friday Harbor, 2014; UCSC, 2012, Endocladia). So while marine algae may compete with other species for space, the algae also facilitates other species by offering refuge from physical stress, predation, competition, and improves the availability of resources. Facilitation, however, as a principle in the ecology of intertidal communities is only one of several conditions that might determine how a community comes together, prevails, and persists over time; variations in physical environment and seasonal changes, among others, create a changing “mosaic” of conditions that determine the character of each community (Forde and Raimondi, 2003; Foster et al, 2003).

Some dynamics of marine biodiversity. When recruitment and settlement of marine invertebrates is so completely dependent on environment, conditions of ocean water, and drift of the currents, how can it be any wonder that one place has barnacles and limpets, but no mussels or shore crabs, and further up the coast another place has shore crabs by the thousands in one tiny cove, and shoals of mussels along open coast. What life can settle onto rock surfaces from out of plankton may depend on whether a substrate is sedimentary or granitic (Raimondi, 2003), a new surface or weathered (Hunt, 2006), solid rock shelves or boulders that can be overturned by waves, or rock fractured or scoured by sand and cobble (Foster et al, 2003). Surface water temperatures change with exposure and have varying influence in settlement of open and protected coast (Raimondi, 2003; Hunt, 2006). South facing exposures have more direct sunlight than north facing exposures and may accelerate desiccation on one side of a cove and have no effect on the other side.

As for the apparent lack of biodiversity at our survey sites, while it would seem to be more than just coincidence that so many keystone species could not be found, the fact remains that each of our surveys was a static view, one look at a location over a matter of hours. The intertidal at any one location, as well as along the entire coast, is a dynamic macrocosm different from place to place according to a variety of conditions. Substrates, environmental conditions, seasonal variations, habitat change, human impact, or crucial events like contagion, harmful algae blooms, or global climate change, even El Nino all have an impact on individual intertidal communities. Changing one or more of these variables can result in a different community.

Biodiversity on land. How different is the case for biodiversity on land? We have been watching a High Sierra meadow in Quincy CA for two weeks now, much longer than any of these surveys, and we have seen many critters, on different days mind you. Deer on Monday, foxes on Wednesdays (washdays), Canada geese and robins nearly every day, and smelled skunks prowling some nights. Tuesdays and Thursdays when we drop by the meadow is usually empty. We have not seen rabbits, ravens, or Steller’s jays (which jays, by the way, were our companions at Timber Cove CA, elevation 100 feet). There are any number of good reasons why there are no cottontails here, and at present no way of knowing why. We do know why there are no bison, or mammoths. Down the road a ways, in Portola, there may be rabbits, Steller’s jays, and elk, and there would also be reasons for their presence there and not in Quincy (especially on Wednesdays).

So, how would we explain the slow return of bunnies to a Quincy meadow, should it happen, or the gradual reappearance of thousands of purple shore crabs off the Coral Street intertidal in Pacific Grove? Perhaps at some point we might be able to explain such marvels, yet some things we may never be able to explain no matter how close our attention over time. Trouble is, we have no way of telling which is which, so we must keep close, patient watch on them all, over a length of time sufficient to solve their mysteries, and try to be there when things change.

Trends and temporary reversals. Eventually, the fundamental question becomes one of whether the apparent consistent decline in present biodiversity noted in our surveys is a trend or a temporary reversal. We can record the diversity of populations by location, but learning the reasons for the variations in diversity is much more difficult. Some of those reasons can be discovered with long-term detailed monitoring of ocean conditions and population changes at specific locations. And so we have long-term projects, like those conducted by universities and public agencies under the auspices of California’s 1999 Marine Life Protection Act (MPA) to set aside marine reserves and monitor conditions there, in order to accumulate historical marine demographics not available from a single season’s surveys by amateur naturalists.


Forde, Samantha E. and Peter T. Raimondi, An experimental test of the effects of variation in recruitment intensity on intertidal community composition, University of California, Santa Cruz, 2003.

Foster, Michael S. et. al., Temporal variation and succession in an algal-dominated high intertidal assemblage, Journal of Experimental Marine Biology and Ecology 289, 2003.

Friday Harbor Laboratories (FHL), University of Washington, “Endocladia muricata, The Brillo Pad Algae,” Friday Harbor Labs Marine Botany website, 2014.

Glynn, P., Ecological studies on the Endocladia muricata-Balanus glandula association in the intertidal zone in Monterey Bay, California, Beaufortia 12: 1-198, 1965.

Hunt, Luke John Hoot, The Rise Of Endocladia Muricata: Punctuated Change At An Abrupt Range Edge, PhD Dissertation, Stanford University, 2006.

Ocean Science Trust, State of the California Central Coast: Results from Baseline Monitoring of Marine Protected Areas 2007–2012, California Ocean Science Trust and California Department of Fish and Wildlife, California, USA. February 2013.

Ricketts, Edward F. and Jack Calvin, Between Pacific Tides, Stanford University Press, Stanford, CA, 1985.

Steinbeck, John and Ed Ricketts, Sea of Cortez, A Leisurely Journal of Travel and Research, J. J. Little and Ives, New York, 1941.

UCSC, Shell Beach Long-Term trends, Pacific Rocky Intertidal Monitoring: Trends and Synthesis website, UCSC website, 2012.

UCSC, Endocladia (Turfweed), Pacific Rocky Intertidal Monitoring: Trends and Synthesis, UCSC website, 2012.

Walder, Ronald K., The persistence of open space in the rocky intertidal Endocladia muricata assemblage, Master’s Thesis, San Francisco State University, 1999.


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