Photos and trip report by Sheila Byers
Ecosystems change with time whether it be seasonally or over a period of years. A visit to the rocky intertidal zone off Figurehead Point in Stanley Park on Sunday, June 16, 2019 is a good example of changes to inhabitants within this community and this is my perspective on what’s happening.
It was Father’s Day and a Dad and his daughter joined us on our beach exploration on the sandy and rocky intertidal areas. There is usually a good turnout on Father’s Day, but a faux pas on my part to not know that the 35thAnnual World Fundraiser to Alleviate Global Poverty Congress was scheduled at Lumberman’s Arch likely prevented others who had planned to attend but were deterred by the throngs of people and traffic. Nonetheless, the exploration was an important one in terms of the ecosystem changes that we observed.
The very low Spring tide (0.5 m) exposed several surprises. By June, the low intertidal zone is generally densely covered with algae like sugar wrack kelp, narrow-winged kelp, seersucker kelp, young bull kelp, and lots of sea lettuce (Ulva lactuca?); the latter especially prevalent at the mid intertidal elevation. Although all these species were present on Sunday, the density and coverage of the algae seemed smaller, leaving patches of substrate with no algal cover at the lower elevation.
Many large thatched barnacles were attached to the boulders during a May visit but very few were seen in June. Perhaps a more thorough investigation is required; or, had they reached the end of their life span (10-15 years)? Multitudes of the predatory wrinkled or frilled dogwinkle were seen in May. Fewer were observed in June although patches of their hatched, oat-sized egg capsules were still evident. Their presence and population, however, has been steadily increasing since first observed here in 2013.
Common barnacle spat (< 1-year old juveniles), were plastered over all the rocks, large or small, with little evidence of the Pacific blue mussel (< 1-year olds) except in the cracks and crevices. Competition for space in the intertidal is fierce and the barnacles appear to have outcompeted the Pacific blue mussel recruitment this year. Interestingly, last year in May 2018 large blue mussels that covered an estimated 50% of the rocks are now gone. Also, in 2008, 2009, the blue mussels were ‘king of the intertidal rock castles’; so much so that the dead mussel shells covered the substrate between the rocks like a crunchy blue carpet.
Common barnacles live for about 10 years while the life span of blue mussels is about two years. It appears that 2019 community depicts the coincidental aftermath of a die-off of both the 10-year class of barnacles along with the 2-year class of mussels, leaving the rocky intertidal real estate up for grabs. The barnacle spat monopolized the space; not the blue mussels.
Other than life spans and competition, are there other factors involved? Certainly – predation and potentially warming oceans due to climate change. Part of the success of barnacles, however, is that their cyprid larvae are known to be attracted to empty adult shells when settling out of the water column onto rocks. Tangentially, is there some physical or chemical deterrent of the calcified base left behind by barnacles that acts as a deterrent to the settlement or attachment of mussels to these same rocks? Time will tell.
It was reassuring to find several dozen purple sea stars tucked out of the sun under the rock ledges, several leather stars and mottled stars. A six-armed leather star was a first-time observation for me. Sadly, one purple star (~5 cm radius) that was found on the sandy area showed white lesions characteristic of Sea Star Wasting Disease (SSWD). The SSWD associated densovirus has been prevalent in BC since at least 2013 and is obviously still affecting the local sea star populations.
Life on the intertidal zone is complex. This account represents changes to only a few species that inhabit the rocky intertidal zone off Figurehead Point. I wonder what surprises 2020 will bring.