Pacific Rocky Intertidal Monitoring: Trends and Synthesis home

Mussel Shoals Long-Term trends

See below for trend graphs

In order to standardize species resolution across all MARINe groups, and over time, some species (typically rare) were lumped for graphical presentation of Long-Term monitoring data. See lumped categories for definitions (some variation occurs between methods and over time).

The anemone plots at Mussel Shoals consist primarily of the colonial anemone, Anthopleura elegantissima rather than the solitary anemone Anthopleura sola (note that these were considered a single species prior to the past several years and they are not distinguished in our sampling methods).  Within the anemone plots, the mean cover of Anthopleura began close to 100% in 1994, declined gradually to under 60% by 1998, then hovered around 80% for several years through 2004.  Since then, two additional cycles of lesser decline and recovery have occurred with the mean cover back in the 80% range presently.  Much of the variability seen here is the result of a single plot which, since its inception, has alternated back and forth between anemone dominated and mussel dominated cover.  Of the remaining four plots, two have exhibited less drastic variability, with periods of increased turf algae cover, while the other two plots have changed very little with Anthopleura clones covering nearly 100% of the plots.  In fact, one of these plots has had a consistent anemone clone separation line in the same position for over ten years.

The barnacle plots at this site consist of a mixture of Chthamalus dalli/fissus (note that species were not distinguished until 2001) and Balanus glandula with the former dominating some plots and a more even mixture in others.  These plots are located on sloping riprap boulders in an area subjected to recurring scour by sand and cobble.  This is reflected in the data which show barnacle cover (and, inversely, bare rock), fluctuating extensively and repeatedly throughout the years.  Barnacle cover has been reduced to near zero twice during the monitoring period (spring of 1996 and spring of 2007) with less significant declines to around 40% occurring four other times.  In each case, high recruitment has allowed these plots to rebound within a single sampling season with cover usually reaching levels of 80 to 90%.  One plot disappeared completely in the late 1990's as the boulder it was housed on became dislodged and overturned.  A replacement plot was installed, but shortly thereafter, a subsequent storm flipped the boulder over again allowing the sampling of that missing plot to resume.  While the orientation of that plot has changed somewhat, making it more susceptible to encroachment by mussels, anemones and turf algae, it continues to be sampled along with its replacement plot. Motile invertebrate counts at this site began seasonally in the fall of 2000 and were changed to annual sampling in 2004.  Within the barnacle plots, littorines and limpets are both highly abundant with the former varying in the neighborhood of 500 individuals per plot and the latter around 50 per plot.  Like barnacles, limpets were scoured to nearly zero in the spring of 2006.

The mussel plots at have been fairly stable throughout the years with mussel cover in the 80 to 90% range most of that time.  A single decline occurred in 2005 and 2006 reducing mussel cover to about 60%.  Since then, mussel cover gradually increased to near 100% in 2009 and has been just a bit lower since then.  These plots are mostly comprised of small and tightly packed mussels, and in addition are located on the shoreward side of a large rock ridge, presumably making them more resistant to scour or breakout. The data from the Motile invertebrate sampling show that snails (Nucella spp. and Tegula funebralis) were more common in the mussel plots in the first few years of sampling.  Shore crabs (Pachygrapsus crassipes) were also a bit higher then as well.  In recent years, the abundance of these motiles, along with littorines, has been low (less than ten individuals per plot).  Limpets, on the other hand, started out low in the early years, but with increases to over 50 individuals per plot in the later years.  This may be partly due to methodological changes:  prior to 2006, only limpets on rock were included in the counts whereas limpets occurring on mussel shells were added after that. 

Rock ("Above Barnacles") plots were added to this site in the fall of 2008 to document any upward spread of intertidal species as a result of global climate change or other factors.  These plots are expected to remain dominated by bare substrates unless barnacles or other species begin to encroach upon them.  As of the spring of 2010, no barnacles had been recorded, although a few small recruits had been observed within these plots.  

The mean cover of Surfgrass (Phyllospadix spp.) at this site has declined from around 80% to near 40% since the inception of these transects in 1994.  This decline occurred precipitously between 1995 and 1998 after which the cover remained near 40% for around seven years.  Then an additional decline proceeded gradually with the mean cover hitting a low of around 20% in the spring of 2009.  Since then, the cover has rebounded back to around 40%.  The initial decline in the late 1990's was met with an increase in bare rock, and Egregia has had periods of higher cover, but species of red algae (primarily Chondracanthus sp., Gastroclonium sp. and filamentous species) have come to fill in most of the habitat that had previously been occupied by Phyllospadix

Seastar (Pisaster ochraceus) abundance has been variable at this site from over 90 stars counted and measured within the plots to around ten.  Seastar numbers peaked in 1996, were particularly low from 1998 to 2001, and have shown seasonal fluctuations (lower in spring, higher in fall) since then with numbers on the lower end of the range as of spring 2010.  Seastar sizes have been measured since the fall of 2000.  Their sizes tend to be relatively large at this site compared to others with relatively low recruitment of small (<50mm) individuals in the earlier sampling seasons, and almost no individuals smaller than 90mm observed since the spring of 2004.   This suggests that, despite extensive sand barriers offshore of the sites, the seastar population at the site is maintained through migration in and out of the site rather than through recruitment.

Photo Plots

Long-Term methods Photo Plot thumbnail

Below are the trends observed for each Photo Plot target species at this site. Long-Term percent cover trend graphs show all species that reached a minimum of 25% cover during any single point in time within a given target species assemblage. Breaks in trend lines represent missed sampling events. For additional species observed that did not meet this 25% threshold, please use the Interactive Map.

For motile invertebrate Species Counts, a mean across all plots was calculated, and only those species with a value of at least 5 individuals for at least one sample are shown. For motile invertebrate size trend graphs by site, please use the Interactive Map.

Anthopleura (Anemones)

Mussel Shoals barnacle trend plot

Chthamalus/Balanus (Acorn Barnacles) - percent cover

Mussel Shoals Mytilus trend plot

Chthamalus/Balanus (Acorn Barnacles) - motile invertebrate counts

Mussel Shoals Mytilus trend plot

Mytilus (California Mussel) - percent cover

Mussel Shoals Mytilus trend plot

Mytilus (California Mussel) - motile invertebrate counts

Mussel Shoals Mytilus trend plot

Rock (Above Barnacles)

Mussel Shoals Mytilus trend plot


Long-Term methods Transects thumbnail

Below are the trends observed for each Transect target species at this site. Long-Term trend graphs show all species that reached a minimum of 25% cover during any single point in time within a given target species assemblage. Breaks in trend lines represent missed sampling events.

Phyllospadix (Surfgrass)

Mussel Shoals surfgrass trend plot

Species Counts and Sizes

Long-Term methods Counts thumbnail

Species Counts and Sizes (where recorded) for Pisaster are shown below for this site. The sum of all individuals across all plots is displayed.

Pisaster (Ochre Star) - counts

Mussel Shoals Pisaster trend plot

Pisaster (Ochre Star) - sizes

Mussel Shoals Pisaster size plot

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