Pacific Rocky Intertidal Monitoring: Trends and Synthesis

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Alegria 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 Alegria 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).  Anthopleura cover declined steadily from plot establishment in 1992 through 1999, after which there have been fluctuations between 40% and 80% cover.  Cover of Anthopleura varied inversely with rock and/or sand with only low cover of other taxa (mostly barnacles and turf algae) within one or more plots.

The barnacle plots at Alegria 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.  One of the original plots at this site was lost completely due to a rock breakout while another became dominated by a persistent Lottia gigantea farm.  Two plots were added in recent years (spring 2007 and spring 2008) as replacements. Barnacle cover varied inversely with rock as periodic scour episodes have occurred.  In recent years, Endocladia, mussels and anemones have begun encroaching on three of the plots, but the average cover has yet to meet the threshold for inclusion in the graphs. Motile invertebrate counts at this site began seasonally in the Fall of 2000 (with littorines added the following fall of 2001) and were changed to annual sampling in 2004.  Both littorines and limpets have been steadily abundant in the Chthamalus plots with the former ranging around 1000 individuals per plot and the latter ranging more variably between 10 and 100 individuals per plot.  Whelks (Nucella sp.) have also been present in low numbers except in 2002 and 2006 when the numbers dropped to zero. 

The Pollicipes plots at this site have remained stable throughout the years with around 20% cover Pollicipes, 40-50% cover mussels, and rock varying from about 20-40%. Motile invertebrate counts show that limpets have been common and highly variable within the Pollicipes plots with abundance as high as 80 individuals per plot on two occasions.  Littorines have been less common in these plots at less than 40 individuals per plot.  In addition, whelk snails (Nucella spp.) and shore crabs (Pachygrapsus crassipes) have been present in low numbers.

Mytilus cover in the mussel plots began around 80% on average in 1992, declined gradually to about half that in 1998, then gradually rebounded to over 60% by 2004.  Then a reef-wide crash occurred in 2006 with cover dropping to zero in, and in the vicinity of, the photoplots. These plots are located on a wide, low-relief reef flat that was presumably scoured during a large winter storm.  Mussels are still present in other, more high-relief areas within the site, such as the reef where the Pollicipes plots are located. The reef flat population has yet to recover.  In the meantime, the plots have become dominated by a mixture of barnacles, colonial anemones, prostrate Phragmatopoma, articulated corallines and other turf algae, and patches of Phyllospadix.  One small recruitment event offered some promise of recovery in the spring of 2011, but those mussels had disappeared by the following fall sampling. Motile invertebrate sampling show frequent but low numbers of whelks (Nucella spp), shore crabs (Pachygrapsus crassipes) and volcano limpets (Fissurella volcano).  On the other hand, Tegula funebralis snails have been more common and increased  to large numbers (over 120 individuals per plot) until 2006 when the mussel population, and the Tegula population with it, crashed.  Following the mussel crash, limpet numbers began to increase as bare rock became more prevalent.  The numbers of Tegula for the spring of 2004 were similar to the preceding and following sampling seasons.

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.   Other than a few spots of tar, these plots have yet to see any barnacle recruitment.

Surfgrass (Phyllospadix) transects were added to Alegria in the fall of 2001.  Since that time the mean cover of surfgrass has hovered in the 70-80% range with seasonal oscillation (lower cover in the spring following winter storms that ripped out plants and abraded leaves; recovery by fall).  There have been no long-term trends in surfgrass cover.  These transects were established at the inshore margin of an extensive, thick surfgrass bed which extends into the subtidal.

Seastar (Pisaster ochraceus) plots were added to this site in the spring of 2002 and consist of three large irregular plots surrounding an area of high relief rock pinnacles and deep tidepools.  At the first sampling, the total number of seastars counted and measured was around 120.  That number increased to over 200 by the following fall, but declined sharply thereafter and has been hovering around 40 seastars since 2006.  While a few smaller (<50mm radius) stars have been found at this site throughout the years, those encountered tend to be larger (>60mm).  General observations, along with the size distribution depicted in the trend graph, suggest that stars are moving in and out from the subtidal rather than recruiting to the monitoring site.

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)

Alegria Anthopleura trend plot

Chthamalus/Balanus (Acorn Barnacles) - percent cover

Alegria barnacle trend plot

Chthamalus/Balanus (Acorn Barnacles) - motile invertebrate counts

Alegria barnacle trend plot

Pollicipes (Goose Barnacle) - percent cover

Alegria Pollicipes trend plot

Pollicipes (Goose Barnacle) - motile invertebrate counts

Alegria Pollicipes trend plot

Mytilus (California Mussel) - percent cover

Alegria Mytilus trend plot

Mytilus (California Mussel) - motile invertebrate counts

Alegria Mytilus trend plot

Rock (Above Barnacles)

Alegria rock trend plot

Transects

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)

Alegria 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 ochraceus (Ochre Star) - counts

Alegria Pisaster trend plot

Pisaster ochraceus (Ochre Star) - sizes

Alegria Pisaster size plot

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Interactive Map

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