Pacific Rocky Intertidal Monitoring: Trends and Synthesis

pacificrockyintertidal.org home

Boathouse 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).

Anemone plots at Boathouse are mainly a mixture of medium-sized, solitary Anthopleura sola, and several small species of red algae (primarily Chondracanthus canaliculatus). Chondracanthus canaliculatus has been recorded as having high cover at this site overall by the biodiversity surveys. Anemone cover has been fairly low but constant over the nearly 20 year span that this long-lived species has been monitored.

Barnacle plots at Boathouse mainly consist of Chthamalus dalli/fissus, although Balanus glandula has occasionally reached cover levels as high as 15-20% in three of the five plots (note that species were not distinguished until 2001). Cover of Chthamalus/Balanus in the barnacle plots at Boathouse was initially high (approximately 80%) and then plummeted to around 20% for the next several years, replaced largely by the red turf alga, Endocladia. Endocladia cover remained fairly high in most plots until around 2001, when it experienced a sharp decline in all five photoplots. Endocladia cover remained low in all but one plot, which is somewhat lower in tidal elevation, and thus desiccation stress on the alga is likely lower. The decline in Endocladia cover was accompanied by an increase in Chthamalus/Balanus cover, which steadily rose until 2004, when it stabilized around 80%.

A similar swapping of species occurred in the Endocladia plots, where declines of the red turf alga in 1992 and 1998 were followed by steady increases in the rockweed, Silvetia. Silvetia cover experienced a sharp decline in 2008, and it appears that Endocladia is beginning to fill the available bare space.

Silvetia cover in Silvetia plots remained high over much of the nearly 20 year monitoring period, although cover declined somewhat around 2008/2009. In some plots this open space remains bare, whereas in others it has been filled in somewhat by Endocladia and the red blade alga Mazzaella. Silvetia plots were not sampled in fall 1995.

Further investigation into the dynamic interaction among barnacles, Endocladia, and Silvetia over space in the upper intertidal has revealed that the barnacle zone has experienced periods where it shifted upward, into areas that were previously bare rock. It had long been assumed that the upper limits of species’ zones were set by physical factors such as temperature and emersion time, and were thus stable, while the lower limits were set by biological interactions (e.g. competition and predation). However, the upward shifts of species zones documented in this study suggest that facilitation, a biological factor, is important for establishing species’ upper limits. In this case, barnacles are facilitating the upward movement of Endocladia by providing favorable settlement substrate and a refuge from grazers, and Endocladia is providing suitable habitat for Silvetia, above the zone where it previously occurred. The process is likely “reset” every so often by an extreme event (e.g. large storm waves or a period of above average warm weather), which removes Endocladia and Silvetia living above their “normal” tidal level, freeing up bare space for new settlement of barnacles.

Limpets and littorines were the dominant motile invertebrates in the barnacle and Endocladia plots. These species were also present in the Silvetia plots, but generally to a lesser degree. A third species, Tegula funebralis was common in all three plot types, increasing in abundance with decreasing tidal height, with counts generally highest in Silvetia plots and lowest in barnacle plots. A fourth species, Pagurus spp. was common only in Silvetia plots.

Mytilus cover remained relatively high throughout most of the nearly 20 year period that mussel plots have been monitored at Boathouse. One exception was plot 4, which was accidently cleared in winter 2005 by another researcher working at the site. Recovery of Mytilus has been slow in this plot, likely because a number of owl limpets (Lottia gigantea) moved into the open space. These large grazers maintain algal “farms”, within which they remove most newly settled organisms in order to provide a large area for diatoms to grow.  Limpets were by far the most common motile invertebrate counted in the mussel plots. These were largely small individuals that we do not identify to species, but as stated above, there has been an increase in Lottia gigantea numbers in these plots, particularly in plot 4.

Seastars at Boathouse were variable in number over time, but have shown no long-term trend of decline or increase. High variation in abundance is expected among samples for highly mobile species such as seastars. Relatively small (<50mm) Pisaster ochraceus are common at this site, indicating that recruitment to the region must be fairly constant. Boathouse is one of the few sites monitored by MARINe where the bat star, Patiria miniata, is found consistently in seastar plots. This species typically occurs in the subtidal, or very low intertidal, below our monitoring plots.

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)

Boathouse Anthopleura trend plot

Chthamalus/Balanus (Acorn Barnacles) - percent cover

Boathouse barnacle trend plot

Chthamalus/Balanus (Acorn Barnacles) - motile invertebrate counts

Boathouse barnacle trend plot

Mytilus (California Mussel) - percent cover

Boathouse Mytilus trend plot

Mytilus (California Mussel) - motile invertebrate counts

Boathouse Mytilus trend plot

Silvetia (Golden Rockweed) - percent cover

Boathouse Silvetia trend plot

Silvetia (Golden Rockweed) - motile invertebrate counts

Boathouse Silvetia trend plot

Endocladia (Turfweed) - percent cover

Boathouse Endocladia trend plot

Endocladia (Turfweed) - motile invertebrate counts

Boathouse Endocladia 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

Boathouse Pisaster trend plot

Pisaster (Ochre Star) - sizes

Boathouse Pisaster size plot

Sites home

Interactive Map

pacificrockyintertidal.org home

See Also