Pacific Rocky Intertidal Monitoring: Trends and Synthesis

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Treasure Island 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).

Barnacle cover at Treasure Island exhibited a general decline over the course of the monitoring period, particularly between 1999 and 2003. Following low cover values of ~40% in 2003, there was a recovery period in which barnacles increased to ~80% cover, but subsequently abundance declined again to 40% by fall 2010. During periods of low barnacle cover, there were concurrent increases in the cover of bare rock. From 2009-2010, cover consisted of approximately half bare rock and half barnacles. Cover of the ephemeral alga Ulva cover was very low in all samples from 1996-2005.  However, in two plots, there were large inundations of Ulva in fall 2005, spring 2006, and spring 2007 with additional increases in Ulva in one plot in spring 2008 and spring 2010. Non-coralline crusts exhibited two peaks in cover in spring 2003 (17%) and spring 2008 (9%), reducing bare rock cover during those periods. Littorina spp. counts in barnacle plots are extremely high with only occasional limpets found during certain sampling seasons.

Mussel cover at this site was relatively low at initiation of the monitoring and decreased during subsequent sampling into the 1997-1998 El Niño. Cover remained low through 2002 but recovered to moderate levels in 2003 – 2004 with an increase of 40% during that year. Since 2004, mussel abundance declined slightly. Unlike the other sites in the region, the severe 2007 storm only resulted in a 10% loss of mussel cover in monitored plots, although mussel loss did occur at this site in the lower intertidal zone (below the area of the fixed photoplots). Bare rock cover was often high during periods of low mussel cover, with exception of seasons when barnacle cover increased, such as in fall 1999 and spring 2002. Other taxa found in the mussel plots included crustose corallines, Tetraclita, and non-coralline crusts but cover of these groups was highly temporally variable.  In spring 2002, the large increase in ‘other brown algae’ (15% cover) was driven mostly by an increase in Petrospongium rugosum. Limpet counts in mussel plots varied greatly over time with peaks in 2004, while other motile invertebrates were relatively low in numbers.

Rockweed cover showed seasonal fluctuations typical of this region with higher cover in the fall than in spring sampling periods. Rockweeds exhibited highly variable cover over time with a general pattern of decline since 2004. The lowest cover (~38%) occurred in spring 2006.  At this time, barnacle cover reached its peak (~40%); aside from this high value, barnacles generally covered 5-10% of the surface in rockweed plots (on average).  Cover of bare rock was highly variable, reaching peaks (20-25%) in 1999 and between 2007 and 2009. Mussel cover was very low in rockweed plots from 1996 to 2003 but increased to 4-9% cover after 2004. Hesperophycus was absent from 1996 to 2002, but abundance increased for a short period in 2003 with a peak of 8% cover in spring; this increase was short-lived, as it was followed by a die off in fall. Hesperphycus cover remained low until fall 2009 and the alga was not found in Silvetia plots at this site in 2010. Articulated corallines, crustose corallines, non-coralline crusts, other red algae, and Ulva were also common in Silvetia plots but highly variable in cover over time. Motile invertebrate counts were dominated by Littorina spp. and limpets.

Sea star (Pisaster) counts, which were low from 1996 to 2004, increased dramatically in 2006. After this 2006 peak, sea star abundance varied greatly among sampling periods from 2006 – 2010.

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.

Chthamalus/Balanus (Acorn Barnacles) - percent cover

Treasure Island barnacle trend plot

Chthamalus/Balanus (Acorn Barnacles) - motile invertebrate counts

Treasure Island barnacle trend plot

Mytilus (California Mussel) - percent cover

Treasure Island Mytilus trend plot

Mytilus (California Mussel) - motile invertebrate counts

Treasure Island Mytilus trend plot

Silvetia (Golden Rockweed) - percent cover

Treasure Island Silvetia trend plot

Silvetia (Golden Rockweed) - motile invertebrate counts

Treasure Island Silvetia 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)

Treasure Island Pisaster trend plot

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Interactive Map

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