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Status and Trends of the Rocky Intertidal Community of the Farallon

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Status and Trends of the Rocky Intertidal Community of the Farallon Monographs of the Western North American Naturalist 7, © 2014, pp. 260–275 STATUS AND TRENDS OF THE ROCKY INTERTIDAL COMMUNITY ON THE FARALLON ISLANDS Jan Roletto1, Scott Kimura2, Natalie Cosentino-Manning3, Ryan Berger4, and Russell Bradley4 ABSTRACT.—The Farallon Islands in the Gulf of the Farallones National Marine Sanctuary (GFNMS) is a 7-island chain located 48 km west of San Francisco, California. Since 1993, GFNMS biologists and associates have monitored algal and invertebrate species abundances on the intertidal shores of the 2 South Farallon Islands. The monitoring occurred 1–3 times yearly in 6 study areas. In each study area, 3–4 permanent, 0.15-m2 quadrats located between the upper and midintertidal zones were sampled for algal and sessile invertebrate cover and invertebrate counts. Taxonomic surveys were also completed to document other species in the vicinity of the sampling quadrats and to further charac- terize the sampling areas. Here we report monitoring results for the period 1993 to 2011. While species richness has remained relatively stable and high compared to the nearest mainland sites (Sonoma County through San Mateo County), there has been a slow, long-term net decline in the abundance of algal species and mussels at various sites on the islands. Causes for the declines remain unknown, but increased trampling from rising numbers of pinnipeds and increased waste from pinnipeds and seabirds are among the influences suspected to be important. RESUMEN.—Las Islas Farallon en el Santuario Nacional Marino Golfo de Farallones (SNMGF) es un archipiélago de siete islas situado a 48 km al oeste de San Francisco, California. Desde 1993, los biólogos y asociados del SNMGF han monitoreado la abundancia de especies de algas e invertebrados en las costas intermareales de las dos Islas Farallon del sur. El seguimiento se produjo una a tres veces al año en seis áreas de estudio, en los que se estudiaron de tres a cuatro cuadrantes permanentes de 0.15 m2 situados entre las zonas intermareales superior y media donde se muestrearon la cubierta de algas y el número de invertebrados sésiles. Adicionalmente, se realizaron estudios taxonómicos para docu- mentar otras especies y caracterizar las áreas de muestreo en las proximidades de los cuadrantes. Aquí incluimos los resultados del monitoreo para el período entre 1993 y 2011. Mientras que la riqueza de especies se ha mantenido relati- vamente estable y elevada en comparación con otros sitios de muestreo cercanos en el continente (Condado de Sonoma a al Condado de San Mateo), se ha observado un lento descenso neto a largo plazo en la abundancia de especies de algas y mejillones en varios sitios en las islas. Las causas de este descenso siguen siendo desconocidas, pero se sospecha que la influencia del aumentó en el pisoteo del creciente número de pinnípedos y el aumento de los residuos de pinnípedos y aves marinas son importantes. The Farallon Islands in the Gulf of the Far- the islands are within the Farallon National allones National Marine Sanctuary (GFNMS) Wildlife Refuge. Below the mean high-tide is a chain of 7 islands and emergent rocky level, the intertidal and subtidal areas are pinnacles located 48 km (30 mi) west of San within GFNMS and are recognized and Francisco, California, 37°42፱N and 123°00፱W designated by the State Water Resources (Fig. 1). The islands and pinnacles are part of Control Board as an Area of Special Biological a granitic submarine ridge flanking the con - Significance (ASBS). Furthermore, portions of tinental shelf (Blankinship and Keeler 1892, the islands’ intertidal and subtidal are desig- Hanna 1951). They are recognized as an nated as State Marine Protected Areas (MPAs), ecosystem unique for its location and diver - which were created by the Marine Life Pro- sity of species across a broad range of bio - tection Act passed by the California State logical communities. For these reasons, the Legislature in 1999. At the Farallon Islands habitats and natural resources at the islands are 2 state marine reserves, 2 special closure and emergent pinnacles are afforded many areas, and one state marine conservation levels of resource management, protection, area. Also, the islands are not open to public conservation, and oversight. Above the mean visitation, and access is by permit and for high-tide level, the terrestrial portions of scientific purposes only. 1Gulf of the Farallones National Marine Sanctuary, 991 Marine Dr., San Francisco, CA 94129. E-mail: [email protected] 2Tenera Environmental, 141 Suburban Rd., Suite A2, San Luis Obispo, CA 93401. 3National Oceanic and Atmospheric Administration, Fisheries Restoration Center, 777 Sonoma Ave., Santa Rosa, CA 95404. 4Point Blue (formerly PRBO) Conservation Science, 3820 Cypress Drive #11, Petaluma, CA 94954. 260 2014] INTERTIDAL FARALLON 261 Fig. 1. Sanctuary Ecosystem Assessment Surveys (SEAS) rocky intertidal study areas on the South Farallon Islands, California (37°42፱ N, 123°00፱ W). Dots indicate locations of the 6 study areas. Map data (left panel): ©2012 Google, LDEO-Columbia, NSF, NOAA, SIO, NOAA, U.S. Navy, NGA, GEBCO, MBARI, Image Landsat. Map data (right panel): ©2008 Google, SIO, NOAA, U.S. Navy, NGA, GEBCO, CSUMB SFML, CA OPC. Blankinship and Keeler (1892) completed Islands), which are the 2 largest of the 7 the first survey of the intertidal community islands (Fig. 1). Together these islands are 44 on the Farallon Islands. Their work provided ha in size and are separated by a narrow surge a general description of the island’s geology channel. A narrow and discontinuous reef and biota and a temporal snapshot of the inter- characterizes the intertidal zone, exposed only tidal community. The next intertidal survey on minus tides. The landward rise is often was 87 years later, completed by the California steep, highly worn, and characterized by cracks State Water Resources Control Board as a and crevices, surge channels, and sea caves reconnaissance survey to consider designation (Hanna 1951). No rocks or boulder fields are of the islands as an ASBS (CSWRCB 1979). found in the upper intertidal, and the only The only other published investigations in - sand is coarse grained, with cobbles deposited volving assessment or documentation of the at the heads of surge channels. Intertidal zones intertidal habitat on the islands focused on were categorized based on species composi- the distribution of Foraminifera (Grivetti 1962) tion. Many locations on the islands are used as and the systematics of Porifera (Klontz 1989). seasonal and year-round haul-outs for pin- Since 1993 and as part of the Sanctuary nipeds. As such, the study areas selected were Eco system Assessment Surveys (SEAS), the accessible and disturbance to pinnipeds (and GFNMS has monitored intertidal algal and seabirds) was minimized, as required by the invertebrate species abundances on the rocky Sanctuary and Refuge permits. The number shores of the 2 South Farallon Islands (Southeast of study areas and number of quadrats in Farallon and Maintop Islands). The long-term each study area were also selected based on monitoring program was created to characterize logistics and funding. See Appendix 1 for the the intertidal habitat and to maintain an ongoing physical descriptions of each study area and database of species abundances in the event of quadrat. oil spills. Here we present data spanning August Sampling Design 1993–February 2011 summarizing macroalgal and invertebrate changes on the Farallon Islands. Three to four permanent, 30 × 50-cm quadrats (0.15 m2) between the upper and METHODS midintertidal zones (Ricketts et al. 1985) were sampled in each of the 6 study areas (Fig. 1). Study Areas The quadrat locations were marked with There are 6 study areas on the 2 South Far- marine epoxy on the rock substrate. Sampling allon Islands (Southeast Farallon and Maintop was completed up to 3 times annually (August, 262 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST [Volume 7 November, February), beginning in 1993. three are presently considered to be rare in Sampling was not scheduled to occur during the sampling region or outside their normal the peak algal growth season (May–July) to range: Branchioglossum undulatum and Myri- minimize and avoid disturbance to breeding ogramme variegata have not been previously seabirds and pinnipeds that typically use the documented north of Carmel Bay, California, sampling areas. and Ulva conglobata is considered an intro- All surveys included taking photographs of duced species. each quadrat followed by point-intercept sam- Species abundances, averaged across all 6 pling, which consisted of sampling 50 random study areas for 2010 and 2011 (the most recent points for algal and sessile invertebrate cover. sampling years), revealed that the top 10 All algal and sessile invertebrate species under species comprised >90% of the total upright each sampling point were identified and re - algal cover for those 2 years combined. Species corded to the lowest taxonomic level practical abundance was variable across the 6 study (Foster et al. 1991, Dethier et al. 1993). Multiple areas, except for the articulated coralline algal layers of the same species (taxon) at a single species Corallina vancouveriensis, which was sampling point were tallied as a single occur- either the first or second most abundant rence, but layers of multiple species under a species (>20% mean cover) in the study areas single point resulted in multiple tallies (con- (Table 1). The Mazzaella flaccida-complex, a tacts) per sampling point. As such, total algal foliose red algal assemblage, was also abun- cover (all species tallies combined for a quadrat) dant, except in the quadrats at Mussel Flat, could exceed 100% cover for highly layered where Anthopleura elegantissima covered large quadrats. Point-intercept assessments in each amounts of the substrate. The Mazzaella flac- quadrat also included a tally of dead animals cida-complex consisted of several species of (i.e., empty barnacle tests or shells and per- Mazzaella, with M.
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