Intertidal community structure and oceanographic patterns around Santa Cruz Island , CA , USA

Recent studies suggest that nearshore oceanographic conditions can have important effects on the structure of benthic communities. On Santa Cruz Island (SCI), CA, USA there is a persistent difference in mean annual sea surface temperature (SST) around the island due to its location at the confluence of opposing cold and warm ocean current systems. Over the course of a 4-year study (1997–2001) seawater nutrient and chl-a concentrations, algal tissue C:N ratios, recruitment and growth of filter-feeders (barnacles and mussels), and intertidal community structure were measured at six intertidal sites around the island. There were strong associations between remotely sensed SST and patterns of community structure. Macrophyte abundance was highest at sites with persistently low SST, while recruitment, abundance, and growth of filter-feeding invertebrates were strongly, positively correlated with SST. The cold-water sites were associated with higher nutrient concentrations and lower algal C:N ratios, particularly in the winter months. Values of chl-a were generally low and variable among sites, and were not correlated with the predominant SST gradient. Recruitment of barnacles and mussels was positively correlated with adult abundance across all sites. While detailed experimental studies are needed to further evaluate the mechanisms underlying community dynamics, these results indicate that the confluence of coldand warm-water masses around SCI may determine the contrasting patterns of intertidal community structure. Introduction A major focus of current ecological research is to understand the factors producing variation in community structure at a variety of spatial and temporal scales. Traditional marine ecological studies have focused on processes operating at relatively small spatial scales (1–10 m), but the generality of these findings is increasingly being challenged (Dayton and Tegner 1984; Carpenter 1996; Polis and Hurd 1996; Menge et al. 1997b, Schiel 2004). As broad scale issues are increasingly dominating questions in conservation and management, marine ecologists have been pressed to examine linkages between patterns and processes operating at larger spatial scales. In marine systems, this ‘‘scaling-up’’ has resulted in a significant conceptual shift in our understanding of the connections among populations and communities and the importance of benthic–pelagic linkages. Benthic communities are inextricably linked to the oceanic environment through the delivery of food, nutrients, and propagules. The oceanographic processes driving the delivery of these constituents span large spatial scales and thereby connect distant communities (Gaines et al. 1985; Roughgarden et al. 1988; Bustamante et al. 1995b; Menge et al. 1997b; Schiel 2004). Recent ecological studies from eastern boundary upwelling ecosystems around the world support a strong coupling between nearshore oceanographic patterns and intertidal benthic community structure. In South Africa, it has been established that large-scale (100’s of km) patterns of intertidal community structure covary with nearshore productivity and wave exposure (Bustamante et al. 1995b; Bustamante and Branch 1996). Along the coast of Chile, several studies provide evidence that the abundance of dominant intertidal functional groups is strongly correlated to persistent differences in nearshore sea surface temperatures (SSTs; Broitman et al. 2001; Nielsen and Navarrete 2004). Around the south island of New Zealand, community structure and dynamics in Communicated by J.P. Grassle, New Brunswick C. A. Blanchette (&) Æ B. R. Broitman Æ S. D. Gaines Marine Science Institute, University of California, Santa Barbara, CA 93106, USA E-mail: [email protected] Fax: +1-805-8938062 B. R. Broitman Æ S. D. Gaines Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106, USA Marine Biology (2006) 149: 689–701 DOI 10.1007/s00227-005-0239-3 the rocky intertidal have been shown to be related to island-scale differences in upwelling intensity around the south island (Menge et al. 1999, 2003). Similarly, on the US West coast, nearshore oceanographic conditions have been shown to determine community structure along the coast of Oregon where enhanced chlorophyll and elevated mussel growth rates are linked to coastal upwelling (Menge et al. 1997a, b). One major mechanistic hypothesis linking nearshore oceanography to community pattern relates to the spatial and temporal variability in coastal upwelling. Latitudinal variation in the upwelling intensity has been suggested to play an important role in the recruitment of benthic invertebrates along the US West coast. The upwelling/ relaxation model is based on the idea that planktonic larvae are swept offshore by Ekman transport where they accumulate along coastal fronts and are occasionally returned to shore when upwelling-favorable winds relax (Roughgarden et al. 1988; Shanks et al. 2000). Variable upwelling and frequent relaxations along the Oregon coast are correlated with high rates of larval arrival in barnacles and mussels (Menge 1991; Connolly et al. 2001). In contrast, consistently strong upwelling along much of the California coast has been proposed to limit invertebrate recruitment (Gaines et al. 1985; Gaines and Roughgarden 1985; Roughgarden et al. 1988; Strub and James 1995). This large-scale variation from Oregon to central California in the recruitment of competitively dominant invertebrates such as mussels and barnacles has been proposed to underlie variation in postsettlement processes such as competition and predation and ultimately community structure (Connolly and Roughgarden 1998). Regions of strong coastal upwelling may also influence community structure through positive direct effects on macroalgae, which compete for space with mussels and barnacles (Schiel 2004). Coastal upwelling delivers cold, nutrient-rich water to shore, and locations of strong upwelling are characterized by high nutrient concentrations which have been shown to affect the abundance (Bustamante et al. 1995b; Broitman et al. 2001) and growth rates (Blanchette et al. 2002; Nielsen and Navarrete 2004) of benthic macroalgae. Persistent upwelling does not describe the entire California coast. Point Conception (34.5 N) divides the coast into two regions with very different nearshore circulation patterns. The northern region is typified by consistently strong coastal upwelling bringing cold, nutrient-rich waters to the surface resulting in both cold SSTs along the coast and high nutrient concentrations (Brink et al. 1984; Abbott and Zion 1985). The Santa Barbara Channel, immediately southeast of Point Conception is typified by weak and infrequent seasonal upwelling, which tends to occur in the winter months (Hickey 1993; Blanchette et al. 2002; Winant et al. 2003). The California Channel Islands, just offshore from Point Conception lie within this oceanographic transition region. These islands have recently received attention as a focal region of biological and oceanographic diversity (Airame et al. 2003). The broad range of SSTs and oceanographic variability found across the entire Point Conception region is found at a much smaller geographic scale within this island group. These islands experience variable mixing between the cold waters of the California current and warmer, nearshore waters of the Southern California Countercurrent (Huyer 1983; Hickey et al. 2003) Santa Cruz Island (SCI) is the largest of the California Channel Islands and lies directly in the center of this transition region (Fig. 1a). The biogeographical significance of the exposure of these offshore islands to different temperature regimes and distinctive water masses was recognized early by Hewatt (1946) who was among the first to describe the marine invertebrate faunas around SCI and their biogeographical affinities with the faunal elements characteristic of mainland intertidal habitats north and south of Point Conception. Although this early documentation was mainly descriptive and contains only single point measures of seawater temperature, Hewatt (1946) noted the potential importance of ocean temperature differences across the island to variability in assemblages of invertebrates around the island. Hewatt’s biogeographic observations were later supported through quantitative studies of macroalgal communities across the Southern California Bight (Neushul et al. 1967; Murray and Littler 1981). Recent work has shed some light on potential mechanisms linking oceanographic variability to larval delivery around SCI. Broitman et al. (2005) examined the connections between long-term mean SST and recruitment of mussels and barnacles around SCI. Sites located on the west end of the island experienced low, but variable SSTs, while eastern sites were warmer (by 1–1.5 C) and less variable. This oceanographic variability was linked to the dynamics of recruitment of mussels and barnacles around the island. Recruitment of both barnacles and mussels was highest at the eastern, warm-water sites. Western sites were dominated by an energetic flux of cold, recently upwelled water relatively depleted of larvae, while eastern sites received high numbers of larvae associated with the influx of warm surface waters (Broitman et al. 2005). The spatial gradient in ocean temperatures around SCI provides a unique opportunity to examine the relationship between nearshore oceanographic conditions and biological community structure at a relatively small spatial scale. Here, we examine the potential community-level consequences of the documented gradient in recruitment of barnacles and mussels at six intertidal sites spanning the major oceanographic gradient across SCI (Broitman et al. 2005). Our aim in this paper is to document patterns of intertidal community structure across the oceanographic transition zone spanning SCI. We evaluate our main hypothesis, which is that the competitively dominant invertebrate filterfeeders (barnacles and mussels) are favored under warmwater conditions, and macrophytes (seaweeds and surfgrass) are favored under cold-water conditions. We discuss several potential mechanisms to link observed 690