Study System

I conducted research at wave-exposed rocky intertidal sites within Neptune State Park (44°15'N, 124°07'W), south of Cape Perpetua on the central Oregon coast. This 4 km stretch of coastline is composed of extensive rocky benches, outcrops, pools, and surge chan-nels.The communities here are similar to those of many other wave-exposed regions along the northern Pacific coast of North America (Dayton 1971, Paine 1980, Menge et al. 1994).The high intertidal zone is characterized by fucoid algae and barnacles, the mid zone by dense beds of the California mussel (Mytilus californianus), and the low zone by a diverse mixture of algae, seagrass, and invertebrates, including the ochre sea star, Pisaster ochraceus (Fig. 4.2).

Imagenes Ponpeya

Figure 4.2. The ochre sea star, Pisaster ochraceus feeding at the lower edge of a mussel bed on the Oregon coast. At low tide, sea stars are frequently observed hunched over their prey, including the California mussels (Mytilus californianus) shown here. In the absence of sea star predation, this mussel species overgrows and outcompetes other sessile species in the low intertidal zone.

Figure 4.2. The ochre sea star, Pisaster ochraceus feeding at the lower edge of a mussel bed on the Oregon coast. At low tide, sea stars are frequently observed hunched over their prey, including the California mussels (Mytilus californianus) shown here. In the absence of sea star predation, this mussel species overgrows and outcompetes other sessile species in the low intertidal zone.

Numerous experimental studies during the past 30 years have identified a subset of strong interactions that maintain the composition and diversity of rocky intertidal communities in the Pacific Northwest (Paine 1966, 1969, 1992, Connell 1970, Dayton 1971, Menge 1992, Menge et al. 1994, Navarrete and Menge 1996). In particular, Paine's classic experiments (Paine 1966, 1969) in Washington State demonstrated that predation by the sea star Pisaster ochraceus prevents the mussel Mytilus californianus from dominating the low intertidal zone. When sea stars were removed from experimental areas, primary space in the low intertidal zone shifted from a diverse assemblage of invertebrates and algae to a monoculture of M. californianus. Paine coined the term keystone predator to describe a single predator species (like Pisaster) that determines most patterns of community structure. Experimental studies have confirmed that Pisaster plays a similar keystone role in wave-swept communities on the central Oregon coast (Menge et al. 1994).

This keystone interaction occurs in a system that is ideally suited to testing the influence of slight temperature change on trophic dynamics. In many terrestrial and aquatic systems, there is a high degree of spatial heterogeneity in the thermal environment (e.g., shaded microhabitats, stratified lakes, above vs. below-ground habitats, etc.). Thus, many ectothermic consumers (e.g., fishes, lizards, insects) may be able to mediate the effects of minor temperature change by selecting preferred microhabitats or limiting activity to certain times (Caulton 1982, Christian et al. 1983, Whicker and Tracy 1987, Gates 1993). These changes in behavior could have important ecological effects, for example by changing where and when consumers are foraging (Rubenstein 1992, Dunham 1993). In addition, changes in behavior complicate attempts to predict the body temperature of consumers, since it is difficult to assess if (or how much) behavioral thermoregulation might offset changes in ambient temperature.

In contrast, most marine benthic consumers (e.g., sea stars, whelks, herbivorous mollusks, sea urchins, etc.) have little opportunity for behavioral thermoregulation when submerged. Sea temperatures along the coast typically vary little over several hundred meters, which is a scale far larger than these organisms' daily movement. Thus, changes in water temperature are experienced as direct changes in the body temperatures of these consumers.

Water temperatures along the Pacific coast vary seasonally, but also vary over shorter time scales in response to coastal upwelling. Episodes of upwelling lasting from several days to three or more weeks are common along the Oregon coast from May to September (Menge et al. 1997). Persistent, strong, southward winds combine with the Coriolis effect to push surface waters offshore (Fig. 4.3), and water temperatures typically drop 3 to 5°C as cold, nutrient-rich water rises from below.

In the Pacific Northwest, densities of Pisaster and its impact in the intertidal zone are highest between May and October (Mauzey 1966, Paine 1974, Robles et al. 1995, Sanford 1999b). However, preliminary observations in Oregon suggested that within this season, intertidal sea star activity underwent marked fluctuations associated with oceanographic conditions. During periods of cold-water upwelling, many sea stars appeared to become inactive in low zone channels or shallow subtidal waters (Sanford 1999b). Upwelling patterns along the Pacific coast have changed substantially in recent decades, and some climate models predict further

North Wind

North Wind

Figure 4.3. Mechanics of coastal upwelling. Strong northerly winds and the Coriolis effect cause surface waters to flow offshore, drawing up cold, nutrient-rich waters from below (after Bakun 1990).

changes associated with global warming (see discussion under "Predicting Changes in Upwelling Intensity"). I therefore conducted experiments to examine how variation in upwelling patterns might alter intertidal communities via effects on this keystone predator.

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