The Gulf of California: Biodiversity and Conservation

3. Reefs That Rock and Roll: Biology and Conservation of Rhodolith Beds in the Gulf of California

CHAPTER 3 Reefs That Rock and Roll Biology and Conservation of Rhodolith Beds in the Gulf of California rafael riosmena-rodríguez, diana l. steller, gustavo hinojosa-arango, and michael s. foster Introduction Imagine an underwater field of closely packed, purple-pink spheres about the size of golf balls, each composed of numerous calcareous branches radiating out from the center of the sphere. The spheres and hundreds of species of animals and seaweeds living on and in them move with the motion of waves and currents. These spheres are rhodoliths, and beds of these calcareous red algal spheres comprise a rarely mentioned but common habitat in global nearshore environments. Rhodoliths are not generally included in natural history and diving guides. The rhodoliths themselves are attractive because of their color and range of morphologies and because of the variety of organisms associated with the beds. Local people view rhodolith beds in terms of the associated rich fisheries or as recruitment sites for harvestable species. Rhodoliths are morphologically diverse, free-living, nongeniculate, coralline red algae (Rhodophyta: Corallinales) that form extensive living beds worldwide (color plate 2A) and are abundant in fossil deposits from the early Cretaceous to the Pleistocene (Aguirre et al. 2000; Foster 2001). As with many attached reef-forming coralline species (Steneck and Adey 1976), rhodolith size, shape, and branching vary among species and—primarily as a result of variation in water motion—within species (Bosence 1976; Foster et al. 1997). Much like the maerl beds common to the northeastern Atlantic , rhodolith beds are benthic communities dominated by rhodoliths 50 / riosmena-rodríguez et al. that collectively create a fragile, structured biogenic matrix (color plate 2B). This matrix provides habitat for diverse assemblages of invertebrates and algae (Cabioch 1969; Keegan 1974; Bosence 1979; Steller et al. 2003). Rare, unusual, and endemic species in rhodolith beds have been reported from geographically diverse locations including the northeastern Atlantic (De Grave 1999), Norwegian fjords (Freiwald et al. 1991), the Mediterranean (Ballesteros 1988), the tropical Atlantic (Ballantine et al. 2000), the Indian Ocean (Weber-Van Bosse and Foslie 1904; Scoffin et al. 1985), and the tropical and subtropical Pacific, including the Gulf of California (Reyes-Bonilla et al. 1997; Clark 2000; James et al. 2006; Foster et al. 2007). Living rhodoliths occur as an upper layer of pigmented, generally rounded thalli that overlay carbonate sediments (color plate 2C). The latter are often the result of long-term accumulation of dead rhodoliths plus hard parts from other calcified organisms such as corals and molluscs (Minoura and Nakamori 1982; Steller and Foster 1995; Boscence and Wilson 2003). In the northeastern Atlantic, unconsolidated rhodolith deposits have long been harvested for a variety of human uses including as a soil amendment, and in many other locations the beds are harvested for clams and scallops (Blunden et al. 1977; Briand 1991; review in Steller et al. 2003). Recent studies in the northeastern Atlantic and Mediterranean have shown that these common benthic environments are not resilient to direct disturbances caused by harvesting (Blunden et al. 1981) and fisheries trawling (HallSpencer and Moore 2000; Bordehore et al. 2003) or to indirect impacts from activities that reduce water quality through siltation and eutrophication (Blunden et al. 1981; Hily et al. 1992; Grall and Glemarec 1997). Slow growth rates (review in Foster 2001; Steller et al. 2007a) combined with the negative impacts of burial make rhodoliths particularly vulnerable to both disturbance and extraction (Adey and McKibben 1970; Potin et al. 1990). These community attributes have led to the recent listing of rhodolith beds as threatened and protected in New Zealand (Department of Conservation 1998), Australian (Director of National Parks 2005), and European (Birkett et al. 1998) coastal habitats. Rhodoliths in the Gulf of California have been known to science since the late 1880s (Hariot 1895), but the exceptional abundance of fossil and living beds (fig. 3.1) dominated by these algae, their high diversity and importance to nearshore ecology, and the need for their conservation in the 3.1. Known locations of living rhodolith beds in the Gulf of California. A rhodolith bed consists of an area of 10% or greater cover of rhodoliths. All beds except those designated E.Y.D (E. Yale Dawson; Dawson 1960b) or O (other) were observed by one or more of the authors (contact the authors for details). Dawson’s observations were based on dredged material or were not specified. Numbers...