Kelp Forest and Rocky Subtital Habitats
III. Invertebrate and Vertebrate Assemblages Associated with Kelp Forests
The holdfasts of giant kelp and mats of geniculate coralline algae provide microhabitats for an abundant and species rich association of invertebrates (Andrews 1945, Foster and Schiel 1985, Dearn 1987). Andrews (1945) found approximately 23,000 individuals from nine invertebrate phyla residing in five giant kelp holdfasts collected from the Monterey and Carmel Bays, the most common of which were polychaetes, amphipods, decapods, gastropods and ophiuroids.
Outside the holdfasts, sponges, tunicates, anemones, cup corals and bryozoans are probably the most commonly occurring sessile animals within kelp forests (Foster and Schiel 1985; Table 1). McLean (1962) observed 204 species of invertebrates living in a bull kelp forest along an exposed coast south of Carmel during 30 SCUBA dives. In addition to these bottom dwelling species, a large number of invertebrates, such as the isopod Idotea resecata and the bryozoan Membranipora tuberculata occur within the canopies, while diverse assemblage of planktonic species such as jellyfish, crustaceans and fish larvae live in the water column of the kelp forests (Figure 1).
A wide variety of motile grazers, the majority of which do not remove entire kelp plants but graze upon their tissue and other associated algae (Foster and Schiel 1985), also occur in the forests. Some species, such as sea urchins (Strongylocentrotus franciscanus and S. purpuratus), may completely remove entire kelp plants by grazing through their holdfasts (Pearse and Hines 1979, Foster and Schiel 1985, Kenner 1992). However, these species usually do not graze on attached plants under normal circumstances, but rather feed on drift algae (Ebeling et al. 1985, Foster and Schiel 1985, Watanabe and Harrold 1991). Other species, like the gastropods Tegula spp., graze along the entire thallus of the kelp plants from the substrate to the surface (Watanabe 1984). In addition to herbivores, several species of predatory sea stars, snails and crabs inhabit the kelp forests, but comparatively little is known about the dynamics of these organisms (Foster and Schiel 1985).
Although they are not limited to kelp forests, a variety of fish occur within them, including many of economic importance. The commercial fisheries associated with kelp forest fishes are reviewed in Leet, Dewees and Haugen (1992). In addition to the commercial fisheries, a large sport fishery is associated with kelp forests, particularly with rockfish, Sebastes spp. (Karpov et al. 1995).
The heterogeneous environment of the forest provides an important source of food and shelter for many species (Foster and Schiel 1985, Bodkin 1988;Table 1). These can be categorized according to where they reside in the forest (Foster and Schiel 1985). Midwater species of the kelp canopy, such as the senorita (Oxyjulius californica) and the surfperch (Brachyistius frenatus) browse on the small crustaceans associated with both the kelp fronds and canopies. Other midwater predatory fishes including the common plankton-feeding blue rockfish (S. mystinus), the blacksmith (Chromus punctipinnus) and juveniles of the predatory kelp rockfish (S. atrovirens), olive rockfish (S. serranoides) and black rockfish (S. melanops). Compared to southern California kelp forests, MBNMS kelp forests have relatively few tropically-derived fish species and even fewer families, but generally host more species per family (see Foster and Schiel 1985 for a review of kelp forest fishes).
Along the central California coast, fish diversity and abundance decrease in areas where the kelp canopies have been removed (Bodkin 1988). In addition, mass mortality of kelp forest fishes, particularly Sebastes spp. have been observed as a result of large oceanographic waves at the southern end of the MBNMS at San Simeon (Bodkin et al. 1987). Variations in fish abundance may have significant impacts on other communities. For example, juvenile rockfishes associated with kelp forests in Monterey Bay can reduce the amount of barnacle larvae reaching the intertidal to 2% of the level found in the absence of fish (Gaines and Roughgarden 1986).
(also see Marine Mammals and Seabirds & Shorebirds sections)
Harbor seals (Phoca vitulina) and California sea lions (Zalophus californianus) are common in and around MBNMS kelp forests. Harbor seals feed on shallow-dwelling kelp forest fishes, while California sea lions, which feed mainly on pelagic fishes, probably limit their use of the forests to transitory feeding (Foster and Schiel 1985). Gray whales (Eschrichtius robustus) have been observed entering kelp forests to escape predation from killer whales (Orcinus orca, Baldridge 1972) and also to feed on invertebrates such as midwater crustacean swarms (Nerini 1984).
One of the most recognizable marine mammals associated with the kelp forests in the MBNMS is the sea otter (Enhydra lutris). The current California sea otter population occupies a linear coastal range of about 280 km in central California. Most of the population occurs within the boundaries of the MBNMS. Otters feed on invertebrates associated with kelp forests (Foster and Schiel 1985; 1988, Estes et al. 1986), and must consume 25% of their body weight per day to meet their energy needs (Costa 1978). Otters also use kelp forests as a refuge from predation by white sharks and winter storms, and as nursery areas for females with pups (Foster and Schiel 1985).
Otters can have significant impacts on sea urchin populations, which in turn may affect the kelp forests themselves (Foster and Schiel 1988). In this respect otters have been called a "keystone species," suggesting that they occur high in the food web and by controlling their prey species (sea urchins) they greatly alter the community as a whole (Estes and Palmisano 1974, Estes and Duggins 1995), although this interaction may not be as important in California kelp forests (Foster and Schiel 1988, Foster 1990).Ostfeld (1978) observed otters colonizing a new kelp forest off Santa Cruz ate mostly red sea urchins (Strongylocentrotus franciscanus) at first, but as urchin densities declined, they switched to kelp crabs and clams.
A wide variety of birds use MBNMS kelp forests although their relationship with the forests is poorly known (Table 1). Foster and Schiel (1985) report that kelp provides three distinct habitats used by birds: the kelp forest made up of living attached plants associated with rocky substrata, drift kelp floating in the open sea, and the kelp wrack, i.e. detached kelp deposited on the beach by water motion. Kelp forests provide a large potential source of invertebrate and fish prey as well as a refuge from storms. Birds commonly observed in this habitat are gulls, terns, snowy egrets, great blue herons and cormorants (Foster and Schiel 1985). Birds associated with drift kelp, like phalaropes, feed on the associated plankton and larvae. The kelp wrack provides an important food source and habitat for kelp flies, maggots and small crustaceans on which several species of shore birds, starlings, common crows, black phoebes and warblers feed (Davis and Baldridge 1980).
Section II. Algal Assemblages Associated with Kelp Forests