Ebeling1985.Pdf

Marine Biology 84, 287-294 (1985) Marine ............ Biology Springer-Verlag 1985 Severe storm disturbances and reversal of community structure in a southern California kelp forest A. W. Ebeling, D. R. Laur and R. J. Rowley Department of Biological Sciences and Marine Science Institute, University of California at Santa Barbara; Santa Barbara, California 93106, USA Abstract amounts of primary substrate were cleared and settled by new growths of sessile organisms, and change in community Regular observations made over a period of 5 yr in four structure varied with degree of exposure to the full force of permanent transects provided data on plant, sea urchin, the waves. Such changes will persist only if the disturbance and fish densities which indicate that two unusually severe rate exceeds recovery time. Hence, geographic differences winter storms in t980 ("Storm I") and 1983 ("Storm It") in storm frequency may account for observed differences had different effects on a southern California kelp-forest in the community structure (Goreau, 1959). However, in community. Storm I removed all canopies of the giant kelp places where severe storms are rare, such as Jamaica Macrocystis pyrifera, but spared most understory kelps, (Woodley etal., 1981) and southern California (Dayton mainly Pterygophora californica. Hence, the previously and Tegner, 1984), the community may either undergo large accumulation of detached drift kelp, mostly cycles of alteration and recovery (Connell and Sousa, M. pyrifera, disappeared. Denied their preferred diet of 1983) or persist in the altered state if recovery time is long drift kelp, the sea urchins Strongylocentrotusfranciscanus (Sutherland, 1981). and S.purpuratus then emerged from shelters to find Recovery time of subtidal kelp forests in temperate alternative food. Without effective predators, they con- waters may be delayed by grazing sea urchins if the sumed most living plants, including the surviving under- urchins have neither adequate detrital food nor effective story kelps. This weakened the important detritus-based predator control. In a mature forest, most of the annual food chain, as indicated indirectly by declining abundances production of kelps forms a litter of detached drift (Gerard, of algal turf and fish (Embiotocidae) that eat small 1976). Urchins readily eat such drift (Vadas, 1977) and animals living in turf. In 1983, Storm II reversed the usually spare living plants wherever sufficient supplies are process by eliminating exposed urchins, while clearing available (e.g. Lowry and Pearse, 1973; Rosenthal etal., rock surfaces for widespread kelp settlement and growth. 1974). If the forest is destroyed by severe storms or other By summer 1984, the kelp grew to maturity to form agencies, the urchins may have no drift to eat (see Harrold extensive canopies despite elevated water temperatures and Reed, 1984). If uncontrolled by predators, disease, or during summer and fall of 1983. Thus, severe storms may water motion, the urchins will then move out over the reef have vastly different effects on community structure, to graze back new growth and form a "barrens" covered depending on the state of the community before the with little more than crustose algae and patches of low turf disturbance. (Lawrence, 1975). Urchins are able to remain in a barrens because they can subsist on little food (Ebert, 1967; Lang and Mann, 1976). This prevents recovery of the important detritus-based food chain (see Mann, 1982), which gener- Introduction ates much of the small invertebrate prey that fish eat (Mann, 1977; Simenstad etal., 1978; Laur and Ebeling, The frequency, magnitude, and scale of major storm 1983). Frequency-dependent foraging by sea otters disturbances may determine the structure of many kinds (Enhydra lutris) effectively controls numbers of exposed, of natural communities, including those of marine reefs grazing urchins in dense kelp forests off the Aleutian (Goreau, 1959; Conneli, I978; Paine, I979). For exampIe, Islands (e.g. Estes eta[., 1978), Alaska (Duggins, 1980), Woodley etaL (1981) described hurricane-induced per- and central California (Lowry and Pearse, 1973; Reed and turbations of a Jamaican coral-reef community: large Foster, 1984). Yet kelp grows abundantly off southern 288 A. W. Ebeling et al.: Storm disturbances of a kelp forest California where otters are absent and Where exposed NM in Fig. 1 D. This included intervals before and after urchins occur abundantly in barrens patches (e.g. Leighton the large storms of 1980 ("Storm I") and 1983 ("Storm II"). et al., 1966; Pearse et al., 1970; North, 1971; Tegner, 1980). Two transect sites (NE, SE) were in an extensive stand of Under such circumstances, kelp forests may be less resilient understory kelp over the reef base. Substrate at the NE and occur more sporadically (North, 1971), site was originally a mosaic of sand, cobble, and partly Thus, mature kelp forests and their productive detrital encrusted rock, but was scoured by Storm I. Throughout food chains may persist off southern California only most of the study the more protected SE site remained because storms powerful enough to destroy them and covered with sand merging with cobble. The two other eliminate drift kelp are rare here (see Dayton and Tegner, transect sites (M, NM) were near the reef top at 7 to 8 m 1984). Once destroyed, however, forests without a resource depth, among crests and rills with high urchin-densities, of drift may be slow to recover unless sea urchins are rich algal turf, and intense foraging by surfperch. Site M controlled (North, 1971; Wilson etal., 1980). Yet a storm was in a rill; Site NM was located along the north slope of strong enough to remove kelp may also remove urchins or the reef. Permanent 60 m transects of polypropylene rope alter their behavior, especially exposed individuals in had been anchored and maintained at Sites NE and NM barren areas (Cowen et al., 1982; Harrold and Reed, 1984). for fish censusing and other studies (see Ebeling and Laur, Thus, the process may be reversible. 1984). A similar transect was constructed at Site SE for In our study, we show that two unusually severe monitoring a persistent stand of understory keIp. At storms, which surprisingly struck coastal southern Cali- Midreef Site M, steel pins marked ends of a 13 m transect fornia only three years apart, started reversible processes. for measuring macroinvertebrate densities along the gently The first storm initiated a process leading to deforestation sloping side of a rill. of our study reef: drift kelp was eliminated, hungry Since the study was opportunistic, we could not pre- urchins left shelter, regrowth was grazed back, secondary design all sampling, and had to extend new observations production probably fell, and fish abundance declined. from previous ones made for other studies. Hence, not all The second storm, of similar force, initiated the process of monitoring was quantitative and not all measurements reforestation: exposed urchins were decimated, kelp were made at each transect site. The state of the kelp forest regrew, drift again accumulated on the bottom, and the before and after Storms I and II was mapped from field detritus food chain was restored. We attribute much of this notes and measurements made during reefwide surveys reversal in community form to marked changes in the throughout the 5 yr interval (Fig. 1). We feel that this is behavior, distribution, and abundance of grazing sea adequate because many changes were widespread and urchins. dramatic: for example, from a thick, continuous kelp forest We documented this by monitoring four important to an exposed "barrens". At least once a season, we components of the detrital food chain accounting for much surveyed the reef and redefined limits of kelp stands and of the obvious community structure at our study site: (1) substrate type. We counted kelp-sporophyte recruits in kelps, both giant kelp Macrocystis pyrifera which forms a 0.25 m 2 quadrats near our transect sites during times of surface canopy, and the understory kelps Pterygophora maximum settlement. Water temperature was measured californica and Larninariafarfowii which rise less than 1 m semi-weekly at the surface and t3m depth during off the bottom; (2) algal turf which houses many of the 1979-1981, but not during 1982, and irregularly during secondary producers such as small crustaceans, mollusks, 1983. and worms; (3) grazing sea urchins Strongylocentrotus ReguLar observations of plant cover and of sea urchin franciscanus and S. yurpuratus; and (4) epibenthic fishes in and fish densities were averaged per bimonthly interval. the surfperch family Embiotocidae which are the principal Percent understory kelp cover was estimated once every exploiters of small invertebrate prey living in the turf two months in thirty 0.75 m 2 quadrats placed randomly (Laur and Ebeling, 1983). Results of this monitoring and along the NE transect both before and after Storm I, and other work provide a unique view of a reef community as along the SE transects after Storm I only. Similarly, we forest, as barrens, and in transition between the two condi- estimated algal turf cover in 15 quadrats along the NM tions. transect before and after Storm I. Sea urchin densities were measured 1 to 7 times within bimonthly intervals before and after Storm I at Transect Site M and occasionally Materials and methods after Storm I at Site NE. Since abundances of the two urchin species were strongly correlated (r= 0.85, n = 69, We monitored densities of these four groups of organisms P<0.001), we pooled densities for counts of total adults at Naples Reef, an isolated outcrop of shale located 1.6 km in a 13 X 1 m band delimited by a meter bar pushed along offshore about 23 km west of Santa Barbara, southern the side of a nylon line stretched between two pins. California (34~ 119~ The reef covers 2.2 ha, Surfperch densities were of total adult and subadult ranges in depth from 5 to 15 m, and consists of a series _~ individuals of the 5 principal species inhabiting the reef rills and ridges running parallel to the coast (Ebeling et al., (see Laur and Ebeling, 1983; Ebeling and Laur, 1984): 1980).

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