EXPERIMENTAL STUDIES OF ALGAL CANOPY INTERACTIONS 



IN A SEA OTTER-DOMINATED KELP COMMUNITY AT 



AMCHITKA ISLAND, ALASKA 



Paul K. Dayton' 



ABSTRACT 



Studies on the results of competitive interactions between three kelp canopy guilds were conducted in 

 a community in which herbivorous invertebrates have been largely removed from shallow water 

 (approximately 20 m) by sea otters. Small sea urchins observed in the haptera of kelps all disappeared 

 following the canopy removal, suggesting that the canopy itself offers a modest refuge from their 

 predators. Experiments prove that the largest alga, Alariafistulosa, behaves as a fugitive species with 

 respect to Laminaria and Agarum species in spite of the structural dominance of a floating canopy. 

 Vegetative regeneration may give Laminaria longipes an advantage over other Laminaria species, 

 Alaria, and presumably Agarum cribrosum following disturbances in very shallow water {<5 m). 

 Laminaria species suppress Agarum growth (and recruitment) in moderate depths (5-20 m) where 

 either Laminaria or Agarum suppresses growth of red algal turf beneath them, and where both 

 Laminaria and Agarum must be removed to allow recruitment and growth of Alaria fistulosa. 

 Although kelps were observed to depths of 30 m, their lower distribution appears primarily limited by 

 sea urchin grazing. 



Few natural communities are so influenced by one 

 population as is the nearshore marine community 

 dominated by the sea otter, Enhydra lutris Linn. 

 The nearshore community at Amchitka Island, 

 Alaska, is especially interesting in this regard 

 because for almost 40 yr it has had a sizable sea 

 otter population. This population has been at or 

 near its carrying capacity for at least 20 yr 

 (Kenyon 1969; Estes and Smith 1973), and is thus 

 one of the few localities where the sea otter can be 

 found in a natural balance with the rest of its 

 community. The sea otter exerts its powerful 

 influence in shallow water, where its predation on 

 diverse kinds of invertebrates is remarkably 

 efficient. In addition to drastically reducing 

 populations of motile herbivores (McLean 1962; 

 Ebert 1968; Lowry and Pearse 1973; Estes and 

 Palmisano 1974), the sea otters eat many sessile 

 animals and may release the algae from potential 

 space competition with many potentially compe- 

 titively important species such as the bivalves 

 Mytilus edulis, Modiolus modiolus, and Pododes- 

 mus macroschisma, and the barnacles Balanus 

 spp. The algal community at Amchitka Island, 

 then, offers unusual opportunities to evaluate al- 



' Scripps Institution of Oceanography, P.O. Box 1529, La Jolla, 

 CA 92037. 



gal-algal interactions in the natural absence of 

 herbivores and animal space competitors. Such 

 interactions might suggest important competitive 

 components of the algal "niches." 



The sublittoral association of perennial algae at 

 Amchitka has four separate canopies (Figure 1). 

 Alariafistulosa P. et R. is a conspicuous kelp with 

 long floating fronds that form a canopy on the 

 surface (Kibbe 1915). The thickest Alaria canopy is 

 usually found in relatively shallow (< 5 m) water. 

 The second canopy level is composed of the 

 following stipitate Laminaria species: L. 

 groenlandica Rosenvinge, L. dentigera Kjellman, 

 L. yezoensis Miyabe, and L. longipes Bory. This 

 canopy can be found from the intertidal to depths 

 of approximately 20 m. The third canopy is usually 

 composed of Agarum cribrosum Bory with short 

 stripes and large broad fronds lying prostrate on 

 the substratum. This canopy of prostrate kelp oc- 

 curs between 10 and 20 m. Finally there is a turf 

 composed of numerous species of red algae and 

 occasional clumps of green algae, especially 

 Codium ritteri Setch. et Gardn. and Cladophora 

 spp. The fact that the canopies tend to occupy 

 nonoverlapping patches in shallow (< 10 m) water 

 suggests that there are competitive interactions 

 between the species comprising the canopies. This 

 paper discusses tests of a series of hypotheses 



Manuscript accepted June 1974. 



FISHERY BULLETIN: VOL. 73, NO. 2, 1975. 



230 



