DAYTON: EXPERIMENTAL STUDIES OF ALGAL CANOPY 



Alaria forming a 75% canopy were removed from 

 a 25 m^ area and no significant change was ob- 

 served in the Laminaria or red algal turf canopies 

 by April 1972. But the removal of an 87% cover of 

 Laminaria produced dramatic (5-100%) increases 

 in the Alaria cover and a significant (P< 0.001) 

 increase in the red algal turf covers (<-test run on 

 data normalized with an arcsine transformation). 

 The 100% Laminaria cover in Control no. 1 suf- 

 fered heavy damage when two large boulders, 

 rolled about by winter storms, reduced Laminaria 

 densities and resulted in significant increases in 

 recruitment of Alaria and red algal turf covers 

 (P<0.01). In addition to the extremely heavy 

 Alaria recruitment in the Laminaria removal 

 areas, there were also patches of Rhodymenia 

 palmata (L.) Greville, Ptilota spp., Desmarestia 

 spp., Cymathere triplicata, Chaetomorpha 

 melagonium (Weber et Mohr) Jutz., and 

 Coilodesme spp. No significant changes were ob- 

 served in Control no. 2. To a certain extent these 

 observations could be explained by a very slow 

 growth rate of Laminaria spp. But certainly the 

 hypothesis that Alaria dominates in competition 

 over Laminaria was negated, and these data 

 strongly support the conclusion that despite the 

 expected competitive advantage gained by form- 

 ing a surface canopy, Alaria fistulosa is not a 

 competitive dominant, but a fugitive species 

 colonizing areas released from competition with 

 the dominant Laminaria canopy. 



Hypothesis II 



The rhizoidal growth pattern of Laminaria 

 longipes allows an efficient recovery following a 

 disturbance (Markham 1968). The hypothesis sug- 

 gests that the removal of an L. longipes canopy 

 results in the area being succeeded by its own 

 extensive vegetative regrowth, in contrast to the 

 invasion of many individuals of fugitive species 

 seen following the removal of a mixed species 

 canopy of Laminaria groenlandica, L. yezoensis, 

 and L. dentigera. This hypothesis was tested by 

 cutting the stipes near the holdfasts of a 100% 

 cover of L. longipes from a 10 m^ patch at a depth of 

 3 m on 7 July 1971. Fifteen V4 m" quadrats observed 

 after the 100% canopy was removed showed the 

 following mean substratum covers: 57% (± 4.9, 

 SE) L. longipes holdfasts, 7% {± 1.8, SE) sponges 

 and compound tunicates, and 22% (± 5.2, SE) 

 coralline algae, mainly Clathromorphum spp. 



They also showed mean Va m^ densities of the sea 

 urchin, Strongylocentrotus sp., of 17.5 {+_ 3.8, SE) 

 and the asteroid, Leptasterias aleutica, of 1.0 

 (±. 0.3, SE). Spores of the three other Laminaria 

 species and of Alaria were potentially available 

 from many plants on rocks on three sides of the 

 clearing. 



By April 1972, the clearing had been completely 

 recolonized by L. longipes, despite the proximity of 

 large plants of the other species. The recovery was 

 so complete that the clearing could only be recog- 

 nized after a long search located a few "land- 

 marks" (sponges, compound tunicates, and a 

 Laminaria yezoensis holdfast with the stipe cut by 

 pruning shears) photographed the previous year. 

 This strongly supports the hypothesis that the 

 rhizoidal growth pattern of L. longipes is an ef- 

 fective adaptation for the recovery of its canopy 

 following a disturbance and is in marked contrast 

 to the heavy Alaria recruitment following the 

 removal of a nearby Laminaria spp. canopy. I was 

 unable to test the obvious hypothesis that this 

 capacity for vegetative growth gives L. longipes 

 an advantage over the other Laminaria spp. in a 

 disturbed area, but loses a competitive advantage 

 in less disturbed areas because the other 

 Laminaria species have a higher, more effective 

 canopy. 



Offshore Area 



An exploratory dive was made on the deeper 

 offshore reef to investigate the relationship 

 between sea urchin densities and the various algal 

 canopies. Samples were taken from haphazardly 

 placed V4 m^ quadrats. Five samples taken in the 

 12-15 m range showed means of 44% (± 23.3, SE) 

 cover of Laminaria spp. and 62% (± 15.7, SE) 

 cover of Agarum crihrosum, and a mean density 

 of 11.2 (± 3.8, SE) sea urchins per V4 ml In the 

 15-21 m depth range five samples provided means 

 of 36% (± 13.0, SE) canopy cover of Laminaria 

 and 80% (± 4.9, SE) canopy cover of Agarum with 

 a mean sea urchin density of 6.4 (± 3.2, SE) per Vi 

 ml Few identifiable foliose algae were seen below 

 21 m, but there was a high mean sea urchin density 

 of 30.4 ( +. 3.7, SE) per V4 m-. In these deeper areas 

 there was almost a complete substratum cover of 

 the encrusting coralline algae Clathromorphum 

 spp. and the green alga, Codium ritteri. Only four 

 Alaria plants were encountered in these 17 

 samples; all were growing from the top portion of 

 one Laminaria stipe at 11 m. 



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