DAYTON: EXPERIMENTAL STUDIES OF ALGAL CANOPY 



plots at 9.1- and 16.8-m depths in July 1971. In both 

 cases a 100% canopy of Laminaria persisted 

 throughout the experiment. A slight recovery of 

 the Agarum population was observed the follow- 

 ing April (Table 2), but no significant differences 

 were observed in the numbers or percent cover of 

 the other species. Thus there is, at present, no 

 reason to negate the hypothesis. 



Hypothesis V 



The Agarum cribrosum canopy in the absence 

 of the Laminaria canopy has an important effect 

 on the other species of algae. This hypothesis was 

 tested by removing both Agarum and Laminaria 

 canopies from 4 m- plots at 9.1- and 16.8-m depths. 

 These clearings were then compared to those in 

 the adjacent Laminaria-on\y removal ex- 

 periments at the same depths (Figure 2C). A 

 strict interpretation of this comparison suggests 

 that either a Laminaria or Agarum canopy or 

 both is sufficient to prevent an increase of red algal 

 turf cover because there is, at those two particular 

 depths, no significant increase of red algal turf in 

 either the Laminaria-on\y or Agarum-on\y 

 removal experiments (Figure 2C, Table 2). This 

 interpretation is equivocal, however, as 

 Hypothesis III demonstrated a slight but sig- 

 nificant Laminaria effect on the red algal turf. 

 There is no equivocation regarding the effect of 

 the combined Laminaria and Agarum canopies on 

 the red algal turf which increased from 7 to 49% at 

 9.1 m and 1 to 38% at 16.8 m (Table 2). These are 

 much more dramatic increases than were observed 

 in the Laminaria-on\y removal areas and con- 

 vincingly argue for a strong Agarum effect in the 

 absence of Laminaria. Some of the red algae in 

 this experiment were Ptilota asplenoides (Esper) 

 C. Ag., Laingia aleutica Wynne, Hypophyllum 



ruprechtianum Zinova, Constantinea rosa- 

 marina (Gmelin) P. et R., Pantoneura juergensii 

 (J. Ag.) Kylin, Cirrulicarpus gmelini (Grunow) 

 Tokida et Masaki, Turnerella sp., Callophyllis 

 flabellulata Harvey, and Nienburgia prolifera 

 Wynne. 



The most impressive effect of the Agarum 

 canopy in the absence of Laminaria was its 

 inhibition of Alaria recruitment. In each of the 

 two quadrats from which both Laminaria and 

 Agarum were removed, the Alaria cover, consist- 

 ing of a heavy recruitment of juvenile plants, 

 increased from to 100% canopy cover (Table 2). 

 The Alaria response was particularly impressive 

 because the dense Alaria recruitment completely 

 filled, but was perfectly contained within, the 

 Agarum-and-Laminaria removal patches. The 

 mean density increased from to 22.8 Alaria 

 plants per 1/16 m- (± 3.5, SE). In contrast to this 

 result in the Agarum-a,nd- Laminaria removal 

 area, there was no Alaria recruitment in the 

 rather extensive area from which Laminaria 

 alone was removed (Figure 2). This result also 

 contrasts sharply with those of the shallow 

 Laminaria removal experiments (Table 1), in 

 which no Agarum canopy level existed. An ad- 

 jacent control was monitored for each experimen- 

 tal clearing; no changes were observed in any of 

 the controls. 



The above comparisons demonstrate that both 

 the secondary Laminaria canopy and the tertiary 

 Agarum canopy individually can significantly 

 reduce the recruitment of Alaria, the species 

 which forms the primary surface canopy. Further 

 evidence of the intense competition in the deeper 

 area where both understory canopies exist is 

 provided by the observation that, of 100 Alaria 

 plants surveyed, 79 were utilizing secondary sub- 

 strata with their holdfasts attached high on 

 Laminaria stipes (Figure 1). 



Table 2.— Effects of Agarum cribrosum and combined Agarum-Laminaria spp. canopies on each other, red algal turf, and Alaria 

 fistuloita at 9.1-m and 16.8-m depths in the offshore study site. Each experimental clearing area was 4 m-. The data are presented as 

 percent cover with the variance presented as the 95% confidence interval about the mean; data presented without variance are visual 

 estimates. 



iSignifies that the canopy was experimentally removed. 



235 



