The Effects of Irradiance in Determining the Vertical Distribution of Elk Kelp Pelagophycus porra
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The Effects of Irradiance in Determining the Vertical Distribution of Elk Kelp Pelagophycus porra


Elk Kelp, Pelagophycus porra, is commonly observed in deep (20-30 m) water along the outer edge of Giant Kelp, Macrocystis pyrifera, beds in southern California, USA and northern Baja California, MEX, but rarely occurs in shallower water or within beds of M. pyrifera. Due to the nature of P. porra’s heteromorphic life history that alternates between a macroscopic diploid sporophyte and a microscopic haploid gametophyte, investigations of both life history stages were needed to understand P. porra’ apparent inability to encroach into Macrocystis beds along the southern California coast.

Juvenile P. porra sporophytes were transplanted (1)within the Pelagophycus zone along the offshore edge of the M. pyrifera bed at 20 m, (2) within the center of the M. pyrifera bed at 15 m and (3) along the inshore edge of the M. pyrifera bed at 8 m. Transplanted P. porra exhibited similar growth across all depths, but the onset of reproductive maturity was observed only at shallower depths. Stipe length at the onset of maturity differed between depths, increasing significantly from inshore and within the center of M. pyrifera beds to offshore within naturally occurring P. porra beds. Photosynthetic measurements of P. porra blades using PAM fluorometry indicated that although P. porra initially exhibits characteristics of a low-light adapted species (20m depth), individuals are able to photoacclimate to increasing light levels showing traits of high-light adapted species in the midwater zone and at the surface.

When the presence of P. porra propagules was increased, P. porra was unable to recruit within M. pyrifera beds. No P. porra recruits were observed near sori bags placed within M. pyrifera beds while heavy recruitment (6.5 ± 0.604 SE and 2.5 ± 0.394S E for 1m and 2m away from sori bag respectively) was observed near the sori bags within the P. porra bed. These seeding experiments indicated that a factor other than spore dispersal is limiting P. porra distribution to deeper depths.

Culture experiments were carried out in the laboratory using microscopic gametophytes and embryonic sporophytes of P. porra to investigate the effects of the higher light levels found within M. pyrifera beds. Cultures were grown under low light (2 μmol photons m-2 s-1) conditions and then moved to high light conditions (16 μmol photons m-2 s-1) at which time both embryonic sporophytes and gametophytes experienced 100% mortality. When grown under constantly higher (18 μmol photons m-2 s1 ) light conditions, the ability of P. porra to photoacclimate decreased. The vunerability of P. porra microscopic stages to higher irradiances appears to be the major limiting factor inhibiting P. porra from becoming established within Macrocystis beds and stresses the importance of a multiple life-history approach when investigating species distributions.

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