Summer 2015

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


Morphological plasticity is common among seaweeds and such form alteration often results in the modification of other physiological processes, such as growth or reproduction. This study explored the consequences of morphological plasticity by comparing two of the common growth forms of the giant kelp Macrocystis, an ecologically important genus in nearshore temperate ecosystems. The aclonal pyrifera morphology of Macrocystis grows in deep water and reproduces via the production and release of microscopic zoospores, while the clonal integrifolia morphology grows in shallow water and reproduces primarily by vegetative growth of its rhizome. The effects of morphology on reproduction, biomass and growth were studied using laboratory and field surveys and experiments. Surveys of frond densities by depth found that Macrocystis morphology could be quantified by standardizing frond densities with the coefficient of variation. Higher coefficient of variation values indicated that fronds are significantly more clumped in deep water, indicative of the pyrifera morphology. The coefficient of variation also increased significantly with depth, stairstepping between the morphologies. Secondarily, seasonal reproductive sampling showed that the pyrifera morphology invested more in reproductive area, resulting in higher total individual reproduction, which was also true at greater depths. The year-round reproductive potential of Macrocystis was observed in this study, with reproduction varying throughout the year and peaking in October. Thirdly, the pyrifera morphology was found to have significantly higher biomass on average, while reproductive area and total reproduction correlated positively with frond biomass. Finally, clearings in the shallow integrifolia bed showed that the integrifolia morphology was unable to regenerate removed fronds during certain times of year, calling into question the storage capabilities of the Macrocystis rhizome. Macrocystis sexual recruitment was not observed into the clearings, leaving only encroachment from bordering individuals to recolonize the disturbed space, which occurred at an extremely slow rate, resulting in potential recovery times of 30 years for the small clearings and 100 years for the large clearings. The inability of Macrocystis to recruit into shallow areas suggests that the integrifolia morphology population persists primarily through the rare recruitment of single individuals that live for long periods of time, growing vegetatively and fragmenting. The findings of this thesis suggest that these two forms, though genetically identical, are variable from one another morphologically, reproductively and physiologically and that switches in morphology have physiological and biological consequences.