Date

Spring 2025

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

The bull kelp, Nereocystis luetkeana, is the predominant canopy forming kelp along much of the northeastern Pacific. However, it remains largely understudied due to the dynamic nature of its niche coupled with its annual life history. For populations to persist, reproductive sporophytes must release substantial spores to facilitate successful sexual fertilization among male and female gametophytes that will develop into the next cohort of sporophytes. High densities of spores increase the likelihood of completing the alternation of generation life cycle characteristic of kelp genera. Yet those high densities of spores have the potential to result in a high density of sporophytes, competing for light to reach the surface canopy. It is currently unknown how density-dependence will alter the morphological development and reproductive investment of Nereocystis, thereby affecting population persistence. To address this, I investigated how density affects the morphology and reproductive traits of Nereocystis across its life history. Laboratory studies were conducted to test the effects of density on gametogenesis, gametophyte growth, female egg production, and sporophyte production. Additional studies were conducted on juvenile whole sporophytes and crowned sporophytes (i.e., modified to retain a small portion of the pneumatocyst) in land-based tumble culture to assess the effects of density on survivorship, stipe growth, pneumatocyst growth, blade growth, blade development (number), and reproductive investment (soral number, blades bearing sori, and soral size). Low spore density treatments resulted in significantly larger female gametophytes, but smaller male gametophytes. Intermediate spore densities yielded higher eggs per female, yet there were no significant differences across density treatments. Female egg production did not significantly differ among spore densities. Sporophyte density was significantly greater at higher spore densities. Juvenile sporophytes grown in tumble culture showed no significant difference in survivorship as a function of sporophyte density. High densities delayed sporophyte development compared to sporophytes grown in low density treatments, which were characterized by long stipes and numerous blades. Density- dependent effects on crowned sporophytes were similar to juvenile sporophytes with low densities promoting increased growth and development, while high densities inhibited normal development. At 4 weeks, sporophytes shifted from allocating resources from growth to reproduction. Soral (i.e., reproductive tissue) size and soral number exhibited a significant negative density-dependent relationship. At the individual scale, low density treatments displayed significantly greater reproductive investment. However, scaling reproduction to the population resulted in intermediate densities having the highest cumulative reproductive potential.

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