Energy fixed by primary producers supports the vast majority of life on earth. The giant kelp Macrocystis pyrifera is the largest marine alga in the world and supports one of the most productive ecosystems on earth. Carbon fixed by Macrocystis pyrifera on temparate rocky reefs not only provides essential habitat for an entire community of associated species, but also provides carbon subsidies to nearby ecological communities. Net primary productivity (NPP) is often used to quantify energy fixation by autotrophs, and researchers often measure NPP by summing the incremental increases in biomass and foliar losses. While the processes governing incremental increases in biomass have been well studied, the processes that drive the loss of foliar biomass are poorly understood.

The main theme of my research is the investigation of the drivers that regulate the lifespan of foliar biomass of giant kelp beds near Santa Barbara, California, USA. Tissue lifespan in marine macroalgae has not received much attention from either plant ecologists or algal ecologists, despite its potential importance to the dynamics of primary productivity. Macrocystis is an ideal species for investigations on tissue lifespan in macroalgae for two main reasons. First, it is well studied and much of its biology is already known, and second, Macrocystis is locally abundant, grows fast and turns over frequently. In addition, due to relatively benign environmental conditions for Macrocystis growth, the coastal margin along the Santa Barbara Channel is an ideal location to study intrinsic properties in the absence of extreme environmental forcing (such as frequent temperature spikes and severe storm events) that could overwhelm potentially important patterns.

I investigated drivers that regulate frond and blade lifespan in Macrocystis, and the consequences of limited frond and blade lifespans. The first chapter is a long-term, multi-site analysis of frond lifetimes and exploration of internal and external drivers that could affect frond lifetime. This study provides broad spatial and temporal scope and identifies progressive senescence as an important driver of Macrocystis biomass dynamics. The second chapter employs a more focused field study to investigate whether spatial variability in light within a kelp forests affects lifespan, size, thickness, nitrogen content and pigment content of Macrocystis pyrifera blades in ways that are predicted by theory developed to predict leaf traits of vascular plants. The last chapter is a mathematical model of the system, which I parameterized with field data, to explicitly quantify the loss of blade tissue via erosion and quantify the amount of biomass not captured in traditional surveys of net primary productivity.

I found that the natural course of progressive senescence in fronds can explain much of the variability in frond loss throughout a typical year in a Santa Barbara kelp bed, that kelp blades that have more access to light have shorter lifetimes (as predicted by leaf lifespan theory), and that ignoring the sub-lethal blade area losses can result in significant underestimates of net primary productivity. I believe that internal regulation of tissue turnover is an important mechanism by which giant kelp maximizes carbon gain in a changing environment and that adaptations that increase photosynthetic efficiency may be an important factor in the widespread success and dominance of Macrocystis..