UC Santa Barbara

This thesis examined the capacity of the foundation species Macrocystis pyrifera, giant kelp, to exhibit surge uptake as a mechanism to enhance nitrogen assimilation during seasons when nitrate is depleted. The ability of M. pyrifera to rapidly take in pulses of 15N-labeled nitrate, ammonium, and urea, was assessed in the spring and summer using whole blade incubation experiments coupled with mass spectrometry analysis. The incubation experiments showed evidence of surge uptake only for ammonium, and only during the summer. However, given the ephemeral nature of potential ammonium pulses, the ecological importance of this physiological adaptation is questionable. Rather it seems more likely that a combination of uptake of ammonium and urea at ambient concentrations combined with uptake of nitrate during longer duration pulses of high concentrations (e.g., 30 – 60 minutes during internal waves), sustains giant kelp during seasons when ambient concentrations of nitrate are low.