Episodic nutrient supply to a kelp forest ecosystem in Southern California

Temporal patterns of nutrient input into a Southern California kelp forest were measured using traditional hydrocast sampling coupled with high frequency temperature profiling. Patterns of nutrient input were related to growth rates of Macrocystis pyrifera located in an adjacent kelp forest. There were 2 distinct components to the pattern of nutrient availability. The long term, or seasonal, component was consistent with large-scale storm-induced mixing and horizontal advection during winter months. In addition, vertical motions of the thermocline, bringing nutrients into the kelp forest, occurred throughout the year with a frequency of about 2 per day and were strongest during the summer months. Weekly hydrocast sampling methods were inadequate for measuring these episodic events, and high frequency sampling was required to resolve the pattern of nutrient input accurately. Although measurable, nutrient input from vertical thermocline motion was inadequate to sustain maximum growth of Macrocystis pyrifera at 10 m depth during the summer months. Thus, the major component of nutrient input came during the winter. These results indicate that nitrate limitation of M. pyrifera is a likely cause of reduced summer growth. Further, high frequency sampling is necessary to predict nutrient availability in nearshore ecosystems dominated by benthic macrophytes where the pattern of nutrient input is dominated by episodic events of short duration.