A demographic study of the elk kelp, Pelagophycus porra (Laminariales, Lessoniaceae), with notes on Pelagophycus × Macrocystis hybrids

Seasonality, survivorship and individual growth patterns of Pelagophycus porra were studied. Bi-weekly measurements were obtained from a cohort of seventy plants growing at 17–21 m depth off Santa Catalina Island, California, beginning with the undivided lamina stage and continuing until plant death or loss. Comparisons were made with a group of thirty-two co-occurring adults. Most sporophytes first appeared from May to July. Pelagophycus is an annual, as 86% of the cohort disappeared within 12 months. Stipe elongation rates of the cohorts displayed a sigmoid growth pattern with a maximum mean rate of 0.7 cm/d from July through November. Terminal stipe length was highly variable, both within and between year classes. The zone of greatest blade elongation was 10–30 cm from the blade–stipule junction. Blade growth ranged from 5.1–6.5 cm/d during July and August, and 2.7–3.2 cm/d from January through March. Rates of stipe elongation and blade growth in naturally occurring Pelagophycus × Macrocystis hybrids were similar to Pelagophycus, but longevity was lower.