Factors influencing recruitment and appearance of bull kelp, Nereocystis luetkeana (phylum Ochrophyta).

Abstract

The dynamics of annual species are strongly tied to their capacity for recruitment each year. We examined how competition and propagule availability influence recruitment and appearance and tracked survivorship of an annual species of marine macroalgae, the bull kelp (Nereocystis luetkeana), which serves as major biogenic habitat in the Salish Sea of Washington State. We hypothesized that (i) juvenile N.luetkeana would exhibit a seasonal appearance as a cohort in the spring and (ii) competition for space would be more limiting than propagules (spores) to recruitment at sites adjacent to established N.luetkeana beds. We tagged N.luetkeana recruits in the field to track appearance and survivorship across seasons (spring, summer, fall, and winter), using a two-factor crossed design to assess effects of competition and propagule availability on appearance of new N.luetkeana sporophytes. Survivorship of N.luetkeana recruits was low and, whereas most new individuals arose in the spring, some appeared in every season. New N.luetkeana recruits also appeared the earliest (median 8weeks vs. >20weeks) after experimental "seeding" in the spring as compared to other seasons. Eliminating macroalgal competitors ("clearing") influenced the appearance of recruits more than enhancement of propagules in the spring. An improved understanding of factors regulating the seasonal appearance of new N.luetkeana sporophytes furthers our understanding of this crucial foundation species' appearance and persistence across seasons, which is increasingly important as global ocean conditions change, and highlights the importance of studying organisms with complex life histories across multiple stages and geographical regions.