Seasonal life-cycle dynamics of Macrocystis pyrifera in a chronically polluted coastal ecosystem
摘要
The giant kelp Macrocystis pyrifera is a foundational species in marine coastal ecosystems, yet its microscopic life stages are particularly vulnerable to warming and pollution. We examined its seasonal phenology over an annual cycle initiated under the 2023—2024 El Niño warming at Horcón (central Chile), a coastal site strongly influenced by long-term industrial activity. Sori were collected in austral spring and summer to winter, and in vitro cultures were used to quantify settled spores, germination, gametophyte and sporophyte development, sex ratio, fecundity, and reproductive success. Sea surface temperature (SST) data showed strong seasonal contrasts, with warm anomalies in summer-autumn linked to the coastal El Niño, followed by markedly cooler conditions in winter. Winter cultures, after El Niño waned, reached ~ 220,500 spores cm⁻2, nearly ten times higher than in spring and autumn and twenty times higher than in summer. Germination peaked in winter (96—100% by day 7) but was lower in spring and autumn (53% and 83%, respectively), with no germination further than day 7. Female gametophytes dominated across seasons (≤ 49%) until day 14 of development, while males remained consistently scarce (≤ 20%). Sporophyte development differed markedly: by day 14, autumn reached 34%, spring 2%, and winter none; by day 28, however, winter reached 46%, surpassing the other seasons, though juveniles were smaller due to density effects. The decoupling of male and female gametophytes during winter reduced fecundity and reproductive success despite high settlement. Summer cultures and field restocking collapsed early, underscoring the fragility of M. pyrifera under stress. Overall, our findings reveal pronounced seasonal shifts in reproductive dynamics, underscoring the sensitivity of kelps to warming in a coastal pollution site.